Interconnecting_Cisco_Networking_Devices.pdf

Interconnecting Cisco
Networking Devices,
Part 1
Volume 1
Version 2.0
ICND1
Student Guide
Part Number: 97-3242-01

Americas Headquarters
Cisco Systems, Inc.
San Jose, CA
Asia Pacific Headquarters
Cisco Systems (USA) Pte. Ltd.
Singapore
Europe Headquarters
Cisco Systems International BV
Amsterdam,
The Netherlands
Cisco has more than 200 offices worldwide. Addresses, phone numbers, and fax numbers are listed on the Cisco Website at
www.cisco.com/go/offices.
Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To
view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third party trademarks mentioned are the property
of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other
company. (1110R)
DISCLAIMER WARRANTY: THIS CONTENT IS BEING PROVIDED “AS IS.” CISCO MAKES AND YOU RECEIVE NO
WARRANTIES IN CONNECTION WITH THE CONTENT PROVIDED HEREUNDER, EXPRESS, IMPLIED, STATUTORY OR IN
ANY OTHER PROVISION OF THIS CONTENT OR COMMUNICATION BETWEEN CISCO AND YOU. CISCO SPECIFICALLY
DISCLAIMS ALL IMPLIED WARRANTIES, INCLUDING WARRANTIES OF MERCHANTABILITY, NON-INFRINGEMENT AND
FITNESS FOR A PARTICULAR PURPOSE, OR ARISING FROM A COURSE OF DEALING, USAGE OR TRADE PRACTICE.
This learning product may contain early release content, and while Cisco believes it to be accurate, it falls subject to the
disclaimer above.
© 2013 Cisco Systems, Inc.

Welcome Students
Note Students, this letter describes important course evaluation access information.
Welcome to Cisco Systems Learning. Through the Cisco Learning Partner Program, Cisco is committed to
bringing you the highest-quality training in the industry. Cisco learning products are designed to advance
your professional goals and give you the expertise that you need to build and maintain strategic networks.
Cisco relies on customer feedback to guide business decisions; therefore, your valuable input will help
shape future Cisco course curricula, products, and training offerings. Please complete a brief Cisco online
course evaluation of your instructor and the course materials in this student kit. On the final day of class,
your instructor will provide you with a URL directing you to a short postcourse evaluation. If there is no
Internet access in the classroom, please complete the evaluation within the next 48 hours or as soon as you
can access the web.
On behalf of Cisco, thank you for choosing Cisco Learning Partners for your Internet technology training.
Sincerely,
Cisco Systems Learning

The Cisco M-Learning Test and Study App
The Cisco M-Learning Test and Study app is the ideal on-the-go study application for those preparing for
Cisco certifications.
Scan the following QR code to get the free Cisco M-Learning Test and Study app along with the 20 free
exam questions and free TCP/IP Architecture video.

Table of Contents
Course Introduction I-1
Overview I-1
Course Goal and Objectives I-2
Course Flow I-3
Your Training Curriculum I-4
Additional References I-7
Building a Simple Network 1-1
Exploring the Functions of Networking 1-3
What Is a Network? 1-4
Physical Components of a Network 1-6
Interpreting a Network Diagram 1-7
Impact of User Applications on the Network 1-8
Characteristics of a Network 1-9
Physical vs. Logical Topologies 1-11
Summary 1-13
Understanding the Host-to-Host Communications Model 1-15
Introducing Host-to-Host Communications 1-16
OSI Reference Model 1-18
TCP/IP Protocol Suite 1-20
Encapsulation and De-Encapsulation 1-21
Peer-to-Peer Communications 1-23
Summary 1-24
Introducing LANs 1-25
Local Area Networks 1-26
LAN Components 1-27
Need for Switches 1-29
Switches 1-31
Summary 1-32
Operating Cisco IOS Software 1-33
Cisco IOS Software Features and Functions 1-34
Cisco IOS CLI Functions 1-35
User EXEC Mode 1-36
Privileged EXEC Mode 1-37
Help Functions in the CLI 1-38
CLI Error Messages 1-40
Managing Cisco IOS Configurations 1-42
Improving the User Experience in the CLI 1-47
Summary 1-50
Starting a Switch 1-51
Switch Installation 1-52
Switch LED Indicators 1-53
Connecting to a Console Port 1-54

Basic Switch Configuration 1-56
Verifying the Switch Initial Startup Status 1-59
Summary 1-62
Understanding Ethernet and Switch Operation 1-63
Ethernet LAN Connection Media 1-64
Ethernet Frame Structure 1-70
MAC Addresses 1-71
Switching Operation 1-74
Duplex Communication 1-76
Configuring Duplex and Speed Options 1-78
Summary 1-81
Troubleshooting Common Switch Media Issues 1-83
Common Troubleshooting Tools 1-84
Media Issues 1-86
Troubleshooting Switch Media Issues 1-88
Port Issues 1-92
Troubleshooting Port Issues 1-95
Summary 1-97
Module Summary 1-99
Module Self-Check 1-101
Establishing Internet Connectivity 2-1
Understanding the TCP/IP Internet Layer 2-3
Internet Protocol 2-4
IPv4 Address Representation 2-6
IPv4 Header Address Fields 2-7
Decimal and Binary Systems 2-8
Decimal-to-Binary Conversion 2-9
IP Address Classes 2-11
Reserved IPv4 Addresses 2-13
Domain Name System 2-15
Verifying the IPv4 Address of a Host 2-16
Summary 2-18
Understanding IP Addressing and Subnets 2-19
Subnets 2-20
Subnet Masks 2-22
Octet Values of a Subnet Mask 2-24
Default Gateways 2-27
Computing Usable Subnetworks and Hosts 2-29
Applying Subnet Masks 2-31
Determining the Network Addressing Scheme 2-33
Example: Addressing Scheme 2-37
Variable-Length Subnet Mask 2-39
VLSM Example 2-41
Summary 2-44
ii Interconnecting Cisco Networking Devices, Part 1 © 2013 Cisco Systems, Inc.

Understanding the TCP/IP Transport Layer 2-45
TCP/IP Transport Layer Functions 2-47
Reliable vs. Best-Effort Transport 2-49
TCP vs. UDP Analogy 2-51
UDP Characteristics 2-52
TCP Characteristics 2-54
TCP/IP Applications 2-56
Summary 2-58
Exploring the Functions of Routing 2-59
Role of a Router 2-60
Router Characteristics 2-61
Router Functions 2-63
Path Determination 2-65
Routing Table 2-66
Types of Routes 2-67
Dynamic Routing Protocols 2-69
Summary 2-71
Configuring a Cisco Router 2-73
Initial Router Startup 2-74
Initial Router Setup 2-75
Configuring Router Interfaces 2-76
Configuring the Cisco Router IP Address 2-78
Verifying Interface Configuration and Status 2-79
Exploring Connected Devices 2-82
Cisco Discovery Protocol 2-83
Discovering Neighbors Using Cisco Discovery Protocol 2-84
Summary 2-86
Exploring the Packet Delivery Process 2-87
Layer 2 Addressing 2-88
Layer 3 Addressing 2-90
Address Resolution Protocol 2-92
Host-to-Host Packet Delivery 2-95
Role of a Switch in Packet Delivery 2-104
Summary 2-107
Enabling Static Routing 2-109
Routing Operations 2-110
Static and Dynamic Routing Comparison 2-112
When to Use Static Routing 2-113
Static Route Configuration 2-114
Default Routes 2-116
Static Route Configuration Verification 2-118
Summary 2-120
Managing Traffic Using ACLs 2-121
Using ACLs 2-122
ACL Operation 2-123
© 2013 Cisco Systems, Inc. Student Guide iii

ACL Wildcard Masking 2-124
Wildcard Bit Mask Abbreviations 2-127
Types of ACLs 2-128
Testing an IP Packet Against a Numbered Standard Access List 2-130
Basic Configuration of Numbered Standard IPv4 ACLs 2-131
Summary 2-133
Enabling Internet Connectivity 2-135
The Demarcation Point 2-137
Dynamic Host Configuration Protocol 2-138
Options for Configuring a Provider-Assigned IP Address 2-140
Configuring a Static Provider-Assigned IP Address 2-141
Configuring a DHCP Client 2-142
Public vs. Private IPv4 Addresses 2-143
Introducing NAT 2-145
Types of Addresses in NAT 2-147
Types of NAT 2-149
Understanding Static NAT 2-150
Configuring Static NAT 2-151
Verifying Static NAT Configuration 2-152
Understanding Dynamic NAT 2-153
Configuring Dynamic NAT 2-155
Verifying Dynamic NAT Configuration 2-156
Understanding PAT 2-157
Configuring PAT 2-159
Verifying PAT Configuration 2-160
Troubleshooting NAT 2-161
Troubleshooting NAT Case Study 2-165
Summary 2-169
Module Summary 2-171
Managing Network Device Security 3-1
Securing Administrative Access 3-3
Network Device Security Overview 3-4
Securing Access to Privileged EXEC Mode 3-5
Securing Console Access 3-7
Securing Remote Access 3-8
Enabling Remote Access Connectivity 3-11
Limiting Remote Access with ACLs 3-12
External Authentication Options 3-13
Configuring the Login Banner 3-14
Summary 3-15
Implementing Device Hardening 3-17
Securing Unused Ports 3-18
Port Security 3-20
iv Interconnecting Cisco Networking Devices, Part 1 © 2013 Cisco Systems, Inc.

Port Security Configuration 3-23
Port Security Verification 3-25
Disabling Unused Services 3-28
Network Time Protocol 3-31
Configuring NTP 3-33
Verifying NTP 3-34
Summary 3-35
Implementing Traffic Filtering with ACLs 3-37
Using ACLs to Filter Network Traffic 3-38
ACL Operation 3-39
Applying ACLs to Interfaces 3-40
The Need for Extended ACLs 3-42
Configuring Numbered, Extended IPv4 ACLs 3-44
Configuring Named ACLs 3-46
ACL Configuration Guidelines 3-48
Monitoring ACLs 3-49
Troubleshooting Common ACL Errors 3-50
Summary 3-57
Module Summary 3-59
Glossary G-1
© 2013 Cisco Systems, Inc. Student Guide v

Course Introduction
Overview
You may be asking yourself, “What do I need to know to support my network?” The answer to this question
depends on the size and complexity of the network. Regardless of its size and complexity, the starting point
for learning to support a network is the same. This course is intended to be that starting point. This course
focuses on providing the skills and knowledge necessary to implement and operate a small- to medium-
sized network.
Learner Skills and Knowledge
This subtopic lists the skills and knowledge that learners must possess to benefit fully from the course.
Learner Skills and Knowledge
Basic computer literacy
Basic PC operating system navigation skills
Basic Internet usage skills
Basic IP addressing knowledge

Course Goal and Objectives
This topic describes the course goal and objectives.
Course Goal
To provide students with the knowledge and skills necessary
to install, configure, and operate small- to medium-sized
networks.
Upon completing this course, you will be able to meet these objectives:
Describe network fundamentals and build simple LANs
Establish Internet connectivity
Manage network device security
Expand small- to medium-sized networks with WAN connectivity
Describe IPv6 basics
I-2 Interconnecting Cisco Networking Devices, Part 1 © 2013 Cisco Systems, Inc.

Course Flow
This topic presents the suggested flow of the course materials.
Course Flow
Day 1 Day 2 Day 3 Day 4 Day 5
AM Course
Introduction
Building a
Simple
Network
Establishing
Internet
Connectivity
Managing
Network
Device
Security
Building a
Medium-
Sized
Network
Introducing
IPv6
PM
LUNCH
Building a
Simple
Network
(Cont.)
Establishing
Internet
Connectivity
(Cont.)
Managing
Network
Device
Security
(Cont.)
Building a
Medium-
Sized
Network
(Cont.)
ICND1
Superlab
The schedule reflects the recommended structure for this course. This structure allows enough time for the
instructor to present the course information and for you to work through the lab activities. The exact timing
of the subject materials and labs depends on the pace of your specific class.
© 2013 Cisco Systems, Inc. Student Guide I-3

Your Training Curriculum
This topic presents the training curriculum for this course.
Additional information is available at http://learningnetwork.cisco.com.
Cisco Career Certifications
Cisco Certifications
www.cisco.com/go/certifications
You are encouraged to join the Cisco Certification Community, a discussion forum open to anyone holding
a valid Cisco Career Certification (such as Cisco CCIE®, CCNA R&S®, CCDA®, CCNP®, CCDP®, CCNP
Security®, and CCNP Voice®, and others). It provides a gathering place for Cisco certified professionals to
share questions, suggestions, and information about Cisco Career Certification programs and other
certification-related topics. For more information, visit http://www.cisco.com/go/certifications.

Cisco Career Certifications
Expand Your Professional Options, Advance Your Career.
Cisco Certified Network Associate
Recommended Training Through
Cisco Learning Partners
§ Interconnecting Cisco Networking Devices
Part 1
Part 2
Cisco Certified Entry Network
Technician
Recommended Training Through
Cisco Learning Partners
Part 1
Entry Technician
Professional
Associate
CCNA Prep Center
http://learningnetwork.cisco.com

Learner Introductions
Learner Introductions
Your name
Your company
Job responsibilities
Skills and knowledge
Brief history
Objective

Additional References
This topic presents the Cisco icons and symbols that are used in this course, as well as information on
where to find additional technical references.
Cisco Icons and Symbols
Router Workgroup Switch
Line: Serial Line: Ethernet
Multilayer Switch Home Office
Wireless Conectivity
Cisco Glossary of Terms
For additional information on Cisco terminology, refer to the Cisco Internetworking Terms and Acronyms
glossary of terms at
http://docwiki.cisco.com/wiki/Category:Internetworking_Terms_and_Acronyms_(ITA).

Module 1
Building a Simple Network
This module provides a high-level overview of basic networking components and their functions. The need
for a communication module is explained, followed by an overview of the TCP/IP protocol stack. Cisco
IOS Software is introduced, and its basic functions and features are described. Basic switch configuration is
described, with configuration examples so that learners can perform switch startup and initial configuration
in the associated lab. LANs are introduced, as well as the Ethernet standard. The operation and role of
switches within LANs is described. Finally, the module provides an overview of common switch media
issues and lists recommended troubleshooting steps.
Objectives
Upon completing this module, you will be able to meet these objectives:
Identify the components of a computer network and describe their basic characteristics
Understand the model of host-to-host communications
Describe LANs and the role of switches within LANs
Describe the features and functions of Cisco IOS Software
Install a switch and perform the initial configuration
Describe Ethernet as the network access layer of TCP/IP and describe the operation of switches
Identify and resolve common switched network issues

Lesson 1
Exploring the Functions of
Networking
Overview
Understanding the benefits of computer networks and how they function is important in maximizing
communication channels among end users. This lesson describes the concept of computer networking,
introduces the components of a computer network, and explains how users benefit from using networks.
Objectives
Upon completing this lesson, you will be able to meet these objectives:
Define a network and describe examples of networks
Identify common networking components by function
Interpret network diagrams
Describe the impact of user applications on the network
List the characteristics of a network
Compare and contrast logical and physical topologies

What Is a Network?
This topic provides examples of networks and describes their characteristics.
What Is a Network?
Network
Home Office Mobile Users
Main Office
Branch Office
A network is a connected collection of devices and end systems, such as computers and servers, which can
communicate with each other. Networks carry data in many types of environments, including homes, small
businesses, and large enterprises. Large enterprise networks may have a number of locations that need to
communicate with each other. Based on where workers are situated, these locations are as follows:
Main office: A main office is a site where everyone is connected via a network and where most
corporate information is located. A main office can have hundreds or even thousands of people who
depend on network access to do their jobs. A main office may use several connected networks that can
span many floors in an office building or cover a campus that contains several buildings.
Remote locations: A variety of remote access locations use networks to connect to the main office or
to each other.
Branch offices: In branch offices, smaller groups of people work and communicate with each
other via a network. Although some corporate information may be stored at a branch office, it is
more likely that branch offices have local network resources, such as printers, but must access
information directly from the main office.
Home offices: When individuals work from home, the location is called a home office. Home-
office workers often require on-demand connections to the main office or branch offices to access
information or to use network resources such as file servers.
Mobile users: Mobile users connect to the main office network while at the main office, at the
branch office, or traveling. The location of the mobile users determines their network access
requirements.

You may use a network in your home office to communicate via the Internet to locate information, place
orders for merchandise, and send messages to friends. You may also have a small office that is set up with a
network that connects other computers and printers in the office. Similarly, you may work in a large
enterprise with many computers, printers, storage devices, and servers that are used to communicate and
store information from many departments over large geographic areas.
© 2013 Cisco Systems, Inc. Student Guide 1-5

Physical Components of a Network
This topic describes the typical physical components of a network, including PCs, interconnections,
switches, and routers.
Physical Components of a Network
Router
h
c
t
i
w
S
h
c
t
i
w
S
PC Printer Server Desktop Laptop
There are four major categories of physical components in a computer network:
Endpoints: Computers serve as endpoints in the network. They send and receive data. Printers and
servers are also endpoints in the network.
Interconnections: The interconnections consist of components that provide a means for data to travel
from one point to another point in the network. Interconnections include components such as these:
NICs, which translate computer data into a format that can be transmitted over the local network
Network media, such as cables or wireless media, which provide the means by which signals are
transmitted between networked devices
Connectors, which provide the connection points for the media
Switches: Switches are devices that provide network attachment to the end systems and provide
intelligent switching of the data within the local network.
Routers: Routers interconnect networks and choose the best paths between networks.

Interpreting a Network Diagram
This topic describes the typical icons that represent the components of a network, including PCs, switches,
and routers.
Interpreting a Network Diagram
PC
Internet
PC
PC Server
Fa0/5
Fa0/7 Fa0/11
Gi0/1
Fa0/3
.1 .2
Gi0/0
192.168.1.0/24 Gi0/1
S0/0
S0/1
The network diagram captures network-related information. The amount of information and the detail
differs from organization to organization. A series of lines and icons commonly represents the network
topology. Some of the more common networking icons used in this diagram include the following:
Network, such as the
Internet, an access
network, a lab network, and
so on
Router Workgroup LAN switch Server
End-user desktop PC End-user laptop Ethernet link Serial link
Other information may be included in the network diagram if space allows. For example, it is common to
identify the interface on a device in the S0/0/0 format for a serial interface, Fa0/0 for a Fast Ethernet
interface, or Gi0/1 for a Gigabit Ethernet interface. It is also common to include the network address of the
segment in the 192.168.1.0/24 format. In the example shown in the figure, 192.168.1.0 indicates the
network address, /24 indicates the subnet mask, and .1 and .2 at the device ends indicate IP addresses on
interfaces (.1 corresponds to 192.168.1.1).

Impact of User Applications on the Network
Applications can affect network performance and, conversely, network performance can affect applications.
This topic describes common interactions between user applications and the network.
Impact of User Applications on the Network
Batch applications:
FTP, TFTP, inventory updates
No direct human interaction
Bandwidth important, but not critical
Interactive applications:
Inventory inquiry, database update
Human-to-machine interaction
Human waiting for response, response time
important but not critical, unless wait becomes
excessive
Real-time applications:
VoIP, video
Human-to-human interaction
End-to-end latency critical
Batch Applications
Interactive Applications
Real-Time Applications
Historically, when considering the interaction between the network and applications that ran on the
network, bandwidth was the main concern. Batch applications, such as FTP, TFTP, and inventory updates,
were initiated by a user and then run to completion by the software, with no further direct human
interaction. For batch applications, bandwidth was important but not critical, as long as the time needed for
completion was not excessive. Interactive applications, such as inventory inquiries and database updates,
required more human interaction. The user would request some type of information from the server and
then wait for a reply. Bandwidth became more important because users became impatient with slow
responses. However, because response time was more dependent on the server than on the network,
bandwidth was still not critical. In most cases, QoS features could overcome bandwidth limitations by
giving interactive applications preference over batch applications.
Like interactive applications, real-time applications such as VoIP and video applications involve human
interaction. Because of the amount of information that is transmitted, bandwidth has become critical. In
addition, because these applications are time-critical, latency (delay through the network) is critical.
Variations in the amount of latency can affect the network. Not only is sufficient bandwidth mandatory,
QoS is mandatory. VoIP and video applications must be given the highest priority.
Today, VoIP is promoted as a way for organizations to save money and is said to be as easy as installing a
VoIP router into the network. While the benefits of VoIP are easily realized in the home network, VoIP can
result in a disaster in a small-office network. Simply installing a VoIP router in a network does not ensure
sufficient bandwidth, nor does it provide a proper QoS scheme. Applications that worked correctly in the
past may begin to run so slowly that they are unusable when someone is on the phone. Additionally, voice
quality may be poor. You can overcome both of these issues, bandwidth and QoS, with good network
design.

Characteristics of a Network
This topic describes the characteristics of a network.
Characteristics of a Network
Topology
Speed
Cost
Security
Availability
Scalability
Reliability
You can describe a network according to performance and structure:
Topology: In networks, there are physical and logical topologies. The physical topology is the
arrangement of the cables, network devices, and end systems. The logical topology is the path over
which the data is transferred in a network. For example, a physical topology describes how the network
devices are actually interconnected with wires and cables. A logical topology describes how the
network devices appear connected to network users.
Speed: Speed is a measure of the data rate in bits per second of a given link in the network.
Cost: Cost indicates the general expense for the purchasing of network components and installation
and maintenance of the network.
Security: Security indicates how protected the network is, including the information that is transmitted
over the network. The subject of security is important, and techniques and practices are constantly
evolving. You should consider security whenever you take actions that affect the network.
Availability: Availability is a measure of the probability that the network will be available for use
when it is required. For networks that are meant to be used 24 hours per day, 7 days per week, 365 days
per year, availability is calculated by dividing the time that it is actually available by the total time in a
year and then multiplying by 100 to get a percentage.
For example, if a network is unavailable for 15 minutes per year because of network outages, you can
calculate its percentage availability as follows:
([Number of minutes in a year – down time] / [number of minutes in a year]) * 100 = percentage
availability
([525600 – 15] / [525600]) * 100 = 99.9971

Scalability: Scalability indicates how easily the network can accommodate more users and data
transmission requirements. If you design and optimize a network for only the current requirements, it
can be very expensive and difficult to meet new needs when the network grows.
Reliability: Reliability indicates the dependability of the components that make up the network, such
as the routers, switches, PCs, and servers. Reliability is often measured as a probability of failure or as
MTBF.
These characteristics and attributes provide a means to compare various networking solutions.

Physical vs. Logical Topologies
This topic describes the physical and logical topologies of networks.
Physical Topologies
Physical layout of the devices and cabling
Three primary categories (bus, star, and mesh)
Bus Topology Star Topology Mesh Topology
Each type of network has a physical and a logical topology.
The physical topology of a network refers to the physical layout of the devices and cabling. You must
match the appropriate physical topology to the type of cabling that you will install, such as twisted pair,
coaxial, or fiber. Understanding the type of cabling that is used is important in understanding each type of
physical topology. These are the primary categories of physical topologies:
Bus: In early bus topologies, computers and other network devices were cabled together in a line using
coaxial cable. Modern bus topologies establish the bus in a hardware device and connect the host
devices to the bus using twisted-pair wiring.
Star: A central cabling device connects the computers and other network devices. The physical
connection is commonly made using twisted-pair wiring.
Mesh: Every network device is cabled with many others. Redundant links offer reliability and self-
healing. The physical connection is commonly made using fiber or twisted-pair wiring.

Logical Topologies
Logical paths that the signals use to travel from one point on
the network to another
Router
A
Server
Router
Switch
B C
D
G
PC A to Server
Logical Link
Router
Router
Switch
E
F
Switch
The logical paths that the signals (data) use to travel between points in the network define the way in which
data accesses the network media and transmits packets across it.
The physical and logical topologies of a network can be the same. For example, in a network that is
physically shaped like a linear bus, the data travels along the length of the cable. Therefore, the network has
both a physical bus topology and a logical bus topology.
On the other hand, a network can have physical and logical topologies that are quite different. For example,
data sent from PC A to a server can take a different path from the shortest path, as indicated in the figure. It
is not always possible to predict how data travels in a network simply by observing its physical layout, so
engineers often document logical topologies as well as physical topologies.

Summary
This topic summarizes the key points that were discussed in this lesson.
Summary
A network is a connected collection of devices that can communicate
with each other.
There are four major categories of physical components in a computer
network: computers, interconnections, switches, and routers.
Icons are used to represent the components of a network in a network
diagram.
Common network user applications can be grouped into batch,
interactive, and real-time applications.
The ways in which networks can be described include characteristics
that address network performance and structure: topology, speed, cost,
security, availability, scalability, and reliability.
A physical topology describes the layout for wiring the physical devices.
A logical topology describes how information flows through a network.

Lesson 2
Understanding the Host-to-
Host Communications Model
Overview
Host-to-host communications models were created to help define how network processes function,
including the various components of networks and the transmission of data. Understanding the structure and
purpose of the most commonly used protocol stack, TCP/IP, is important for understanding how one host
communicates with another host. This lesson introduces the OSI model and describes the TCP/IP protocol
stack and its layers.
Objectives
Identify the requirements of a host-to-host communications model
Define the OSI reference model
Describe the functions of the TCP/IP layers
Describe the processes of encapsulation and de-encapsulation
Describe how peer-to-peer communications work

Introducing Host-to-Host Communications
Host-to-host communications require a consistent model. The model addresses hardware, software, and data
transmission. This topic describes the host-to-host communications model.
Introducing Host-to-Host Communications
Two different types of host-to-host models:
Older model:
Proprietary
Applications and combinations of software controlled by one vendor
Standards-based model:
Multivendor software
Layered approach
Examples: OSI, TCP/IP
The network devices that people are most familiar with are called end devices. End devices form the
interface between the human network and the underlying communications network. In the context of a
network, end devices are called hosts. A host device is either the source or the destination of a message that
is transmitted over the network. Communication begins with a message, or information, that must be sent
from one device to another device. The message then flows through the network and arrives at the end
device.
Successful communication between hosts on a network requires the interaction of many different protocols.
A protocol is a set of rules that govern communications. Networking protocols describe the functions that
occur during network communications. Protocols are implemented in the software and hardware of each
host and other devices.
Original host-to-host communications models were proprietary. Each vendor controlled its own application
and embedded communications software. An application that was written by one vendor would not function
on a network that was developed by another vendor. In the computer industry, “proprietary” is the opposite
of “open.” Proprietary means that one company or small group of companies controls all use of the
technology. Open means that use of the technology is available and is free to the public.
Business drivers and technology advances led to a multivendor solution. The first step is to separate
application software from communications software, which allows new communications technologies to be
implemented without requiring new applications. However, it still requires a single-vendor solution for
communications software and hardware.

It became apparent that a multivendor solution for communications software and hardware would require a
layered approach with clearly defined rules for interlayer interaction. Within a layered model, various
vendors provide solutions for separate layers. Hardware vendors could design hardware and software to
support emerging physical-level technologies (that is, Ethernet, Token Ring, Frame Relay, and so on).
Other vendors could write software to be used by network operating systems that control host
communications.
Examples of such standards-based models are TCP/IP and OSI.

OSI Reference Model
This topic describes the OSI reference model, which provides a means of describing how data is transmitted
over a network. The model addresses hardware, software, and data transmission.
OSI Reference Model
Physical
Application
Presentation
Session
Transport
Network
Data Link
7
6
1
2
3
4
5
To address the problem of networks being incompatible and unable to communicate with each other, the
ISO researched different network schemes. As a result of this research, the ISO created a model to serve as
a framework on which to build a suite of open systems protocols. The vision was that this set of protocols
would be used to develop an international network that would not be dependent on proprietary systems.
As a reference, the OSI model provides an extensive list of functions and services that can occur at each
layer. It also describes the interaction of each layer with the layers directly above and below it. More
importantly, the OSI model facilitates an understanding of how information travels throughout a network. It
provides vendors with a set of standards that ensures compatibility and interoperability between the various
types of network technologies that are produced by companies around the world. It is also used for data
network design, operation specifications, and troubleshooting.
The OSI reference model separates network functions into seven categories. This separation of networking
functions is called layering. The OSI reference model has seven numbered layers, each one illustrating a
particular network function.
The physical layer (Layer 1): The physical layer defines the electrical, mechanical, procedural, and
functional specifications for activating, maintaining, and deactivating the physical link for bit
transmission between end devices. Physical layer specifications are defining characteristics such as
voltage levels, timing of voltage changes, physical data rates, maximum transmission distances,
physical connectors, and other similar attributes.
The data link layer (Layer 2): The data link layer defines how data is formatted for transmission and
how access to physical media is controlled. This layer also typically includes error detection and
correction to ensure reliable delivery of the data.

The network layer (Layer 3): The network layer provides connectivity and path selection between
two host systems that may be located on geographically separated networks. The growth of the Internet
has increased the number of users that access information from sites around the world. The network
layer is the layer that manages the connectivity of these users by providing logical addressing.
The transport layer (Layer 4): The transport layer defines services to segment, transfer, and
reassemble the data for individual communications between the end devices. For example, business
users in large corporations often transfer large files from field locations to a corporate site. Reliable
delivery of the files is important, so the transport layer breaks down large files into smaller segments
that are less likely to incur transmission problems.
The session layer (Layer 5): The session layer establishes, manages, and terminates sessions between
two communicating hosts. The session layer also synchronizes dialog between the presentation layers
of the two hosts and manages their data exchange. For example, web servers have many users, so there
are many communication processes open at a given time. It is important, then, to keep track of which
user communicates on which path. In addition to session regulation, the session layer offers provisions
for efficient data transfer, CoS, and exception reporting of session layer, presentation layer, and
application layer problems.
The presentation layer (Layer 6): The presentation layer ensures that the information that is sent at
the application layer of one system is readable by the application layer of another system. For example,
a PC program communicates with another computer. One PC is using EBCDIC and the other PC is
using ASCII to represent the same characters. If necessary, the presentation layer translates among
multiple data formats by using a common format.
The application layer (Layer 7): The application layer is the OSI layer that is closest to the user. This
layer provides network services to the applications of the user, such as email, file transfer, and terminal
emulation. The application layer differs from the other layers in that it does not provide services to any
other OSI layer. It provides services only to applications outside the OSI model. The application layer
establishes the availability of intended communication partners, and it synchronizes and establishes
agreement on procedures for error recovery and control of data integrity.

TCP/IP Protocol Suite
Although OSI reference model layer names are often used, the OSI protocol stack is not the most
commonly used reference model. The TCP/IP protocol suite, which was defined at approximately the same
time as the OSI reference model, has become the most commonly used reference. Within the set of various
individual communication protocols in the TCP/IP protocol suite, the two most important protocols are TCP
and IP.
TCP/IP Protocol Suite
Physical
Application
Presentation
Session
Transport
Network
Data Link
Application
Transport
Link
Internet
TCP/IP Stack
OSI Reference Model
7
6
1
2
3
4
5
A TCP/IP protocol suite is the most popular protocol stack used in networks. It specifies end-to-end
connectivity, describing how data should be formatted, addressed, routed, and transmitted. Functions are
organized into the following four layers:
Link layer: The link layer covers the same processes as the two lower OSI layers, the data link and
physical layers. The link layer describes the physical characteristics of a link, how access is controlled,
and how data is formatted for transmission.
Internet layer: The internet layer provides routing of data from the source to the destination by
defining the packet and the addressing schemes, moving data between the link layer and transport
layers, routing packets of data to remote hosts, and performing fragmentation and reassembly of data
packets.
Transport layer: The transport layer is the core of the TCP/IP architecture. It provides communication
services directly to the application processes that are running on network hosts.
Application layer: The application layer provides applications for file transfer, network
troubleshooting, and Internet activities. It also supports network APIs, which allow programs that have
been created for a particular operating system to access the network.
Note Although this course refers to the TCP/IP stack, it has become common in the industry to shorten this
term to "IP stack."

Encapsulation and De-Encapsulation
Information that is transmitted over a network must undergo a process of conversion at the sending and
receiving ends of the communication. The conversion process is known as encapsulation and de-
encapsulation of data. This topic describes the encapsulation and de-encapsulation processes.
Data Encapsulation
Application
Transport
Internet
Network Access
L2
HDR
L3
HDR
L4
HDR
User Data FCS
L3
HDR
L4
HDR
User Data
L4
HDR
Other HDR User Data
User Data
Sender
HDR = Header
Other HDR
Other HDR
Encapsulation
The information that is sent on a network is referred to as data or data packets. As application data is passed
down the protocol stack on its way to be transmitted across the network media, various protocols add
information to it at each level. This process is commonly known as the encapsulation process. Each layer
adds a header (and a trailer, if applicable) to the data before passing it down to a lower layer. The headers
and trailers contain control information for the network devices and the receiver, to ensure proper delivery
of the data and to ensure that the receiver can correctly interpret the data.
The figure shows how encapsulation occurs. It shows how data travels through the layers. The data is
encapsulated as follows:
1. The user data is sent from an application to the application layer.
2. The transport layer adds the transport layer header (Layer 4 header) to the data. The Layer 4 header and
the previous data become the data that is passed down to the internet layer.
3. The internet layer adds the internet layer header (Layer 3 header) to the data. The Layer 3 header and
the previous data become the data that is passed down to the link layer.
4. The link layer adds the Layer 2 header and trailer to the data. A Layer 2 trailer is usually the FCS,
which is used by the receiver to detect whether the data is in error.
Encapsulation is like sending a package through a postal service. The first step is to put the contents of the
package into a container. Next, you write the address to which you want to send the package on the outside
of the container. Then you put the addressed package into the postal service collection bin, and the package
begins its route toward its destination.

Data De-Encapsulation
Application
Transport
Internet
Network Access
L2
HDR
L3
HDR
L4
HDR
User Data FCS
L3
HDR
L4
HDR
User Data
L4
HDR
Other HDR User Data
User Data
Receiver
Other HDR
Other HDR
HDR = Header
De-Encapsulation
When receiving messages on a network, the protocol stack on a host operates from the bottom to the top.
The process of encapsulation is reversed at the receiving host. The data is de-encapsulated as it moves up
the stack toward the end-user application.
When the remote device receives a sequence of bits, the data is de-encapsulated as follows:
1. The link layer checks the trailer (the FCS) to see if the data is in error. The frame may be discarded or
the link layer may ask for the data to be retransmitted.
2. If the data is not in error, the link layer reads and interprets the control information in the Layer 2
header.
3. The link layer strips the Layer 2 header and trailer and then passes the remaining data up to the internet
layer, based on the control information in the link layer header.
Each subsequent layer performs a similar de-encapsulation process.
The de-encapsulation process is like reading the address on a package to see if it is addressed to you and
then removing the contents of the package.

Peer-to-Peer Communications
Each layer of the OSI model at the source must communicate with its peer layer at the destination so that
data packets can travel from the source to the destination. This topic describes the process of peer-to-peer
communications.
Peer-to-Peer Communications
Receiver
Sender
Application
Transport
Network
Link
Application
Transport
Network
Link
Data
Segment
Packet
Frame
PDUs
During the process of peer-to-peer communication, the protocols at each layer exchange packets of
information, called PDUs, between peer layers.
These data packets originate at a source on a network and then travel to a destination. To provide a service,
each layer depends on the TCP/IP layer below it. To perform its service function, the lower layer uses
encapsulation to put the PDU from the upper layer into the lower layer data field. During encapsulation,
each succeeding layer encapsulates the PDU that it receives from the layer above, in accordance with the
protocol being used. At each stage of the process, a PDU has a different name to reflect its new appearance.
Although there is no universal naming convention for PDUs, in this course, PDUs are named according to
the protocols of the protocol suite:
Data: The general term for the PDU used at the application layer
Segment: A transport layer PDU
Packet: An internet layer PDU
Frame: A link layer PDU
The application layer protocol begins the process by delivering application data to the transport layer, which
adds the necessary header.
The internet layer moves the data through the internetwork by encapsulating the data and attaching a header
to create a packet. The header information is required to complete the transfer, and includes such
information as source and destination logical addresses.
The link layer provides a service to the network layer by encapsulating the network layer packet into a
frame. The frame header contains the physical addresses that are required to complete the data link
functions, and the frame trailer contains the FCS.

Summary
Summary
Successful communication between hosts on a network requires the
interaction of many different protocols.
The TCP/IP protocol suite, the most commonly used protocol model,
enables an understanding of how networks function.
TCP/IP organizes the functions that must occur for communications to
be successful into four layers (link, Internet, transport, and application).
As application data within the TCP/IP suite is passed down the protocol
stack on its way to be transmitted across the network media, various
protocols add information to it at each level. This process is commonly
known as the encapsulation process. The process is reversed at the
receiving host and is known as de-encapsulation.
During the process of peer-to-peer communication, the protocols at
each layer exchange packets of information called PDUs. Protocols,
which are implemented on the sending and receiving hosts, interact to
provide end-to-end delivery of applications over a network.

Lesson 3
Introducing LANs
Overview
LANs are a relatively low-cost way of sharing expensive resources. LANs allow multiple users in a
relatively small geographic area to exchange files and messages and to access shared resources such as file
servers. LANs have rapidly evolved into support systems that are critical to communications within an
organization. This lesson describes LAN components and switches and explains their roles in local
networks.
Objectives
Define a LAN
Identify the components of a LAN
Identify the need for switches in a LAN
List the characteristics and features of a switch

Local Area Networks
This topic defines LANs.
Local Area Networks
A LAN is a network of computers and other components that
are located relatively close together in a limited area.
Small Office LAN
Large Office LAN
LANs can vary widely in size. A LAN may consist of only two computers in a home office or small
business, or it may include hundreds of computers in a large corporate office or multiple buildings.
The defining characteristics of LANs, in contrast to WANs, include their typically higher data transfer rates,
smaller geographic area, and lack of a need for leased telecommunication lines.
Examples: A Small-Office LAN and a Large-Office LAN
A small home business or a small-office environment can use a small LAN to connect two or more
computers and to connect the computers to one or more shared peripheral devices, such as printers.
A large corporate office can use multiple LANs to accommodate hundreds of computers and shared
peripheral devices, spanning many floors in an office complex.

LAN Components
Every LAN has specific components, including hardware, interconnections, and software. This topic
describes the components of a LAN.
LAN Components
Computers:
PCs
Servers
Interconnections:
NICs
Media
Network devices:
Switches
Routers
Protocols:
Ethernet
IP
ARP
DHCP
Switch
PC Server
PC
Router

Regardless of its size, a LAN requires these fundamental components for its operation:
Computers: Computers serve as the endpoints in the network. They send and receive data.
Interconnections: Interconnections allow data to travel from one point to another in the network.
Interconnections include these components:
NICs: NICs translate the data that is produced by the computer into a format that can be
transmitted over the LAN.
Network media: Network media, such as cables or wireless media, transmit signals from one
device on the LAN to another.
Network devices: A LAN requires these network devices:
Ethernet switches: Ethernet switches form the aggregation point for LANs. Ethernet switches
provide intelligent distribution of frames within the LAN.
Routers: Routers, sometimes called gateways, provide a means to connect LAN segments.
Protocols: Protocols are sets of rules that govern data transmission over a LAN and include the
following:
Ethernet protocols (IEEE 802.2 and IEEE 802.3)
IP
ARP
DHCP

Need for Switches
This topic describes the need for switches.
Need for Switches
Broadcast Domain
Collision
Domain 2
Collision Domain 1 Collision Domain 3
When you connect three or more devices together, you need a dedicated network device to enable
communication between hosts.
Historically, when network devices had few Ethernet segments, end host devices had to compete for the
same bandwidth, and only one device was able to transmit data at a time. Network segments that share the
same bandwidth are known as collision domains, because when two or more devices within that segment try
to communicate at the same time, collisions may occur.
Today, it is common to use switches as network devices, operating at the link layer of the TCP/IP protocol
suite, to divide a network into segments and reduce the number of devices that compete for bandwidth.
Each new segment, then, results in a new collision domain. More bandwidth is available to the devices on a
segment, and collisions in one collision domain do not interfere with the working of the other segments.
As shown in the figure, each switch port connects to a single PC or server. Each switch port represents a
unique collision domain.
The broadcast domain is another key concept. The filtering of frames by switches, based on their MAC
addresses, does not extend to filtering broadcast frames. By their nature, broadcast frames must be
forwarded. Therefore, a port on a switch forms a single broadcast domain. It takes a Layer 3 entity, such as
a router, to terminate a Layer 2 broadcast domain.

Need for Switches (Cont.)
Switches have these functions:
Operate at the link layer of the TCP/IP protocol suite
Forward, filter, or flood frames based on MAC table entries
Have many full-duplex ports to segment a large LAN into many smaller
segments
Are fast and support various port speeds
Switch
Ethernet switches selectively forward individual frames from a receiving port to the port where the
destination node is connected. This selective forwarding process can be thought of as establishing a
momentary point-to-point connection between the transmitting and receiving nodes. The connection is
made only long enough to forward a single frame. During this instant, the two nodes have a full-bandwidth
connection between them and represent a logical point-to-point connection.
The switch builds and maintains a table, called a MAC table, that matches a destination MAC address with
the port that is used to connect to a node. For each incoming frame, the destination MAC address in the
frame header is compared to the list of addresses in the MAC table. Switches then use MAC addresses as
they decide whether to filter, forward, or flood frames.
The table shows how switches process unicast frames.
How Switches Process Unicast Frames on an Ethernet LAN
Step Action
1 When a unicast frame is received on a port, the switch compares the destination MAC address to the
MAC addresses contained in its tables.
2 If the switch determines that the destination MAC address of the frame resides on the same network
segment as the source, it does not forward the frame. This process is called filtering, and by performing
this process, switches can significantly reduce the amount of traffic going between network segments by
eliminating the unnecessary frames.
3 If the switch determines that the destination MAC address of the frame is not in the same network
segment as the source, it forwards the frame to the appropriate segment.
4 If the switch does not have an entry for the destination address, it transmits the frame out of all ports
except the port on which it received the frame. This process is called flooding.

Switches
LAN switches have special characteristics that make them effective in alleviating network congestion. This
topic describes the characteristics of switches.
Switches
LAN switch
characteristics:
High port density
Large frame buffers
Mixture of port speeds
Fast internal switching
Low per-port cost
Switches have become a fundamental part of most networks. They allow the segmentation of a LAN into
separate collision domains. Each port of the switch represents a separate collision domain and provides the
full media to the node or nodes that are connected on that port. The introduction of full-duplex
communications (a connection that can carry transmitted and received signals at the same time) has enabled
1 Gb/s Ethernet and beyond.
Switches connect LAN segments, use a table of MAC addresses to determine the segment to which the data
is to be sent, and reduce network traffic. The following are some important characteristics of switches:
High port density: Switches have high port densities: 24- and 48-port switches operate at speeds of
100 Mb/s, 1 Gb/s, and 10 Gb/s. Large enterprise switches may support hundreds of ports.
Large frame buffers: The ability to store more received frames before needing to start dropping them
is useful, particularly when there may be congested ports to servers or other parts of the network.
Port speed: Depending on the cost of a switch, it may be possible to support a mixture of speeds. Ports
of 100 Mb/s are expected, but switches offering ports supporting 1 or 10 Gb/s are more common.
Fast internal switching: Having fast internal switching allows many speeds: 100 Mb/s, 1 Gb/s, and 10
Gb/s. The method that is used may be a fast internal bus or shared memory, which affects the overall
performance of the switch.
Low per-port cost: Switches provide high port density at a lower cost. For this reason, LAN switches
can accommodate network designs featuring fewer users per segment, therefore increasing the average
available bandwidth per user.

Summary
Summary
A LAN is a network that is located in a limited area, with the computers
and other components that are part of this network located relatively
close together.
Regardless of its size, several fundamental components are required for
the operation of a LAN, including computers, interconnections, network
devices, and protocols.
Ethernet switches enable the exchange of frames between devices.
They selectively forward individual frames from a receiving port to the
destination port.
Switches support high-throughput performance, high port density, and
have low per-port cost.

Lesson 4
Operating Cisco IOS
Software
Overview
Cisco IOS Software is a feature-rich network system software that provides network intelligence to meet all
current networking demands. This lesson describes Cisco IOS Software and the basic Cisco IOS CLI
functions and operations. This lesson also describes how to navigate the Cisco IOS CLI configuration
modes, how to use embedded keyboard help, how to manage configurations, and how to use additional
Cisco IOS features to improve the user experience in the CLI.
Objectives
List the features and functions of Cisco IOS Software
Define the functions and use of the Cisco IOS CLI
Describe the user EXEC mode
Describe privileged EXEC mode
Describe CLI help functions
Explain the role of CLI error messages
Describe how to manage Cisco IOS configurations
Describe how to improve the user experience in the Cisco IOS CLI

Cisco IOS Software Features and Functions
Cisco IOS Software is the industry-leading and most widely deployed network system software. This topic
describes the features and functions of Cisco IOS Software.
Cisco IOS Software Features and Functions
Cisco IOS Software delivers networking services in Cisco
products:
Connectivity, for high-speed traffic between devices
Security, to control access and prohibit unauthorized network use
Scalability, to add interfaces and capability for network growth
Reliability, to ensure access to networked resources
Consistency, to experience among various device types
Like a computer, a switch or router cannot function without an operating system. Without an operating
system, the hardware does not have any capabilities. Cisco IOS Software is the system software in Cisco
devices. It is the core technology that extends across most of the Cisco product line. Cisco IOS Software is
used for most Cisco devices, regardless of the size and type of the device. It is used for routers, LAN
switches, small wireless access points, large routers with numerous interfaces, and many other devices.
The Cisco IOS Software operational details vary on different internetworking devices, depending on the
purpose and feature set on the device.
The services that are provided by Cisco IOS Software are generally accessed using the CLI. The CLI is
accessed through a console connection, a modem connection, or a Telnet or SSH session. Regardless of
which connection method is used, access to the Cisco IOS CLI is generally referred to as an EXEC session.
The look and feel of the CLI is similar among various device types, although features that are accessible
through the CLI vary, based on the version of Cisco IOS Software and the type of device.

Cisco IOS CLI Functions
Cisco IOS Software uses a CLI via the console as its traditional environment to enter commands. While
Cisco IOS Software is a core technology that extends across many products, the details of its operation vary
on different internetworking devices. This topic describes the functions of the Cisco IOS CLI.
Cisco IOS CLI Functions
The CLI is used to enter commands.
Operations vary on different internetworking
devices.
Users type or copy-and-paste entries in the
console command modes.
Command modes have distinctive prompts.
Pressing Enter instructs the device to parse
(translate) and execute the command.
The two primary EXEC modes are user mode
and privileged mode.
Cisco IOS Software is designed as a modal operating system. The term “modal” describes a system in
which there are various modes of operation, each having its own domain of operation. The CLI uses a
hierarchical structure for the modes.
To enter commands into the CLI, type or copy-and-paste the entries within one of the several console
command modes. Each command mode is indicated with a distinctive prompt. The term “prompt” is used
because the system is prompting you to make an entry. By default, every prompt begins with the device
name. Following the name, the remainder of the prompt indicates the mode. As commands are used and
modes are changed, the prompt changes to reflect the current context. Pressing Enter instructs the device to
parse and execute the command.
Cisco IOS Software uses a hierarchy of commands in its command-mode structure. Each command mode
supports specific Cisco IOS commands that are related to a type of operation on the device.
As a security feature, Cisco IOS Software separates EXEC sessions into two access levels:
User EXEC: Allows a person to access only a limited number of basic monitoring commands
Privileged EXEC: Allows a person to access all device commands, such as those used for
configuration and management, and can be password-protected to allow only authorized users to access
the device

User EXEC Mode
The user executive mode, or user EXEC, has limited capabilities but is useful for some basic operations.
This topic describes the user EXEC mode.
User EXEC Mode
User EXEC mode allows limited examination of a switch or a
router
Command prompt: hostname>
Switch>?
Exec commands:
access-enable Create a temporary Access-List entry
access-profile Apply user-profile to interface
clear Reset functions
connect Open a terminal connection
crypto Encryption related commands
disable Turn off privileged commands
disconnect Disconnect an existing network connection
enable Turn on privileged commands
exit Exit from the EXEC
<output omitted>
The table lists the procedure for enabling user EXEC mode on a Cisco device.
Step Action Results and Notes
1 Log into the device with a username and
password (if a login has been
configured). This action displays a user
EXEC mode prompt.
A hostname> prompt appears, signifying that you have
entered user EXEC mode.
The right arrow (>) in the prompt indicates that the device is at
the user EXEC level.
Enter exit to close the session from user EXEC mode.
2 Enter the ? command at the user EXEC
prompt to display the command options
that are available in user EXEC mode.
The ? command in user EXEC mode reveals fewer command
options than it does at privileged EXEC level. This feature is
referred to as context-sensitive help.
User EXEC mode allows only a limited number of basic monitoring commands. This mode is often referred
to as view-only mode. The user EXEC level does not allow for the execution of any commands that might
change the configuration or control the operation of the device or switch. For example, user EXEC mode
does not allow reloading of the device or switch.
By default, no authentication is required to access user EXEC mode from the console. It is a good practice
to ensure that authentication is configured during the initial configuration.

Privileged EXEC Mode
Cisco IOS Software supports two EXEC command modes: user and privileged. This topic describes the
privileged EXEC mode.
Privileged EXEC Mode
Privileged EXEC mode allows detailed examination of a switch
or a router
Enables configuration and debugging
Prerequisite for other configuration modes
Change from user EXEC mode to privileged EXEC mode using the
enable command
Privileged EXEC Mode (Cont.)
Command prompt: hostname#
Switch>enable
Switch#?
Exec commands:
access-enable Create a temporary Access-List entry
access-profile Apply user-profile to interface
access-template Create a temporary Access-List entry
archive Manage archive files
beep Blocks Extensible Exchange Protocol commands
call-home Call-Home commands
cd Change current directory
clear Reset functions
clock Manage the system clock
cns CNS agents
<output omitted>
Critical commands, such as those related to configuration and management, require that you are logged into
privileged EXEC mode.
To return to the user EXEC level, enter the disable command at the hostname# prompt.

Help Functions in the CLI
Cisco devices use Cisco IOS Software with extensive command-line input help functions, including
context-sensitive help. This topic describes the CLI keyboard help that is available on Cisco IOS devices.
Help Functions in the CLI
Type of CLI Help Description
Context-sensitive
help
Provides a list of commands and the arguments that are
associated with a specific command
Console error
messages
Identifies problems with commands that are incorrectly
entered so that they can be altered or corrected
The Cisco IOS CLI on Cisco devices offers two types of context-sensitive help:
Word help: Enter the character sequence, followed immediately by a question mark. Do not include a
space before the question mark. The device will display a list of commands that start with the
characters that you entered. For example, enter the sh? command to get a list of commands that begin
with the character sequence “sh”.
Command syntax help: Enter the ? command to get help on the syntax needed to complete a
command. Enter the question mark in place of a keyword or argument, following the command name.
Include a space before the question mark. The network device will then display a list of available
command options, with <cr> standing for carriage return. For example, enter show ? to get a list of the
command options available for the show command.
Note Cisco routers and Cisco Catalyst switches have similar command-line help functions. All of the help
functions that are mentioned in this section for devices also apply to Catalyst switches, unless otherwise
stated.
When you submit a command by pressing Enter, the command-line interpreter parses the command from
left to right to determine which action is being requested. If the interpreter understands the command, the
requested action is executed and the CLI returns to the appropriate prompt. However, if the interpreter
cannot understand the command being entered, it provides feedback describing what is wrong with the
command.

Help Functions in the CLI (Cont.)
This sequence of commands shows how CLI context-sensitive
help can be used:
SwitchX#cl?
clear clock
SwitchX#clock ?
set Set the time and date
SwitchX#clock set ?
hh:mm:ss Current time
SwitchX#clock set 19:50:00 ?
<1-31> Day of the month
MONTH Month of the year
SwitchX#clock set 19:50:00 25 June ?
<1993-2035> Year
SwitchX#clock set 19:50:00 25 June 2012
SwitchX#
Context-sensitive help provides a list of commands and the arguments that are associated with those
commands within the context of the current mode. To access context-sensitive help, enter a question mark
(?) at any prompt. There is an immediate response. You do not need to press Enter.
One use of context-sensitive help is to get a list of available commands. This list can be used when you are
unsure of the name for a command or you want to see if Cisco IOS Software supports a particular command
in a particular mode.
For example, to list the commands available at the user EXEC level, enter ? at the prompt.
Another use of context-sensitive help is to display a list of commands or keywords that start with a specific
character or characters. Enter a character sequence, then immediately enter a question mark (without a
space before it). Cisco IOS Software displays a list of commands or keywords for this context that start with
the characters that you entered.
For example, enter sh? to get a list of commands that begin with the character sequence “sh”.
If the word “clock” is misspelled as it is entered, the system performs a symbolic translation of the
misspelled command as parsed by Cisco IOS Software. If no CLI command matches the string that was
entered, an error message is returned. Furthermore, if there is no Cisco IOS Software command that begins
with the letters entered, the device interprets the command as a host name and attempts to resolve the host
name to an IP address. Then the device tries to establish a Telnet session to that host.
A final type of context-sensitive help is used to determine which options, keywords, or arguments are
matched with a specific command. Enter a command, followed by a space and then a question mark, to
determine what can or should be entered next.
As shown in the example, after entering the command clock set 19:50:00, you can enter a space and then a
question mark to determine the options or keywords that are available with this command.

CLI Error Messages
This topic describes CLI error messages.
CLI Error Messages
SwitchX#c
% Ambiguous command:'c'
Not enough characters were entered.
SwitchX#clock set
% Incomplete command
SwitchX#clock set 19:50:00
% Incomplete command
Required arguments or keywords were left off the end of the command.
SwitchX#>clock set 19:50:00 25 6
^
% Invalid input detected at "^" marker
The caret (^) indicates where the command interpreter cannot decipher the command.
There are three types of console error messages:
Ambiguous command
Incomplete command
Incorrect command
Error Message Meaning How to Get Help
% Ambiguous
command: “show
con”
You did not enter enough
characters for your device to
recognize the command.
Re-enter the command, followed by ? without a space
before it.
The possible keywords that you can enter with the
command are displayed.
% Incomplete
command
You did not enter all the keywords
or values that are required by this
command.
Re-enter the command, followed by ? with a space
before it.
% Invalid input
detected at ‘^’
marker
You entered the command
incorrectly. The ^ marks the point
of the error.
Enter ? to display all of the commands or parameters
that are available.
The command history buffer stores the commands that have been most recently entered. To see these
commands, enter the Cisco IOS show history command.
You can use context-sensitive help to determine the syntax of a particular command. For example, if the
device clock needs to be set but you are not sure of the clock command syntax, the context-sensitive help
provides a means to check the syntax.
Context-sensitive help supplies the whole command even if you enter just the first part of the command,
such as cl?.

If you enter the command clock but an error message is displayed, indicating that the command is
incomplete, enter the ? command (preceded by a space) to determine which arguments are required for the
command. In the clock ? example, the help output shows that the keyword set is required after clock.
If you now enter the command clock set but another error message appears, indicating that the command is
still incomplete, press the Up arrow to repeat the command entry. Then, add a space and enter the ?
command to display a list of arguments that are available for the command.
The example shows that after the last command recall, the administrator used the ? command to reveal the
additional arguments, which involve entering the current time using hours, minutes, and seconds.
The figure continues to illustrate how to set the device clock.
If, after entering the current time, you still see the Cisco IOS Software error message indicating that the
command entered is incomplete, recall the command, add a space, and enter the ? command to display a list
of arguments that are available for the command. In this example, enter the day, month, and year using the
correct syntax and then press Enter to execute the command.
Syntax checking uses the caret symbol (^) as an error-location indicator. It appears at the point in the
command string where an incorrect command, keyword, or argument has been entered. The error-location
indicator and interactive help system provide a way to easily find and correct syntax errors. In the clock
example, the caret symbol indicates that the month was entered incorrectly as a number. The parser is
expecting the month to be spelled out.

Managing Cisco IOS Configurations
This topic describes various Cisco IOS Software configurations and how to save, back up, and load
configurations.
Managing Cisco IOS Configurations
There are two general types of configuration:
Running configuration: This state reflects the current configuration of
the device.
Startup configuration: This file is used to load the saved configuration
after powering up. If no configuration is present, enter setup mode or
load a blank configuration.
Running Configuration:
RAM
Configuration Configuration
Startup Configuration:
NVRAM
Cisco switches and routers have three primary types of memory:
RAM: Stores routing tables (router), fast-switching cache, running configuration, and so on
NVRAM: Used for writable permanent storage of the startup configuration
Flash: Provides permanent storage of the Cisco IOS Software image, backup configurations, and any
other files via memory cards
After the Cisco IOS Software image is loaded and started, the router or switch must be configured to be
useful. If there is a saved configuration file (startup configuration) in NVRAM, it is executed. If there is no
saved configuration file in NVRAM, the router or switch enters the setup utility. The setup utility prompts
you at the console for specific configuration information to create a basic initial configuration on the router
or switch.
You can also interrupt the setup utility and start configuring a router or switch by manually entering all the
needed commands. After you enter and execute a command, the command is stored in the running
configuration. The running configuration reflects the current configuration of the device. It is stored in
RAM and is therefore deleted at reboot. You have to copy the running configuration to the startup
configuration to preserve the configuration across a router or switch reboot.

Managing Cisco IOS Configurations (Cont.)
This is how you investigate the current running configuration (RAM):
Switch#show running-config
Building configuration...
Current configuration : 1707 bytes
!
! Last configuration change at 04:45:43 UTC Fri Aug 17 2012
! NVRAM config last updated at 04:45:43 UTC Fri Aug 17 2012
!
version 15.0
no service pad
service timestamps debug datetime msec
<output omitted>
Managing Cisco IOS Configuration (Cont.)
This is how you investigate the saved configuration (NVRAM):
Switch#show startup-config
Using 1707 out of 65536 bytes
!
!
version 15.0
no service pad
service timestamps debug datetime msec
<output omitted>
The show running-config command displays the current running configuration in RAM. The “Building
configuration…” output indicates that the running configuration is being built from the active
configurations stored in RAM.
After the running configuration is built from RAM, the “Current configuration: 1707 bytes” message
appears, indicating that this is the current configuration running in RAM and the size of the current running
configuration, in bytes.

The show running-config command also has various command options to filter the running configurations
that will be displayed. For example, you can use the show running-config interface GigabitEthernet0/1
command to display only the running configuration of interface GigabitEthernet0/1.
The show startup-config command displays the saved configuration in NVRAM.
The first line of the command output indicates the amount of NVRAM used to store the configuration. For
example, “Using 1707 out of 65536 bytes” indicates that the total size of the NVRAM is 65536 bytes and
the current configuration stored in NVRAM takes up 1707 bytes.
A configuration can be backed up to an external server:
External Server
RAM
Configuration Configuration
NVRAM
tftp
ftp
scp
http
…
You can copy configuration files from the router or switch to a file server using FTP, SCP, HTTP, TFTP,
and others. For example, you can copy configuration files to back up a current configuration file to a server
before changing its contents, thereby allowing the original configuration file to be restored from the server.
The protocol that is used depends on which type of server is used.
You can copy configuration files of the router or switch from an external server to the running configuration
in RAM or to the startup configuration file in NVRAM for one of the following reasons:
To restore a backed-up configuration file.
To use the configuration file for another router or switch. For example, you may add another router or
switch to the network and want it to have a configuration similar to the original router or switch. By
copying the file to the network server and making the changes to reflect the configuration requirements
of the new router or switch, you can save time.
To load the same configuration commands onto all the routers or switches in the network so that all the
routers or switches have similar configurations.

Configuration
NVRAM
TFTP Server
copy running startup
copy startup running ( merge)
configure terminal ( merge)
RAM
copy tftp: running ( merge)
copy run tftp:
copy tftp: start
copy start tftp: Blank
erase start
ge)
Configuration
In addition to using the setup utility or the CLI to load or create a configuration, there are several other
configurations that you can use.
You can use the Cisco IOS copy command to move configurations from one component or device to
another. The syntax of the copy command requires that the first argument indicate the source (where the
configuration is to be copied from), followed by the destination (where the configuration is to be copied to).
For example, in the copy running-config tftp: command, the running configuration in RAM is copied to a
TFTP server.
Use the copy running-config startup-config command after a configuration change is made in the RAM
that must be saved to the startup configuration file in NVRAM. Similarly, copy the startup configuration
file in NVRAM back into RAM with the copy startup-config running-config command. Notice that you
can abbreviate the commands.
Similar commands exist for copying between a TFTP server and either NVRAM or RAM.
Use the configure terminal command to interactively create configurations in RAM from the console or
remote terminal.
Use the erase startup-config command to delete the saved startup configuration file in NVRAM.
When a configuration is copied into RAM from any source, the configuration merges with the existing
configuration in RAM. New configuration parameters are added, and changes to existing parameters
overwrite the old parameters. Configuration commands that exist in RAM for which there is no
corresponding command in NVRAM remain unaffected. Copying the running configuration from RAM into
the startup configuration file in NVRAM overwrites the startup configuration file in NVRAM.

This is how you can save a device configuration:
Switch#copy running-config startup-config
Destination filename [startup-config]?
[OK]
This is how you back up a configuration to the TFTP server:
Switch#copy running-config tftp:
Address or name of remote host []? 10.1.1.1
Destination filename [running-config]? config.cfg
!!!
1684 bytes copied in 13.300 secs (129 bytes/sec)
After the commands to configure the router or switch have been entered, you must save the running
configuration to NVRAM with the copy running-config startup-config command. If the configuration is
not saved to NVRAM and the router or switch is reloaded, the configuration will be lost and the router or
switch will revert to the last configuration saved in NVRAM.
You can use the TFTP servers to store configurations in a central place, allowing centralized management
and updating. Regardless of the size of the network, there should always be a copy of the current running
configuration online as a backup.
The copy running-config tftp: command allows the current configuration to be uploaded and saved to a
TFTP server. The IP address or name of the TFTP server and the destination filename must be supplied.
During the copying process, a series of exclamation marks shows the progress of the upload.
The copy tftp: running-config command downloads a configuration file from the TFTP server to the
running configuration of the RAM. Again, the address or name of the TFTP server and the source and
destination filenames must be supplied. In this case, because you are copying the file to the running
configuration, the destination filename should be running-config. This is a merge process, not an overwrite
process.

Improving the User Experience in the CLI
The Cisco IOS Software CLI includes many features that make the configuration process easier and faster.
This topic describes Cisco IOS CLI features that enhance a configuration process.
Command-
Line Editing
Key
Sequence
Description
Tab Completes the remainder of the command or keyword
Ctrl-A Moves the cursor to the beginning of the command line
Ctrl-E Moves the cursor to the end of the command line
Backspace Removes one character to the left of the cursor
Ctrl-U Erases a line
Ctrl-Shift-6 Allows the user to abort a Cisco IOS process such as ping or
traceroute
Ctrl-C Aborts the current command and exits the configuration mode
Ctrl-Z Ends configuration mode and returns to the EXEC prompt
The key sequences indicated in the figure are shortcuts or hot keys that are provided by the CLI. Use these
key sequences to move the cursor around on the command line for corrections or changes and to make
configuring, monitoring, and troubleshooting easier.
(Cont.)
Switch#terminal history size 50
Sets session command buffer size to 50 lines
Switch#show history
enable
terminal history size 50
show history
Displays the contents of the command buffer

With the command history feature, you can complete these tasks:
Display the contents of the command buffer.
Set the command history buffer size.
Recall previously entered commands that are stored in the history buffer. There is a buffer for EXEC
mode and another buffer for configuration mode.
By default, command history is enabled, and the system records the last 10 command lines in its history
buffer.
To change the number of command lines that the system will record during the current terminal session
only, use the terminal history command in user EXEC mode.
To recall older commands in the history buffer, press Ctrl-P or the Up Arrow key. The command output
will begin with the most recent command. Repeat the key sequence to recall successively older commands.
To return to more recent commands in the history buffer (after recalling older commands with Ctrl-P or the
Up Arrow key), press Ctrl-N or the Down Arrow key. Repeat the key sequence to successively recall
more recent commands.
On most computers, there are additional select and copy functions available. Copy a previous command
string, and then paste or insert it as the current command entry and press Enter.
(Cont.)
Switch#show running-config
Current configuration : 1707 bytes
!
!
<output omitted>
!
--More--
The Cisco IOS CLI pauses after a specific number of lines is displayed.
Switch#terminal length 100
Sets the number of lines on the current terminal screen
When you use show commands (for example, show running-config), Cisco IOS Software automatically
pauses its display of the output after a specified number of lines and displays "--More--" text. It waits for
user input to continue with the display. You must press Spacebar to display a set of subsequent lines, or
press Enter to display a single line.
You can use the terminal length command followed by a number to control the number of lines that are
displayed without pause. A value of zero prevents the router from pausing between screens of output. By
default, the value is set to 24.

(Cont.)
You can filter show outputs using the pipe (|) character and a
filtering parameter.
Use the include parameter to display configuration commands that include a
specific word:
Switch#show running-config | include hostname
hostname Switch
Use the section parameter to display a section of the configuration:
Switch#show running-config | section FastEthernet0/11
interface FastEthernet0/11
switchport access vlan 100
switchport mode access
switchport port-security
Another useful feature that improves the user experience in the CLI is filtering of show outputs. Using
filtering, you can display only the parts of show outputs that you want to display. You can filter outputs by
providing the pipe (|) character after a show command, followed by a filtering parameter and a filtering
expression.
The table describes filtering parameters that are available for output filtering:
Parameter Description
begin Shows all the output lines from a certain point, starting
with the line that matches the filtering expression
exclude Excludes all output lines that match the filtering
expression
include Includes all output lines that match the filtering
expression
section Shows entire section that starts with the filtering
expression
Note You can use output filters in combination with any show command. The show running-config
command in the example is used for the sake of simplicity only.

Summary
Summary
Cisco IOS Software provides network services to Cisco products to
perform various internetworking functions.
The Cisco IOS CLI uses a hierarchy of commands in its command-mode
structure.
Two basic configuration modes are user EXEC mode and privileged
EXEC mode.
Context-sensitive help and console error messages are available in the
Cisco IOS CLI to help you configure Cisco devices.
Two general configurations that are used by Cisco routers and switches
are the running configuration and the startup configuration.
You can use hot keys and shortcuts, command history, and output filters
to improve the CLI user experience.

Lesson 5
Starting a Switch
Overview
Before you start a Cisco Catalyst switch, the physical installation must meet operational conditions. After
the switch is turned on and startup is complete, you can configure the initial software settings. Verifying
that the switch startup has been completed without error is the first step in deploying a Catalyst switch. The
switch must start successfully and have a default configuration to operate on the network. This lesson
describes switch installation and how to verify the initial operation and configuration.
Objectives
Identify physical installation requirements
Identify the conditions that are reflected by the LEDs on switches
Connect to a switch console port
Configure a Cisco IOS switch from the CLI
Verify initial switch operation

Switch Installation
Before you physically install a Catalyst switch, you must have the correct power and operating
environment. When the cables are correctly connected, the switch can be powered up. This topic describes
the physical installation and startup of a Catalyst switch.
Switch Installation
Before installing a switch, verify the power and cooling requirements.
Physically install the switch:
Rack mount
Wall mount
Table or shelf mount
Verify the network cabling.
Attach the power cable plug to start the switch.
System startup routines perform POST and initiate the switch software.
Physical installation and startup of a Catalyst switch requires completion of these steps:
1. Before performing physical installation, verify the following:
Switch power requirements
Switch operating environment requirements (operational temperature and humidity)
2. Use the appropriate installation procedures for rack mounting, wall mounting, or table or shelf
mounting.
3. Before starting the switch, verify that network cable connections are secure.
4. Attach the power cable plug to the power supply socket of the switch. The switch will start. Some
Catalyst switches, including the Cisco Catalyst 2960 Series, do not have power buttons.
5. Observe the boot sequence:
When the switch is on, POST begins. During POST, the switch LED indicators blink while a series
of tests determine that the switch is functioning properly.
The Cisco IOS Software output text is displayed on the console.
When all startup procedures are finished, the switch is ready to configure.

Switch LED Indicators
Cisco Catalyst switches have several status LEDs that are generally lit in green when the switch is
functioning normally. The LEDs are lit in amber when there is a malfunction. This topic describes the states
of the LEDs on Catalyst 2960 Series Switches.
Switch LED Indicators
1—System LED
2—Remote Power
Supply LED
3–6—Port Mode LEDs
7—Mode Button
8—Port Status LED
The front panel of a switch has several lights, called LEDs, to help monitor system activity and
performance. LED indicators provide only a quick overview of the status of the switch. For a more detailed
view, you can use Cisco IOS commands.
The front of the switch features these LEDs:
System LED: Indicates whether the system is receiving power and functioning correctly.
Remote Power Supply LED: Indicates whether the remote power supply is in use.
Port Mode LEDs: Indicate the current state of the Mode button, which is used to switch between
various modes. The modes are used to determine how the Port Status LEDs are interpreted.
Port Status LEDs: Have various meanings, depending on the current value of the Mode LED.
For more details about LED indicators and their functions, refer to the Cisco Switch Hardware Installation
Guide. It is recommended that you refer to the model-specific documentation. The reference for the Catalyst
2960 switches can be found at http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/hardware/
installation/guide/higover.html#wp1021241.

Connecting to a Console Port
Upon initial installation, you can configure the switch from a PC that is connected directly through the
console port on the switch. This topic describes how to connect to the console port and log in to a Cisco
Catalyst switch to begin the initial configuration.
Connecting to a Console Port
Console port
Console cable
USB-to-serial port adapter
To perform initial switch configuration, connect to the switch through the console port.
This equipment is needed to access a Cisco device through the console port:
RJ-45-to-DB-9 console cable
PC or equivalent with serial port and communications software, such as HyperTerminal, configured
with these settings:
Speed: 9600 b/s
Data bits: 8
Parity: None
Stop bit: 1
Flow control: None
Modern computers and notebooks rarely include built-in serial ports. Often a USB-to-RS-232-compatible
serial port adapter is used instead.
On newer Cisco network devices, a USB serial console connection is also supported. A suitable USB cable
(USB Type A-to-5-pin mini Type B) and operating system device driver are needed to establish
connectivity.
Note Only one console port can be active at a time. When a cable is plugged into the USB console port, the
RJ-45 port becomes inactive. When the USB cable is removed from the USB port, the RJ-45 port
becomes active.

When a console connection is established, you gain access to user EXEC mode, by default. To start
configuration, you must enter privileged EXEC mode by using the enable command.

Basic Switch Configuration
Cisco IOS Software on a Catalyst switch has various configuration modes, including global configuration
mode and interface configuration mode. This topic describes how to complete the initial switch
configuration using both modes.
Basic Switch Configuration
Configuration modes:
Global configuration mode
Switch#configure terminal
Switch(config)#
Interface configuration mode
Switch(config)#interface FastEthernet0/1
Switch(config-if)#
Cisco IOS Software CLI modes are hierarchically structured.
When you are in privileged EXEC mode, you can access other configuration modes. Each mode is used to
accomplish particular tasks and has a specific set of commands that are available in that mode. For example,
to configure a switch interface, you must be in interface configuration mode. All configurations that are
entered in interface configuration mode apply only to that interface.
To begin, you will explore how to navigate two frequently used configuration modes: global configuration
mode and interface configuration mode.
The example starts with the switch in privileged EXEC mode. To configure global switch parameters, such
as the switch hostname or the switch IP address, which are used for switch management, use global
configuration mode. To access global configuration mode, enter the configure terminal command in
privileged EXEC mode:
Switch#configure terminal
After the command is executed, the prompt changes to show that the router is in global configuration mode.
Switch(config)#
Configuring interface-specific parameters is a common task. To access interface configuration mode for the
Fast Ethernet 0/1 interface from global configuration mode, enter the interface FastEthernet 0/1
command:
Switch(config)#interface FastEthernet 0/1
Or, you can use a shorter version of the same command:
Switch(config)#interface fa 0/1

The prompt changes:
Switch(config-if)#
To exit interface configuration mode, use the exit command.
Sometimes, you need to execute commands on multiple subinterfaces at the same time. Use the interface
range command in global configuration mode for this task:
Switch(config)#interface range FastEthernet 0/1 - 24
Switch(config-if-range)#
This example shows how to configure interfaces FastEthernet 0/1 through 0/24, all at the same time.
Basic Switch Configuration (Cont.)
Set the local identity for the switch.
Switch(config)#hostname SwitchX
SwitchX(config)#
Naming the switch enables you to better manage the network by being able to uniquely identify each switch
within the network. The name of the switch is considered to be the hostname and is the name that is
displayed at the system prompt. The switch name is assigned in global configuration mode. In the example
that is shown in the figure, the switch name is set to "SwitchX" with the use of the hostname command. If
the hostname is not explicitly configured, a switch has the factory-assigned default hostname of "Switch."

Basic Switch Configuration (Cont.)
Assign the IP address and subnet mask for the switch.
SwitchX(config)#interface vlan 1
SwitchX(config-if)#ip address 172.20.137.5 255.255.255.0
SwitchX(config-if)#no shutdown
PC SwitchX
Console
Note: Most software versions require use of the no shutdown command to
make the interface operational.
To manage a switch remotely, you need to assign an IP address to the switch. In this example, you want to
manage SwitchX from a PC, a computer that is used for managing the network, so you need to assign an IP
address to SwitchX. This IP address is assigned to a virtual interface called a VLAN. The default
configuration on the switch is to have switch management controlled through VLAN 1. Therefore, the IP
address in the example is assigned to VLAN 1.
To configure an IP address and subnet mask for the switch, you must be in VLAN 1 interface configuration
mode and then use the ip address configuration command.
You must use the no shutdown interface configuration command to make the interface operational.
The operational VLAN interface with the assigned IP address can be used, for example, in a remote
management Telnet connection, or, if you use SNMP, to manage the switch.
After you enter the initial commands to configure the switch, you must save the running configuration to
NVRAM with the copy running-config startup-config command. If the configuration is not saved to
NVRAM and the switch is reloaded, the configuration is lost and the router reverts to the last configuration
saved in NVRAM.

Verifying the Switch Initial Startup Status
After you log in to a Catalyst switch, you can verify the switch software and hardware status using
verification commands executed from privileged EXEC mode. This topic describes the switch show
commands that can be used to verify the initial switch operation.
SwitchX#show version
Cisco IOS Software, C2960 Software (C2960-LANBASEK9-M), Version 15.0(1)SE3,
RELEASE SOFTWARE (fc1)
<output omitted>
SwitchX uptime is 15 hours, 30 minutes
System returned to ROM by power-on
System restarted at 15:06:49 UTC Tue Aug 21 2012
System image file is "flash:/c2960-lanbasek9-mz.150-1.SE3/c2960-lanbasek9-mz.
150-1.SE3.bin"
<output omitted>
cisco WS-C2960-24TT-L (PowerPC405) processor (revision D0) with
65536K bytes of memory.
Processor board ID FOC1141Z8YW
<output omitted>
Displays the configuration of the system hardware, software version, and boot images
Use the show version EXEC command to display the configuration of the system hardware and the
software version information. The table describes some of the output fields of the show version command.
Output Description
Cisco IOS Software
Release
Identification of the software by name and release number
Always specify the complete release number when reporting a possible software
problem. In this example, the switch is running Cisco IOS Release 15.0(1)SE3.
Switch uptime Current days and time since the system was last booted
In this example, the switch uptime is 15 hours and 30 minutes.
System image file is Show location and file name of used system image
Switch platform Hardware platform information, including revision and amount of RAM
Processor board ID Device serial number
The information in this figure is from a Cisco Catalyst WS-C2960-24 switch with 24 Fast Ethernet ports.

(Cont.)
SwitchX#show interfaces FastEthernet 0/1
FastEthernet0/1 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 001e.147c.bd01 (bia 001e.147c.bd01)
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, media type is 10/100BaseTX
<output omitted>
5 minute input rate 31000 bits/sec, 33 packets/sec
<output omitted>
--More--
Displays statistics for selected interfaces on the switch
The show interfaces command displays status and statistical information for the network interfaces of the
switch. The resulting output varies, depending on the network for which an interface has been configured.
Usually, this command is entered with the options type and slot/number. The type option allows values such
as FastEthernet and GigabitEthernet, and the slot/number option indicates slot 0 and the port number on the
selected interface (for example, fa0/1). The table shows some of the fields in the display that are useful for
verifying fundamental switch details.
Output Description
FastEthernet0/1 is up Indicates the status of the interface hardware. In this example, it is functioning
correctly. The hardware status is followed by the status of the line protocol, which in
this example is also operational and active.
Full-duplex, 100 Mb/s Shows the type and mode of connection. Other possibilities include half-duplex, 10
Mb/s.
5 minute input rate 31000
bits/sec
Reports interface traffic statistics for average input rate.
The show interfaces command is frequently used when you are configuring and monitoring network
devices.

(Cont.)
SwitchX#show running-config
<output omitted>
!
hostname SwitchX
!
<output omitted>
!
interface Vlan1
ip address 172.20.137.5 255.255.255.0
!
ip default-gateway 172.20.137.1
!
<output omitted>
Displays the current active configuration file of the switch
The show running-config command displays the current running (active) configuration file of the switch.
This command requires privileged EXEC mode access. The IP address, subnet mask, and default gateway
settings are displayed here.

Summary
Summary
Physical switch installation must meet power and environmental
requirements.
Cisco IOS switches have several status LEDs that are generally green
when the switch is functioning normally but turn amber when there is a
malfunction.
A console cable and port are needed to access the switch console and
perform the initial configuration.
Cisco IOS switches can be configured in the CLI using the global
configuration mode and other configuration modes.
You can verify switch hardware, software, and operational status using
the show version, show interfaces, and show running-config
commands.

Lesson 6
Understanding Ethernet and
Switch Operation
Overview
This lesson describes various Ethernet media options (copper and fiber), including the most common
connectors and cable types. Ethernet frame structure and MAC addresses are discussed. In the second part
of the lesson, switch operation, or switch frame processing, is described. Duplex options are described,
collision domains are explained, and configuration examples for duplex settings are provided.
Objectives
Describe the types of Ethernet LAN connection media
Describe the fields of an Ethernet frame
Define the structure and function of MAC addresses
Describe how switches operate
Compare half-duplex and full-duplex operation
Configure speed and duplex on an interface

Ethernet LAN Connection Media
This topic defines the types of Ethernet LAN connection media.
Ethernet LAN Connection Media
The mechanical properties of Ethernet depend on the type of
physical medium:
Coaxial (not used anymore)
Copper
Fiber-optic
Ethernet was originally based on the concept of computers
communicating over a shared coaxial cable, sharing files and
applications.
The mechanical properties of Ethernet depend on the type of physical medium in use, with coaxial, copper,
fiber, and wireless media available. Although wireless is increasing in popularity for desktop connectivity,
copper and fiber are the most popular physical layer media for network deployments.
Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as
a broadcast transmission medium. Originally, the shared coaxial cable of an Ethernet (the shared medium)
connected every attached machine in a building or campus. A scheme known as CSMA/CD governed the
way in which the computers shared the channel.
Through the first half of the 1980s, the 10BASE5 implementation of Ethernet used a coaxial cable with a
diameter of 9.5 mm (0.375 inches), later called “thick Ethernet” or “thicknet.” Its successor, 10BASE2, also
called “thin Ethernet” or “thinnet,” used a cable similar to the television cable of the era. The emphasis was
on making installation of the cable easier and less costly. Modifying Ethernet to conform to the twisted-pair
telephone wiring already installed in commercial buildings provided another opportunity to lower costs,
expand the installed base, and leverage building design. Thus, twisted-pair Ethernet was the next logical
development in the mid-1980s, beginning with StarLAN. UTP-based Ethernet became widely deployed
with the 10BASE-T standard. This system replaced coaxial cable systems with a system of full-duplex
switches linked via UTP.

Ethernet LAN Connection Media (Cont.)
Modifying Ethernet to conform to existing twisted-pair telephone wiring
enabled cost reduction.
UTP-based Ethernet, which uses copper, became widely deployed after
the 10BASE-T standard.
Fiber-optic variants of Ethernet offer high performance, electrical
isolation, and long distance (tens of kilometers with some versions).
With the introduction of the 10BASE-T standard in 1990, Ethernet switches supplemented the half-duplex
CSMA/CD scheme with a full-duplex system that offered higher performance at a lower cost than routers.
With the arrival of 100BASE-T, Ethernet switches capable of mixed-speed and mixed-duplex operation
were built.
The Ethernet physical layer evolved over a considerable time and now encompasses many physical media
interfaces and several magnitudes of speed. Fiber-optic variants of Ethernet offer high performance,
electrical isolation, and long distance (tens of kilometers with some versions). In general, network protocol
stack software works similarly on all varieties.
Unshielded Twisted-Pair Cable
RJ-45 Connector
Outer Jacket Twisted-Pair Cable
Color-Coded Plastic Insulation
Characteristic Value
Speed and throughput From 10 Mb/s to 10 Gb/s
Average cost per node Least expensive
Media and connector size Small
Maximum cable length Varies

Ethernet over twisted-pair technologies use twisted-pair cables for the physical layer of an Ethernet
computer network. Twisted-pair cabling is a type of wiring in which two conductors (the forward and return
conductors of a single circuit) are twisted together for the purposes of canceling EMI from external sources
(for example, electromagnetic radiation from UTP cables and crosstalk between neighboring pairs).
A UTP cable is a four-pair wire. Each of the eight individual copper wires in a UTP cable is covered by an
insulating material. In addition, the wires in each pair are twisted around each other. The advantage of a
UTP cable is its ability to cancel interference, because the twisted-wire pairs limit signal degradation from
EMI and RFI. To further reduce crosstalk between the pairs in a UTP cable, the number of twists in the wire
pairs varies. Cables must follow precise specifications regarding how many twists or braids are permitted
per meter.
UTP cable is used in a variety of types of networks. When used as a networking medium, a UTP cable has
four pairs of either 22- or 24-gauge copper wire. A UTP used as a networking medium has an impedance of
100 ohms, differentiating it from other types of twisted-pair wiring such as that used for telephone wiring.
A UTP cable has an external diameter of approximately 0.43 cm (0.17 inches), and its small size can be
advantageous during installation.
Several categories of UTP cable exist:
Category 1: Used for telephone communications, not suitable for transmitting data
Category 2: Capable of transmitting data at speeds of up to 4 Mb/s
Category 3: Used in 10BASE-T networks—can transmit data at speeds up to 10 Mb/s
Category 4: Used in Token Ring networks—can transmit data at speeds up to 16 Mb/s
Category 5: Capable of transmitting data at speeds up to 100 Mb/s
Category 5e: Used in networks running at speeds up to 1000 Mb/s (1 Gb/s)
Category 6: Consists of four pairs of 24-gauge copper wires, which can transmit data at speeds of up
to 1 Gb/s
Category 6a: Used in networks running at speeds up to 10 Gb/s
RJ-45 Connector and Jack

The RJ-45 plug is the male component, crimped at the end of the cable. As you look at the male connector
from the front, as shown in the figure, the pin locations are numbered from 8 on the left to 1 on the right.
The jack is the female component in a network device, wall, cubicle partition outlet, or patch panel. As you
look at the female connector from the front, as shown in the figure, the pin locations are numbered from 1
on the left to 8 on the right.
Optical Fiber (Single Mode)
Dimensions are in μm (10–6 meters)
9 125 250
Core
Cladding
Buffer (or Coating)
An optical fiber is a flexible, transparent fiber that is made of very pure glass (silica) and is not much larger
than a human hair. It acts as a waveguide, or “light pipe,” to transmit light between the two ends of the
fiber. Optical fibers are widely used in fiber-optic communication, which permits transmission over longer
distances and at higher bandwidths (data rates) than other forms of communication. Fibers are used instead
of metal wires because signals travel along them with less loss and with immunity to electromagnetic
interference.
The two fundamental components that allow a fiber to confine light are the core and the cladding. Most of
the light travels from the beginning to the end inside the core. The cladding around the core provides
confinement. The diameters of the core and cladding are shown in the figure, but the core diameter may
vary for various fiber types. In this case, the core diameter of 9 µm is very small—the diameter of a human
hair is about 50 µm. The outer diameter of the cladding is a standard size of 125 µm. Standardizing the size
means that component manufacturers can make connectors for all fiber-optic cables.
The third element in this picture is the buffer (coating), which has nothing to do with the confinement of the
light in the fiber. Its purpose is to protect the glass from scratches and moisture. The fiber-optic cable can be
easily scratched and broken, like a glass pane. If the fiber is scratched, the scratch could propagate and
break the fiber. Another important aspect is the need to keep the fiber dry.

Fiber Types
n2
n1
Cladding
Core
n2
n1
Cladding
Core
Multimode Fiber (MMF) Single-Mode Fiber (SMF)
The most significant difference between SMF and MMF is in the ability of the fiber to send light over a
long distance at high bit rates. In general, MMF is used for shorter distances at a lower bit rate than SMF.
For long distance communications, SMF is preferred. There are many variations of fiber for both MMF and
SMF.
The most significant physical difference is in the size of the core. The glass in the two fibers is the same,
and the index of refraction change is similar. The core diameter can make a major difference. The diameter
of the fiber cladding is universal for matching fiber ends.
The effect of having different-size cores in fiber is that the two fiber types will support various ways for the
light to get through the fiber. MMF supports multiple ways for the light from one source to travel through
the fiber (the source of the designation "multimode"). Each path can be thought of as a mode.
For SMF, the possible ways for light to get through the fiber have been reduced to one, a “single mode.” It
is not exactly one, but that is a useful approximation.
The table summarizes MMF and SMF characteristics.
MMF and SMF Characteristics
MMF Characteristics SMF Characteristics
LED transmitter usually used Laser transmitter usually used
Lower bandwidth and speed Higher bandwidth and speed
Shorter distances Longer distances
Less expensive More expensive

Fiber Connector Types
An optical fiber connector terminates the end of an optical fiber. A variety of optical fiber connectors are
available. The main differences among the types of connectors are dimensions and methods of mechanical
coupling. Generally, organizations standardize on one type of connector, depending on the equipment that
they commonly use, or they standardize per type of fiber (one for MMF, one for SMF). There are about 70
connector types in use today.
There are three types of connectors:
Threaded
Bayonet
Push-pull
These materials are used for connectors:
Metal
Plastic sleeve
These are the most common types of connectors and their typical uses:
ST: For patch panels (for their durability)
FC: For patch panels, used by service providers
SC: For enterprise equipment
LC: For enterprise equipment, commonly used on SFP modules
In data communications and telecommunications applications today, small-form-factor connectors (for
example, LCs) are replacing the traditional connectors (for example, SCs), mainly to pack more connectors
on the faceplate and thus reduce system footprints.

Ethernet Frame Structure
Bits that are transmitted over an Ethernet LAN are organized into frames. This topic describes the structure
of an Ethernet frame.
Ethernet Frame Structure
Typical Ethernet Frame
8 bytes 6 6 2 46-1500 4
Preamble Destination
Address
Source
Address
Type Data FCS
FCS = frame check sequence
Field length is stated in bytes.
In Ethernet terminology, the container into which data is placed for transmission is called a frame. The
frame contains header information, trailer information, and the actual data that is being transmitted.
The figure shows the most important fields of a MAC layer of the Ethernet frame:
Preamble: This field consists of 8 bytes of alternating 1s and 0s that are used to synchronize the
signals of the communicating computers.
Destination address: This field contains the address of the NIC on the local network to which the
packet is being sent.
Source address: This field contains the address of the NIC of the sending computer.
Type or length: In the Ethernet II standard, this field contains a code that identifies the network layer
protocol. In the 802.3 standard, this field specifies the length of the data field. Therefore, the protocol
information is contained in 802.2 fields, which are at the LLC layer that is contained in the 802.2
header and data field.
Data and pad: This field contains the data that is received from the network layer on the transmitting
computer. This data is then sent to the same protocol on the destination computer. If the data is shorter
than the minimum length of 46 bytes, a string of extraneous bits is used to pad the field.
FCS: The FCS field includes a checking mechanism to ensure that the packet of data has been
transmitted without corruption.

MAC Addresses
Communication in a network occurs in three ways: unicast, broadcast, and multicast. Ethernet frames are
addressed accordingly. This topic describes the relationship of Ethernet frames to methods of network
communications.
MAC Addresses
Unicast
Broadcast
Multicast Client Group
Clien
There are three major types of network communications:
Unicast: Communication in which a frame is sent from one host and addressed to one specific
destination. In a unicast transmission, there is just one sender and one receiver. Unicast transmission is
the predominant form of transmission on LANs and within the Internet.
Broadcast: Communication in which a frame is sent from one address to all other addresses. In this
case, there is just one sender, but the information is sent to all the connected receivers. Broadcast
transmission is essential for sending the same message to all devices on the LAN.
Multicast: Communication in which information is sent to a specific group of devices or clients.
Unlike broadcast transmission, in multicast transmission, clients must be members of a multicast group
to receive the information.

MAC Addresses (Cont.)
OUI
Vendor
Assigned
48 Bits
MAC Address
24 Bits 24 Bits
The address that is on the NIC is the MAC address, which is often referred to as the BIA. Some vendors
allow modification of this address to meet local needs. There are two components of a 48-bit Ethernet MAC
address:
24-bit OUI: The OUI identifies the manufacturer of the NIC. The IEEE regulates the assignment of
OUI numbers.
24-bit vendor-assigned end-station address: This portion uniquely identifies the Ethernet hardware.
MAC Addresses (Cont.)
Different display formats:
0000.0c43.2e08
00:00:0c:43:2e:08
00-00-0C-43-2E-08

The MAC sublayer of the TCP/IP link layer processes physical addressing issues. The physical address is a
number in hexadecimal format that is actually burned into the NIC. This address is referred to as the MAC
address, and it is expressed as groups of hexadecimal digits that are organized in pairs or quads, such as
0000.0c43.2e08, 00:00:0c:43:2e:08, or 00-00-0C-43-2E-08. Note that various systems use various display
formats.
Each device on a LAN must have a unique MAC address to participate in the network. The MAC address
identifies the location of a specific computer on a LAN. Unlike other kinds of addresses that are used in
networks, the MAC address should not be changed unless there is some specific need to do so.

Switching Operation
This topic describes how switches process and transmit frames based on MAC addresses.
Switching Operation
3
Frame
3
1
2
1
Frame
PC B
PC C
PC A
3
1
2
Frame
PC B
PC A
PC C
MAC Table
Port: 1: MAC PC A
Port: 2: Empty
Port: 3: Empty
2
MAC Table
Port: 1: MAC PC A
Port: 2: Empty
Port: 3: Empty
e
C
C
Switching Operation (Cont.)
3
1
2
4
Frame
PC B
PC C
PC A
3
1
2
Frame
6
PC B
PC A
PC C
MAC Table
Port: 1: MAC PC A
Port: 2: Empty
Port: 3: Empty
5
MAC Table
Port: 1: MAC PC A
Port: 2: Empty
Port: 3: MAC PC B
me
C
C
The switch builds and maintains a table, called a MAC table, that matches a destination MAC address with
the port used to connect to a node. The MAC table is stored in CAM, which enables very fast lookups.
For each incoming frame, the destination MAC address in the frame header is compared to the list of
addresses in the MAC table. Switches then use MAC addresses as they decide whether to filter, forward, or
flood frames.

The switch creates and maintains a table using the source MAC addresses of incoming frames and the port
number through which the frame entered the switch. When an address is not known, a switch learns the
network topology by analyzing the source address of incoming frames from all of the attached networks.
The table describes the switching process:
Switching Frames Procedure
Step Action
1 The switch receives a frame from PC A on port 1.
2 The switch enters the source MAC address and the switch port that received the frame into the MAC
table.
3 The switch checks the table for the destination MAC address. Because the destination address is not
known, the switch floods the frame to all the ports except the port on which it received the frame.
4 The destination device with the matching MAC address replies to the broadcast with a unicast frame
addressed to PC A.
5 The switch enters the source MAC address of PC B and the port number of the switch port that received
the frame into the MAC table. The destination address of the frame and its associated port is found in the
MAC table.
6 The switch can now forward frames between the source and destination devices without flooding because
it has entries in the MAC table that identify the associated ports.

Duplex Communication
Full-duplex communication increases effective bandwidth by allowing both ends of the connection to
transmit simultaneously. However, this method of optimizing network performance requires
microsegmentation before full-duplex communication can occur. This topic describes half-duplex and full-
duplex communication and illustrates how full-duplex communication can improve the performance of a
switched LAN.
Duplex Communication
Half-duplex operation:
Unidirectional data flow
Higher potential for collision
Legacy connectivity
Switch
Server
Half-duplex communication relies on a unidirectional data flow, where sending and receiving data are not
performed at the same time. This behavior is similar to walkie-talkies or two-way radios in that only one
person can talk at a time. If someone talks while another person is speaking, a collision occurs. As a result,
half-duplex communication implements Ethernet CSMA/CD to help reduce the potential for collisions and
to detect them when they do happen.
Half-duplex communication has performance issues that stem from the constant waiting, because data can
flow in only one direction at a time. Half-duplex connections are typically seen in older hardware such as
hubs. Nodes that are attached to hubs that share their connection to a switch port must operate in half-
duplex mode because the end computers must be able to detect collisions. Nodes can operate in a half-
duplex mode if the NIC cannot be configured for full-duplex operations. In this case, the port on the switch
operates in half-duplex mode as well. Because of these limitations, full-duplex communication has replaced
half duplex in more current hardware.

Duplex Communication (Cont.)
Full-duplex operation:
Point-to-point only
Attached to dedicated switched port
Requires full-duplex support on both ends
In full-duplex communication, the data flow is bidirectional, so data can be sent and received at the same
time. The bidirectional support enhances performance by reducing the wait time between transmissions.
Most Ethernet, Fast Ethernet, and Gigabit Ethernet NICs offer full-duplex capability. Frames that are sent
by the two connected end nodes cannot collide, because the end nodes use two separate circuits in the
network cable. Each full-duplex connection uses only one port. Full-duplex connections require a switch
that supports full duplex or a direct connection between two nodes that each support full duplex. Nodes that
are directly attached to a dedicated switch port with NICs that support full duplex should be connected to
switch ports that are configured to operate in full-duplex mode.

Configuring Duplex and Speed Options
This topic describes how to configure speed and duplex on an interface.
Configuring Duplex and Speed Options
Setting full duplex and 100-Mb/s speed settings on the FastEthernet0/1
Setting auto-duplex and auto-speed settings on the FastEthernet0/5
PC1 SwitchY PC2
SwitchX
Fa0/5 Fa0/1 Fa0/1 Fa0/3
SwitchX(config)#interface FastEthernet0/1
SwitchX(config-if)#duplex full
SwitchX(config-if)#speed 100
SwitchX(config-if)#interface FastEthernet0/5
SwitchX(config-if)#duplex auto
SwitchX(config-if)#speed auto
General recommendation:
Use manual settings on infrastructure links.
Use auto settings on ports toward end devices.
The duplex parameters on the Cisco Catalyst 2960 Series Switch are as follows:
The auto option sets autonegotiation of duplex mode. With autonegotiation enabled, the two ports
communicate to decide the best mode of operation.
The full option sets full-duplex mode.
The half option sets half-duplex mode.
For Fast Ethernet and 10/100/1000 ports, the default option is auto. For optical 100BASE-FX ports, the
default is full. The 10/100/1000 ports operate in either half- or full-duplex mode when they are set to 10 or
100 Mb/s, but when they are set to 1000 Mb/s, they operate only in full-duplex mode.
A general recommendation is to manually set the duplex mode and speed on ports between infrastructure
devices. In the figure, ports between the switches are manually set. On ports toward users and on end
devices, autonegotiation is suggested. This simplifies the administration when users frequently connect
different end devices to the network. The port toward PC1 in the figure is configured with autonegotiation,
which is also the default option for Fast Ethernet ports.

Configuring Duplex and Speed Options (Cont.)
Overview of duplex settings and the resulting operation
Duplex settings Half Full Auto
Half Half Mismatch Half
Full Mismatch Full Full
Auto Half Full Full
The table in the figure shows combinations of duplex settings on a link between two devices. A mismatch
state makes a link nonoperational, while half-duplex enables low performance connectivity. If devices
support it, full-duplex connectivity is preferred.
Autonegotiation can sometimes produce unpredictable results. By default, when autonegotiation fails, the
Catalyst switch sets the corresponding Fast Ethernet switch port to half-duplex mode. This type of failure
happens when an attached device does not support autonegotiation. If the device is manually configured to
operate in half-duplex mode, it matches the default mode of the switch. However, autonegotiation errors
can happen if the device is manually configured to operate in full-duplex mode. This configuration, half
duplex on one end and full duplex on the other end, causes late collision errors at the half-duplex end. To
avoid this situation, manually set the duplex parameters of the switch to match the attached device.

Configuring Duplex and Speed Options (Cont.)
Verify the duplex and speed settings on the FastEthernet0/5 interface.
SwitchX#show interfaces FastEthernet0/5
Hardware is Fast Ethernet, address is 0022.91c4.0e01 (bia 0022.91c4.0e01)
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
input flow-control is off, output flow-control is unsupported
<output omitted>
5 minute input rate 0 bits/sec, 0 packets/secl
5 minute output rate 0 bits/sec, 0 packets/sec
7289 packets input, 927927 bytes, 0 no buffer
Received 184 broadcasts (1380 multicasts
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 1380 multicast, 0 pause input
0 input packets with dribble condition detected
39965 packets output, 7985339 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out
Verify the duplex settings by using the show interfaces command on the Cisco Catalyst 2960 Series
Switch. The show interfaces privileged EXEC command displays statistics and the status for all interfaces
or specified interfaces. The figure shows the duplex and speed settings of a Fast Ethernet interface.
If the switch port is in full-duplex mode and the attached device is in half-duplex mode, check for CRC
errors on the full-duplex switch port.

Summary
Summary
Ethernet over twisted-pair technologies use twisted-pair cables for the
physical layer of an Ethernet computer network. Optical fiber permits
transmission over longer distances and at higher data rates.
The Ethernet frame contains header information, trailer information, and
the actual data that is being transmitted.
An Ethernet MAC address consists of two parts: OUI and a vendor-
assigned end-station address.
The switch creates and maintains a MAC address table by using the
source MAC addresses of incoming frames and the port number through
which the frame entered the switch.
Full-duplex communication increases effective bandwidth by allowing
both ends of the connection to transmit simultaneously.

Lesson 7
Troubleshooting Common
Switch Media Issues
Overview
Most issues that affect a switched network are encountered during the original implementation. After a
network is installed, it should continue to operate without problems. However, that is only true in theory.
Cabling becomes damaged, configurations change, and new devices are connected to the switch that require
switch configuration changes. Ongoing maintenance is necessary. This lesson describes the most common
media and port issues and how to troubleshoot them.
Objectives
Describe common tools for connectivity verification
Identify common switched network media issues
Troubleshoot switched network media issues
Identify common access port issues
Troubleshoot common access port issues

Common Troubleshooting Tools
This topic describes common troubleshooting tools that can be used when verifying network connectivity.
Common Troubleshooting Tools
ping command
Switch#ping 10.1.1.100
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.100, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/203/1007 ms
C:>ping example.com
Pinging example.com [192.0.43.10] with 32 bytes of data:
Reply from 192.0.43.10: bytes=32 time=107ms TTL=243
Ping statistics for 192.0.43.10:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 107ms, Maximum = 137ms, Average = 114ms
The ping command is a utility for testing IP connectivity between hosts. It sends out requests for responses
from a specified host address. The ping command uses a Layer 3 protocol that is a part of the TCP/IP suite
called ICMP, and it uses the ICMP echo request and ICMP echo reply packets.
If the host at the specified address receives the ICMP echo request, it responds with an ICMP echo reply
packet. For each packet sent, the ping command measures the time that is required to receive the response.
As each response is received, the ping command displays the time between the request being sent and when
the response is received. By using interval timing and response rates, the ping command estimates the RTT,
generally in milliseconds, and the packet-loss rate between hosts. The RTT is a measure of the network
performance.
The ping command is supported on most devices connected to the network, including network devices such
as switches and routers, and different operating systems running on a PC such as Windows, Mac, or Linux.
In the figure, the first output illustrates the use of the ping command on the Cisco Catalyst switch, while the
second output shows ping verification on a PC running Windows.

Common Troubleshooting Tools (Cont.)
Telnet
Switch1#telnet 10.1.20.1
Trying 10.1.20.1 ... Open
Switch2>
Telnet provides the capability to remotely access another computer, servers, and networking devices. Telnet
enables a user to log in to a remote host and execute commands. It is also used to verify connectivity with a
remote host.
Cisco IOS devices can also be used as a Telnet client to connect to other devices. You can initiate a Telnet
session with the telnet command.
Some operating systems on a PC include built-in Telnet clients. On others, you can install a client as an
add-on or use a dedicated application that includes a Telnet client. An application named PuTTY, shown in
the figure, is one example of a free open-source Telnet client.

Media Issues
This topic describes the methods of identifying common switched network media issues.
Media Issues – Copper
Copper media issues have several possible sources:
Wiring becomes damaged.
New EMI sources are introduced.
Traffic patterns change.
New equipment is installed.
Media issues are common. Here are some examples of common situations that can cause media issues:
In an environment using Category 3 wiring, the maintenance crew installs a new air conditioning
system that introduces new EMI sources into the environment.
In an environment using Category 5 wiring, cabling is run too close to an elevator motor.
Poor cable management puts a strain on some RJ-45 connectors, causing one or more wires to break.
New applications change traffic patterns.
When new equipment is connected to a switch, the connection operates in half-duplex mode, or a
duplex mismatch occurs, which could lead to an excessive number of collisions.

Media Issues – Fiber
Fiber media issues have these possible sources:
Macrobend losses:
Bending the fiber in too small a radius causes light to escape.
Light strikes the core or cladding at less than the critical angle.
Total internal reflection no longer happens, and light leaks out.
Splice losses
Light
Fiber Core
Radius
Greater Than
25–30 mm
= No Loss
There are several ways in which light can be lost from the fiber. Some of these are manufacturing problems
(for example, microbends, macrobends, and splicing fibers that do not have their cores centered), while
others are physics problems (back reflections), because light reflects whenever it encounters a change in the
index of refraction.
Macrobends are typically applied to the fiber during fiber installation.
There is another explanation for why light leaks out at a macrobend. Part of the traveling wave, called the
evanescent wave, travels inside the cladding. Around the bend, part of the evanescent wave must travel
faster than the speed of light in the material. This is not possible, so that part radiates out of the fiber.
Bend losses can be minimized by designing a larger index difference between the core and the cladding.
Another approach is to operate at the shortest possible wavelength and perform good installations.
Splices are a way to connect two fibers by fusing their ends. The best way to align the fiber core is by using
the outside diameter of the fiber as a guide. If the core is at the center of the fiber, a good splice can be
achieved. If the core is off center, then it is impossible to create a good splice. You would have to cut the
fiber further upstream and test again.
Another possibility is that the fibers to be spliced could have dirt on their ends. Dirt can cause many
problems, particularly if the dirt intercepts some or all of the light from the core. Recall that the core for
SMF is only 9 µm.

Troubleshooting Switch Media Issues
This topic describes how to troubleshoot switched network media issues.
Troubleshooting Switch Media Issues
Verify
interface
status
Check for
excessive
noise
No or bad
connection
Successful
connection
Operational
Fix damaged
cabling or
connectors
Remove source
of noise
Check cable length
Down
No
Yes
show
interface
show
interface
Check for
excessive
collisions
show
interface
No
Verify and fix
duplex
settings
Yes
No
To troubleshoot switch media issues when you have no connection or a bad connection between a switch
and another device, follow this process:
1. Use the show interface command to check the interface status. If the interface is not operational, check
the cable and connectors for damage.
2. Use the show interface command to check for excessive noise. If there is excessive noise, first find
and remove the source of the noise, if possible. Verify that the cable does not exceed the maximum
cable length and check the type of cable that is used. For copper cable, it is recommended that you use
at least Category 5.
3. Use the show interface command to check for excessive collisions. If there are collisions or late
collisions, verify the duplex settings on both ends of the connection.
Note A counter shows excessive values when it is constantly increasing while performing sequential
verification of counter values.

Troubleshooting Switch Media Issues (Cont.)
Verify interface status.
Switch#show interface FastEthernet0/1
FastEthernet0/1 is up, line protocol is up
<output omitted>
Interface Status Line Protocol Status Link State
Up Up Operational
Up Down Connection problem
Down Down (not connected) Cable unplugged; other end of
the link disconnected or
interface in shutdown mode
Down Down Interface problem
Administratively
down
Down Disabled
One of the most important elements of the show interface command output is the display of the line and
data link protocol status. This figure highlights the key summary line to check in the command output, and
the status meanings for an interface.
The first parameter (Interface Status) refers to the hardware layer and, essentially, reflects whether the
interface is receiving the carrier detect signal from the other end. The second parameter (Line Protocol
Status) refers to the data link layer and reflects whether the data link layer protocol keepalives are being
received.
Based on the output of the show interface command, possible problems can be defined as follows:
If the interface is up and the line protocol is down, a problem exists. There could be an encapsulation
type mismatch, the interface on the other end could be error-disabled, or there could be a hardware
problem.
If the line protocol and the interface are both down, a cable is not attached or some other interface
problem exists. For example, in a back-to-back connection, the other end of the connection may be
administratively down.
If the interface is administratively down, it has been manually disabled (the shutdown command has
been issued) in the active configuration.

Interface Status Verification
Excessive noise:
Presence of many CRC errors
Inspect the cable for damage and correct length, and search for noise
sources
Excessive collisions:
Normal in half-duplex operations
Configure the link to use full-duplex
Excessive late collisions:
Indicates duplex mismatch
Configure both ends of the link to use the same duplex settings
Based on the output of the show interface command, you can find, diagnose, and correct the problem.
If you see many CRC errors, there is too much noise on the link and you should inspect the cable for
damage and length. You should also search for and eliminate noise sources, if possible.
Collisions will occur in networks where half duplex is being used. Collisions in half-duplex operations are
normal and you should not be concerned about them, as long as you are pleased with the half-duplex
operations. However, you should never see collisions in a correctly designed and configured network that
uses full duplex. It is highly recommended that you use full duplex unless you have older or legacy
equipment that does not support full duplex.
Late collision refers to a collision that occurs after the preamble is transmitted. Late collisions occur when
you have serious problems in a switched network, such as duplex misconfiguration. For example, you could
have one end of a connection configured for full duplex and the other for half duplex. You would see late
collisions on an interface that is configured for half duplex. In that case, you must configure the same
duplex setting on both ends.

Interface Status Verification (Cont.)
Switch#show interface FastEthernet0/1
FastEthernet0/1 is up, line protocol is up
<output omitted> .
2295197 packets input, 305539992 bytes, 0 no buffer
Received 1925500 broadcasts, 0 runts, 0 giants, 0 throttles
3 input errors, 3 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 68 multicast, 0 pause input
0 input packets with dribble condition detected
3594664 packets output, 436549843 bytes, 0 underruns
1935 output errors,1790 collisions,10 interface resets
0 unknown protocol drops
0 babbles, 135 late collision, 0 deferred
<output omitted>
Displays interface status and statistics
The figure shows an example of show interface command output. The example shows counters and
statistics for the FastEthernet0/1 interface.
The table describes some of the parameters in the show interface command output.
Runts Packets that are discarded because they are smaller
than the minimum packet size for the medium. For
instance, any Ethernet packet that is less than 64 bytes
is considered a runt.
Giants Packets that are discarded because they exceed the
maximum packet size for the medium. For example, any
Ethernet packet that is greater than 1518 bytes is
considered a giant.
Input errors Total number of errors. It includes runts, giants, no
buffer, CRC, frame, overrun, and ignored counts.
CRC CRC errors are generated when the calculated
checksum is not the same as the checksum received.
Output errors Sum of all errors that prevented the final transmission of
datagrams out of the interface that is being examined.
Collisions Number of messages retransmitted because of an
Ethernet collision. This often happens on half-duplex
links when two devices transmit frames at the same
time.
Late collisions Jammed signal could not reach to ends. Caused by
duplex mismatch or by exceeded Ethernet cable length
limits.

Port Issues
This topic describes the methods of identifying common port access issues.
Port Issues
Most common port issues are related to duplex and speed
issues.
Duplex-related issues result from a mismatch in duplex settings:
full auto
Speed-related issues result from a mismatch in speed settings:
auto
100
A common issue with speed and duplex occurs when the duplex settings are mismatched between two
switches, between a switch and a router, or between a switch and a workstation or server. This mismatch
can occur when you manually hard-code the speed and duplex, or from autonegotiation issues between the
two devices.
Port Issues (Cont.)
These are examples of duplex-related issues:
One end set to full duplex and the other set to half duplex results in a
mismatch.
One end is set to full duplex and the other is set to autonegotiation:
If autonegotiation fails, and that end reverts to half duplex, it results in a
mismatch.
One end is set to half duplex and the other is set to autonegotiation:
If autonegotiation fails, and that end reverts to half duplex.
Both ends are set to half duplex, and there is no mismatch.

Port Issues (Cont.)
More examples of duplex-related issues:
Autonegotiation is set on both ends:
One end fails to full duplex, and the other end fails to half duplex.
Example: A Gigabit Ethernet interface defaults to full duplex, while a 10/100
defaults to half duplex.
Autonegotiation fails on both ends, and they both revert to half duplex.
Both ends are set to half duplex, and there is no mismatch.
full half
?
Duplex mismatch is a situation in which the switch operates at full duplex and the connected device
operates at half duplex, or vice versa. The result of a duplex mismatch is extremely slow performance,
intermittent connectivity, and loss of connection. Other possible causes of data-link errors at full duplex are
bad cables, a faulty switch port, or NIC software or hardware issues.
Use the show interface command to verify the duplex settings.
If the mismatch occurs between two Cisco devices with Cisco Discovery Protocol enabled, you will see
Cisco Discovery Protocol error messages on the console or in the logging buffer of both devices. Cisco
Discovery Protocol is useful to detect errors as well as to gather port and system statistics on nearby Cisco
devices. Whenever there is a duplex mismatch (in this example, on the FastEthernet0/1 interface), these
error messages are displayed on the switch consoles of Catalyst switches that run Cisco IOS Software:
%CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/1 (not half duplex)
Additionally, for switches with Cisco IOS Software, these messages appear for link up or down situations
(in this example, on the FastEthernet0/1 interface):
%LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
%LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to down

Port Issues (Cont.)
These are examples of speed-related issues:
One end is set to one speed and the other is set to another speed,
resulting in a mismatch.
One end is set to a higher speed and autonegotiation is enabled on the
other end:
If autonegotiation fails, switch senses what the other end is using and reverts
to optimal speed.
Autonegotiation fails on both ends, and they revert to their lowest speed.
Both ends are set at lowest speed, and there is no mismatch.
auto
100 ?
Use the show interface command to verify the speed settings.

Troubleshooting Port Issues
This topic describes how to troubleshoot common port access issues.
Troubleshooting Port Issues
Troubleshooting duplex-related issues:
Check for
speed
mismatch
Check for
duplex
mismatch
No or bad
connection
Successful
connection
No
Manually fix the speed
on both sides
Manually fix the duplex
setting on both sides
Yes Yes
show
interface
No
show
interface
%CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/1 (not
half duplex)
Duplex mismatch detected by Cisco Discovery Protocol
To troubleshoot switch port issues when you have no connection or a bad connection between a switch and
another device, use this general process:
1. Use the show interface command to check whether there is a speed mismatch between the switch and
a device on the other side (switch, router, server, and so on). If there is a speed mismatch, set the speed
on both sides to the same value.
2. Use the show interface command to check whether there is a duplex mismatch between the switch and
a device on the other side. It is recommended that you use full duplex if both sides support it.

Troubleshooting Port Issues (Cont.)
Switch#show interfaces FastEthernet 0/1
Hardware is Fast Ethernet, address is 001e.147c.6f01 (bia 001e.147c.6f01)
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:28, output 00:00:01, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
Displays duplex and speed statistics
The figure shows an example of show interface command output. The example highlights duplex and
speed settings for interface FastEthernet0/1.
Based on the output of the show interface command, you can find, diagnose, and correct the duplex or
speed mismatch between the switch and the device on the other side.

Summary
Summary
Use ping and telnet commands to verify connectivity.
Switch media issues are common and have several possible sources,
such as damaged copper wiring, EMI, and macrobend and splice losses
in fiber media.
Speed or duplex mismatch on a link leads to serious performance
degradation.
Use the show interface verification command when troubleshooting
speed and duplex port issues.

Module Summary
This topic summarizes the key points that were discussed in this module.
Module Summary
A network is a connected collection of devices (computers,
interconnections, routers, and switches) that can communicate with
each other, providing the means for users to share hardware and
applications.
TCP/IP defines four categories of functions that must occur for
communications to be successful (the network access, Internet,
transport, and application layers).
A LAN is a network that is located in a limited area, with the computers
and other components that are part of this network located relatively
close together.
Ethernet switches divide collision domains and reduce the number of
devices that are competing for bandwidth. Ethernet switches selectively
forward individual frames from a receiving port to the destination port.

Module Summary (Cont.)
Cisco IOS Software provides network services to Cisco products to
perform various internetworking functions.
Ethernet over twisted-pair technologies use twisted-pair cables for the
physical layer of an Ethernet computer network. Optical fibers permit
transmission over longer distances and at higher data rates.
The switch creates and maintains a MAC address table by using the
source MAC addresses of incoming frames and the port number through
which the frame entered the switch.
Switch media issues are common and have several possible sources,
such as damaged copper wiring, EMI sources, and macrobend and
splice losses in fiber media.
Speed or duplex mismatch on a link leads to serious performance
degradation.

Module Self-Check
Questions
Use the questions here to review what you learned in this module. The correct answers and solutions are
found in the Module Self-Check Answer Key.
1. Which three statements about networks are accurate? (Choose three.) (Source: Exploring the
Functions of Networking)
A. Networks are used to transmit data in various environments, including homes, small businesses,
and large enterprises.
B. A main office can have hundreds or even thousands of people who depend on network access to
do their jobs.
C. A network is a connected collection of devices that can communicate with each other.
D. A main office usually has one large network to connect users.
E. The purpose of a network is to create a means to provide workers with access to all information
and components that are accessible via the network.
F. Remote locations cannot connect to a main office through a network.
2. Match each network characteristic to its definition. (Source: Exploring the Functions of Networking)
A. security indicates the protection level of the network itself and the
data that is transmitted
B. reliability indicates how well the network can accommodate more
users or data transmission requirements
C. speed indicates the dependability of the network
D. availability indicates how fast data is transmitted over the network
E. cost indicates the general price of components, installation, and
maintenance of the network
F. topology indicates the structure of the network
G. scalability indicates the probability that the network will be available
for use

3. Which two statements about the purpose of the TCP/IP model are accurate? (Choose two.) (Source:
Understanding the Host-to-Host Communications Model)
A. The TCP/IP model defines the network functions that occur at each layer.
B. The TCP/IP model facilitates an understanding of how information travels throughout a network.
C. The TCP/IP model ensures reliable data delivery through its layered approach.
D. The TCP/IP model defines seven layers.
4. Match each TCP/IP layer to its function. (Source: Understanding the Host-to-Host Communications
Model)
A. application layer provides logical addressing and determines the best path
through the network
B. internet layer deals with human interaction and the implementation of
software applications and related protocols
C. transport layer controls the hardware devices and media that make up the
network
D. link layer supports communication between end devices across
diverse networks
5. Which two statements about switches are accurate? (Choose two.) (Source: Introducing LANs)
A. They operate only at the internet layer of the TCP/IP model.
B. They forward, filter, or flood frames based on MAC table entries.
C. A hub is a newer and better version of a switch.
D. Their three major functions are forwarding, filtering, and flooding.
6. Which CLI prompt indicates that you are working in privileged EXEC mode? (Source: Operating
Cisco IOS Software)
A. hostname#
B. hostname>
C. hostname-exec>
D. hostname-config
7. Which command would you enter in privileged EXEC mode to list the command options? (Source:
Operating Cisco IOS Software)
A. ?
B. init
C. enable
D. login
8. Which CLI command would you enter to display a list of commands that begin with the letter “c”?
(Source: Operating Cisco IOS Software)
A. c?
B. c ?
C. help c
D. help C

9. Match each step of the physical Cisco Catalyst switch startup process to its description. (Source:
Starting a Switch)
A. Step 2 Verify that all cable connections are secure, the terminal is
connected to the console port, and the console terminal
application is selected.
B. Step 1 Observe the boot sequence, including the Cisco IOS
Software output text on the console.
C. Step 3 Attach the power cable plug to the power supply socket of
the switch.
10. Which configuration mode would you use to configure a particular port on a switch? (Source:
Starting a Switch)
A. user EXEC mode
B. global configuration mode
C. interface configuration mode
D. controller configuration mode
11. Match the types of traffic with their description. (Source: Understanding Ethernet and Switch
Operation)
A. multicast A frame is sent from one address to all other addresses.
B. unicast A frame is sent from one address to multiple addresses.
C. broadcast A frame is sent from one address to a single address.
12. Which CLI prompt indicates the correct way to set the port duplex operation to full? (Source:
Understanding Ethernet and Switch Operation)
A. SwitchX(config-if)#duplex half
B. SwitchX>duplex full
C. SwitchX(config-if)#duplex full
D. SwitchX(config)#full duplex
13. Which Cisco IOS command is the most useful for troubleshooting media issues? (Source:
Troubleshooting Common Switch Media Issues)
A. show controller
B. show troubles
C. show counters
D. show interface

14. Issuing a show interface FastEthernet 0/1 command resulted in output that stated "FastEthernet0/1
is up, line protocol is down." What does this message mean? (Source: Troubleshooting Common
Switch Media Issues)
A. The link is fully functional.
B. The interface has been manually disabled (the shutdown command has been issued) in the active
configuration.
C. There could be an encapsulation type mismatch, the interface on the other end could be error-
disabled, or there could be a hardware problem.
D. A cable is not attached, or some other interface problem exists. For example, in a back-to-back
connection, the other end of the connection may be administratively down.

Answer Key
1. A, B, C
2. A. speed indicates how fast data is transmitted over the network
B. cost indicates the general price of components, installation, and
maintenance of the network
C. security indicates the protection level of the network itself and the
data that is transmitted
D. availability indicates the probability that the network will be available
for use
E. scalability indicates how well the network can accommodate more
users or data transmission requirements
F. reliability indicates the dependability of the network
G. topology indicates the structure of the network
3. A, B
4. A. application layer deals with human interaction and the implementation of
software applications and related protocols
B. transport layer supports communication between end devices across
diverse networks
C. internet layer provides logical addressing and determines the best path
through the network
D. link layer controls the hardware devices and media that make up the
network
5. B, D
6. A
7. A
8. A
9. A. Step 1 Verify that all cable connections are secure, the terminal is
connected to the console port, and the console terminal
application is selected.
B. Step 2 Attach the power cable plug to the power supply socket of
the switch.
C. Step 3 Observe the boot sequence, including the Cisco IOS
Software output text on the console.
10. C

11. A. unicast A frame is sent from one address to a single address.
B. broadcast A frame is sent from one address to all other addresses.
C. multicast A frame is sent from one address to multiple addresses.
12. C
13. D
14. C

Module 2
Establishing Internet
Connectivity
When you fully understand how a small network is built, you can more easily understand how a router
connects your network to the Internet. Connectivity with the outside world is the primary function of
networks. Internet access is usually provided by ISPs using various technologies, from DSL and cable to
various wireless technologies such as IEEE 802.11 and 3G or 4G.
When you connect to the Internet, it is important to understand how to use ACLs for filtering and
classification of traffic. IP uses packets to carry information through the network. The transport layer is
responsible for the overall end-to-end transfer of application data. Networks are usually divided by
administrators into subnets to provide network scalability.
NAT and PAT translate IP addresses and eliminate the need for host renumbering. This module describes
their configuration.
Objectives
Describe IPv4 and its addressing scheme
Describe subnets, subnetting, and the role of subnet masks
Describe the TCP/IP transport layer
Describe the functions of routing in the network model
Implement basic configuration on a Cisco router
Understand host-to-host communications across switches and routers
Describe the operation, benefits, and limitations of static routing
Describe the operation of ACLs and their applications in the network
Configure Internet access using DHCP clients, NAT, and PAT on Cisco routers

Lesson 1
Understanding the TCP/IP
Internet Layer
Overview
There are various aspects to IP addressing, including calculations for constructing an IP address, classes of
IP addresses designated for specific routing purposes, and public versus private IP addresses. There are also
two types of IP addresses: IPv4 and IPv6. This lesson introduces the concepts behind IPv4 addresses.
Objectives
List the characteristics of IP
Describe the components of an IPv4 address
Identify the source and the destination fields within the IPv4 header
Describe the decimal and binary number systems
Convert a decimal number to a binary number
List the classes of IPv4 addresses
Describe reserved IPv4 addresses
Define the function of DNS
Verify the IPv4 address of a host

Internet Protocol
The IP component of TCP/IP determines where packets of data are routed, based on their destination
addresses. IP has certain characteristics that are related to how it manages this function. This topic describes
some of the key functions of IP.
Internet Protocol
IP characteristics:
Operates at the internet layer of the TCP/IP
stack
Connectionless protocol
Packets treated independently
Hierarchical addressing
Best-effort delivery
No data-recovery features
Media-independent
Two variants: IPv4 and IPv6
Application
Transport
Internet
Link
IP
IP uses packets to carry information through the network. A packet is a self-contained, independent entity
that contains data and sufficient information to be routed from the source to the destination without reliance
on earlier exchanges.
IP has these characteristics:
IP operates at Layer 3 of the OSI model (network layer) and at the internet layer of the TCP/IP stack.
IP is a connectionless protocol in which a one-way datagram is sent to the destination without advance
notification to the destination device. The destination device receives the data and does not return any
status information to the sending device.
Each packet is treated independently, which means that each packet can travel a different way to the
destination.
IP uses hierarchical addressing, in which the network ID is the equivalent of a street and the host ID is
the equivalent of a house or office building on that street.
IP provides service on a best-effort basis and does not guarantee packet delivery. A packet can be
misdirected, duplicated, or lost on the way to its destination.
IP does not provide any special features that recover corrupted packets. These services are instead
provided by the end systems of the network.
IP operates independently of the medium that is carrying the data.
There are two types of IP addresses: IPv4 and IPv6.

Example: Delivering a Letter Through a Postal Service
An analogy for IP services would be mail delivery by a postal service. For example, you live in San
Francisco and your mother lives in New York. You write three letters to your mother. You seal each letter
in a separate envelope, address each letter, and write your return address in the upper left-hand corner of
each envelope.
You deposit the three letters in the outgoing mail slot at your local post office. The postal service makes its
best attempt to deliver all three letters to your mother in New York. However, the postal service will not
guarantee that the letters will arrive at their destination. It will not guarantee that all three letters will be
processed by the same carrier or take the same route. And it will not guarantee that the letters will arrive in
the order in which you mailed them.

IPv4 Address Representation
This topic describes the components of an IPv4 address.
IPv4 Address Representation
Every host (computer, networking device, peripheral) must have a
unique address.
An IP address consists of two parts:
Network ID:
Identifies the network of which the host is a part
Used by routers to maintain information about routes
Host ID:
Identifies the individual host
Assigned by organizations to individual devices
32 Bits
Network Host
172.16.12.22
Physical street addresses are necessary to identify the locations of specific homes and businesses so that
mail can reach them efficiently. In the same way, logical IP addresses are used to identify the location of
specific devices on an IP network so that data can reach those network locations. Every host, computer,
networking device, or peripheral device connected to the Internet has a unique 32-bit IP address that
identifies it. Without a structure for allocating all of those IP addresses, it would be impossible to route
packets efficiently. Learning how IP addresses are structured and how they function in the operation of a
network provides an understanding of how IP packets are forwarded over networks using TCP/IP.
The IPv4 address is the most common type of address that is currently used on the Internet. IPv4 addresses
are 32-bit numbers that describe the location of a network device.
An IP address is hierarchical and consists of two parts:
The network address portion (network ID) describes the network of which this IP address is a part.
The host address component (host ID) identifies a specific endpoint. These endpoints are the servers,
computers, and other devices that are connected to the network. Host IDs are assigned to individual
devices (end-user devices, printers, network devices, and so on).

IPv4 Header Address Fields
This topic identifies the source and destination address fields within the IPv4 header.
IPv4 Header Address Fields
Ver. IHL Service Type Total Length
Identification Flag Fragment Offset
Time to Live Protocol Header Checksum
Source Address
Destination Address
Options Padding
The IPv4 header has several fields. These are the two fields that are most important for you at this point:
Source Address: Specifies the 32-bit binary value that represents the IP address of the sending
endpoint
Destination Address: Specifies the 32-bit binary value that represents the IP address of the receiving
endpoint
Other fields in the header are the following:
Version: Describes the version of Internet Protocol
IHL: Internet Header Length, describes the length of the header
Service Type: Provides information of desired quality of service
Total Length: Describes the length of a packet, including header and data
Identification: Used for unique fragment identification
Flag: Sets various control flags regarding fragmentation
Fragment Offset: Indicates where specific fragment belongs
Time to Live: Limits lifetime of a packet
Protocol: Indicates the protocol used in the data portion of a IP packet
Header Checksum: Used for header error detection
Options: Includes optional parameters
Padding: Used to ensure that the header ends on a 32-bit boundary
If you would like to learn more about the IPv4 header fields, go to http://tools.ietf.org/html/rfc791.

Decimal and Binary Systems
The decimal (base 10) system is the numbering system that is used in everyday mathematics, and the binary
(base 2) system is the foundation of computer operations. Network device addresses use the binary system
to define their location on the network. The IP address is based upon a dotted-decimal notation of a binary
number. Having a basic understanding of the mathematical properties of a binary system helps you to
understand networking. This topic describes the decimal and binary systems.
Decimal and Binary Systems
Decimal numbers are represented by the numbers 0 through 9.
Binary numbers are represented by a series of 1s and 0s.
1001
9
1000
8
111
7
110
6
101
5
100
4
11
3
10
2
1
1
0
0
Binary
Decimal
10011
19
10010
18
10001
17
10000
16
1111
15
1110
14
1101
13
1100
12
1011
11
1010
10
Binary
Decimal
In the decimal system, the digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. When quantities higher than 9 are
required, the decimal system begins with 10 and continues all the way to 99. Then the decimal system
begins again with 100, and so on, with each column to the left raising the exponent by 1.
The binary system uses only the digits 0 and 1. Therefore, the first digit is 0, followed by 1. If a quantity
higher than 1 is required, the binary system goes to 10, followed by 11. The binary system continues with
100, 101, 110, 111, then 1000, and so on. This figure shows the binary equivalent of the decimal numbers 0
through 19.

Decimal-to-Binary Conversion
Decimal numbers can be converted to binary numbers through a specific process. This topic describes how
to convert decimal numbers to binary numbers.
Decimal-to-Binary Conversion
BaseExponent 27 26 25 24 23 22 21 20
Place Value 128 64 32 16 8 4 2 1
Example:
Convert
decimal
35 to binary
0 0 1 0 0 0 1 1
35 = (27*0
)+
(26*0
)+
(25*1
)+
(24*0
)+
(23*0
)+
(22*0
)+
(21*1
)+
(20*1)
35 = (32*1) + (2*1) + (1*1)
35 = 0 + 0 + 1 + 0 + 0 + 0 + 1 + 1
35 =
00100011
This figure shows a simple binary conversion of the decimal number 35. The base exponent line shows
base-2 numbers and their exponents (2 * 2 = 4 * 2 = 8 and so on). The decimal value of the base exponent
number is listed in the second row, and the binary number is displayed in the third row. The table describes
the steps to determine the binary number. Note that the first 2 bits of the binary number are 0s. These 0s are
known as leading 0s. In reality, the decimal number 35 would only be a 6-bit binary number. Because IP
addresses are laid out as four sets of octets, the binary number is made into an octet by placing 0s to the left
of the 6-bit number.
The table shows the steps for converting the number 35 to a binary number.
Procedure for Converting a Decimal Number to a Binary Number
Step Action
1. Looking at the figure, what is the greatest power of 2 that is less than or equal to 35? 128 does not go into
35, so place a 0 in that column.
2. 64 does not go into 35, so place a 0 in that column.
3. 25 (32) is smaller than 35. 32 goes into 35 one time. Place a 1 in that column.
4. Calculate how much is left over by subtracting 32 from 35. The result is 3.
5. Check to see if 16 (the next lower power of 2) fits into 3. Because it does not, a 0 is placed in that column.
6. The value of the next number is 8, which is larger than 3, so a 0 is placed in that column also.
7. The next value is 4, which is still larger than 3, so it, too, receives a 0.
8. The next value is 2, which is smaller than 3. Because 2 fits into 3 one time, place a 1 in that column.

Step Action
9. Subtract 2 from 3, and the result is 1.
10. The decimal value of the last bit is 1, which fits into the remaining number. Therefore, place a 1 in the last
column. The binary equivalent of the decimal number 35 is 00100011.

IP Address Classes
To accommodate networks of various sizes and to aid in classifying them, IP addresses are divided into
categories that are called classes. This topic describes the IP address classes and the structure of the IP
addresses within them.
IP Address Classes
A B C … Easy as 1 2 3
Host
. Host
. Host
.
Network
. Host
. Host
.
Network
. Network
. Host
.
Class A … First 1 Bit Fixed 0 x x x x x x x
Class B … First 2 Bits Fixed 1 0 x x x x x x
Class C … First 3 Bits Fixed 1 1 0 x x x x x
Assigning IP addresses to classes is known as classful addressing. The classes were determined during the
early days of the Internet by the IANA.
Each IP address is broken down into a network ID and a host ID. In addition, a bit or bit sequence at the
start of each address determines the class of the address. The figure shows three of the five IP classes.
Class A
A Class A address block is designed to support extremely large networks with more than 16 million host
addresses. The Class A address uses only the first octet (8 bits) of the 32-bit number to indicate the network
address. The remaining three octets of the 32-bit number are used for host addresses. The first bit of a Class
A address is always a 0. Because the first bit is a 0, the lowest number that can be represented is 00000000
(decimal 0), and the highest number that can be represented is 01111111 (decimal 127). However, these two
network numbers, 0 and 127, are reserved and cannot be used as network addresses. Any address that starts
with a value between 1 and 126 in the first octet of the 32-bit number is a Class A address.
Class B
The Class B address space is designed to support the needs of moderate to large networks with more than
65,000 hosts. The Class B address uses two of the four octets (16 bits) to indicate the network address. The
remaining two octets specify host addresses. The first two bits of the first octet of a Class B address are
always binary 10. Starting the first octet with binary 10 ensures that the Class B space is separated from the
upper levels of the Class A space. The remaining six bits in the first octet may be populated with either 1s
or 0s. Therefore, the lowest number that can be represented with a Class B address is 10000000 (decimal
128), and the highest number that can be represented is 10111111 (decimal 191). Any address that starts
with a value in the range of 128 to 191 in the first octet is a Class B address.

Class C
The Class C address space is the most commonly available address class. This address space is intended to
provide addresses for small networks with a maximum of 254 hosts. In a Class C address, the first three
octets (24 bits) of the IP address identify the network portion, with the remaining octet reserved for the host
portion. A Class C address begins with binary 110. Therefore, the lowest number that can be represented is
11000000 (decimal 192), and the highest number that can be represented is 11011111 (decimal 223). If an
address contains a number in the range of 192 to 223 in the first octet, it is a Class C address.
Note Additionally, there is also Class D address space, dedicated for multicast addressing and Class E
address space, which is reserved for experimental use and research. The Class D address space range
goes from 224.0.0.0 to 239.0.0.0, while the Class E range uses IPs from 240.0.0.0 to 254.0.0.0.
IP Address Classes (Cont.)
IP Address Ranges
IP Address
Class
First Octet
Decimal
Value
First Octet Binary Value
Possible
Number of
Hosts
Class A 1–126 00000001 to 01111110* 16,777,214
Class B 128–191 10000000 to 10111111 65,534
Class C 192–223 11000000 to 11011111 254
*127 (01111111) is a Class A address reserved for loopback testing and cannot
be assigned to a network.
The table shows the IP address range of the first octet (in decimal and binary) for Class A, B, and C IP
addresses, as well as the number of host addresses available for each class of addresses.
Each class of network allows a fixed number of hosts. In a Class A network, the first octet is assigned to the
network, leaving the last three octets to be assigned to hosts. The first host address in each network (all 0s)
is reserved for the actual network address, and the final host address in each network (all 1s) is reserved for
broadcasts. The maximum number of hosts in a Class A network is 224 – 2 (subtracting the network and
broadcast reserved addresses), or 16,777,214.
In a Class B network, the first two octets are assigned to the network. The final two octets are assigned to
hosts. The maximum number of hosts in a Class B network is 216 – 2, or 65,534.
In a Class C network, the first three octets are assigned to the network. The final octet can be assigned to
hosts, so the maximum number of hosts is 28 – 2, or 254.
Note RFC 1700 grouped the unicast ranges into specific sizes that are called Class A, Class B, and Class C
addresses, as presented in this topic. It also defined Class D (multicast) and Class E (experimental)
addresses.

Reserved IPv4 Addresses
Certain IP addresses are reserved and cannot be assigned to individual devices on a network. These reserved
addresses include a network address, which is used to identify the network itself, and a broadcast address,
which is used for broadcasting packets to all of the devices on a network. This topic describes the types of
reserved IP addresses and provides examples of each.
Reserved IPv4 Address
These are reserved IPv4 addresses:
Network address
Directed broadcast address
Local broadcast address
Local loopback address
All zeros address
Network Address
The network address is a standard way to refer to a network. An IP address that has binary 0s in all of the
host bit positions is reserved for the network address. For example, in a Class A network, 10.0.0.0 is the IP
address of the network containing the host 10.1.2.3. All hosts in 10.0.0.0 will have the same network bits.
The IP address 172.16.0.0 is a Class B network address, and 192.16.1.0 is a Class C network address. A
router uses the network IP address when it searches its IP routing table for the destination network location.
The decimal numbers that constitute the first two octets in a Class B network address are assigned. The last
two octets contain 0s because those 16 bits are for host numbers and are used for devices that are attached
to the network. In the IP address 172.16.0.0, the first two octets are reserved for the network address and are
never used as an address for any device that is attached to it. An example of an IP address for a device on
the 172.16.0.0 network is 172.16.16.1. In this example, 172.16 is the network address portion and 16.1 is
the host address portion.

Directed Broadcast Address
The broadcast IP address of a network is a special address for each network that allows communication to
all of the hosts in that network. To send data to all of the hosts in a network, a host can send a single packet
that is addressed to the broadcast address of the network. The broadcast address uses the highest address in
the network range. This is the address in which the bits in the host portion are all 1s. For network 10.0.0.0,
with 8 network bits, the broadcast address would be 10.255.255.255. This address is also referred to as the
directed broadcast.
For the network address 172.16.0.0, the last 16 bits make up the host field (or host part of the address). The
broadcast that would be sent out to all of the devices on that network would include a destination address of
172.16.255.255.
The directed broadcast address can be routed. However, for some versions of the Cisco IOS operating
system, routing directed broadcasts is not the default behavior.
Local Broadcast Address
If an IP device wants to communicate with all of the devices on the local network, it sets the destination
address to all 1s (255.255.255.255) and transmits the packet. For example, hosts that do not know their
network number and are asking a server for it may use this address. The local broadcast is never routed.
Local Loopback Address
A local loopback address is used to let the system send a message to itself for testing. The loopback address
creates a shortcut method for TCP/IP applications and services that run on the same device to communicate
with each other. A typical local loopback IP address is 127.0.0.1.
All zeros address
Address 0.0.0.0 indicates the host in "this" network and is used only as a source address. An example use
case is the DHCP assignment process before the host has a valid IP address.
For more information about reserved IPv4 addresses, refer to RFC 5735, at http://tools.ietf.org/html/
rfc5735.

Domain Name System
DNS provides an efficient way to convert human-readable names of IP end systems into machine-readable
IP addresses that are necessary for routing. This topic describes the function of DNS.
Domain Name System
I need to access cisco.com.
What is its IP address?
Domain name cisco.com
maps to 198.133.219.25.
The domain name cisco.com is not in my database.
I‘ll forward request to another DNS Server.
DNS Server
DNS Server
Web Server
cisco.com
198.133.219.25
1
2
3
4
5
In data networks, devices are labeled with numeric IP addresses so that they can send and receive messages
over the network. However, most people find it difficult to remember this numeric address. Therefore,
domain names were created to convert the numeric address into a simple, recognizable name.
DNS was created for domain name-to-address resolution for networks. DNS uses a set of servers to resolve
the names that are associated with numbered addresses. The DNS protocol defines an automated service
that matches resource names with the required numeric network address.

Verifying the IPv4 Address of a Host
Most operating systems and devices connected to a network provide a set of tools that can be used to verify
host addressing. This topic focuses on the tools available on the Microsoft Windows platform. Additionally,
it also describes how to verify the IP address of a switch acting as a host.
Verifying the IPv4 Address of a Host
Windows Platform
The following describes the syntax for the ipconfig command:
On a PC with the Microsoft Windows operating system, the ipconfig command can be used to display all
current TCP/IP network configuration values at the command line. Using various parameters, the command
can also be used to refresh DHCP and DNS settings. Used without parameters, the ipconfig command
displays the IP address, subnet mask, and default gateway for all adapters.
ipconfig [/all] [/renew [Adapter]] [/release [Adapter]] [/flushdns]
These parameters are commonly used:
/all: Displays the complete TCP/IP configuration for all adapters. Without this parameter, the ipconfig
command displays only the IP address, subnet mask, and default gateway values for each adapter.
Adapters can represent physical interfaces, such as installed network adapters, or logical interfaces,
such as dial-up connections.
/renew [Adapter]: Renews the DHCP configuration for all of the adapters (if an adapter is not
specified) or for a specific adapter if the Adapter parameter is included. This parameter is available
only on computers with adapters that are configured to obtain an IP address automatically. To specify
an adapter name, type the adapter name that appears when you use the ipconfig command without
parameters.
/release [Adapter]: Sends a DHCPRELEASE message to the DHCP server to release the current
DHCP configuration and discard the IP address configuration for either all of the adapters (if an adapter
is not specified) or for a specific adapter if the Adapter parameter is included. This parameter disables
TCP/IP for adapters that are configured to obtain an IP address automatically. To specify an adapter
name, type the adapter name that appears when you use the ipconfig command without parameters.

/?: Displays help at the command prompt.
Additional information and the command syntax can be found at http://technet.microsoft.com/en-us/library/
dd197434.
Verifying the IPv4 Address of a Host (Cont.)
Verifying IP address of a switch
Switch#show ip interface brief
Interface IP-Address OK? Method Status Protocol
Vlan1 10.1.1.11 YES manual up up
FastEthernet0/1 unassigned YES unset up up
FastEthernet0/2 unassigned YES unset down down
FastEthernet0/5 unassigned YES unset down down
<output omitted>
Although a switch does not need an IP address for its operation of forwarding frames, it uses one for
management purposes. You can verify the IP address settings with the show ip interface brief command.

Summary
Summary
IP is a Layer 3 media-independent connectionless protocol that uses
hierarchical logical addressing and provides service in a best-effort
manner.
Every node that is connected to the Internet has a unique IP address
that identifies it. An IP address consists of two parts: the network ID and
the host ID.
Every packet that travels through the network contains a source address
and a destination address.
Certain IP addresses (for example, network and broadcast addresses)
are reserved and cannot be assigned to individual network devices.
DNS is an application that is specified in the TCP/IP suite. It provides a
means to translate human-readable names into IP addresses.

Lesson 2
Understanding IP Addressing
and Subnets
Overview
Subnetworks, or subnets, are common in all but the smallest network environments, segmenting networks
into smaller divisions that have their own addresses. To create subnet addresses, some of the bits that are
used for the host portion of an IP address are "borrowed" to create the subnet address. This lesson describes
how subnets function and how they are computed.
Objectives
Describe the purposes and functions of subnets and their addressing schemes
Explain the role of a subnet mask
Describe the octet values of a subnet mask
Describe how end systems use subnet masks and default gateways
Determine the number of subnets and hosts
Identify a procedure to determine subnet addresses
Show how to determine a subnet address
Determine an addressing scheme
Describe the role of VLSM
Use VLSM to subnet a network

Subnets
Network administrators often need to divide networks, especially large networks, into subnetworks, or
subnets, to provide scalability. This topic describes the purposes and functions of subnets and their
addressing schemes.
Subnets
There can be problems within a single broadcast domain:
The domain relies on MAC addresses for packet delivery.
Larger amounts of broadcast traffic consume resources.
All devices share the same broadcast domain.
A company that occupies a three-story building might have a network that is divided by floors, with each
floor divided into offices. Think of the building as the network, the floors as the three subnets, and the
offices as the individual host addresses.
A subnet segments the hosts within the network. Without subnets, the network has a flat topology. A flat
topology has a short routing table and relies on MAC addresses to deliver packets. MAC addresses have no
hierarchical structure. As the network grows, the use of the network bandwidth becomes less efficient.
There are other disadvantages to a flat network. All devices share the same bandwidth and broadcast
domain, and it is difficult to apply security policies because there are no boundaries between devices.
On a switch-connected network, the host sees all of the broadcasts in the broadcast domain. When traffic is
heavy, collisions caused by two or more devices that are transmitting simultaneously may be frequent. The
devices detect the collisions, stop transmitting, and then begin transmitting later, at random intervals. To
users, this process is perceived as the network slowing down. The switches are improving network
performance, but routers are a better choice in these situations. Routers can be used to separate networks by
breaking the network into multiple subnets.

Subnets (Cont.)
Solution: Subnetworks
Smaller networks are easier to manage.
Overall traffic is reduced.
You can apply network security policies more easily.
Finance
Subnetwork 3
Manufacturing
Subnetwork 2
Engineering
Subnetwork 1
WAN Interface
The advantages of subnetting a network are as follows:
Smaller networks are easier to manage and map to geographical or functional requirements.
Overall network traffic is reduced, which can improve performance.
You can more easily apply network security measures at the interconnections between subnets than
within a large single network.
In multiple-network environments, each subnetwork may be connected to the Internet by a single router.
This figure shows one router connecting multiple subnetworks to the Internet. The details of the internal
network environment and how the network is divided into multiple subnetworks are inconsequential to
other IP networks.
The IP addressing that is used in the flat network must be modified to accommodate the required
segmentation. A subnet mask identifies the network-significant portion of an IP address. The network-
significant portion of an IP address is simply the part that identifies the network that the host device is on.
This part is called the network address and defines every subnetwork. The use of segmentation is important
for the routing operation to be efficient.

Subnet Masks
This topic describes the role of the subnet mask within the network.
Subnet Masks
A subnet mask:
Defines the number of bits that represent the network and subnet part of
the address
Used by end systems to identify the destination IP address as either
local or remote
Used by Layer 3 devices to determine network path
Subnet mask: 255.255.0.0 or /16
IP Address: 172.16.55.87
?
Network Host
Subnet
How much of this address is the
network and subnet part?
How do you know how many bits represent the network portion of the address and how many bits represent
the host portion? When you express an IPv4 network address, you add a prefix length to the network
address. The prefix length is the number of bits in the address that give the network portion. For example, in
172.16.55.87 /16, /16 is the prefix length. It tells you that the first 16 bits are the network address. This
leaves the remaining 16 bits, the last octet, as the host portion. The entity that is used to specify the network
portion of an IPv4 address to the network devices is called the subnet mask. The subnet mask consists of 32
bits, just as the address does, and uses 1s and 0s to indicate which bits of the address are network bits and
which bits are host bits. You express the subnet mask in the same dotted decimal format as the IPv4
address. The subnet mask is created by placing a binary 1 in each bit position that represents the network
portion and placing a binary 0 in each bit position that represents the host portion. A /16 prefix is expressed
as a subnet mask of 255.255.0.0 (11111111.11111111.00000000.00000000). The remaining bits (low order)
of the subnet mask are zeroes, indicating the host address within the network.
The subnet mask is configured on a host with the IPv4 address to define the network portion of that address.
Networks are not always assigned the same prefix. Depending on the number of hosts on the network, the
prefix that is assigned may be different. Having a different prefix number changes the host range and
broadcast address for each network.
For example, look at the host 10.1.20.70/26:
Address:
10.1.20.70
00001010.00000001.00010100.01000100
Subnet mask:
255.255.255.196
11111111.11111111.11111111.11000000

Network address:
10.1.20.64
00001010.00000001.00010100.01000000

Octet Values of a Subnet Mask
This topic describes the octet values of a subnet mask.
Octet Values of a Subnet Mask
Subnet masks, like IP addresses, are represented in the dotted decimal
format, such as 255.255.255.0.
The binary 1 reflects the network and subnetwork part of the IP address.
Octet Values of a Subnet Mask (Cont.)
128 64 32 16 8 4 2 1
0 0 0 0 0 0 0 0 = 0
1 0 0 0 0 0 0 0 = 128
1 1 0 0 0 0 0 0 = 192
1 1 1 0 0 0 0 0 = 224
1 1 1 1 0 0 0 0 = 240
1 1 1 1 1 0 0 0 = 248
1 1 1 1 1 1 0 0 = 252
1 1 1 1 1 1 1 0 = 254
1 1 1 1 1 1 1 1 = 255
Although subnet masks use the same format as IP addresses, they are not IP addresses themselves. Each
subnet mask is 32 bits long, divided into four octets, and is usually represented in dotted decimal notation
like IP addresses. In their binary representation, subnet masks have all 1s in the network and subnetwork
portions, and have all 0s in the host portion.

Because the high-order bits of the subnet masks are contiguous 1s, there is only a limited number of subnet
values within an octet. Recall that you only need to expand an octet if the network and host division falls
within that octet. Therefore, the number of 8-bit patterns that are used in address masks is limited.
The subnet field always immediately follows the network number. The bits that are borrowed to create a
subnet mask must be the first n bits in that octet. The borrowed bits must start with the MSB of the default
host field. This figure shows the borrowed n bits, where value n is from 1 to 8. The subnet mask is the tool
that is used by the router to determine which bits are routing (network and subnet) bits and which bits are
host bits.
The last column in the figure shows the decimal representation of the subnet mask for each combination of
borrowed bits. If the subnet mask for an octet is represented by 255, then all the equivalent bits in that octet
of the address are network bits. Similarly, if the subnet mask for an octet is represented by 0, then all the
equivalent bits in that octet of the address are host bits. In each of these cases, it is not necessary to expand
this octet to binary to determine the network and host portions.
The figure shows only one octet because the decimal representation of each octet is the same. In Class A,
the default subnet address is 255.0.0.0 or 11111111.00000000.00000000.00000000. If the three highest-
order bits from the next highest-order host octet are borrowed, they add up to 224. This value translates to
255.224.0.0 or 11111111.11100000.00000000.00000000. The same approach applies to the other classes.
The only difference is in the default subnet mask that is used for the various classes. In Class B, the default
subnet address is 255.255.0.0 or 11111111.11111111.00000000.00000000. In Class C, the default subnet
address is 255.255.255.0 or 11111111.11111111.11111111.00000000.
Default Subnet Masks, Class A
Example Class A address (decimal): 10.0.0.0
Example Class A address (binary): 00001010
.00000000.00000000.00000000
Default Class A mask (binary): 11111111
.00000000.00000000.00000000
Default Class A mask (decimal): 255.0.0.0
Default classful prefix length: /8

Default Subnet Masks, Class B
Example Class B address (decimal): 172.16.0.0
Example Class B address (binary): 10101100
.00010000.00000000.00000000
Default Class B mask (binary): 11111111
.11111111.00000000.00000000
Default Class B mask (decimal): 255.255.0.0
Default Subnet Masks, Class C
Example Class C address (decimal): 192.168.42.0
Example Class C address (binary): 11000000
.10101000.00101010.00000000
Default Class C mask (binary): 11111111
.11111111.11111111.00000000
Default Class C mask (decimal): 255.255.255.0
In IP addressing, the subnet mask identifies the addressing information that is necessary in order to send
packets toward their final destinations. The subnet mask identifies which bits within the IP address are the
network and subnet bits.
The figure shows the default subnet masks and the default classful prefix length for Class A, Class B, and
Class C addresses. The subnet mask itself is indicated with 1s in the binary notation for the mask. All other
bits are indicated as 0s.

Default Gateways
The source host is able to communicate directly with the destination host only if the two hosts are on the
same network. If the two hosts are on different networks, the sending host must send the data to the default
gateway, which will forward the data to the destination. This topic describes the function of a default
gateway.
Default Gateways
IP: 10.1.1.1
Subnet Mask:
255.255.255.0
IP: 192.168.3.2
Subnet Mask:
255.255.255.0
Default
Gateway
10.1.1.254
OK, I have some data to send to 10.1.1.2.
The destination host is in my local network, so I can
send data directly using the correct MAC address.
IP: 10.1.1.2
Subnet Mask:
255.255.255.0
Before an end system can send a packet to its destination, it must first determine if the destination address is
in the local network. The subnet mask defines the network part of the IP address. The end system compares
the network portion of the local network address with the destination network address of the packet to be
sent. If the network portion of the local network address is the same as the destination network address, then
the end system can deliver packets directly. If the network portion of the local network address is not the
same as the destination network address, then the packets must be forwarded to some other network.

Default Gateways (Cont.)
The default gateway is needed to send a packet out of the local network. If the network portion of the
destination address of the packet is different from the network of the originating host, the packet has to be
routed outside of the original network. To do this, the packet is sent to the default gateway. This default
gateway is a router interface that is connected to the local network. The default gateway interface has a
network layer address that matches the network address of the hosts. The hosts are configured to recognize
that address as the default gateway.
On a Windows computer, the Internet Protocol (TCP/IP) Properties tools are used to enter the default
gateway IP address. The host IP address as well as the default gateway address must have the same network
portion of their respective addresses.

Computing Usable Subnetworks and Hosts
One of the decisions you must make when creating subnets is to determine the optimal number of subnets
and hosts. This topic describes a process for planning subnets.
Possible Subnets and Hosts for a Class B
Network
Network Network
. . .
Bits to Borrow
Bits
Borrowed (s)
Subnets
Possible (2s)
Bits Remaining in
Host ID (h=16–s)
Hosts Possible per
Subnet (2h–2)
1 2 15 32,766
2 4 14 16,382
3 8 13 8,190
... ... ... ...
13 8192 3 6
14 16384 2 2
15 32768 1 0
16 65536 0 0
Consider a Class B network address in which 16 bits are used for the network ID and 16 bits are used for
the host ID. Therefore, there are 65,536 (216) possible addresses that are available to assign to hosts. Of
these, 65,534 are usable addresses after subtracting the two addresses that cannot be used—the broadcast
and subnet addresses.
Now, imagine that this Class B network is divided into subnets. If 2 bits are borrowed from the default 16-
bit host field, the size of the host field decreases to 14 bits. All possible combinations of 0s and 1s that
could occur in the remaining 14 bits produce a total number of possible hosts that could be assigned in each
subnet. Thus, the number of hosts that are assigned to each subnet is now 16,382.
In the same Class B network, if 3 bits are borrowed, the size of the host field decreases to 13 bits, and the
total number of assignable hosts for each subnet decreases to 8192 (213). The number of usable host
numbers decreases to 8190 (8192 – 2). This last example shows six (8 – 2) usable subnets in a Class B
network. Each of these six subnets can have 8190 (8192 – 2) usable host addresses.
The table shows all possible subnets and hosts for a Class B network.
Subnetting a Class B Network
Number of Bits
Borrowed
(s)
Number of Possible
Subnets
2s
Number of Bits
Remaining in Host ID (24
– s = h)
Number of Hosts
Possible per Subnet (2h –
2)
1 2 15 32766
2 4 14 16382
3 8 13 8190

Number of Bits
Borrowed
(s)
Number of Possible
Subnets
2s
Number of Bits
Remaining in Host ID (24
– s = h)
Number of Hosts
Possible per Subnet (2h –
2)
4 16 12 4094
5 32 11 2046
6 64 10 1022
7 128 9 510
8 256 8 254
9 512 7 126
10 1024 6 62
11 2048 5 30
12 4096 4 14
13 8192 3 6
14 16384 2 2
15 32768 1 0
16 65536 0 0

Applying Subnet Masks
Most network administrators work with existing networks, complete with subnets and subnet masks in
place. Network administrators need to be able to determine which part of an existing IP address is the
network and which part of the address is the subnet. Subnet masks provide this information. This topic
describes how to apply subnet masks.
Applying Subnet Masks
Procedure for implementing subnets:
1. Determine the IP address space.
2. Based on the organizational and administrative structure, determine the
number of subnets that are required.
3. Based on the address class and required number of subnets, determine
the number of bits that you need to borrow from the host ID.
4. Determine the binary and decimal value of the subnet mask.
5. Apply the subnet mask to the network IP address to determine the
subnet and host addresses.
6. Assign subnet addresses to specific interfaces for all devices that are
connected to the network.
The procedure that is described in the table explains how to select the number of subnets that you need for a
particular network and then apply a mask to implement subnets.
Procedure for Implementing Subnets
Step Action Example
1 Determine the IP address to be used for your
network.
Assume that you are assigned an address of
172.16.0.0 with a subnet mask of 255.255.0.0.
2 Based on your organizational and
administrative structure, determine the number
of subnets that are required for the network. Be
sure to plan for growth.
Assume that you are managing a worldwide network
in 25 countries. Each country has an average of eight
locations. Therefore, you will need 200 subnets.
3 Based on the address class and the number of
subnets that you selected, determine the
number of bits that you need to borrow from the
host ID.
To create 200 subnets, you need to borrow 8 bits (28
= 256).
4 Determine the binary and decimal values of the
subnet mask that you select.
For an address with 16 bits in the network ID, from
which you borrow 8 bits, the mask is /24.
The binary value of the mask is
11111111.11111111.11111111.00000000.
The decimal value of the mask is 255.255.255.0.

Step Action Example
5 Apply the subnet mask for the network IP
address to determine the subnet and host
addresses. You will also determine the network
and broadcast addresses for each subnet.
The first subnet is 172.16.0.0 with a subnet mask
255.255.255.0.
The network address is the first one in the subnet, the
broadcast address is the last one. The network
address is 172.16.0.0 while the broadcast address is
172.16.0.255. The next available subnet is
172.16.1.0 /24.
6 Assign subnet addresses to specific interfaces
on all of the devices that are connected to the
network.
Apply the first host address within the subnet to the
selected host or interface. The first available host
address within the first subnet is 172.16.0.1.

Determining the Network Addressing Scheme
This topic describes how to determine the network addressing scheme.
Example 1: The IP address with subnet mask is
172.16.36.42/24.
The following tables show the eight steps that are used to
determine the subnet addresses of a given IP address. In this
example, the IP address and subnet mask are as follows:
IP address: 172.16.36.42
Subnet mask: 255.255.255.0
(Cont.)
Step Description Example
1 Write down the octet that is
being split and all remaining
octets on the right in binary.
Third and fourth octet (36.42):
00100100.00101010
2 Write down the mask or classful
prefix length in binary.
Assigned mask (/24):
11111111.11111111.11111111.0000
0000
3 Draw a line to delineate the
subnet and host bits in the
assigned IP address. Write the
IP address and the mask on top
of each other so that you are
able to identify the significant
bits in the IP address.
Split octet (binary): 00100100 |
00101010
Split mask (binary): 11111111 |
00000000

(Cont.)
4 Copy the subnet bits four times. 00100100.00000000 (subnet
address)
00100100.00000001 (first address in
subnet)
00100100.11111110 (last address in
subnet)
00100100.11111111 (broadcast
address)
5 In the first line, define the
network address by placing all 0s
in the host bits.
6 In the last line, define the
broadcast address by placing all
1s in the host bits.
7 In the middle lines, define the
first and last host number.
8 Increment the subnet bits by 1 to
determine the next subnet.
00100101.00000000
When subnet addresses are defined, the host part includes two addresses that cannot be used for any host.
The first address consists of all 0s in the host part. All 0s define the subnet itself. The second address that
cannot be used for host addressing is all 1s in the host part of the address. All 1s define the broadcast
address on that subnet.
(Cont.)
After converting the addresses from binary to decimal, the
addresses for the subnets are as follows:
Subnet address: 172.16.36.0
First host address: 172.16.36.1
Last host address: 172.16.36.254
Broadcast address: 172.16.36.255
Next subnet address: 172.16.37.0

(Cont.)
Example 2: The IP address with subnet mask is
192.168.221.37/29.
The following tables show the eight steps that are used to
determine the subnet addresses of a given IP address. In this
example, the IP address and subnet mask are as follows:
IP address: 192.168.221.37
Subnet mask: 255.255.255.248
(Cont.)
1 Write down the octet that is being
split and all remaining octets on
the right in binary.
Fourth octet (37): 00100101
2 Write down the mask or classful
prefix length in binary.
Assigned mask (/29):
11111111.11111111.11111111.1111
1000
3 Draw a line to delineate the
subnet and host bits in the
assigned IP address. Write the IP
address and the mask on top of
each other so that you are able
to identify the significant bits in
the IP address.
Split octet (binary): 00100 | 101
Split mask (binary): 11111 | 000

(Cont.)
4 Copy the subnet bits four times. 00100000 (network address)
00100001 (first address in subnet)
00100110 (last address in subnet)
00100111 (broadcast address)
5 In the first line, define the
network address by placing all 0s
in the host bits.
6 In the last line, define the
broadcast address by placing all
1s in the host bits.
7 In the middle lines, define the
first and last host number.
8 Increment the subnet bits by 1 to
determine the next subnet.
00101000
(Cont.)
After converting the addresses from binary to decimal, the
addresses for the subnets are as follows:
Subnet address: 192.168.221.32
First host address: 192.168.221.33
Last host address: 192.168.221.38
Broadcast address: 192.168.221.39
Next subnet address: 192.168.221.40

Example: Addressing Scheme
This topic provides an example of determining a network addressing scheme.
Example: Addressing Scheme
The IP address with subnet mask is 192.168.5.139/27.
IP Address 192 168 5 139
IP Address 11000000 10101000 00000101 100|01011
Subnet Mask 11111111 11111111 11111111 111|00000
Network (2) 11000000 10101000 00000101 100|00000
Network (10) 192 168 5 128
First Host 192 168 5 100|00001 = 129
Last Host 192 168 5 100|11110 = 158
Directed Broadcast 192 168 5 100|11111 = 159
Next Network 192 168 5 101|00000 = 160
Steps to Determine Subnet Addresses
1 Write down the octet that is being split in binary. 10001011
2 Write down the mask or classful prefix length in binary. 11100000
3 Draw a line to delineate the significant bits in the
assigned IP address.
Cross out the mask so that you can view the significant
bits in the IP address.
100 | 01011
111 | 00000
4 Copy the significant bits four times. 100 00000 (first subnet address)
100 00001 (first host address)
100 11110 (last host address)
100 11111 (broadcast address)
5 In the first line, define the network address by placing
0s in the remaining host bits.
6 In the last line, define the directed broadcast address
by placing 1s in the host bits.
7 In the middle lines, define the first and last host ID for
this subnet.
8 Increment the subnet bits by 1 to determine the next
subnet address.
Repeat Steps 4 through 8 for all subnets.
101 00000 (next subnet address)

Subnet Addresses Table
Subnet
Number
Subnet ID Host Range Broadcast Address
All 0s 192.168.5.0 192.168.5.1 to 192.168.5.30 192.168.5.31
1 192.168.5.32 192.168.5.33 to 192.168.5.62 192.168.5.63
2 192.168.5.64 192.168.5.65 to 192.168.5.94 192.168.5.95
3 192.168.5.96 192.168.5.97 to 192.168.5.126 192.168.5.127
4 192.168.5.128 192.168.5.129 to 192.168.5.158 192.168.5.159
5 192.168.5.160 192.168.5.161 to 192.168.5.190 192.168.5.191
6 192.168.5.192 192.168.5.193 to 192.168.5.222 192.168.5.223
All 1s 192.168.5.224 192.168.5.225 to 192.168.5.254 192.168.5.255

Variable-Length Subnet Mask
This topic describes the purpose of a VLSM.
Variable-Length Subnet Masking
Network using fixed-length subnet masking:
172.16.5.0/24
172.16.6.0/24
172.16.7.0/24
A
B
C
HQ
172.16.1.0/24
172.16.3.0/24
172.16.0.0/24
172.16.0.0/16
The figure illustrates a network that uses fixed-length prefixes to address various segments. Address space
is wasted because large subnets are used for addressing point-to-point segments.
Variable-Length Subnet Masking (Cont.)
Network using VLSM:
The subnet 172.16.14.0/24 is divided into smaller subnets.
One subnet has a subnet mask /27.
Further subnetting of one of the unused /27 subnets into multiple /30
subnets.
172.16.14.32/27
172.16.14.64/27
172.16.14.96/27
A
B
C
HQ
172.16.2.0/24
172.16.14.136.0/30
172.16.1.0/24
172.16.0.0/16

VLSM affords the options of including more than one subnet mask within a network and of subnetting an
already subnetted network address. VLSM offers these benefits:
More efficient use of IP addresses: Without the use of VLSM, companies must implement a single
subnet mask within an entire Class A, B, or C network number.
For example, consider the 172.16.0.0/16 network address that is divided into subnetworks using /24
masking. One of the subnetworks in this range, 172.16.14.0/24, is further divided into smaller
subnetworks using /27 masking. These smaller subnetworks range from 172.16.14.0/27 to
172.16.14.224/27.
In the figure, one of these smaller subnets, 172.16.14.128/27, is further divided using the /30 prefix,
which creates subnets with only two hosts, to be used on the WAN links. The /30 subnets range from
172.16.14.128/30 to 172.16.14.156/30. The WAN links used the 172.16.14.132/30, 172.16.14.136/30,
and 172.16.14.140/30 subnets out of the range.
Better-defined network hierarchical levels: VLSM allows more hierarchical levels within an
addressing plan, which enables easier aggregation of network addresses. For example, in the figure,
subnet 172.16.14.0/24 describes all of the addresses that are further subnets of 172.16.14.0, including
those from subnet 172.16.14.0/27 to subnet 172.16.14.128/30.

VLSM Example
This topic provides an example of VLSM.
VLSM Example
?
50 Hosts
?
50 Hosts
?
50 Hosts
?
50 Hosts
Entire Region Subnet
172.16.32.0/20
In the figure, the subnet address 172.16.32.0/20, used for this portion of the enterprise network, is generated
from subnetting the 172.16.0.0/16 network into multiple /20 subnets.
VLSM Example (Cont.)
1st subnet: 172 . 16 .0010 0000.00 000000 = 172.16.32.0/26
2nd subnet: 172 . 16 .0010 0000.01 000000 = 172.16.32.64/26
3rd subnet: 172 . 16 .0010 0000.10 000000 = 172.16.32.128/26
4th subnet: 172 . 16 .0010 0000.11 000000 = 172.16.32.192/26
5th subnet: 172 . 16 .0010 0001.00 000000 = 172.16.33.0/26
Network Subnet VLSM
Subnet
Host
Subnetted address: 172.16.32.0/20
In binary: 10101100.00010000.00100000.00000000
VLSM address: 172.16.32.0/26
In binary: 10101100.00010000.00100000.00000000

By using VLSM, you can further subnet an already subnetted address. Consider, for example, that your
region of the enterprise network has a subnet address of 172.16.32.0/20 and that you need to assign
addresses to multiple LANs. Additionally, each LAN must have 50 hosts within your region. With VLSM,
you can further subnet address 172.16.32.0/20 to give you more network addresses and fewer hosts per
network. For example, if you subnet 172.16.32.0/20 to 172.16.32.0/26, you gain 64 (26) subnets, each of
which could support 62 (26 – 2) hosts.
172.16.32.0/26
50 Hosts
172.16.32.64/26
50 Hosts
172.16.32.128/26
50 Hosts
172.16.32.192/26
50 Hosts
?
2 Hosts
172.16.32.0/20
LAN Subnets
Derived from
172.16.32.0/20 ?
2 Hosts
2 Hosts
?
2 Hosts
?
1
1
1
1
1
1
In the figure, the subnet addresses that are used on the Ethernet LANs are generated from subdividing the
172.16.32.0/20 subnet into multiple /26 subnets.
To calculate the subnet addresses that are used on the WAN links, further subnet one of the unused /26
subnets.

1st subnet: 172 . 16 .33 .0000 00 00 = 172.16.33.0/30
2nd subnet: 172 . 16 .33 .0000 01 00 = 172.16.33.4/30
3rd subnet: 172 . 16 .33 .0000 10 00 = 172.16.33.8/30
4th subnet: 172 . 16 .33 .0000 11 00 = 172.16.33.12/30
Network Subnet VLSM
Subnet
Host
Subnetted address: 172.16.33.0/26
In binary: 10101100.00010000.00100001.00000000
VLSM address: 172.16.33.0/30
In binary: 10101100.00010000.00100001.00000000
For each WAN link, you need only two IP addresses, one for each end of a connection. Therefore, only two
bits are needed in the host portion of an IP address to create the appropriate subnet. Subnet mask /30 is used
for each WAN subnet.
172.16.32.0/26
50 Hosts
172.16.32.64/26
50 Hosts
172.16.32.128/26
50 Hosts
172.16.32.192/26
50 Hosts
172.16.33.030
2 Hosts
172.16.32.0/20
LAN Subnets
Derived from
172.16.32.0/20 172.16.33.4/30
2 Hosts
2 Hosts
172.16.33.8/30
2 Hosts
172.16.33.12/30
1
1
1
1
1
1
WAN Subnets
Derived from
172.16.33.0/26
In the figure, the subnet addresses that are used on the WAN links are generated from subdividing the
172.16.33.0/26 subnet into multiple /30 subnets. This mechanism provides 16 (24) subnets and 2 (22 – 2)
hosts for each of the WANs.

Summary
Summary
Networks, particularly large networks, are often divided into smaller
subnetworks, or subnets, which can improve network performance and
control.
The subnet mask defines the number of bits that represent the network
part or subnet part of an IP address.
End systems use subnet masks to identify the destination IP address as
either local or remote.
A default gateway is needed to send a packet out of the local network.
Determining the optimal number of subnets and hosts depends on the
type of network and the number of host addresses required.
The algorithm for computing a number of subnets is 2n, where n is the
number of subnet bits.
VLSM lets you allocate IP addresses more efficiently by adding multiple
layers to the addressing hierarchy.

Lesson 3
Understanding the TCP/IP
Transport Layer
Overview
Data networks and the Internet provide seamless and reliable communications, locally and world-wide.
Applications such as email, web browsers, and instant messaging allow you to use computers and networks
to send messages and find information. Data from applications is packaged, transported, and delivered to
the appropriate server or application on the destination device. The processes that are described in the
TCP/IP transport layer accept data from the application layer and prepare it for addressing at the internet
layer. The transport layer is responsible for the overall end-to-end transfer of application data. The transport
layer also provides functions to enable multiple applications to communicate over the network at the same
time on a single device. This layer uses error-processing mechanisms (if required) to ensure that all of the
data is received reliably and in order by the correct application.
For the Internet and internal networks to function correctly, data must be delivered reliably. You can ensure
reliable delivery of data through development of an application and by using the services that are provided
by the network protocol. In the TCP/IP and OSI models, the transport layer manages the process of reliable
data delivery. The transport layer hides the details of any network-dependent information from the higher
layers to provide transparent data transfer. TCP/IP UDP and TCP therefore operate between the network
layer and the application layer. Learning how UDP and TCP function between the network layer and the
transport layer provides a more complete understanding of how data is transmitted in a TCP/IP networking
environment. This lesson describes the function of the transport layer and how UDP and TCP operate.
Objectives
Explain the purpose and major functions of the TCP/IP transport layer
Contrast connection-oriented transport with connectionless transport
Explain the basic difference between TCP and UDP

List the characteristics of UDP
List the characteristics of TCP
List the common applications that are provided by TCP/IP

TCP/IP Transport Layer Functions
Residing between the application and internet layers, the transport layer is fundamental to the operation of
the TCP/IP layered network architecture. This topic describes the functions of the transport layer.
TCP/IP Transport Layer Functions
Application
Transport
Internet
Link
TCP UDP
• Session multiplexing
• Identification of different applications
• Segmentation*
• Flow control*
• Connection-oriented*
• Reliability*
*When Required
The TCP/IP internet layer directs information to its destination, but it cannot guarantee that the information
will arrive in the correct order, free of errors, or even that it will arrive at all. The two most common
transport layer protocols of the TCP/IP protocol suite are TCP and UDP. Both protocols manage the
communication of multiple applications and provide communication services directly to the application
process on the host. The basic service that is provided by the transport layer is tracking the individual
communications between applications on the source and destination hosts. This service is called session
multiplexing, and it is performed by both UDP and TCP. The premium service that is provided by the
transport layer is ensuring reliable delivery, which is performed only by TCP.
The primary duty of the transport layer is to track the individual communications between applications on
the source and destination hosts. This tracking is provided by both UDP and TCP. To pass data streams to
the proper applications, the transport layer must identify the target application. If TCP is used, the transport
layer has the further responsibilities of establishing end-to-end operations, segmenting data and managing
each piece, reassembling the segments into streams of application data, managing flow control, and
applying reliability mechanisms.
Session Multiplexing
Session multiplexing is an activity in which a single computer with a single IP address is able to support
multiple sessions simultaneously. A session is created when a source machine needs to send data to a
destination machine. Most often, this process involves a reply, but a reply is not mandatory.

Identifying the Applications
To pass data streams to the proper applications, the transport layer must identify the target application. The
transport layer assigns an identifier to an application. The TCP/IP protocols call this identifier a port
number. Each software process that needs to access the network is assigned a port number that is unique in
that host. This port number is used in the transport layer header to indicate to which application that piece
of data is associated.
Segmentation
TCP takes data chunks from the application layers and prepares them for shipment onto the network. Each
chunk is broken up into smaller segments that will fit the MTU of the underlying network layers. UDP is
simpler; it does no checking or negotiating and it expects the application process to give it data that will
work.
Flow Control
If a sender transmits data faster than the receiver can receive it, the receiver drops the data and requires it to
be retransmitted. Retransmission can waste time and network resources, which is why most flow control
methods try to maximize the transfer rate and minimize the required retransmissions.
Basic flow implementation in TCP uses acknowledgments that are generated by the receiver. For every data
chunk sent, the sender waits for this acknowledgment from the receiver before sending the next part.
However, if the RTT is significant, the overall transmission rate may slow to an unacceptable level. A
mechanism called windowing increases network efficiency when combined with basic flow control.
Windowing allows a receiving computer to advertise how much data it is able to receive before transmitting
an acknowledgment to the sending computer.
Connection-Oriented Transport Protocol
Within the transport layer, a connection-oriented protocol, such as TCP, establishes the session connection
and then maintains the connection during the entire transmission. When the transmission is complete, the
session is terminated.
Reliability
TCP reliability has these three main objectives:
Recognition and correction of data loss
Recognition and correction of duplicate or out-of-order data
Avoidance of congestion in the network
Reliability is not always necessary. For example, in a video stream, if a packet is dropped and then
retransmitted, it will appear out of order. That result would be frustrating and confusing to the audience and
serve no useful purpose. In real-time applications, such as voice and video streaming, dropped packets can
be tolerated as long as the overall percentage of dropped packets is low.

Reliable vs. Best-Effort Transport
“Reliable” and “best effort” are terms that describe two types of connections between computers. Each type
has advantages and disadvantages. This topic compares the two connection types.
Reliable vs. Best-Effort Transport
Reliable Best Effort
Protocol TCP UDP
Connection Type Connection-oriented Connectionless
Sequencing Yes No
Uses Email
File sharing
Downloading
Voice streaming
Video streaming
Reliable (Connection-Oriented)
Applications such as databases, web pages, and email require that all of the sent data must arrive at the
destination in its original condition in order for the data to be useful. Any missing data could cause a
corrupt communication that is either incomplete or unreadable. Therefore, these applications are designed to
use a transport layer protocol that implements reliability.
Some applications that use TCP are as follows:
Web browsers
Email
File transfers
TCP is the reliable protocol at the transport layer. To support reliability, a connection is established.
Establishing a connection ensures that the application is ready to receive the data. During the initial process
of connection establishment, information is exchanged about the capabilities of the receiver and starting
parameters are agreed to. These parameters are then used for tracking the data transfer for the duration of
the connection.
When the sending computer transmits data, it gives the data a sequence number. The receiver then responds
with an acknowledgment number equal to the next expected sequence number. This exchange of sequence
and acknowledgment numbers allows the protocol to recognize when data has been lost or duplicated or has
arrived out of order. TCP is a complex transport layer protocol.

Best Effort (Connectionless)
Other applications are more tolerant of the loss of small amounts of data. For example, if one or two
segments of a video stream fail to arrive, it would only create a momentary disruption in the stream. This
disruption might appear as distortion in the image, but it might not even be noticeable to the user.
Some applications that use UDP are as follows:
Video streaming
VoIP
UDP, providing best-effort delivery, does not need or want to keep information about previously sent data.
Therefore, UDP does not need to establish any connection with the receiver and is termed connectionless.
There are many situations in which best-effort delivery is more desirable than reliable delivery. A
connectionless protocol is desirable for applications that require faster communication without verification
of receipt.

TCP vs. UDP Analogy
This topic describes the basic difference between TCP and UDP, using a postal service analogy.
TCP vs. UDP Analogy
TCP
Registered
Letter
Letter
Received
Source Destination
UDP
Regular
Letter
Best-Effort
Delivery
Source Destination
No ACK
ACK is returned
Example: UDP—Sending Regular Mail
An analogy for UDP services is using the postal service to pay your bills. You address each bill payment to
a specific company address, stamp the envelope, and include your return address. The postal service
guarantees its best effort to deliver each payment. The postal service does not guarantee delivery, and it is
not responsible for telling you that delivery was successful or unsuccessful. Like the postal service, UDP is
a very simple protocol that provides only the most basic data-transfer services.
Example: TCP—Sending Certified Mail
An analogy for TCP services is sending certified mail through the postal service. Imagine that you live in
San Francisco and that you have a book that must be sent to your mother in New York. You discover that
the postal service will only process letters. You copy the pages in the book and put each page in a separate
envelope. To ensure that your mother reassembles the book correctly, you number each envelope. You
address the envelopes and send the first one as certified mail. The postal service delivers it by any truck and
any route but, because it is certified, the carrier who delivers it must get a signature from your mother and
return a certificate of delivery to you.
Sending each page separately is tedious, so you send several envelopes together. The postal service again
delivers each envelope by any truck and any route. Your mother signs a separate receipt for each envelope
in the batch as she receives them. If one envelope is lost in transit, you will not receive a certificate of
delivery for that numbered envelope, and you would resend that page. After receiving all the envelopes,
your mother puts the pages in the right order to recreate the book. Like certified mail, TCP is a complex
protocol that offers precise and traceable data transfer services.

UDP Characteristics
The transport layer of the TCP/IP stack contains protocols that provide addressing information so that data
can be transmitted over a network. UDP is an expansion of the early TCP/IP suite and is one of those
protocols. This topic describes some of the major functions of UDP.
UDP Characteristics
Operates at the transport layer of the TCP/IP stack
Provides applications with access to the network layer without the
overhead of reliability mechanisms
Operates as a connectionless protocol
Provides limited error checking
Provides best-effort delivery
Provides no data recovery features
UDP is a simple protocol that provides the basic transport layer functions.
UDP operates at the transport layer of the TCP/IP stack.
UDP provides applications with access to the network layer without the overhead of reliability
mechanisms.
Like IP, UDP is a connectionless protocol in which a one-way datagram is sent to a destination without
advance notification to the destination device.
UDP is capable of performing very limited error-checking. The UDP datagram includes an optional
checksum value, which the receiving device can use to test the integrity of the data. Additionally, the
UDP datagram includes a pseudo-header that includes the destination address. If the receiving device
sees that the datagram is directed to an inactive port, it returns a message that the port is unreachable.
UDP provides service on a best-effort basis and does not guarantee data delivery, because packets can
be misdirected, duplicated, or lost on the way to their destination.
UDP does not provide any special features that recover lost or corrupted packets. This functionality
does not mean that applications that use UDP are always unreliable. It simply means that these
functions are not provided by the transport layer protocol.

UDP Characteristics (Cont.)
The UDP header:
16-Bit Source Port 16-Bit Destination Port
16-Bit UDP Length 16-Bit UDP Checksum
Data
An analogy to UDP service is using a postal service to send flyers that notify neighbors about a garage sale.
In this example, the seller creates a flyer that advertises the day, time, and location of the garage sale. The
seller addresses each flyer with the specific name and address of each neighbor within a 2-mile (3.2-km)
radius. The postal service delivers each flyer by any truck and any route. However, the seller does not use
certified mail because it is not important if a flyer is lost in transit or if a neighbor acknowledges receipt of
the flyer.
UDP has the advantage of providing low-overhead data delivery. This advantage is why the UDP header
length is always only 64 bits (8 bytes). The figure shows the field definitions in the UDP segment:
Source Port: Number of the calling port (16 bits)
Destination Port: Number of the called port (16 bits)
Length: Length of UDP header and UDP data (16 bits)
Checksum: Calculated checksum of the header and data fields (16 bits)
Data: ULP data (varies in size)
Application layer protocols that use UDP include DNS, SNMP, DHCP, RIP, TFTP, NFS, and online games.

TCP Characteristics
TCP is a connection-oriented protocol that provides data reliability between hosts. TCP has a number of
characteristics that are related to the way in which it accomplishes this transmission. This topic describes
the major characteristics of TCP.
TCP Characteristics
Transport layer of the TCP/IP stack
Access to the network layer for applications
Connection-oriented protocol
Full-duplex mode operation
Error checking
Sequencing of data packets
Reliable delivery—acknowledgment of receipt
Data recovery features
Flow control
TCP is another protocol in the transport layer of the TCP/IP stack that provides addressing information so
that data can be transmitted over a network.
TCP is characterized as follows:
Like UDP, TCP operates at the transport layer of the TCP/IP stack.
Like UDP, TCP provides a service to applications: access to the network layer.
TCP is a connection-oriented protocol where two network devices set up a connection to exchange
data. The end systems synchronize with each other to manage packet flows and adapt to congestion in
the network.
A TCP connection is a pair of virtual circuits, one in each direction, so that it operates in full-duplex
mode.
TCP provides error checking by including a checksum in the datagram to verify that the TCP header
information is not corrupt.
TCP segments are numbered and sequenced so that the destination can reorder segments and determine
if data is missing.
Upon receipt of one or more TCP segments, the receiver returns an acknowledgment to the sender to
indicate that it received the segment. Acknowledgments form the basis of reliability within the TCP
session. When the source receives acknowledgment, it knows that the data has been successfully
delivered. If the source does not receive acknowledgment within a predetermined period, it retransmits
that data to the destination. It may also terminate the connection if it determines that the receiver is no
longer on the connection.
TCP provides recovery services in which the receiver can request retransmission of a segment.

TCP provides mechanisms for flow control. Flow control assists the reliability of TCP transmission by
adjusting the effective rate of data flow between the two services in the session.
Reliable data delivery services are critical for applications such as file transfers, database services,
transaction processing, and other mission-critical applications in which delivery of every packet must be
guaranteed. TCP provides this reliability but sometimes sacrifices speed. UDP provides speed at the
expense of reliability.
TCP Characteristics (Cont.)
The TCP header:
Source Port Destination Port
Sequence Number
Acknowledgment Number
Header
Length
Reserved Flags Window Size
TCP Checksum Urgent Pointer
Options
Data
TCP segments are sent by using IP packets. The TCP header follows the IP header, and supplies
information specific to the TCP protocol. Flow control, reliability, and other TCP characteristics are
achieved by having fields in the TCP segment header, each with a specific function. The fields of the TCP
header include the following:
Source port: Number of the calling port (16 bits)
Destination port: Number of the called port (16 bits)
Sequence number and acknowledgment number: Tools for reliability mechanisms (each 32 bits)
Acknowledgment number: The next TCP octet that is expected by the receiver (32 bits)
Checksum: Calculated checksum of the header and fields that are used for error checking (16 bits)
Data: Upper-layer protocol data (varies in size)
Header length: Number of 32-bit words in the header (4 bits)
Reserved: Set to 0 (3 bits)
Flags: Used in session management and in treatment of segments (9 bits). A single bit that has a
specific meaning is often referred to as a flag.
Window size: Number of octets that the device is willing to accept (16 bits)
Urgent pointer: Indicates the end of the urgent data (16 bits)
Options: The variable-length field, which contains optional headers that you may want to use.
Maximum TCP segment size is currently defined.

TCP/IP Applications
UDP and TCP use internal software ports to support multiple conversations between various network
devices. To differentiate the segments and datagrams for each application, TCP and UDP both have header
fields that uniquely identify these applications. These unique identifiers are the port numbers. This topic
lists various applications along with their port numbers and short descriptions.
TCP/IP Applications
TCP UDP
F
T
P
T
e
l
n
e
t
H
T
T
P
D
N
S
T
F
T
P
S
N
M
P
Application
Layer
Network
Layer
Transport
Layer
Port
Numbers
IPv6 IPv4
Protocols
S
S
H
H
T
T
P
S
21 23 80 443 69 161
22 53
These are some of the applications that TCP/IP supports:
FTP (port 21, TCP): FTP is a reliable, connection-oriented service that uses TCP to transfer files
between systems that support FTP. FTP supports bidirectional binary and ASCII file transfers. In
addition to port 21, which is used for exchange of control, it also uses one additional port for data
transmission.
SSH (port 21, TCP): SSH provides the capability to remotely access other computers, servers, and
networking devices. SSH enables a user to log in to a remote host and execute commands. SSH
messages are encrypted.
Telnet (port 23, TCP): Telnet is a predecessor to SSH. It sends messages in unencrypted cleartext.
Most organizations now use SSH for remote communications.
HTTP (port 80): HTTP defines how messages are formatted and transmitted and what actions
browsers and web servers take in response to various commands. It uses TCP.
HTTPS (port 443, TCP): HTTPS combines HTTP with a security protocol (SSL/TLS).
DNS (port 53, TCP and UDP): DNS is used to resolve Internet names to IP addresses. DNS uses a
distributed set of servers to resolve names that are associated with numbered addresses.
TFTP (port 69, UDP): TFTP is a connectionless service. Routers use TFTP to transfer configuration
files and Cisco IOS images as well as other files between systems that support TFTP.
SNMP (port 161, UDP): SNMP is an application layer protocol and facilitates the exchange of
management information between network devices. SNMP enables network administrators to manage
network performance, find and solve network problems, and plan for network growth.

These are just some applications with their port numbers. Go to the Service Name and Transport Protocol
Port Number Registry for a complete list at http://www.iana.org/assignments/service-names-port-numbers/
service-names-port-numbers.xml.

Summary
Summary
The purpose of the transport layer is to hide the network requirements
from the application layer and to ensure end-to-end transfer of
application data.
Connection-oriented transport provides reliable transport.
Connectionless transport provides best-effort transport.
UDP is a protocol that operates at the transport layer and provides
applications with access to the network layer without the overhead of the
reliability mechanisms of TCP. UDP is a connectionless, best-effort
delivery protocol.
TCP is a protocol that operates at the transport layer and provides
applications with access to the network layer. TCP is connection-
oriented and provides reliable transport.
Port numbers identify applications.

Lesson 4
Exploring the Functions of
Routing
Overview
Routing is the process that forwards data packets between networks or subnetworks, using a TCP/IP
internet layer device, a router. The routing process uses network routing tables, protocols, and algorithms to
determine the most efficient path for forwarding an IP packet. Routers gather routing information and
update other routers about changes in the network. Routers greatly expand the scalability of networks by
terminating Layer 2 collisions and broadcast domains. Understanding how routers function will help you to
understand the broader topic of how networks are connected and how data is transmitted over networks.
This lesson describes the operation of routing.
Objectives
Describe the role of a router in the IP packet delivery process
Describe the physical characteristics of a router
Describe the functions of a router
Describe the method that is used to determine the optimal path for forwarding IP packets
Describe the functions of the routing table in the routing process
List types of routes
Describe the function of dynamic routing protocols

Role of a Router
This topic describes the role of a router in internetwork communication.
Role of a Router
Routers are required to reach hosts that are not in the local network.
Routers use a routing table to route between networks.
Host A
Fa0/0 Fa0/1
Routing Table:
192.168.1.0/24 Fa0/0
192.168.2.0/24 Fa0/1
Host B
Router
192.168.1.0/24 192.168.2.0/24
While switches switch data frames between segments to enable communication within a single network,
routers are required to reach hosts that are not in the local LAN. Routers enable internetwork
communication by placing the interface of each router in the network of the other routers. They use routing
tables to route traffic between different networks.

Router Characteristics
This topic describes common characteristics of routers.
Router Characteristics
Router components:
CPU
Motherboard
Memory
Ports
Management: For the connection of a
terminal used for management
Network: Various LAN or WAN media
ports
Router
CPU Motherboard
Memory
Ports Network
Management
(Console, Ethernet, AUX)
RAM ROM Flash
Routers are essential components of large networks that use TCP/IP, because routers can accommodate
growth across wide geographical areas. These characteristics are common to all routers:
Routers have these components, which are also found in computers and switches:
CPU: The CPU, or processor, is the chip, that is installed on the motherboard, that carries out the
instructions of a computer program. For example, it processes all of the information that is
gathered from other routers or sent to other routers.
Motherboard: The motherboard is the central circuit board, which holds critical electronic
components of the system. The motherboard provides connections to other peripherals and
interfaces.
Memory: There are three types of memory: RAM, ROM, and flash. RAM is memory on the
motherboard that stores data during CPU processing. It is a volatile type of memory in that its
information is lost when power is switched off. ROM is read-only memory on the motherboard.
The content of ROM is not lost when power is switched off. Data stored in ROM cannot be
modified, or it can be modified only slowly or with difficulty, so it is mainly used to distribute
firmware. Flash memory is a nonvolatile storage chip that can be electrically erased and
reprogrammed.
Routers have various network ports to which IP addresses are assigned. Ports are used to connect
routers to other devices in the network.
Routers can have these types of ports:

Management port (console, Ethernet, auxiliary [AUX]): The router uses a console port to attach
to a terminal that is used for management, configuration, and control. High-end routers have a
dedicated Ethernet port used only for management. An IP address is assigned to the Ethernet port
and the router may be accessed from the management subnet. The AUX interface is used for
remote management of the router. Typically, a modem is connected to the AUX interface for dial-
in access. From a security standpoint, enabling the option to connect remotely to a network device
carries with it the responsibility of vigilant device management.
Network port: The router has a number of network ports, including various LAN or WAN media
ports, which may be copper or fiber cable.

Router Functions
This topic describes the two most important functions of routers, path determination and packet forwarding.
Router Functions
Path determination
Packet forwarding
RouterA#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
<output omitted>
Gateway of last resort is not set
172.17.0.0/16 is variably subnetted, 8 subnets
O 172.17.14.0/24 [110/51] via 172.17.100.22, 1d05h
B 172.17.25.0/24 [200/0] via 172.17.100.22, 6d05h
D 172.17.43.0/24 [90/30720] via 172.17.50.4, 3d20h, GigabitEthernet0/0
C 172.17.50.0/24 is directly connected, GigabitEthernet0/0
L 172.17.50.2/32 is directly connected, GigabitEthernet0/0
S 172.17.92.0/24 [1/0] via 172.17.50.4
Routing table on RouterA
Routers are devices that gather routing information from neighboring routers in the network. The routing
information that is processed locally goes into the routing table. The routing table contains a list of all
known destinations to the router and provides information about how to reach them. Routers have these two
important functions:
Path determination: Routers must maintain their own routing tables and ensure that other routers
know about changes in the network. Routers use a routing protocol to communicate network
information to other routers. A routing protocol distributes the information from a local routing table on
the router. Different protocols use different methods to populate the routing table. In the show ip route
output in the figure, the first letter in each line of the routing table indicates which protocol was the
source for the information (for example, O = OSPF). It is possible to statically populate routing tables
by manually configuring static routes. However, statically populating routing tables does not scale well
and leads to problems when the network topology changes. Design changes and outages can also pose
problems.
Packet forwarding: Routers use the routing table to determine where to forward packets. Routers
forward packets through a network interface toward the destination network. Each line of the routing
table indicates which network interface is used to forward a packet. The destination IP address in the
packet defines the packet destination. Routers use their local routing table and compare the entries to
the destination IP address of the packet. The result is a decision about which outgoing interface to use
to send the packet out of the router. If routers do not have a matching entry in their routing tables, the
packets are dropped.
Routers support three packet-forwarding mechanisms:
Process switching: Process switching is the oldest forwarding mechanism that is available in
Cisco routers. Every packet requires full lookup in the routing table, which makes this mechanism
very slow. It is typically not used in modern networks.

Fast switching: To overcome the slow performance of process switching, Cisco IOS platforms
support several switching mechanisms that use a cache to store the most recently used destinations.
The first packet whose destination is not found in the fast-switching cache is process-switched, and
an entry is created in the cache. Subsequent packets are switched in the interrupt code, using the
cache to improve performance.
Cisco Express Forwarding: Cisco Express Forwarding is the most recent and preferred Cisco
IOS packet-forwarding mechanism, which incorporates the best of the previous switching
mechanisms. Generation of cache table entries is not packet-triggered but change-triggered. When
something changes in the network topology, the change is also reflected in the cache table. This
makes Cisco Express Forwarding the fastest forwarding mechanism and the preferred choice.
In the figure, the show ip route command displays the routing table that the Cisco IOS Software of
RouterA is currently using to choose the best path to its destination networks.

Path Determination
During the path-determination portion of transmitting data over a network, routers evaluate the available
paths to remote destinations. This topic describes how routers determine the most efficient path for
forwarding packets.
Path Determination
Routers select the best path to the destination among various sources.
Administrative distance defines the reliability of the route source.
OSPF Best Path
EIGRP Best Path
Should I use the EIGRP or OSPF best path?
Which path?
A routing table holds only one entry per network. More sources of information about a particular
destination might exist. A router must be able to determine which source (routing protocol) it should use if
it has two identical routes to a network from two different sources. Multiple sources result from having
multiple dynamic routing protocols running and from static and default information being available. The
routing process that runs on the router must be able to evaluate all the sources and select the best one to
populate the routing table.
Routing protocols use different metrics to measure the distance and desirability of a path to a destination
network. Administrative distance is the feature that routers use to select the best path when there are two or
more routes to the same destination network from two routing protocols. Administrative distance defines
the reliability of a routing protocol. Each routing protocol is prioritized from most to least reliable
(believable) with the help of an administrative distance value.
For example, in the figure, the router on the far left has two paths to the destination network on the far right.
One of the paths is learned through a dynamic routing protocol, OSPF, and the other path is learned through
EIGRP. Since EIGRP has a better (lower) administrative distance than OSPF, the router on the far left will
use the EIGRP path and publish only the EIGRP path to the destination network in its routing table.

Routing Table
As part of the path-determination procedure, the routing process builds a routing table that identifies known
networks and how to reach them. This topic describes the function of the routing table in the routing
process.
Routing Table
A routing table lists all known destinations and information about how to reach
them.
Network
10.1.3.0
R2
Network
10.1.2.0 Network
10.1.1.0
Network
10.8.3.0
Routing Table
Network Interface or Next Hop
S0/0/0
fa0/0 fa0/1
10.1.2.0
10.1.1.0
10.8.3.0
10.1.3.0
Directly connected – fa0/0
Directly connected – fa0/1
Directly connected – s0/0/0
Via 10.1.2.2 (R2)
Routers forward packets using the information in a routing table. Each router has its own local routing table,
populated from different sources. Routing metrics vary, depending on the routing protocol that is running in
the router. The figure shows how routers maintain a table of information.
Routing Table Information
The routing table consists of an ordered list of known network addresses. Network addresses can be learned
dynamically by the routing process or by being statically configured. All directly connected networks are
added to the routing table automatically. Routing tables also include information about destinations and
next-hop associations. These associations tell a router that a particular destination is either directly
connected to the router or that it can be reached via another router. This router is the next-hop router and is
on the path to the final destination. When a router receives an incoming packet, it uses the destination
address and searches the routing table to find the best path. If no entry can be found, the router discards the
packet after sending an ICMP message to the source address of the packet.
In the figure, the routing table of the router in the middle shows the forwarding rules. When the router
receives a packet with a destination address on the 10.1.3.0 network, it must forward the packet to router R2
(the R2 interface with the IP address 10.1.2.2).
Routing Update Messages
Routers communicate with each other and maintain their routing tables. A number of update messages are
transmitted between routers in order to keep the routing tables updated. The information that is contained in
the routing update messages includes the destination networks that the router can reach and the routing
metric for each destination. By analyzing routing updates from neighboring routers, a router can
dynamically build and maintain its routing table.

Types of Routes
Routers can learn about other networks via directly connected networks, static routes, dynamic routes, and
default routes. This topic describes each of these types of routes.
Types of Routes
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, + - replicated route
Gateway of last resort is 10.1.1.1 to network 0.0.0.0
O 172.16.1.0/24 [110/2] via 192.168.10.2, 00:01:08, GigabitEthernet0/1
D 192.168.20.0/24 [90/156160] via 10.1.1.1, 00:01:23, GigabitEthernet0/0
S 192.168.30.0/24 [1/0] via 192.168.10.2
S* 0.0.0.0/0 [1/0] via 10.1.1.1
The routing table can be populated by these methods:
Directly connected networks: This entry comes from having router interfaces that are directly
attached to network segments. This method is the most certain method of populating a routing table. If
the interface fails or is administratively shut down, the entry for that network is removed from the
routing table. The administrative distance is 0 and therefore preempts all other entries for that
destination network. Entries with the lowest administrative distance are the best, most-trusted sources.
Static routes: A system administrator manually enters static routes directly into the configuration of a
router. The default administrative distance for a static route is 1; therefore, static routes will be included
in the routing table unless there is a direct connection to that network. Static routes can be an effective
method for small, simple networks that do not change frequently. For bigger or unstable networks,
static routes are not a scalable solution.
Dynamic routes: The router learns dynamic routes automatically when a routing protocol is
configured and a neighbor relationship to other routers is established. The information changes with
changes in the network and updates constantly. Larger networks require the dynamic routing method
because there are usually many addresses and constant changes. These changes require updates to
routing tables across all routers in the network, or connectivity is lost.
Default routes: A default route is an optional entry that is used when no explicit path to a destination
is found in the routing table. The default route can be manually inserted or it can be populated from a
dynamic routing protocol.
The figure displays the show ip route command, which is used to show the contents of the routing table in
a router. The first part of the output explains the codes, presenting the letters and the associated source of
the entries in the routing table.
The letter C, which is reserved for directly connected networks, labels the first and seventh entries.

The letter L, which is reserved for local routes and indicating local interfaces within connected
networks, labels the second and eighth entries.
The letter S, which is reserved for static routes, labels the sixth entry. The letter S with an asterisk (*)
indicates a static default route, as shown in the figure.
The letter O, which is reserved for the OSPF routing protocol, labels the third entry.
The letter D, which is reserved for EIGRP, labels the fourth entry. The letter D stands for DUAL, the
update algorithm used by EIGRP.

Dynamic Routing Protocols
Routing protocols use their own rules and metrics to build and update routing tables automatically. This
topic describes the routing metrics and methods that routing protocols use.
Dynamic Routing Protocols
Routing protocols choose different metrics to calculate best
paths.
100 Mb/s
1 Gb/s
1 Gb/s
Bandwidth
Delay
Cost
Hop Count
Host A
Host B
When a routing protocol updates a routing table, the primary objective of the protocol is to determine the
best information to include in the table. The routing algorithm generates a number, called the metric, for
each path through the network. Sophisticated routing protocols can base route selection on multiple metrics,
combining them into a single metric. Typically, the lower the metric value, the better the path.
Metrics can be based on either a single characteristic or on several characteristics of a path. These metrics
are most commonly used by routing protocols:
Bandwidth: The data capacity of a link (the connection between two network devices).
Delay: The length of time that is required to move a packet along each link from the source to the
destination. The delay depends on the bandwidth of intermediate links, port queues at each router,
network congestion, and physical distance.
Cost: An arbitrary value that is assigned by a network administrator, usually based on bandwidth,
administrator preference, or other measurement, such as load or reliability.
Hop count: The number of routers that a packet must travel through before reaching its destination. In
the figure, the hop count from Host A to Host B would be two if the path over the 100-Mb/s link is
used. The hop count would be three if the path over the 1-Gb/s link is used.

Distance Vector vs. Link State
You can divide routing protocols into two groups, based on how they operate and how they calculate the
best path:
Distance vector routing protocols: Distance vector routing protocols have information available that
is similar to the information from a road sign. If you are on the road and you do not have a map, you
must rely on road signs that tell you the direction of the destination and the distance to it. Similarly, in
distance vector routing, a router does not have to know the entire path to every network segment. The
router only has to know the direction, or vector, in which to send the packet. The distance vector
routing approach determines the direction (vector) and distance (hop count) to any destination network.
An example of a distance vector protocol is RIP, which uses hop count as its routing metric.
Link state routing protocols: Link state routing protocols have a complete map of the area. Each
router builds its own internal map of the entire network topology in its link state (topology) database.
Each router on its own runs an SPF algorithm to calculate the shortest path to all known destinations.
An example of a link state routing protocol is OSPF, which uses cost as its routing metric.

Summary
Summary
Routers enable internetwork communication.
Routers include various ports and hardware similar to PCs.
The primary functions of a router are path determination and packet
forwarding.
Routers select the best path from among different sources, based on
administrative distance.
Routing tables provide an ordered list of best paths to known networks.
Routers use various types of routes: directly connected networks and
static, dynamic, and default routes.
Dynamic routing protocols use different metrics to calculate the best
path.

Lesson 5
Configuring a Cisco Router
Overview
After hardware installation, when a Cisco router is turned on, it goes through its startup procedure. After the
operating system is loaded, you can start configuring the router. This lesson describes basic configuration,
how to configure interfaces, and how to use Cisco Discovery Protocol to discover connected neighboring
devices.
Objectives
Describe router startup
Perform initial router setup
Configure router interfaces
Configure a router interface IP address
Verify the router interface configuration
Describe the need for a network discovery protocol
Describe Cisco Discovery Protocol
Use the CLI to discover neighbors on a network using Cisco Discovery Protocol

Initial Router Startup
Starting a Cisco router requires verifying the physical installation, powering up the router, and viewing the
Cisco IOS Software output on the console. This topic describes the initial startup of Cisco routers.
Initial Router Startup
Initial startup:
Before you start the router, verify the power and cooling requirements,
cabling, and console connection.
Push the power switch to On.
System startup routines initiate the router software.
Cisco IOS Software output text appears on the console.
When a Cisco router powers up, it performs a power-on self-test (POST). During the POST, the router
executes diagnostics to verify the basic operation of the CPU, memory, and interface circuitry.
After verifying the hardware functions, the router proceeds with software initialization. During software
initialization, the router finds and loads the Cisco IOS image. When the Cisco IOS image is loaded, the
router finds and loads the configuration file, if one exists.

Initial Router Setup
When the router starts, it looks for a device configuration file. If it does not find one, the router executes a
question-driven initial configuration routine called setup. This topic describes the initial command-line
output and explains how to skip the setup dialog.
Initial Router Setup
Console
RouterX con0 is now available
Press RETURN to get started.
RouterX>
A configured router with an existing configuration displays a user EXEC mode prompt.
--- System Configuration Dialog ---
Would you like to enter the initial configuration dialog? [yes/no]:
A router without an existing configuration enters the system configuration dialog.
After a router completes the POST and loads a Cisco IOS image, it looks for a device configuration file in
its NVRAM. The NVRAM of the router is a type of memory that retains its contents, even when power is
turned off. If the router has a configuration file in NVRAM, the user EXEC mode prompt appears. The
figure shows the RouterX> prompt.
When you start a new Cisco router or a Cisco router without a configuration in NVRAM, there will be no
configuration file. If no valid configuration file exists in NVRAM, the operating system executes a
question-driven initial configuration routine that is referred to as the system configuration dialog or setup
mode.
The system configuration dialog is an interactive CLI mode that prompts you for information that is needed
to build an initial configuration for a Cisco networking device. It is not intended for more advanced
configuration. Rather than using setup mode, you can use various other manual configuration modes to
configure the router.
To skip the system configuration dialog and configure the router manually, answer the first question in the
system configuration dialog with No or press Ctrl-C. If desired, you can enter the system configuration
dialog anytime by issuing the setup command in privileged EXEC mode.

Configuring Router Interfaces
One of the main functions of a router is to forward packets from one network device to another. For the
router to perform this task, you must define the characteristics of the interfaces through which the packets
are received and sent. This topic describes the commands that are used to configure interfaces on Cisco
routers.
Configuring Router Interfaces
RouterX(config)#interface GigabitEthernet 0/0
RouterX(config-if)#
Enters GigabitEthernet 0/0 interface configuration mode
RouterX(config)#interface Serial 0/0/0
RouterX(config-if)#description Link to ISP
Enters Serial 0/0/0 interface configuration mode and adds descriptive text
The router interface characteristics include, but are not limited to, interface description, the IP address of the
interface, the data-link encapsulation method, the media type, the bandwidth, and the clock rate.
You can enable many features on a per-interface basis. Enter interface configuration mode using this
command:
Command Description
interface type module/slot/port Specifies an interface and enters interface configuration
mode. The type values include GigabitEthernet, Serial,
Loopback, Dialer, Tunnel, and others. The module, slot,
and port options identify specific interfaces.
A number that is used to identify each interface is assigned, based on the physical location of the interface
hardware in the router. This identification is critical when there are multiple interfaces of the same type in a
single router.
You can add a description to an interface to help you remember specific information about that interface.
Common descriptions include the network that is serviced by that interface or the customer that is
connected to that interface. The description is meant solely as a comment to help identify how the interface
is being used.

Command Description
description string Adds a text comment to the interface configuration when
in interface configuration mode. The string value is
limited to 238 characters. To remove the description, use
the no form of this command.
To quit interface configuration mode and to move into global configuration mode, enter the exit command.
The description will appear in the output when the configuration of the router is displayed. The same text
will appear in the show interfaces command output:
RouterX#show interfaces
<output omitted>
GigabitEthernet0/0 is up, line protocol is up
Hardware is CN Gigabit Ethernet,
address is 5475.d08e.9ad8 (bia 5475.d08e.9ad8)
Description: Link to ISP
<output omitted>
Configuring Router Interfaces (Cont.)
RouterX#configure terminal
RouterX(config)#interface GigabitEthernet 0/0
RouterX(config-if)#no shutdown
%LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0,
changed state to up
Enables an interface that is administratively shut down
RouterX(config-if)#shutdown
%LINK-5-CHANGED: Interface Serial0/0/0, changed state to
administratively down
%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0/0, changed
state to down
Administratively disables an interface
When an interface is first configured, except in setup mode, you must administratively enable the interface
before it can be used to transmit and receive packets. Use the no shutdown command to allow Cisco IOS
Software to use the interface.
You may want to disable an interface to perform hardware maintenance on a specific interface or a segment
of a network. You may also want to disable an interface if a problem exists on a specific segment of the
network and you must isolate that segment from the rest of the network.
The shutdown command administratively turns off an interface. To restart the interface, use the no
shutdown command.

Configuring the Cisco Router IP Address
Each interface on a Cisco router must have its own IP address to uniquely identify it on the network. This
topic describes how to configure the IP address for each interface on a Cisco router.
Configuring the Cisco Router IP Address
Each router interface needs a unique IP address.
172.18.0.1
172.18.0.2
192.168.1.1 Gi0/0
S0/0/0
RouterX(config-if)#ip address 172.18.0.1 255.255.0.0
Configures an IP address on the Serial 0/0/0 interface on router RouterX
Unique IP addressing is required for communication between hosts and other network devices. Each router
link to a different network is associated to a dedicated and unique subnet. The router needs to have an IP
address configured on each of its links to each network.
To configure an interface on a Cisco router, complete these steps.
Step Action Results and Notes
1 Enter global configuration mode using the configure
terminal command:
Router#configure terminal
Displays a new prompt:
Router(config)#
2 Identify the specific interface that requires an IP address
by using the interface type module/slot/port command:
Router(config)#interface Serial 0/0/0
Displays a new prompt; for example:
Router(config-if)#
3 Set the IP address and subnet mask for the interface by
using the ip address ip-address mask command:
Router(config-if)#ip address 172.18.0.1 255.255.0.0
Configures the IP address and subnet mask
for the selected interface
4 Enable the interface to change state from
administratively down to up by using the no shutdown
command:
Router(config-if)#no shutdown
Enables the current interface
5 Exit configuration mode for the interface by using the
exit command.
Router(config-if)#exit
Displays the global configuration mode prompt:
Router(config)#

Verifying Interface Configuration and Status
When you have completed the router interface configuration, you can verify the configuration by using
various show commands. This topic describes the show commands and the output that you see to verify the
configuration.
Router show ip interface brief Command
RouterX#show ip interface brief
Interface IP-Address OK? Method Status
Protocol
Embedded-Service-Engine0/0 unassigned YES NVRAM administratively down
down
GigabitEthernet0/0 10.1.1.1 YES manual up up
GigabitEthernet0/1 209.165.200.226 YES manual up up
GigabitEthernet0/2 unassigned YES NVRAM administratively down
down
Serial0/0/0 172.18.0.1 YES manual up up
Serial0/0/1 unassigned YES manual administratively down
down
Verifies the status of all interfaces
Router show ip interface brief Command
(Cont.)
Branch#show ip route
<output omitted>
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 172.18.0.0/16 is directly connected, Serial0/0/0
L 172.18.0.1/32 is directly connected, Serial0/0/0
Enabled interfaces populate the routing table
To display a brief summary of the IP information and status of an interface, use the show ip interface brief
command in privileged EXEC mode. The output fields and their meanings are shown in the table.

Output Field Description
Interface Type of interface
IP-Address IP address assigned to the interface
OK? "Yes" means that the IP address is valid, "No" means that the IP address is not
valid
Method Describes how the IP address was obtained or configured
Status Shows the status of the interface
Protocol Shows the operational status of the routing protocol on this interface
For more details about the show ip interface brief command, refer to the Cisco IOS Interface and
Hardware Component Command Reference at http://www.cisco.com/en/US/docs/ios-xml/ios/interface/
command/ir-s5.html
Router show interfaces Command
RouterX#show interfaces
Hardware is CN Gigabit Ethernet, address is f866.f231.7250 (bia
f866.f231.7250)
Description: Link to LAN
Internet address is 10.1.1.1/24
Full Duplex, 100Mbps, media type is RJ45
output flow-control is unsupported, input flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:53, output 00:00:09, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
<output omitted>
Verifies the statistics for all interfaces that are configured on the router
The show interfaces command displays the status and statistics of all of the network interfaces on the
router. The status for a specific interface can be displayed by using the show interfaces type module/slot/
port command. Some of the output fields for a Gigabit Ethernet interface and their meanings are shown in
the table.
Output Description
GigabitEthernet...is {up | down |
administratively down}
Indicates whether the interface hardware is currently active, down, or has been
taken down by an administrator
line protocol is
{up | down}
Indicates whether the software processes that manage the line protocol
consider the interface usable (that is, whether keepalives are successful). If the
interface misses three consecutive keepalives, the line protocol is marked as
down.
hardware Hardware type and MAC address

Output Description
description Displays the configured interface description
Internet address IP address followed by the prefix length (subnet mask)
MTU MTU of the interface
BW Bandwidth of the interface, in kilobits per second. The bandwidth parameter is
used to compute routing protocol metrics and other calculations.
DLY Delay of the interface, in microseconds
rely Reliability of the interface as a fraction of 255 (255/255 is 100 percent
reliability), calculated as an exponential average over 5 minutes
load Load on the interface as a fraction of 255 (255/255 is completely saturated),
calculated as an exponential average over 5 minutes
encapsulation Encapsulation method that is assigned to an interface
5 minute input rate,
5 minute output rate
Average number of bits and packets that were transmitted per second in the last
5 minutes
For more details about the show interfaces command, refer to Cisco IOS Interface and Hardware
Component Command Reference at http://www.cisco.com/en/US/docs/ios-xml/ios/interface/command/ir-
s4.html

Exploring Connected Devices
Most network devices, by definition, do not work in isolation. A Cisco device frequently has other Cisco
devices as neighbors on the network. Being able to obtain information about those other devices is
important to assist with network design decisions, troubleshooting, and completing equipment changes.
This topic describes why network discovery is needed and describes a tool that can be used for discovery.
Exploring Connected Devices
What are the neighboring devices of the router?
172.18.0.1
192.168.1.1 Gi0/0
S0/0/0
?
?
If no documentation about the network topology is available or if the existing documentation is not up to
date, you may need to discover the neighboring devices of a router. You can sometimes do this manually by
inspecting the physical wiring, if the devices are installed next to each other. When neighboring devices are
in other buildings or cities, you must use a different method.
One possibility is to use a dynamic discovery protocol that gathers information about directly connected
devices.
Cisco devices support Cisco Discovery Protocol, which provides information about directly connected
Cisco devices and their functions and capabilities.

Cisco Discovery Protocol
Cisco Discovery Protocol is a powerful network monitoring and troubleshooting tool. It is also an
information-gathering tool that is used by network administrators to obtain information about directly
connected Cisco devices. This topic describes the function of Cisco Discovery Protocol.
A proprietary utility that gathers information about directly connected
Cisco switches, routers, and other Cisco devices
Discovers neighboring devices, regardless of which protocol suite they
are running
LLDP—an alternative standards-based discovery protocol
Upper-layer entry
addresses
IPv4, IPv6, and others
Discovers and displays information about directly
connected Cisco devices
Media LAN, Frame Relay, ATM, others
Cisco Discovery Protocol is a proprietary tool that enables you to access a summary of protocol and address
information about Cisco devices that are directly connected. By default, each Cisco device sends periodic
messages, known as Cisco Discovery Protocol advertisements, to other directly connected Cisco devices.
These advertisements contain information such as the types of devices that are connected, the router
interfaces to which they are connected, the interfaces that are used to make the connections, and the model
numbers of the devices.
Information that is gathered from other devices can assist you in making network design decisions,
troubleshooting, and making changes to equipment. Cisco Discovery Protocol can be used as a network
discovery tool, helping you to build a logical topology of a network when such documentation is missing or
lacking in detail.
Cisco Discovery Protocol runs over the OSI data link layer, connecting the physical media to the upper-
layer protocols. Because Cisco Discovery Protocol operates at the data link layer, two or more Cisco
network devices can learn about each other even if they are using different network layer protocols, if they
are not configured, or if IP addressing, for example, is misconfigured on neighboring routers.
LLDP is a standardized vendor-independent discovery protocol that discovers neighboring devices from
different vendors. It is standardized by IEEE as the 802.1AB standard and it performs functions similar to
Cisco Discovery Protocol.
More information about LLDP and a comparison with Cisco Discovery Protocol can be found at http://
www.cisco.com/en/US/technologies/tk652/tk701/technologies_white_paper0900aecd804cd46d.html.

Discovering Neighbors Using Cisco Discovery
Protocol
The Cisco Discovery Protocol verification commands display information about neighboring devices. This
topic describes the information that is provided by the show cdp neighbors command.
Discovering Neighbors Using Cisco Discovery
Protocol
SW1 Branch HQ
Gi0/0
Fa0/13
192.168.1.2
192.168.1.1
10.1.1.1
10.1.1.11
S0/0/0
S0/0/1
Branch#show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
D - Remote, C - CVTA, M - Two-port Mac Relay
Device ID Local Intrfce Holdtme Capability Platform Port ID
HQ Ser 0/0/0 123 R S I CISCO2901 Ser 0/0/1
SW1 Gig 0/0 124 S I WS-C2960- Fas 0/13
Displays information about neighboring devices discovered with Cisco Discovery
Protocol
The figure shows the output from the show cdp neighbors command for router Branch. For each Cisco
Discovery Protocol neighbor, this information is displayed:
Neighbor device ID
Local interface
Hold time value, in seconds
Neighbor device capability code
Neighbor hardware platform
Neighbor remote port ID
Note The command output in the figure clearly shows OSI Layer 2 and OSI Layer 3 neighbors, a switch and
router, respectively.
The format of the show cdp neighbors output varies among different types of devices, but the available
information is generally consistent across devices.
The show cdp neighbors command can be also used on a Cisco Catalyst switch to display the Cisco
Discovery Protocol updates that were received on the local interfaces. Note that on a switch, the local
interface is referred to as the local port.

Using the show cdp neighbors detail
Command
SW1 Branch HQ
Gi0/0
Fa0/13
192.168.1.2
192.168.1.1
10.1.1.1
10.1.1.11
S0/0/0
S0/0/1
Branch#show cdp neighbors detail
-------------------------
Device ID: HQ
Entry address(es):
IP address: 192.168.1.2
Platform: Cisco CISCO2901/K9, Capabilities: Router Switch IGMP
Interface: Serial0/0/0, Port ID (outgoing port): Serial0/0/1
Holdtime: 132 sec
Version: Cisco IOS Software, C2900 Software (C2900-UNIVERSALK9-M),
Version 15.2(4)M1, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2012 by Cisco Systems, Inc.
Compiled Tue 20-Mar-12 18:57 by prod_rel_team
<output omitted>
Displays detailed information about neighboring devices
The show cdp neighbors detail or show cdp entry * command displays detailed information about
neighboring devices. To display information about a specific neighbor, the command string must include
the IP address or device ID of the neighbor. The show cdp entry command shows the following
information:
Neighbor device ID (router HQ in the figure)
OSI Layer 3 protocol information (for example, IP address 192.168.1.2 in the figure)
Device platform (Cisco 2901 in the figure)
Device capabilities (router, switch, IGMP in the figure)
Local interface type and outgoing remote port ID (Serial 0/0/0 in the figure)
Hold time value, in seconds (132 seconds in the figure)
Cisco IOS Software type and release (C2900 Software [C2900-UNIVERSALK9-M], Version
15.2[4]M1, in the figure)
For more details about show cdp commands, refer to the Cisco IOS Network Management Command
Reference at http://www.cisco.com/en/US/docs/ios/netmgmt/command/reference/nm_12.html#wp1115418

Summary
Summary
The router startup sequence begins with POST, then the Cisco IOS
image is found and loaded. Finally, the configuration file is loaded, if it
exists.
If a router starts without a configuration, the Cisco IOS Software
executes a question-driven configuration dialog, which can be skipped.
The main function of a router is to relay packets from one network
device to another.
Interface characteristics, such as the IP address and description, are
configured using interface configuration mode.
When you have completed router interface configuration, you can verify
it by using the show ip interface brief and show interfaces commands
Summary (Cont.)
Cisco Discovery Protocol is an information-gathering tool used by
network administrators to obtain information about directly connected
devices.
Cisco Discovery Protocol exchanges hardware and software device
information with its directly connected Cisco Discovery Protocol
neighbors.
The show cdp neighbors command displays information about the
Cisco Discovery Protocol neighbors of a router.
The show cdp neighbors detail command displays detailed Cisco
Discovery Protocol information on a Cisco device.

Lesson 6
Exploring the Packet Delivery
Process
Overview
Understanding the packet delivery process is a fundamental part of understanding networking devices. You
must understand host-to-host communications to administer a network. This lesson describes host-to-host
communications through a router by providing a graphic representation. The beginning of the lesson
illustrates the role of Layer 2 and Layer 3 addresses in packet delivery. The role of ARP follows. The lesson
ends with a step-by-step analysis of the packet delivery process that shows all the mechanisms in a network
scenario.
Objectives
Describe Layer 2 addressing
Describe Layer 3 addressing
Explain the role of ARP
Describe the host-to-host packet delivery
Describe the host-to-host packet delivery process through a switch

Layer 2 Addressing
This topic describes where the Layer 2 address fits into the host-to-host packet delivery process.
Layer 2 Addressing
Layer 2 characteristics:
Ethernet uses MAC addresses.
Identifies end devices in the LAN.
Enables the packet to be carried by the local media across each
segment.
Layer 2
MAC Address
Layer 2 defines how data is formatted for transmission and how access to the physical media is controlled.
Layer 2 devices provide an interface with the physical media. Some common examples are an NIC installed
in a host or switch.
Host-to-host communications require Layer 2 addresses. MAC addresses are assigned to end devices such
as hosts.
The OSI data link layer (Layer 2) physical addressing that is implemented as an Ethernet MAC address is
used to transport the frame across the local media. Although unique host addresses are provided, physical
addresses are not hierarchical. They are associated with a particular device, regardless of its location or to
which network it is connected. These Layer 2 addresses have no meaning outside the local network media.
An Ethernet MAC address is a two-part, 48-bit binary value that is expressed as 12 hexadecimal digits. The
address formats might appear like 00-05-9A-3C-78-00, 00:05:9A:3C:78:00, or 0005.9A3C.7800.
All devices that are connected to an Ethernet LAN have MAC-addressed interfaces. The NIC uses the MAC
address to determine if a message should be passed to the upper layers for processing. The MAC address is
permanently encoded into a ROM chip on an NIC. The MAC address is made up of the OUI and the vendor
assignment number.
Switches do not need assigned MAC addresses for frame forwarding. However, they need a MAC address
when remote access management is configured.

Layer 2 Addressing (Cont.)
Layer 2 addressing:
The router has two interfaces directly connected to two PCs.
Each PC and each router interface has its own unique MAC address.
L2 = 0800:0222:2222 L2 = 0800:0222:1111
L2 = 0800:0333:1111
L2 = 0800:0333:2222
L2 = Layer 2
The figure describes Layer 2 addressing in the host-to-host communications model. MAC addresses are
assigned to end devices such as hosts. The physical interfaces on a router provide a Layer 2 function and are
also assigned a MAC address.
As seen in the example, the router has two interfaces directly connected to two PCs. Each PC and each
router interface has its own unique MAC address.

Layer 3 Addressing
This topic describes where Layer 3 devices and addressing fit into the host-to-host communications model.
Layer 3 Addressing
Layer 3 devices and functions:
The network layer provides connectivity and path selection between two
host systems.
In the host, this is the path between the data link layer and the upper
layers.
In the router, it is the actual path across the network.
Layer 3
The network layer provides connectivity and path selection between two host systems that may be located
on geographically separated networks. At the boundary of each local network, an intermediary network
device, usually a router, de-encapsulates the frame to read the destination address that is contained in the
header of the packet (the Layer 3 PDU). Routers use the network identifier portion of this address to
determine which path to use to reach the destination host. Once the path is determined, the router
encapsulates the packet in a new frame and sends it toward the destination end device.
Layer 3 addressing:
Layer 3 addresses must include identifiers that enable intermediary
network devices to locate hosts on different networks.
TCP/IP protocol stack uses IP.
Layer 3

Layer 3 addresses must include identifiers that enable intermediary network devices to locate hosts on
different networks. In the TCP/IP protocol suite, every IP host address contains information about the
network where the host is located.
Intermediary devices that connect networks are called routers. The role of the router is to select paths for
and direct packets toward a destination. This process is known as routing. A router uses a list of paths that
is called a routing table to determine where to send data.
Layer 3 addresses are assigned to hosts and network devices that
provide Layer 3 functions.
Network devices maintain a routing table.
Routing Table
192.168.3.0/24 Interface Gi0/0
192.168.4.0/24 Interface Gi0/1
L3 = 192.168.3.1 L3 = 192.168.3.2 L3 = 192.168.4.1 L3 = 192.168.4.2
Gi 0/0 Gi 0/1
L3 = Layer 3
Layer 3 addresses are assigned to end devices such as hosts and to network devices that provide Layer 3
functions. The router has its own Layer 3 address on each interface. Each network device that provides a
Layer 3 function maintains a routing table.
As seen in the example, the two router interfaces belong to different networks. The left interface and the
directly connected PC belong to the 192.168.3.0/24 network, while the right interface and the directly
connected PC belong to the 192.168.4.0/24 network. For devices in different IP networks, a Layer 3 device
is needed to route traffic.

Address Resolution Protocol
For IP communication on Ethernet-connected networks, the logical (IP) address must be bound to the
physical (MAC) address of its destination. This process is performed by ARP, which is described in this
topic.
Address Resolution Protocol
ARP provides two basic
functions:
Resolving IP addresses to
MAC addresses
Maintaining a cache of
mappings
I need the MAC
address of
172.16.3.2.
I heard that broadcast.
That is me. Here is my
MAC address.
IP: 172.16.3.2 = Ethernet: 0800.0200.1111
IP: 172.16.3.2 = ???
Map IP Ethernet
Local ARP
To send data to a destination, a host on an Ethernet network must know the physical (MAC) address of the
destination. ARP provides the essential service of mapping IP addresses to physical addresses on a network.
The term address resolution refers to the process of binding the network layer IP address of a remote device
to its locally reachable, data link layer MAC address. The address is considered to be resolved when ARP
broadcasts the known information—the target destination IP address and its own IP address. All of the
devices on the Ethernet segment receive the broadcast. When the target recognizes itself by reading the
contents of the ARP request packet, it responds with the required MAC address in its ARP reply. The
address resolution procedure is completed when the originator receives the reply packet, which contains the
required MAC address, from the target and updates the table containing all of the current bindings. This
table is usually called the ARP table or ARP cache. The ARP table is used to maintain a correlation between
each IP address and its corresponding MAC address.

Address Resolution Protocol (Cont.)
The ARP table keeps a record of recent bindings of IP
addresses to MAC addresses.
On the PC:
C:Windowssystem32>arp -a
Interface: 192.168.250.11 --- 0xb
Internet Address Physical Address Type
192.168.250.1 00-1b-0c-5d-91-0f dynamic
192.168.250.12 00-0c-29-13-cc-bf dynamic
On the router:
Branch#show ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.100 5 000c.2993.6a84 ARPA GigabitEthernet0/0
Internet 10.1.1.101 4 000c.2913.ccc9 ARPA GigabitEthernet0/0
Each IP device on a network segment maintains a table in memory: the ARP table. This table maps the IP
addresses of other devices on the network with their physical (MAC) addresses. When a host wants to
transmit data to another host on the same network, it searches the ARP table to see if there is an entry. If
there is an entry, the host uses it. If there is not, ARP is used to get an entry.
Each entry, or row, of the ARP table has a pair of values: an IP address and a MAC address. The
relationship between the two values is a map, which simply means that you can locate an IP address in the
table and discover the corresponding MAC address. The ARP table caches the mapping for the devices on
the local LAN.
The ARP table is created and maintained dynamically. It adds and changes address relationships as they are
used on the local host. The entries in an ARP table on a PC usually expire after 300 seconds, which is the
default value. It takes up to four hours by default on routers. Such timeout ensures that the table does not
contain information for systems that may be switched off or that have been moved. When the local host
wants to transmit data again, the entry in the ARP table is regenerated through the ARP process.
If no device responds to the ARP request, the packet is dropped because a frame cannot be created.
The arp -a command displays the current ARP table for all interfaces on a PC using the Microsoft
Windows operating system.
To display the ARP table on the router, use the show ip arp EXEC command as follows:
show ip arp [ip-address] [host-name] [mac-address] [interface type number]
Syntax Description
ip-address (Optional) Displays ARP entries matching this IP address
host-name (Optional) Hostname

mac-address (Optional) 48-bit MAC address
interface type number (Optional) Displays ARP entries that are learned via this interface type and number

Host-to-Host Packet Delivery
This topic describes the process of delivering an IP packet over a routed network.
Host-to-Host Packet Delivery (Step 1 of 16)
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
APP
DATA
Application: Network, I have some data to send to
192.168.4.2, and I do not need a reliable connection.
Transport: I will use UDP. Send me the data.
Application: Here is the data.
A B
In this example, host 192.168.3.1 has data that it wants to send to host 192.168.4.2. The application does
not need a reliable connection, therefore UDP is selected. Because it is not necessary to set up a session, the
application can start sending data.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
APP
DATA
A B
APP
DATA
UDP
HDR
APP
DATA
UDP
HDR
DST IP
192.168.4.2
SRC IP
192.168.3.1
UDP: I will put it in a UDP header.
UDP: IP, send this to 192.168.4.2.
IP: I will put it in an IP header.

UDP prepends a UDP header (UDP HDR) and passes the PDU to IP (Layer 3) with an instruction to send
the PDU to 192.168.4.2. IP encapsulates the PDU in a Layer 3 packet, setting the source IP address (SRC
IP) of the packet to 192.168.3.1, while the destination IP address (DST IP) is set to 192.168.4.2.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
Layer 3: I am on 192.168.3.0/24 and the destination is
on 192.168.4.0/24, so we are on different segments.
I will have to use the default gateway 192.168.3.2.
Packet
Parking Lot
Layer 2: ARP, do you have a
mapping for 192.168.3.2?
The host sends any packet that is not destined for the local IP network to the default gateway. The default
gateway is the address of the local router, which must be configured on hosts (PCs, servers, and so on). IP
encapsulates the PDU in a Layer 3 packet and passes it to Layer 2 with instructions to forward it to the
default gateway.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
APP
DATA
UDP
HDR
DST IP
192.168.4.2
SRC IP
192.168.3.1
Layer 2: ARP, do you have a mapping for 192.168.3.2?
ARP: No, Layer 2 will have to hold the
packet while I resolve the addressing.

To deliver the packet, the host needs the Layer 2 information of the next-hop device. The ARP table in the
host does not have an entry and must resolve the Layer 2 address (MAC address) of the default gateway.
The default gateway is the next hop for the packet. The packet waits while the host resolves the Layer 2
information.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
Packet
Parking Lot
ARP: The ARP request will say that I am 192.168.3.1.
Are you 192.168.3.2?
SRC MAC
0800:0222:2222
DST MAC
Broadcast
ARP
Request
ARP
Request
SRC MAC
0800:0222:2222
DST MAC
Broadcast
ARP
Request
Since the host does not have a mapping of Layer 2 and Layer 3 addresses for the default gateway, the host
uses the standard ARP process to obtain the mapping. The host sends an ARP request to the router.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
Packet
Parking Lot
ARP
Request
SRC MAC
0800:0222:2222
DST MAC
Broadcast
ARP
Request
Router: I just received an ARP request.
Let me add host 192.168.3.1 to my
ARP table with MAC address
of 0800:0222:2222.

The user has programmed the IP address of 192.168.3.2 as the default gateway. Host 192.168.3.1 sends out
the ARP request, and the router receives it. The ARP request contains information about the host, and the
router adds the information in its ARP table.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
L
L
L
L
L
L
L
L
L
L
L
L
L3
2
A B
Packet
Parking Lot
SRC MAC
0800:0333:2222
DST MAC
0800:0222:2222
ARP
Reply
Router: I will send an ARP reply
that I am 192.168.3.2 with MAC
address 0800:0333:2222.
The router processes the ARP request like any other host and sends the ARP reply with its own information.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
Packet
Parking Lot
SRC MAC
0800:0333:2222
DST MAC
0800:0222:2222
ARP
Reply
ARP
Reply
ARP: I just got an ARP reply from 192.168.3.2.
Let me add its IP and MAC to my ARP table.
ARP: Now I have a mapping for my default gateway.
I can give Layer 2 a mapping for 192.168.3.2.
ARP: Layer 2, I have 192.168.3.2 mapped
to 0800:0333:2222
The host receives an ARP reply to the ARP request and enters the information in its local ARP table.

L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
Layer 2: I can send out that pending frame.
DST MAC
0800:0333:2222
SRC MAC
0800:0222:2222
APP
DATA
UDP
HDR
DST IP
192.168.4.2
SRC IP
192.168.3.1
Now the Layer 2 frame with the application data can be sent to the default gateway. The pending frame is
sent with the local host IP address and MAC address as the source. However, the destination IP address is
that of the remote host, but the destination MAC address is that of the default gateway.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
DST MAC
0800:0333:2222
SRC MAC
0800:0222:2222
APP
DATA
UDP
HDR
DST IP
192.168.4.2
SRC IP
192.168.3.1
APP
DATA
UDP
HDR
DST IP
192.168.4.2
SRC IP
192.168.3.1
Router L3: I need to
forward this packet.
Router L2: I received a frame with my
MAC address. I need to pass it to L3.
Router L3: This is not my
address. It needs to be routed.
When the frame is received by the router, the router recognizes its MAC address and processes the frame.
At Layer 3, the router sees that the destination IP address is not its address. A host Layer 3 device would
discard the frame. However, because this device is a router, it passes all packets that are for unknown
destinations to the routing process. The routing process determines where to send the packet.

Destination Next Hop Interface
192.168.3.0/24 Connected Gi 0/0
192.168.4.0/24 Connected Gi 0/1
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
APP
DATA
UDP
HDR
DST IP
192.168.4.2
SRC IP
192.168.3.1
Router L3: L2, send this packet.
Router L3: I have an interface on the
192.168.4.0/24 segment. I can
forward this packet directly to host.
The routing process looks up the destination IP address in its routing table. In this example, the destination
segment is directly connected. Therefore, the routing process can pass the packet directly to Layer 2 for the
appropriate interface.
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
Packet
Parking Lot
ARP
Request
SRC MAC
0800:0333:1111
DST MAC
Broadcast
ARP
Request
ARP: The ARP request will say
that I am 192.168.4.1.
Are you 192.168.4.2?
Layer 2 uses the ARP process to obtain the mapping for the IP address and the MAC address. The router
asks for the Layer 2 information in the same way as the hosts. An ARP request for the destination Layer 3
address is sent to the link.

L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
Packet
Parking Lot
ARP
Request
SRC MAC
0800:0333:1111
DST MAC
Broadcast
ARP
Request
The destination receives and processes the ARP request.
The host receives the frame that contains the ARP request and passes the request to the ARP process. The
ARP process takes the information about the router from the ARP request and places the information in its
local ARP table. The ARP process generates the ARP reply and sends it back to the router.

The router receives the ARP reply and takes the information that is required for forwarding the packet to the
next hop. The router populates its local ARP table and starts the packet-forwarding process.

L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.4.2
L2 = 0800:0222:1111
L3 = 192.168.4.1
L2 = 0800:0333:1111
L3 = 192.168.3.2
L2 = 0800:0333:2222
A B
DST MAC
0800:0222:1111
SRC MAC
0800:0333:1111
APP
DATA
UDP
HDR
DST IP
192.168.4.2
SRC IP
192.168.3.1
Router L2: I can send out that pending packet.
The frame is forwarded to the destination. Note that the router changed the Layer 2 address as needed, but it
will not change the Layer 3 address.
Note The router changes source and destination MAC addresses, while source and destination IP addresses
remain the same.
Note Remember that a switch does not change the frame in any way. When the switch receives the frame, it
needs to forward it out the proper port according to the MAC address table.

Role of a Switch in Packet Delivery
This topic describes host-to-host communication when data passes through a switch.
Role of a Switch in Packet Delivery (Step 1 of
4)
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.3.2
L2 = 0800:0333:2222
A
Fa0/1 Fa0/3
SRC MAC
0800:0222:2222
DST MAC
Broadcast
ARP
Request
Switch: I just received a frame from a
host that is not in my MAC table. Let
me add it to the table.
Fa0/6
MAC Port
0800:0222:2222 Fa0/1
This example focuses on the role of a switch in the host-to-host packet delivery process. An application on
host PC A wishes to send data to a distant network. Before an IP packet can be forwarded to the default
gateway, its MAC address needs to be obtained. ARP protocol on PC A creates an ARP request and sends it
out. Before the ARP request reaches other devices on a network, the switch receives it.
When the switch receives the frame, it needs to forward it out on the proper port. However, in this example,
the source and the destination MAC addresses are not in the MAC address table of the switch. The switch
can learn the port mapping for the source host from the source MAC address in the frame, so the switch will
add it to the table (0800:0222:2222 = port Fa0/1).

4)
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.3.2
L2 = 0800:0333:2222
A
Fa0/1 Fa0/3
SRC MAC
0800:0222:2222
DST MAC
Broadcast
ARP
Request
Switch: Since the destination address
of a frame is broadcast, I will flood
the frame out on all ports.
Fa0/6
MAC Port
0800:0222:2222 Fa0/1
Because the destination address of the frame is a broadcast, the switch has to flood the packet out to all of
the ports. The only exception is the port on which the switch received the broadcast frame.
4)
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.3.2
L2 = 0800:0333:2222
A
Fa0/1 Fa0/3
SRC MAC
0800:0333:2222
DST MAC
0800:0222:2222
ARP
Reply
Switch: I just received a frame from
a host that is not in my MAC table.
Let me add it to the table.
Fa0/6
A
MAC Port
0800:0222:2222 Fa0/1
0800:0333:2222 Fa0/3
The router replies to the ARP request and sends an ARP reply packet back to the sender as a unicast frame.
The switch learns the port mapping for the new source host from the source MAC address in the frame. The
switch adds it to the MAC address table (0800:0333:2222 = port Fa0/3).

4)
L3 = 192.168.3.1
L2 = 0800:0222:2222
L3 = 192.168.3.2
L2 = 0800:0333:2222
A
Fa0/1 Fa0/3
SRC MAC
0800:0333:2222
DST MAC
0800:0222:2222
ARP
Reply
Switch: The destination MAC is in
my MAC table, so I will send the
frame out on port Fa0/1.
Fa0/6
MAC Port
0800:0222:2222 Fa0/1
0800:0333:2222 Fa0/3
The destination address of the frame is found in the MAC address table, therefore the switch can forward
the frame out on port Fa0/1. In the case when the destination address is not found in the MAC address table,
the switch would need to flood the frame out on all ports.
All frames pass through the switch unchanged. When the MAC address table is built, all unicast frames are
sent directly to a destination host based on the destination MAC address and data stored in MAC address
table.

Summary
Summary
If hosts are not in the same network, the frame is sent to the default
gateway.
Frames sent to the default gateway have the local host source MAC
address and the default gateway destination MAC address.
A router changes the Layer 2 address as needed, but it does not change
the Layer 3 address.
The switch does not change the frame in any way, it just forwards the
frame out on the proper port according to the MAC address table.

Lesson 7
Enabling Static Routing
Overview
Routing is the process by which a packet moves from one location to another. In the terms of computer
networks, it is the process of determining where to send data packets destined for addresses outside of the
local network.
A router is a special-purpose computer that performs packet forwarding by learning about remote networks
and maintaining routing information. The router is the junction or intersection that connects multiple IP
networks. The primary forwarding decision of the router is based on OSI Layer 3 information, which is the
destination IP address.
To effectively manage an IP network, you must understand how both static and dynamic routing operate
and the impact that they have on IP networks. This lesson introduces static IP routing.
Objectives
Describe the basic characteristics of routing operations
Explain the differences between static and dynamic routing
Explain when to use static routing
Configure static routes
Configure default routes
Verify static route configuration

Routing Operations
This topic describes basic static and dynamic routing operations.
Routing Operations
Path identification and selection:
Discovers possible routes to the intended destination
Selects the best route
Maintains and verifies the routing information
Packet forwarding:
Router identifies the destination address
172.16.1.0
10.120.2.0
In networking, a router is the device that is used to route traffic. Routers forward packets by learning about
remote networks and maintaining routing information. To forward packets correctly, a router needs some
basic information about the network and the packet to be forwarded. The primary forwarding decision of
the router is based on Layer 3 information, which is the destination IP address.
To be able to route something, a router or any entity that performs routing must perform these functions:
Path identification and selection:
Identify the sources of routing information: Determine from which sources (other routers) that
the router can learn the paths to the given destinations.
Identify routes: Determine the initial possible routes or paths to the intended destination.
Select routes: Select the best path to the intended destination.
Maintain and verify routing information: Determine whether the known paths to the destination
are the most current.
Packet forwarding:
Identify the destination address: Determine the destination (or address) of the packet that needs
to be routed.

Routing Operations (Cont.)
A router must learn about destinations that are not directly connected to
it.
The routing table is used to determine the best path to the destination.
172.16.1.0
10.120.2.0
Network Protocol Destination Network Exit Interface Next Hop
Connected 10.120.2.0 fa0/0
Learned 172.16.1.0 s0/0/0 172.20.1.2
A router receives routing information from other routers and places it in its routing table. The routing table
is used to find the best match and path between the destination IP of a packet and a network address in the
routing table. The routing table ultimately determines the outgoing interface for forwarding the packet, and
the router encapsulates that packet in the appropriate data-link frame for that outgoing interface.
The routing table stores information about connected and remote networks. A connected network is directly
attached to one of the router interfaces. These interfaces are the gateways for the hosts on various local
networks. If the destination network is directly connected, the router already knows which interface to use
when forwarding packets.

Static and Dynamic Routing Comparison
This topic describes the differences between static and dynamic routing.
Static and Dynamic Routing Comparison
Static routes:
A network administrator manually enters static routes into the router.
A network topology change requires a manual update to the route.
Routing behavior can be precisely controlled.
Dynamic routes:
A network routing protocol automatically adjusts dynamic routes when
the topology or traffic changes.
Routers learn and maintain routes to the remote destinations by
exchanging routing updates.
Routers discover new networks by sharing routing table information.
Based on the router configuration, routers can forward packets over static routes or dynamic routes. Remote
networks are added to the routing table by an administrator either configuring static routes or enabling a
dynamic routing protocol.
There are two ways to tell the router where to forward packets to the networks that are not directly
connected:
Static routes: Routes to remote networks with an associated next hop can be manually configured on
the router. These routes are known as static routes. The administrator must manually update a static
route entry whenever an internetwork topology change requires an update. Static routes are user-
defined routes that specify the path that packets take between a source and a destination. These
administrator-defined routes allow very precise control over the routing behavior of the IP
internetwork.
Dynamic routes: The router dynamically learns routes and adds them to the routing table after an
administrator configures a routing protocol that helps determine routes. After the administrator enables
dynamic routing, the routing process automatically updates route knowledge whenever new topology
information is received. The router learns and maintains routes to remote destinations by exchanging
routing updates with other routers in the internetwork.

When to Use Static Routing
This topic describes when to use static routing.
When to Use Static Routing
Use static routes:
In a small network that requires only simple routing
In a hub-and-spoke network topology
When you want to create a quick ad hoc route
Do not use static routes:
In a large network
When the network is expected to scale
Depending on network size, complexity, desired security, and design, you can choose between a dynamic
routing protocol and configuring static routes.
Static routing has some advantages over dynamic routing:
Conserves router resources: Static routing does not consume network bandwidth and the CPU
resources of the router. When you use a routing protocol, the traffic between routers adds some
overhead as the routers exchange routing updates about remote networks. Depending on the size of the
network, a router requires some CPU cycles to compute the best way to remote networks.
Simple to configure in a small network: Static routes are commonly used in small networks that have
few routers. Many small networks are designed as stub networks (networks that are accessed by a
single route), and static routes are the most appropriate solution. Also, most of these networks are
designed in a hub-and-spoke topology, where you can use default routes for branches pointing to the
hub, which is the gateway to other networks.
The disadvantages of static routing can be summarized as follows:
Scalability: Static routing is appropriate for networks that have fewer than four or five routers.
Dynamic routing is more appropriate for large networks to reduce the probability of errors in routing
configuration.
More manual configuration in larger networks: When the number of routers increases, the number
of static routes also increases. In large networks that use static routing, adding even one router with
only one new network means that you must configure the newly added router with static routes to other
networks, and also that you must configure all existing routers in the network with static routes to the
new network.
Organizations often use a combination of static and dynamic routes.

Static Route Configuration
This topic describes how to configure static routes on Cisco routers.
Static Route Configuration
Configure unidirectional static routes to and from a stub
network to allow communication to occur.
Network
10.0.0.0
s0/0/0
172.16.2.2 172.16.2.1
172.16.1.0
Stub Network
A B
Static routes are commonly used when you are routing from a network to a stub network (a network that is
accessed by a single link). Static routes can also be useful for specifying a “gateway of last resort” to which
all packets with an unknown destination address are sent.
Static Route Configuration (Cont.)
Static route configuration steps:
Define a path to an IP destination network (172.16.1.0 255.255.255.0).
Use the IP address of the next-hop router (172.16.2.1).
Or, use the outbound interface of the local router (serial 0/0/0).
Network
10.0.0.0
s0/0/0
172.16.2.2 172.16.2.1
172.16.1.0
Stub Network
A B
Static route pointing to next-hop IP.
RouterA(config)#ip route 172.16.1.0 255.255.255.0 172.16.2.1
In the figure, router A is configured with a static route to reach the 172.16.1.0 subnet via the next hop IP
172.16.2.1 using the ip route command.

Alternatively, the static route can be configured by pointing to the exit interface instead of using the next-
hop IP address.
RouterA(config)#ip route 172.16.1.0 255.255.255.0 serial0/0/0
The table lists the ip route command parameters for this example.
Command Parameters Description
ip route Identifies the static route
172.16.1.0 IP address of a static route to the destination subnetwork
255.255.255.0 Indicates the subnet mask—there are 8 bits of subnetting in effect
172.16.2.1 IP address of the next-hop router in the path to the destination
Serial 0/0/0 Identifies the interface that will be used to reach the next-hop router
Assigning a static route to reach the stub network 172.16.1.0 is proper for router A because there is only
one way to reach that network.
In the figure, router B also needs to be configured with a static or default route to reach the networks behind
router A via the serial interface of router B.
Note A static route is configured for connectivity to remote networks that are not directly connected to your
router. For end-to-end connectivity, a static route must be configured in both directions.

Default Routes
This topic describes how to configure default route forwarding.
Default Routes
This route allows the stub network to reach all known networks
beyond Router A.
Network
10.0.0.0
s0/0/0
172.16.2.2 172.16.2.1
172.16.1.0
Stub Network
A
2 1
B
s0/0/1
Default route pointing to next-hop IP.
RouterB(config)#ip route 0.0.0.0 0.0.0.0 172.16.2.2
Default route pointing to exit interface.
RouterB(config)#ip route 0.0.0.0 0.0.0.0 Serial0/0/1
Use a default route when the route from a source to a destination is not known or when it is not feasible for
the router to maintain many routes in its routing table.
A default static route is a route that matches all packets. Default static routes are used in these instances:
When no other routes in the routing table match the destination IP address of the packet or when a
more specific match does not exist. A common use for a default static route is to connect the edge
router of a company to an ISP network.
When a router has only one other router to which it is connected. This condition is known as a stub
router.
The syntax for a default static route is like that for any other static route, except that the network address is
0.0.0.0 and the subnet mask is 0.0.0.0.
RouterB(config)#ip route 0.0.0.0 0.0.0.0 172.16.2.2
or
RouterB(config)#ip route 0.0.0.0 0.0.0.0 serial0/0/1
The 0.0.0.0 network address and 0.0.0.0 mask is called a quad-zero route.
In the figure, router B is configured to forward to router A all packets that do not have the destination
network that is listed in the router B routing table.
This table lists the ip route command parameters for this example.
ip route Identifies the static route
0.0.0.0 Routes to networks that are not in the routing table

0.0.0.0 Special mask that indicates the default route
172.16.2.2 IP address of the next-hop router to be used as the default for packet
forwarding

Static Route Configuration Verification
This topic describes how to verify static route configuration.
Static Route Configuration Verification
<output omitted>
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, FastEthernet0/0
S 172.16.1.0/24 [1/0] via 172.16.2.1
Static Route Configuration Verification (Cont.)
To verify static routes in the routing table, examine the routing
table with the show ip route command:
Includes network address and subnet mask as well as IP address of
next-hop router or exit interface
Denoted with the code “S” in the routing table
Routing tables must contain directly connected networks that
are used to connect remote networks before static or dynamic
routing can be used.
Most routing tables contain a combination of static routes and dynamic routes. However, the routing table
must first contain the directly connected networks that are used to access the remote networks before any
static or dynamic routing can be used.

A static route includes the network address and subnet mask of the remote network, along with the IP
address of the next-hop router or exit interface. Static routes are denoted with the code “S” in the routing
table, as shown in the figure.
When you configure a static route to use an exit interface instead of a next-hop IP address, the routing table
entry is changed as follows:
<output omitted>
C 10.0.0.0 is directly connected, FastEthernet0/0
S 172.16.1.0/24 is directly connected, Serial0/0/0
Note that the entry in the routing table no longer refers to the next-hop IP address but refers directly to the
exit interface. This exit interface is the same one to which the static route was resolved when it used the
next-hop IP address. Now that the routing table process has a match for a packet and this static route, it is
able to resolve the route to an exit interface in a single lookup.
Note The static route displays the route as directly connected. It is important to understand that this does not
mean that this route is a directly connected network or a directly connected route. This route is still a
static route.
Verifying the Default Route Configuration
To verify the default route configuration, examine the routing
table on RouterB:
RouterB#show ip route
Codes: L - local, C - connected, S - static,
R - RIP, M - mobile, B - BGP
<output omitted>
C 172.16.1.0/24 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 172.16.2.2
The example in the figure shows the RouterB routing table after configuration of the default route.
The asterisk (*) indicates the last path option that the router will use when forwarding a packet.

Summary
Summary
Routing is the process by which items get from one location to another.
Routers can forward packets over static routes or dynamic routes.
Static routes are entered manually by a network administrator. Dynamic
routes are learned by a routing protocol, and dynamic routes change
automatically when circumstances in the network change.
Unidirectional static routes must be configured to and from a stub
network to allow communication to occur.
The ip route command can be used to configure default route
forwarding.
The show ip route command is used to verify that static routing is
properly configured. Static routes are signified in the command output
by “S” in the first position.

Lesson 8
Managing Traffic Using ACLs
Overview
ACLs enable administrators to identify specific traffic that receives special treatment. The lesson introduces
the concept of ACL operation, followed by an explanation of wildcard masking. While different types of
ACLs are considered, configuration details are shown only for standard numbered ACLs.
Objectives
Describe ACLs as a Cisco IOS tool for traffic identification
Explain how ACLs operate
Describe wildcard masking
Describe wildcard bit mask abbreviations
Compare standard and extended ACLs
Explain what is tested when a packet is tested against a numbered standard access list
Configure and verify standard IPv4 ACLs

Using ACLs
This topic describes how ACLs can be used as a tool to identify traffic.
Understanding ACLs
What is an ACL?
An ACL is a Cisco IOS tool for traffic identification.
An ACL is a list of permit and deny statements.
An ACL identifies traffic based on the information within the IP packet.
After traffic is identified, different actions can be taken.
ACLs can be used on routers and switches.
An ACL is a Cisco IOS feature that is used for traffic identification. The ACL enables an administrator to
create a set of rules in the form of permit and deny statements that describe which traffic should be
identified. Traffic identification is based on the header values in the IP packet. Identified traffic can receive
different treatment, depending on which Cisco IOS function is using the ACLs.
ACLs are supported on a wide range of products, including routers and switches.

ACL Operation
This topic describes how ACLs operate.
ACL Operation
ACL tests:
An ACL consists of a series of
permit and deny statements.
An ACL is consulted in top-down
order.
The first match executes the permit
or deny action and stops further
ACL matching.
There is an implicit deny all
statement at the end of each ACL.
No
No
No
Deny
Deny
Deny
Packet Ready for
ACL Processing
Packet Discarded
Packet
Permitted
Permit
Permit
Permit
Implicit
Deny
Match?
Match?
Match?
ACL statements operate in sequential, logical order. They evaluate packets from the top down, one
statement at a time. If a packet header and an ACL statement match, the rest of the statements in the list are
skipped. The packet is then permitted or denied, as determined by the matched statement. If a packet header
does not match an ACL statement, the packet is tested against the next statement in the list. This matching
process continues until the end of the list is reached.
A final implied statement covers all packets for which conditions did not test true. This final test condition
matches all other packets and results in a deny instruction. The router denies all of these remaining packets.
This final statement is often referred to as the “implicit deny any" statement. Because of this statement, an
ACL should have at least one permit statement in it; otherwise, the ACL denies all packets.

ACL Wildcard Masking
When processing ACLs, a router needs a mechanism to determine which bits of an IP address must match.
A wildcard mask describes which bits of an IP address must match in an IP packet to result in a match for a
permit or deny statement. This topic describes how to use wildcard masks with ACLs.
ACL Wildcard Masking
Wildcard bits—how to check the corresponding address bits:
0 means to match the value of the corresponding address bit.
1 means to ignore the value of the corresponding address bit.
Do Not Check Address
(Ignore Bits in Octet)
=
0 0 1 1 1 1 1 1
128 64 32 16 8 4 2 1
=
0 0 0 0 0 0 0 0
=
0 0 0 0 1 1 1 1
=
1 1 1 1 1 1 0 0
1 1 1 1 1 1 0
=
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1
Octet Bit Position and
Address Value for Bit
Ignore Last 6 Address Bits
Match All Address Bits
(Match All)
Ignore Last 4 Address Bits
Match Last 2 Address Bits
Examples
ACL statements include masks, also called wildcard masks. A wildcard mask is a string of binary digits that
tell the router which parts of the subnet number to look at. Although wildcard masks have no functional
relationship with subnet masks, they do provide a similar function. The mask determines how much of an IP
source or destination address to apply to the address match. The numbers 1 and 0 in the mask identify how
to treat the corresponding IP address bits. However, they are used for different purposes and follow
different rules.
Wildcard masks and subnet masks are both 32 bits long and use binary 1s and 0s. Subnet masks use binary
1s and 0s to identify the network, subnet, and host portion of an IP address. Wildcard masks use binary 1s
and 0s to filter individual or groups of IP addresses to permit or deny access to resources based on an IP
address. By carefully setting wildcard masks, you can permit or deny a single or several IP addresses.
Wildcard masks and subnet masks differ in the way in which they match binary 1s and 0s. Wildcard masks
use the following rules to match binary 1s and 0s:
Wildcard mask bit 0: Match the corresponding bit value in the address.
Wildcard mask bit 1: Ignore the corresponding bit value in the address.
The figure shows how different wildcard masks filter IP addresses. As you look at the example, remember
that binary 0 signifies a match and that binary 1 signifies ignore.
Note A wildcard mask is sometimes referred to as an inverse mask. In a subnet mask, binary 1 is equal to a
match and binary 0 is not a match. The reverse is true for wildcard masks. A 0 in a bit position of the
wildcard mask indicates that the corresponding bit in the address must be matched. A 1 in a bit position
of the wildcard mask indicates that the corresponding bit in the address is not interesting and can be
ignored.

By carefully setting wildcard masks, you can permit or deny with one ACL statement. You can select a
single IP address or many IP addresses.
The figure shows how to check corresponding address bits.
ACL Wildcard Masking (Cont.)
Filter for IP subnets 170.30.16.0/24 to 172.30.31.0/24.
Address and wildcard mask:
172.30.16.0 0.0.15.255
ACL Wildcard Masking (Cont.)
This example shows the wildcard masking process for IP
subnets.
0
Network.Host
172.30.16.0
0 0 0 1 0 0 0
Wildcard Mask: 0 0 0 0 1 1 1 1
|<---- Match ---- >|< Don’t Care >|
0 0 0 1 0 0 0 0 = 16
0 0 0 1 0 0 0 1 = 17
0 0 0 1 0 0 1 0 = 18
: :
0 0 0 1 1 1 1 1 = 31
---- ----
In the figure, an administrator wants to test a range of IP subnets that is to be permitted or denied. Assume
that the IP address is a Class B address (the first two octets are the network number), with 8 bits of
subnetting (the third octet is for subnets). The administrator wants to use the IP wildcard masking bits to
match subnets 172.30.16.0/24 to 172.30.31.0/24.

To use one ACL statement to match this range of subnets, use the IP address 172.30.16.0 in the ACL, which
is the first subnet to be matched, followed by the required wildcard mask.
First, the wildcard mask matches the first two octets (172 and 30) of the IP address using corresponding 0
bits in the first two octets of the wildcard mask.
Because there is no interest in an individual host, the wildcard mask ignores the final octet by using the
corresponding 1 bit in the wildcard mask. For example, the final octet of the wildcard mask is 255 in
decimal.
In the third octet, where the subnet address occurs, the wildcard mask of decimal 15, or binary 00001111,
matches the high-order 4 bits of the IP address. In this case, the wildcard mask matches subnets starting
with the 172.30.16.0/24 subnet. For the final (low-end) 4 bits in this octet, the wildcard mask indicates that
the bits can be ignored. In these positions, the address value can be binary 0 or binary 1. Thus, the wildcard
mask matches subnet 16, 17, 18, and so on, up to subnet 31. The wildcard mask does not match any other
subnets.
In the example, the address 172.30.16.0 with the wildcard mask 0.0.15.255 matches subnets 172.30.16.0/24
to 172.30.31.0/24.
In some cases, you must use more than one ACL statement to match a range of subnets. For example, to
match 10.1.4.0/24 to 10.1.8.0/24, use 10.1.4.0 0.0.3.255 and 10.1.8.0 0.0.0.255.

Wildcard Bit Mask Abbreviations
This topic describes wildcard mask abbreviations.
Wildcard Bit Mask Abbreviations
Using wildcard bit mask abbreviations:
172.30.16.29 0.0.0.0 matches all of the
address bits.
Abbreviate this wildcard mask using the IP
address preceded by the keyword host
(host 172.30.16.29).
0.0.0.0 255.255.255.255 ignores all
address bits.
Abbreviate expression with the keyword
any.
172.30.16.29
0.0.0.0
(Matches All Bits)
Wildcard Mask:
0.0.0.0
255.255.255.255
(Ignores All Bits)
Wildcard Mask:
The 0 and 1 bits in an ACL wildcard mask cause the ACL to either match or ignore the corresponding bit in
the IP address. Working with decimal representations of binary wildcard mask bits can be tedious. For the
most common uses of wildcard masking, you can use abbreviations so that you need not enter as many
numbers when configuring address test conditions.
In the example, instead of entering 172.30.16.29 0.0.0.0, you can use the string host 172.30.16.29. Using
the abbreviation host communicates the same test condition to Cisco IOS Software.
In the example, instead of entering 0.0.0.0 255.255.255.255, you can use the keyword any by itself. Using
the abbreviation any communicates the same test condition to Cisco IOS Software.

Types of ACLs
This topic identifies the two main types of ACLs, standard and extended, and two methods of identifying
ACLs, numbering and naming.
Types of ACLs
Two main types of ACLs:
Standard ACL:
Checks source IP address
Permits or denies entire protocol suite
Extended ACL:
Checks source and destination IP address
Generally permits or denies specific protocols and applications
Two methods that you can use to identify standard and
extended ACLs:
Numbered ACLs
Named ACLs
ACLs can be categorized into the following types:
Standard ACLs: Standard IP ACLs check the source addresses of packets that can be routed. The
result either permits or denies the output for an entire protocol suite, which is based on the source
network, subnet, or host IP address.
Extended ACLs: Extended IP ACLs check both the source and destination packet addresses. They can
also check for specific protocols, port numbers, and other parameters, which allows administrators
more flexibility and control.
There are two methods that you can use to identify standard and extended ACLs:
Numbered ACLs use a number for identification.
Named ACLs use a descriptive name or number for identification.

Types of ACLs (Cont.)
How to identify ACLs:
Numbered standard IPv4 ACLs (1 to 99) test conditions of all IP packets
for source addresses. The expanded range is 1300 to 1999.
Numbered extended IPv4 ACLs (100 to 199) test conditions of source
and destination addresses, specific TCP/IP protocols, and destination
ports. The expanded range is 2000 to 2699.
Named ACLs identify IP standard and extended ACLs with an
alphanumeric string (name).
IPv4 ACL Type Number Range or Identifier
Numbered Standard 1–99, 1300–1999
Numbered Extended 100–199, 2000–2699
Named (Standard and Extended) Name
You can create many ACLs for a protocol. Select a different ACL number for each new ACL within a given
protocol. However, on an interface, you can apply only one ACL per protocol, per direction.
Specifying an ACL number from 1 to 99 or 1300 to 1999 instructs the router to accept numbered standard
IPv4 ACL statements. Specifying an ACL number from 100 to 199 or 2000 to 2699 instructs the router to
accept numbered extended IPv4 ACL statements.

Testing an IP Packet Against a Numbered
Standard Access List
This topic describes how standard numbered ACL operates.
Testing An IP Packet Against a Numbered
Standard Access List
Source
Address
Segment
(For Example,
TCP Header)
Data
Packet
(IP Header)
Frame
Header
(For Example,
HDLC)
Deny Permit
Use ACL
Statements
1–99
1300–1999
Standard IPv4 ACLs, whether numbered (1 to 99 and 1300 to 1999) or named, filter packets based on a
source address and mask, and they permit or deny the entire TCP/IP protocol suite.
Note The standard ACL may not provide the required level of control that you require. You may need a more
precise tool to select network traffic.

Basic Configuration of Numbered Standard IPv4
ACLs
This topic describes how to configure a basic numbered standard IPv4 ACL.
Basic Configuration of Numbered Standard
IPv4 ACLs
Configure a numbered standard IPv4 ACL:
RouterX(config)#access-list 1 permit 172.16.0.0 0.0.255.255
The statement matches any source address that starts with 172.16.x.x.
Standard ACL configuration uses 1 to 99, or 1300 to 1999, for the ACL
number (1 in the example).
The default wildcard mask is 0.0.0.0 (only standard ACL).
Display the current ACLs configured on RouterX:
RouterX#show access-lists
Standard IP access list 1
10 permit 172.16.0.0, wildcard bits 0.0.255.255
You can configure numbered standard IPv4 ACLs on a Cisco router in global configuration mode. The
access-list command creates an entry in a standard IPv4 filter list. The figure shows the syntax of this
command.
In the figure, the output of the show access-list command displays the current ACLs that are configured on
router RouterX.

Basic Configuration of Numbered Standard
IPv4 ACLs (Cont.)
Delete a numbered standard IPv4 ACL:
RouterX(config)#no access-list 1
RouterX(config)#exit
RouterX#show access-lists
RouterX#
Use the no access-list 1 command to remove the entire ACL 1.
To remove the ACL, use the no access-list number command in global configuration mode. Issue the show
access-list command to confirm that ACL 1 has been removed. With numbered ACLs, individual entries
cannot be removed with the no access-list command, because this command removes the entire ACL. The
traditional way of removing or modifying a single numbered ACL entry would be to copy the whole ACL
to a text editor, make the changes that are needed, and remove the entire ACL from the router using the no
access-list command. The modified ACL can then be copied and pasted from the text editor. Newer Cisco
IOS Software versions allow easier editing by using sequence numbering.

Summary
Summary
An ACL is a tool to identify traffic for special handling.
ACLs perform top-down processing and can be configured for incoming
or outgoing traffic.
In a wildcard bit mask, a 0 bit means to match the corresponding
address bit and a 1 bit means to ignore the corresponding address bit.
You can create an ACL using a named or numbered ACL. Named or
numbered ACLs can be configured as standard or extended ACLs,
which determines what they can filter.

Lesson 9
Enabling Internet
Connectivity
Overview
It has become common for small sites to use the Internet to connect to other sites. Internet service is
obtained through an ISP. In some cases, the ISP provides a static IP address for the interface that is
connected to the Internet. In other cases, this IP address is provided using DHCP.
Two scalability challenges for the Internet are the depletion of the registered IPv4 address space and scaling
in routing. Cisco IOS NAT and PAT are mechanisms for conserving registered IP addresses in large
networks, and they also simplify IP addressing tasks. NAT and PAT translate IP addresses within private
internal networks to valid IP addresses for transport over public external networks, such as the Internet,
without requiring a registered subnet address. Incoming traffic is translated for delivery within the inside
network. This translation of IP addresses eliminates the need for host renumbering and allows the same IP
address range to be used in multiple intranets.
Objectives
Describe various connectivity options for Internet access
Explain the demarcation point
Define the function of DHCP
Describe the process of obtaining a public IP address
Configure a router with a static public IP address
Configure a Cisco router as a DHCP client
Compare public and private IPv4 addresses
Describe the features and benefits of NAT

Describe types of NAT addresses
Describe types of NAT
Understand how static NAT operates
Configure static NAT
Verify static NAT configuration
Understand how dynamic NAT works
Describe an example of configuring dynamic NAT
Describe an example of verifying dynamic NAT
Understand how NAT overload operates
Describe an example of configuring PAT
Describe an example of verifying PAT
Describe how to troubleshoot NAT
Troubleshoot NAT

The Demarcation Point
This topic describes the demarcation point and explains its importance.
The Demarcation Point
Router
CPE
Demarcation Point
Wireless
Router
LAN
CSU/DSU
Cable Modem
DSL Modem
Media Converter
ISP
Although functions within the service provider network are not usually of concern to customers, there are
some terms and concepts that you should be familiar with.
Service providers install a connection point (usually in the form of an RJ-45 jack) that physically connects a
circuit to their nearest switching office. This link is known as the demarcation point and represents the
point at which the service provider responsibility is said to end. In other words, the service provider ensures
that the link functions correctly up to that point. The other end of this link connects to the service provider
network. These links are part of what is known as the local loop or last mile. The local loop may consist of
a variety of technologies, including DSL, cable, fiber optics, traditional twisted-pair wiring, and others.
The customer side of the demarcation point is the location of the CPE. The term CPE is often used quite
loosely, but it traditionally refers to equipment that is owned and managed by the customer for the purpose
of connecting to the service provider network. However, many companies lease CPE from their service
providers, and this equipment is still considered to be CPE. Before physically connecting to a service
provider network, a company needs to determine the type of WAN service or connectivity that it requires.
Note The exact demarcation point is different from country to country. The example described is for the United
States.

Dynamic Host Configuration Protocol
Originally, network administrators had to manually configure IP addresses and other network parameters on
equipment connecting to the Internet. DHCP was introduced to automate assignment of network
parameters.
Dynamic Host Configuration Protocol
Understanding DHCP:
DHCP is a client-server model.
A DHCP server allocates network addresses and delivers
configurations.
A DHCP client is a host that requests an IP address and configuration
from a DHCP server.
IP Address Allocation
DHCP Clients DHCP Server
IP Address Pool
192.168.1.1–192.168.1.250
Managing a network can be time-consuming. Network clients break or are moved around, and new clients
appear that need network connectivity—all part of the network administrator job. Manually configuring IP
addresses for all devices in the network is impossible in some cases. DHCP can greatly lighten the
workload of the network administrator. DHCP automatically assigns an IP address from an IP address pool
that is defined by the administrator. However, DHCP is much more than just an IP address allocation
mechanism. A DHCP server can push other initial configuration parameters to the client devices, such as
the time or a log server address.
DHCP is built on a client-server model. The DHCP server hosts allocate network addresses and deliver
configuration parameters to dynamically configured hosts. The term “client” refers to a host that is
requesting initialization parameters from a DHCP server.

Dynamic Host Configuration Protocol (Cont.)
DHCP IP address allocation mechanisms:
Automatic allocation: A permanent IP address is assigned to a client.
Dynamic allocation: A client is assigned an IP address for a limited
time.
Manual allocation: A client is assigned an IP address by the network
administrator.
DHCP supports three mechanisms for IP address allocation. With automatic allocation, DHCP assigns a
permanent IP address to a client. With dynamic allocation, DHCP assigns an IP address to a client for a
limited time (or until the client explicitly relinquishes the address). With manual allocation, a client is
assigned an IP address by the network administrator, and DHCP is used simply to convey the assigned
address to the client.
Dynamic allocation is the only one of the three mechanisms that allows automatic reuse of an address that is
no longer needed by the client to which it was assigned. Dynamic allocation is particularly useful for
assigning an address to a client that will be connected to the network only temporarily or for sharing a
limited pool of IP addresses among a group of clients that do not need permanent IP addresses. Dynamic
allocation may also be a good choice for assigning an IP address to a new client that is being permanently
connected to a network in which IP addresses are so scarce that it is important to reclaim them when clients
are retired.

Options for Configuring a Provider-Assigned IP
Address
This topic describes options for configuring a provider-assigned IP address.
Options for Configuring a Provider-Assigned IP
Address
Options for configuring IP addresses:
Statically assigned
Dynamically assigned through DHCP
Customer LAN Internet
Service Provider
Access Network
DHCP Server
IP Address
The DHCP service enables devices on a network to obtain IP addresses and other information from a DHCP
server. This service automates assignment of IP addresses, subnet masks, gateways, and other IP
networking parameters.
A service provider sometimes provides a static address for an interface that is connected to the Internet. In
other cases, the address is provided using DHCP. On larger local networks or where the user population
changes frequently, DHCP is preferred. New users may arrive with laptops and need a connection. Others
have new workstations that need to be connected. Rather than have the network administrator assign an IP
address for each workstation, it is more efficient to have IP addresses assigned automatically using DHCP.
If an ISP uses DHCP to provide interface addressing, no manual addresses can be configured. Instead, the
interface is configured to operate as a DHCP client. This configuration means that when the router is
connected to a cable modem, for example, it is a DHCP client and requests an IP address from the ISP.

Configuring a Static Provider-Assigned IP
Address
This topic describes how to configure a static provider-assigned IP address on a Cisco router.
Configuring a Static Provider-Assigned IP
Address
ISP Access Network
Company LAN
Gi0/0
Assign an IP address and
create a default route.
Router(config)#interface GigabitEthernet 0/0
Router(config-if)#no shutdown
Configures a public IP address
Router(config)#ip route 0.0.0.0 0.0.0.0 209.165.200.226
Creates a default route that points toward the next-hop IP address
Static provider-assigned IP addresses can be more useful in several respects than dynamic addresses. Static
IP addresses can be linked to a domain name (such as www.cisco.com), and public or private servers can be
run for access by outside users.
For example, your ISP assigns you a static IP address of 209.165.200.225/27. You proceed with a two-step
process. The first step is to configure the static IP address that you were assigned on the outside interface of
the router. The second step is to configure a default route that forwards all traffic that is intended for the
Internet to the outside interface.
Command Description
interface interface Specifies an interface and enters interface configuration
mode
ip address address subnet_mask Sets the IP address and mask of the device
no shutdown Enables an interface
ip route net-prefix prefix-mask next_hop_ip_address Establishes a static route to destination

Configuring a DHCP Client
This topic describes how to configure a router interface so that a device can acquire an IP address from a
DHCP server.
Configuring a DHCP Client
Customer LAN Internet
Service Provider
Access Network
DHCP Server
IP Address
Router(config)#interface GigabitEthernet0/0
Router(config-if)#ip address dhcp
Router automatically injects default route based on optional default gateway parameter
received with assigned IP address
An ISP sometimes provides a static address for an interface that is connected to the Internet. In other cases,
an address is provided using DHCP. If the ISP uses DHCP to provide interface addressing, no manual
address can be configured. Instead, the interface is configured to operate as a DHCP client.
If the router receives an optional DHCP parameter called default gateway with the assigned IP address, the
default route will be injected into the routing table, pointing to the default gateway IP address.
Command Description
interface interface Specifies an interface and enters interface configuration
mode
ip address dhcp Specifies that the interface acquires an IP address
through DHCP

Public vs. Private IPv4 Addresses
Some networks connect to each other through the Internet, while others are private. For instance, the
example addresses used in this course are private, which means that they are not assigned to public use.
Public and private IP addresses are required for both of these network types. This topic describes the
purpose and sources for public and private IP addresses.
Public vs. Private IPv4 Addresses
Class Private Address Range
A 10.0.0.0 to 10.255.255.255
B 172.16.0.0 to 172.31.255.255
C 192.168.0.0 to 192.168.255.255
Class Public Address Range
A 1.0.0.0 to 9.255.255.255
11.0.0.0 to 126.255.255.255
B 128.0.0.0 to 172.15.255.255
172.32.0.0 to 191.255.255.255
C 192.0.0.0 to 192.167.255.255
192.169.0.0 to 223.255.255.255
Private IP Addresses
Internet hosts require a globally unique IP address, but private hosts that are not connected to the Internet
can use any valid address, as long as it is unique within the private network. However, because many
private networks exist alongside public networks, grabbing "just any address" is strongly discouraged.
Three blocks of IP addresses (one Class A network, 16 Class B networks, and 256 Class C networks) are
designated for private, internal use. The figure shows the address ranges for each class. Addresses in these
ranges are not routed on the Internet backbone. Internet routers are configured to discard private addresses.
In a private intranet, these private addresses can be used instead of globally unique addresses.
When a network that is using private addresses must connect to the Internet, it is necessary to translate the
private addresses to public addresses. This translation process is called NAT. A router is often the network
device that performs NAT.

Public IP Addresses
Public IP addresses are used for the hosts that are publicly accessible from the Internet. Internet stability
depends directly on the uniqueness of publicly used network addresses. Therefore, a mechanism is needed
to ensure that addresses are, in fact, unique. This mechanism was originally managed by the InterNIC.
IANA succeeded InterNIC. IANA carefully manages the remaining supply of IP addresses to ensure that
duplication of publicly used addresses does not occur. Duplication would cause instability in the Internet
and would compromise its ability to deliver datagrams to networks using the duplicated addresses.
To obtain a provider-dependent IP address or block of addresses, you must contact an ISP. To obtain
provider-independent IP addresses, you must contact an LIR. LIRs obtain IP address pools from their RIRs:
AfriNIC
APNIC
ARIN
LACNIC
RIPE NCC
With the rapid growth of the Internet, public IP addresses began to run out. New mechanisms such as NAT,
CIDR, VLSM, and IPv6 were developed to help solve the problem.

Introducing NAT
This topic describes why NAT is used.
Introducing NAT
NAT allows private users to access the Internet by sharing one
or more public IP addresses.
Internet
LAN
Translate IP address.
209.165.201.5
10.1.1.100
Small networks are commonly implemented using private IP addressing as defined in RFC 1918. Private
addressing gives enterprises considerable flexibility in network design. This addressing enables
operationally and administratively convenient addressing schemes as well as easier growth. However, you
cannot route private addresses over the Internet, and there are not enough public addresses to allow all
organizations to provide a private address to all of their hosts. Therefore, network administrators need a
mechanism to translate private addresses to public addresses (and back) at the edge of their network.
NAT provides this mechanism. Before NAT, a host with a private address could not access the Internet.
Using NAT, companies can provide some or all of their hosts with private addresses and use NAT to
provide address translation to allow access to the Internet.
NAT is like the receptionist in a large office. Assume that you have left instructions with the receptionist
not to forward any calls to you unless you request it. Later, you call a potential client and leave a message
asking the client to call you back. You tell the receptionist that you are expecting a call from this client, and
you ask the receptionist to put the call through to you. The client calls the main number to your office,
which is the only number that the client knows. When the caller gives the receptionist your name, the
receptionist checks a lookup table that matches your name to your extension. The receptionist knows that
you requested this call and forwards the caller to your extension.
Usually, NAT connects two networks and translates the private (inside local) addresses in the internal
network to public (inside global) addresses before packets are forwarded to another network. You can
configure NAT to advertise only one address for the entire network to the outside world. Advertising only
one address effectively hides the internal network, providing additional security as a side benefit.
The network address translation process of swapping one address for another is separate from the
convention that is used to determine what is public and private, and devices must be configured to
recognize which IP networks are to be translated. This requirement is one of the reasons why NAT can also
be deployed internally when there is a clash of private IP addresses, such as, for example, when two
companies merge.

Benefits of NAT
These are the benefits of NAT:
Eliminates the need to readdress all hosts that require external access, saving time and money.
Conserves addresses through application port-level multiplexing. With PAT, multiple internal hosts can
share a single registered IPv4 address for all external communication. In this type of configuration,
relatively few external addresses are required to support many internal hosts, which conserves IPv4
addresses.
Protects network security. Because private networks do not advertise their addresses or internal
topology, they remain reasonably secure when they gain controlled external access in conjunction with
NAT.
Drawbacks of NAT
These are disadvantages of NAT:
Many IP addresses and applications depend on end-to-end functionality, with unmodified packets
forwarded from the source to the destination. By changing end-to-end addresses, NAT blocks some
applications that use IP addressing. For example, some security applications, such as digital signatures,
fail because the source IP address changes. Applications that use physical addresses instead of a
qualified domain name do not reach destinations that are translated across the NAT router. Sometimes,
this problem can be avoided by implementing static NAT mappings.
End-to-end IP traceability is also lost. It becomes much more difficult to trace packets that undergo
numerous packet address changes over multiple NAT hops, so troubleshooting is challenging. On the
other hand, hackers who want to determine the source of a packet find it difficult to trace or obtain the
original source or destination address.
Using NAT also complicates tunneling protocols, such as IPsec, because NAT modifies the values in
the headers, which interferes with the integrity checks done by IPsec and other tunneling protocols.
Services that require the initiation of TCP connections from the outside network, or stateless protocols
such as those using UDP, can be disrupted. Unless the NAT router makes a specific effort to support
such protocols, incoming packets cannot reach their destination. Some protocols can accommodate one
instance of NAT between participating hosts (passive mode FTP, for example) but fail when both
systems are separated from the Internet by NAT.
The last disadvantage involves performance. NAT increases switching delays because translation of
each IP address within the packet headers takes time. The first packet is process-switched, meaning that
it always goes through the slower path. The router must look at each packet to decide whether it needs
translation. The router needs to alter the IP header and possibly alter the TCP or UDP header.
Remaining packets go through the fast-switched path if a cache entry exists; otherwise, they too are
delayed.

Types of Addresses in NAT
This topic describes NAT address types.
Types of Addresses in NAT
These are the most important types of addresses in NAT:
Inside local: Host on the inside network
Inside global: Usually assigned by an ISP and allows the customer
outside access
Outside global: Host on the outside network
Types of Addresses in NAT (Cont.)
10.1.1.100
10.1.1.101
Internet
Inside Outside
NAT Table
10.1.1.100
10.1.1.101
209.165.201.5
209.165.201.6
Inside Local
IPv4 Address
Outside Global
IPv4 Address
10.1.1.100 209.165.201.5
10.1.1.101 209.165.201.6
In NAT terminology, the inside network is the set of networks that are subject to translation. The outside
network refers to all other addresses. Usually, these are valid addresses that are located on the Internet.

Cisco defines these NAT terms:
Inside local address: The IPv4 address that is assigned to a host on the inside network. The inside
local address is likely not an IPv4 address that is assigned by the network information center or service
provider.
Inside global address: A legitimate IPv4 address that is assigned by the network information center or
service provider that represents one or more inside local IPv4 addresses to the outside world.
Outside global address: The IPv4 address that is assigned to a host on the outside network by the host
owner. The outside global address is allocated from a globally routable address or network space.
Outside local address: The IPv4 address of an outside host as it appears to the inside network. Not
necessarily legitimate, the outside local address is allocated from a routable address space on the inside.
A good way to remember what is local and what is global is to add the word visible. An address that is
locally visible normally implies a private IP address, and an address that is globally visible normally implies
a public IP address. The rest is simple. Inside means internal to your network and outside means external to
your network. So, for example, an inside global address means that the device is physically inside your
network and has an address that is visible from the Internet. It could be a web server, for instance.

Types of NAT
This topic describes the types of NAT.
Types of NAT
These are the types of NAT:
Static NAT: One-to-one address mapping
Dynamic NAT: Many-to-many address mapping
PAT: Many-to-one address mapping
NAT can work in these ways:
Static NAT: Maps an unregistered IPv4 address to a registered IPv4 address (one to one). Static NAT
is particularly useful when a device must be accessible from outside the network. This type of NAT is
used when a company has a server for which it needs a static IP address.
Dynamic NAT: Maps an unregistered IPv4 address to a registered IPv4 address from a group of
registered IPv4 addresses. This type of NAT is used, for example, when two companies merge that are
using the same private address space. With the use of dynamic NAT readdressing, use of the entire
address space is avoided or at least postponed.
PAT: PAT maps multiple unregistered IPv4 addresses to a single registered IPv4 address (many to
one) by using different ports. PAT is also known as NAT overloading. It is a form of dynamic NAT
and is the most common use for NAT. It is used every day in your place of business or your home.
Multiple users of PCs, tablets, and phones are able to access the Internet, even though only one public
IP address is available for that LAN.

Understanding Static NAT
This topic describes how static NAT works.
Understanding Static NAT
209.165.201.5
10.1.1.100
10.1.1.101
Internet
e
d
i
s
t
u
O
e
d
i
s
n
I
NAT Table
Host B
209.165.202.131
10.1.1.101
10.1.1.101
1
2
209.165.201.5
3
4
5
Inside Local
IPv4 Address
Inside Global
IPv4 Address
Outside Global
IPv4 Address
10.1.1.101 209.165.201.5 209.165.202.131
You can translate your own IPv4 addresses into globally unique IPv4 addresses when you are
communicating outside your network.
The figure illustrates a router that is translating a source address inside a network into a source address
outside the network. These are the steps for translating an inside source address:
1. The user at host 10.1.1.101 wants to open a connection to host B (IP address 209.165.202.131).
2. The first packet that the router receives on its NAT inside-enabled interface from host 10.1.1.101
causes the router to check its NAT table.
3. The router replaces the inside local source address of host 10.1.1.101 with the translated inside global
address (209.165.201.5) and forwards the packet.
4. Host B receives the packet and responds to host 10.1.1.101, using the inside global IPv4 destination
address 209.165.201.5.
5. When the router receives the packet on its NAT outside-enabled interface with the inside global IPv4
address of 209.165.201.5, the router performs a NAT table lookup using the inside global address as a
key. The router then translates the address back to the inside local address of host 10.1.1.101 and
forwards the packet to host 10.1.1.101.
6. Host 10.1.1.101 receives the packet and continues the conversation. The router performs Steps 2
through 5 for each packet.

Configuring Static NAT
This topic describes how to configure static NAT.
Configuring Static NAT
Example: Configuring static NAT
Router(config-if)#ip nat outside
Router(config-if)#ip nat inside
Router(config)#ip nat inside source static 10.1.1.2 209.165.201.5
Remember that static NAT is a one-to-one mapping between an inside address and an outside address.
Static NAT allows external devices to initiate connections to internal devices. For instance, you may want
to map an inside global address to a specific inside local address that is assigned to your web server.
Configuring static NAT translations is a simple task. You need to define the addresses to translate and then
configure NAT on the appropriate interfaces. Packets arriving on an inside interface from the identified IP
address are subject to translation. Packets arriving on an outside interface that are addressed to the identified
IP address are also subject to translation.
The figure shows the example of the commands for the steps. You enter static translations directly into the
configuration. Unlike dynamic translations, these translations are always in the NAT table.
Command Description
interface interface Specifies an interface and enters interface
configuration mode
ip address address subnet_mask Sets the IP address and mask of the device
ip nat inside Marks the interface as connected to the
inside network
ip nat outside Marks the interface as connected to the
outside network
ip nat inside source static inside_address outside_address Establishes a static translation between an
inside local and inside global address
For more details about the ip nat inside and ip nat outside and related commands, check the Cisco IOS IP
Addressing Services Command Reference at http://www.cisco.com/en/US/docs/ios/ipaddr/command/
reference/iad_book.html

Verifying Static NAT Configuration
This topic describes verifying static NAT.
Verifying Static NAT Configuration
10.1.1.100
Internet
Verify static NAT configuration.
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
Router#show ip nat translations
Pro Inside global Inside local Outside local Outside global
tcp 209.165.201.5:1031 10.1.1.100:1031 209.165.202.155:23 209.165.202.155:23
--- 209.165.201.5 10.1.1.100 --- ---
Command Description
show ip nat translations Displays active NAT translations
For more details about the ip nat inside, ip nat pool, and show ip nat translations and related commands,
check the Cisco IOS IP Addressing Services Command Reference at http://www.cisco.com/en/US/docs/ios/
ipaddr/command/reference/iad_book.html

Understanding Dynamic NAT
This topic describes how dynamic NAT functions.
Understanding Dynamic NAT
209.165.201.5
10.1.1.100
10.1.1.101
Internet
Inside Outside
NAT Table
Host B
209.165.202.131
10.1.1.101
10.1.1.101
1
2
209.165.201.5
3
4
5
Inside Local
IPv4 Address
Inside Global
IPv4 Address
Outside Global
IPv4 Address
10.1.1.101 209.165.201.5 209.165.202.131
10.1.1.100 209.165.201.6 209.165.202.131
While static NAT provides a permanent mapping between an internal address and a specific public address,
dynamic NAT maps private IP addresses to public addresses. These public IP addresses come from a NAT
pool. Dynamic NAT configuration differs from static NAT, but it also has some similarities. Like static
NAT, it requires the configuration to identify each interface as an inside or outside interface. However,
rather than creating a static map to a single IP address, a pool of inside global addresses is used.
The figure illustrates a router that is translating a source address inside a network into a source address
outside the network. These are the steps for translating an inside source address:
1. The users at hosts 10.1.1.100 and 10.1.1.101 want to open a connection to host B (IP address
209.165.202.131).
2. The first packet that the router receives from host 10.1.1.101 causes the router to check its NAT table.
If no static translation entry exists, the router determines that the source address 10.1.1.101 must be
translated dynamically. The router then selects a legal global address from the dynamic address pool
and creates a translation entry (in this example, 209.165.201.5). This type of entry is called a simple
entry. For the second host, 10.1.1.100, the router selects a legal global address from the dynamic
address pool and creates a second translation entry (in this example, 209.165.201.6).
3. The router replaces the inside local source address of host 10.1.1.101 with the translated inside global
address and forwards the packet.
4. Host B receives the packet and responds to host 209.165.201.5, using the inside global IPv4 destination
address 209.165.201.5. When host B receives the second packet, it responds to host 209.165.201.6,
using the inside global IPv4 destination address 209.165.201.6.

5. When the router receives the packet with the inside global IPv4 address of 209.165.201.5, the router
performs a NAT table lookup using the inside global address as a key. The router then translates the
address back to the inside local address of host 10.1.1.101 and forwards the packet to host 10.1.1.101.
When the router receives the packet with the inside global IPv4 address of 209.165.201.6, the router
performs a NAT table lookup using the inside global address as a key. The router then translates the
address back to the inside local address of host 10.1.1.100 and forwards the packet to host 10.1.1.100.
6. Hosts 10.1.1.100 and 10.1.1.101 receive the packets and continue the conversation. The router
performs Steps 2 through 5 for each packet.

Configuring Dynamic NAT
This topic provides an example of configuring dynamic NAT.
Configuring Dynamic NAT
10.1.1.100
Internet
Configure ACL
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
10.1.1.101
Inside Outside
Router(config)#access-list 1 permit 10.1.1.0 0.0.0.255
Router(config)#ip nat pool NAT-POOL 209.165.201.5 209.165.201.10 netmask
255.255.255.240
Router(config)#ip nat inside source list 1 pool NAT-POOL
Command Description
configuration mode
ip nat pool pool_name start_ip end_ip netmask netmask Defines an IP address pool
ip nat inside source list acl_number pool pool_name Establishes a dynamic source translation by
specifying the ACL and the address pool
ip address address subnet_mask Sets the IP address and mask
inside network
outside network
access-list acl_number permit ip_address netmask Creates an access list that defines the inside
local addresses that are eligible to be
translated
Note The ACL must permit only those addresses that need to be translated. Remember that there is an
implicit deny any statement at the end of each ACL. An ACL that is too permissive can lead to
unpredictable results. Using permit any can result in NAT consuming too many router resources, which
can cause network problems.

Verifying Dynamic NAT Configuration
This topic describes an example of verifying NAT.
Verifying Dynamic NAT Configuration
icmp 209.165.201.5:3 10.1.1.100:3 209.165.202.155:3 209.165.202.155:3
--- 209.165.201.5 10.1.1.100 --- ---
icmp 209.165.201.6:1 10.1.1.101:1 209.165.201.125:1 209.165.201.125:1
tcp 209.165.201.6:1030 10.1.1.101:1030 209.165.201.125:23 209.165.201.125:23
--- 209.165.201.6 10.1.1.101 --- ---
Command Description

Understanding PAT
This topic describes the operation of PAT.
Understanding PAT
209.165.201.5
209.165.201.5
10.1.1.100
10.1.1.101
Internet
Inside Outside
NAT Table
Host B
209.165.202.131
H
Host C
209.165.202.132
10.1.1.101
10.1.1.101
1
2
209.165.201.5
3
5
4
6
Protocol Inside Local
IPv4 Address
Inside Global
IPv4 Address
Outside Global
IPv4 Address
TCP 10.1.1.100:1723 209.165.201.5:1723 209.165.202.131:23
TCP 10.1.1.101:1927 209.165.201.5:1927 209.165.202.132:23
TCP 10.1.1.101:1723 209.165.201.5:1724 209.165.202.131:23
One of the main forms of NAT is PAT, which is also referred to as overload in Cisco IOS configuration.
Several inside local addresses can be translated using NAT into just one or a few inside global addresses by
using PAT. Most home routers operate in this manner. Your ISP assigns one address to your router, yet
several members of your family can simultaneously surf the Internet.
With NAT overload, multiple addresses can be mapped to one or a few addresses because a TCP or UDP
port number tracks each private address. When a client opens a TCP/IP session, the NAT router assigns a
port number to its source address. NAT overload ensures that clients use a different TCP or UDP port
number for each client session with a server on the Internet. When a response comes back from the server,
the source port number (which becomes the destination port number on the return trip) determines the client
to which the router routes the packets. It also validates that the incoming packets were requested, thus
adding a degree of security to the session.
PAT uses unique source port numbers on the inside global IPv4 address to distinguish between
translations. Because the port number is encoded in 16 bits, the total number of internal addresses that
NAT can translate into one external address is, theoretically, as many as 65,536.
PAT attempts to preserve the original source port. If the source port is already allocated, PAT attempts
to find the first available port number. It starts from the beginning of the appropriate port group, 0 to
511, 512 to 1023, or 1024 to 65535 (in the figure, port 2031 is used). If PAT does not find an available
port from the appropriate port group and if more than one external IPv4 address is configured, PAT
moves to the next IPv4 address and tries to allocate the original source port again. PAT continues
trying to allocate the original source port until it runs out of available ports and external IPv4 addresses.
NAT generally translates IP addresses only as a 1:1 correspondence between publicly exposed IP addresses
and privately held IP addresses. NAT overload modifies the private IP address and potentially the port
number of the sender. NAT overload chooses the port numbers that hosts see on the public network.

NAT routes incoming packets to their inside destination by referring to the incoming source IP address
given by the host on the public network. With NAT overload, there is generally only one publicly exposed
IP address (or a very few). Incoming packets from the public network are routed to their destinations on the
private network by referring to a table in the NAT overload device that tracks public and private port pairs.
This mechanism is called connection tracking.
The figure illustrates PAT operation when one inside global address represents multiple inside local
addresses. The TCP port numbers act as differentiators. Both hosts B and C think that they are talking to a
single host at address 209.165.201.5. They are actually talking to different hosts, and the port number is the
differentiator. In fact, many inside hosts could share the inside global IPv4 address by using many port
numbers.
The router performs this process when it overloads inside global addresses:
1. The user at host 10.1.1.100 opens a connection to host B. A second user at host 10.1.1.101 opens a
connection to host B and C.
2. The first packet that the router receives from host 10.1.1.100 causes the router to check its NAT table.
If no translation entry exists, the router determines that address 10.1.1.100 must be translated and sets
up a translation of inside local address 10.1.1.100 into a legal inside global address. If overloading is
enabled and another translation is active, the router reuses the inside global address from that
translation and saves enough information to be able to translate back. This type of entry is called an
extended entry.
3. The router replaces the inside local source address 10.1.1.100 with the selected inside global address
209.165.201.5 and forwards the packet.
4. Host B receives the packet and responds to host 10.1.1.100, using the inside global IPv4 address
209.165.201.5. Host C receives a packet with the same inside global address, even though the packet
originated from host 10.1.1.101.
5. When the router receives the packet with the inside global IPv4 address, the router performs a NAT
table lookup. Using the inside global address and port and outside global address and port as a key, the
router translates the address back into the correct inside local address, 10.1.1.100, and forwards the
packet to host 10.1.1.100.
6. Host 10.1.1.100 receives the packet and continues the conversation. The router performs Steps 2
through 5 for each packet.

Configuring PAT
This topic describes an example of configuring PAT.
Configuring PAT
Configure
ACL.
Inside Outside
10.1.1.100
Internet
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
10.1.1.101
Router(config-if)#interface GigabitEthernet 0/1
Router(config#ip nat outside
Router(config)#ip nat inside source list 1 interface Gi 0/1 overload
Command Description
configuration mode
ip address address subnet_mask Sets the IP address and mask
inside network
outside network
ip nat inside source list access-list-number interface interface
overload
Establishes dynamic source translation,
specifying the ACL
access-list acl_number permit ip_address netmask Creates an ACL that defines the inside local
addresses that are eligible to be translated

Verifying PAT Configuration
This topic describes an example of verifying PAT.
Verifying PAT Configuration
10.1.1.100
Internet
Verify PAT configuration.
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
10.1.1.101
tcp 209.165.201.5:27497 10.1.1.100:27497
209.165.202.155:80 209.165.202.155:80
tcp 209.165.201.5:2597 10.1.1.100:2597 209.165.201.125:443
209.165.201.125:443
Command Description

Troubleshooting NAT
This topic describes how to troubleshoot NAT.
Troubleshooting NAT
Yes
No
Are
translations
occurring?
Are the
translations
occurring but
there is no
connectivity?
No connectivity
to the outside
Connectivity
• Check that NAT ACL is identifying
the correct traffic.
• Check that there are enough
addresses in the pool.
• Check that inside/outside
interfaces are correctly defined.
• Check that there is a traffic-
initiating translation.
Yes
show ip route
show ip nat translations
show ip nat statistics
show access-list
Check that the remote router has
a return route back to the
translated address.
No
When you have IPv4 connectivity problems in a NAT environment, it is often difficult to determine the
cause of the problem. NAT is often blamed, when in reality there is an underlying problem. When you are
trying to determine the cause of an IPv4 connectivity problem, it helps to eliminate NAT as the problem.
Follow these steps to verify that NAT is operating as expected:
1. Verify that translations are occurring:
Use the show ip nat translations command to determine if translations exist in the translation
table.
Verify that the translation is actually occurring, by using the show ip nat statistics and debug ip
nat commands.
Use the show access-list command to verify that the ACL associated with the NAT command is
permitting all necessary networks.
Use the show ip nat statistics command to verify that the router interfaces are appropriately
defined as NAT inside or NAT outside.
If some devices have connectivity but others do not, the NAT pool might be out of addresses.
2. If translations are occurring but there is no connectivity, use show ip route to verify that there is a
return route to the translated address.

Troubleshooting NAT (Cont.)
Are Addresses Being Translated?
Router#show ip nat statistics
Total translations: 5 (0 static, 5 dynamic, 5 extended)
Outside Interfaces: Serial0
Inside Interfaces: Ethernet0 , Ethernet1
Hits: 42 Misses: 44
<output omitted>
Monitors NAT statistics
Router#show access-list
access-list 1 permit 10.1.1.100 0.0.0.255
Verifies that the NAT ACL is permitting all necessary networks
In a simple network environment, it is useful to monitor NAT statistics with the show ip nat statistics
command. The show ip nat statistics command displays information about the total number of active
translations, NAT configuration parameters, the number of addresses in the pool, and the number that have
been allocated. However, in a more complex NAT environment, with several translations taking place, this
show command may not clearly identify the issue. It may be necessary to run debug commands on the
router.
Note You can use the clear ip nat translation * command to clear all dynamic address translation entries. By
default, translation entries time out after 24 hours. When testing the NAT configuration, it can be useful
to clear translations.

To display detailed dynamic data and events, you can use debug
commands.
A debug command can intensively use device resources. Use carefully on
production equipment.
Always turn off debug after troubleshooting with the no debug all command.
Router#debug ip nat
NAT*: s=10.1.1.100->209.165.201.1, d=172.16.1.100 [103]
NAT*: s=172.16.1.100, d=209.165.201.1->10.1.1.100 [103]
NAT*: s=10.1.1.100->209.165.201.1, d=172.16.1.100 [104]
NAT*: s=172.16.1.100, d=209.165.201.1->10.1.1.100 [104]
<output omitted>
Displays information about every packet that is translated by the router
Note The debug command, especially the debug all command, should be used sparingly. These commands
can disrupt router operations. The debug commands are useful when configuring or troubleshooting a
network. However, they can make intensive use of CPU and memory resources. It is recommended that
you run as few debug processes as necessary and disable them immediately when they are no longer
needed. The debug commands should be used with caution on production networks because they can
affect the performance of the device.
The debug ip nat command displays information about every packet that the router translates, which helps
you to verify NAT operation. The debug ip nat detailed command generates a description of each packet
that is considered for translation. This command also provides information about certain errors or exception
conditions, such as the failure to allocate a global address. The debug ip nat detailed command generates
more overhead than the debug ip nat command, but it can provide the detail that you need to troubleshoot
the NAT problem. Always remember to turn off debugging when finished.
The figure shows a sample debug ip nat output. In the output, you can see that inside host 10.1.1.100
initiated traffic to outside host 209.165.202.131 and has been translated to address 209.165.201.5.
For decoding the debug output, note what the following symbols and values indicate:
*: The asterisk next to "NAT" indicates that the translation is occurring in the fast-switched path. The
first packet in a conversation is always process-switched, which is slower. The remaining packets go
through the fast-switched path if a cache entry exists.
s=: Refers to the source IP address.
a.b.c.d->w.x.y.z: Indicates that source address a.b.c.d is translated to w.x.y.z.
d=: Refers to the destination IP address.
[xxxx]: The value in brackets is the IP identification number. This information may be useful for
debugging because it enables correlation with other packet traces from protocol analyzers.
Finally, you should make sure that the ACL that the NAT command references is permitting all of the
necessary networks. Notice that ACLs use wildcard masks and not subnet masks.

If translations are occurring, but there is no connectivity, verify
that the remote router has a route to the translated address.
10.1.1.100
10.1.1.1
209.165.201.1
209.165.201.2
10.1.1.100
HQ Branch
NAT
Branch#show ip route
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, + - replicated route
S* 0.0.0.0/0 [1/0] via 209.165.201.1
If the translations are occurring but a ping to the remote network still fails, the issue might be a missing
route back to the translated address. This problem can arise in NAT between a headquarters and branch
office. It is usually not an issue when connecting to an ISP, because the service provider takes care of
routing all of the necessary traffic back to the customer.

Troubleshooting NAT Case Study
Troubleshooting NAT Case Study
Host A and host B are unable to ping after a new NAT
configuration is put in place.
Host A
10.1.1.100
Internet Host B
209.165.202.131
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
Ping fails!
The figure shows that host A (10.1.1.100) cannot ping host B (209.165.202.131). Relevant part of
configuration is shown in the show running-config output.
Troubleshooting NAT Case Study (Cont.)
Router#show running-config
<output omitted>
ip route 0.0.0.0 0.0.0.0 209.165.201.2
!
access-list 20 permit 0.0.0.0 255.255.255.0
!
interface GigabitEthernet0/0
ip address 10.1.1.1 255.255.255.0
ip nat outside
!
interface GigabitEthernet0/1
ip address 209.165.200.1 255.255.255.254
ip nat inside
!
ip nat inside source list 20 interface GigabitEthernet0/1 overload

Translations are not occurring.
Host A
10.1.1.100
Internet Host B
209.165.202.131
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
Ping fails!
To troubleshoot the problem, you use the show ip nat translation command to see if any translations are
currently in the table. You find that no translations are in the table.
The router interfaces are incorrectly defined as NAT inside and
NAT outside.
Host A
10.1.1.100
Internet Host B
209.165.202.131
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
Ping fails!
Router#show ip nat statistics
Total active translations: 0 (0 static, 0 dynamic; 0 extended)
Outside interfaces:
GigabitEthernet0/0
Inside interfaces:
GigabitEthernet0/1
<output omitted>

How to fix configuration:
Router#configure terminal
Router(config-if)#interface GigabitEthernet 0/1
Next, you must determine whether any translations have ever taken place and identify the interfaces
between which translation should be occurring. You use the show ip nat statistics command.
As shown in the example, you determine that the NAT counters are at 0, verifying that no translation has
occurred. You also find that the router interfaces are incorrectly defined as NAT inside or NAT outside. The
output also shows how to fix configuration.
Verify that the access list is correct.
Host A
10.1.1.100
Internet Host B
209.165.202.131
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
Ping fails!
RouterA#show access-list
How to fix access list:
Router#config terminal
Router(config)#no access-list 20
After you correctly define the NAT inside and outside interfaces, you generate another ping from host A to
host B. The ping still fails. To troubleshoot the problem, you use the show ip nat translations and show ip
nat statistics commands again. You find that translations are still not occurring.

Next, you use the show access-list command to determine whether the ACL that the NAT command
references is permitting all of the necessary networks. You determine that an incorrect wildcard bit mask
has been used in the ACL that defines the addresses that need to be translated.
Verify that translations are occurring and you have connectivity
to the remote network.
Host A
10.1.1.100
Internet Host B
209.165.202.131
Gi0/0 Gi0/1
10.1.1.1 209.165.201.1
Ping successful!
C:>ping 209.165.202.131
Pinging 209.165.202.131 with 32 bytes of data:
<output omitted>
icmp 209.165.201.1:1 10.1.1.100:1 209.165.202.131:1 209.165.202.131:1
After you correct the wildcard mask, you generate another ping from host A to host B. The connectivity test
is now a success. You issue the show ip nat translations command to verify the translation.

Summary
Summary
Provider-assigned IP addresses can be configured on a router statically
or can be dynamically assigned through DHCP.
A DHCP client is a host that requests an IP address and configuration
from a DHCP server.
A DHCP server allocates network addresses and delivers
configurations.
Summary (Cont.)
NAT enables private IP internetworks that use private IP addresses to
connect to the Internet. PAT, or NAT overload, a feature of NAT,
enables several internal addresses to be translated to only one or a few
external addresses.
Static NAT is one-to-one address mapping. Dynamic NAT addresses
are picked from a pool.
PAT allows you to map many inside addresses to one outside address.
Use the show ip nat translations command to display the translation
table and verify that translation has occurred.
To determine whether a current translation entry is being used, use the
show ip nat statistics command to check the hits counter.

Module Summary
Module Summary
IP is a Layer 3 media-independent connectionless protocol that uses
hierarchical logical addressing and provides best-effort service.
Internet hosts require a unique public IP address. Hosts in private
networks can have any valid private IP address that is unique locally in
each network.
Networks, particularly large networks, are often divided into smaller
subnetworks, or subnets. Subnets can improve network performance
and control.
TCP is a connection-oriented protocol that provides reliable transport.
UDP is a connectionless transport protocol that provides best-effort
transport.

The main function of a router is to relay packets from one network
device to another. To do this, you must define the characteristics of the
interfaces through which packets are received and sent. Interface
characteristics, such as the IP address, are configured in interface
configuration mode.
Cisco Discovery Protocol is an information-gathering tool that is used by
network administrators to obtain information about directly connected
devices.
Static routers use a route that a network administrator manually enters
into the router. Dynamic routers use a route that a network routing
protocol adjusts automatically for topology or traffic changes.
ACLs can be used as a Cisco IOS tool to identify traffic that receives
special handling.
NAT enables private IP internetworks that use private IP addresses to
connect to the Internet. PAT, a feature of NAT, enables several internal
addresses to be translated to one external address or a few external
addresses.

Module Self-Check
Questions
1. How many bits are in an IPv4 address? (Source: Understanding the TCP/IP Internet Layer)
A. 4
B. 8
C. 16
D. 32
E. 64
F. 128
2. Which characteristic is similar in TCP and UDP? (Source: Understanding the TCP/IP Transport
Layer)
A. operates at the transport layer of the OSI model and the TCP/IP stack
B. capable of performing error checking
C. provides service on a best-effort basis and does not guarantee packet delivery
D. provides no special features that recover lost or corrupted packets
3. What is the purpose of a router? (Source: Exploring the Functions of Routing)
A. to interconnect networks and choose the best paths between them
B. to provide the connection points for the media
C. to serve as the endpoint in the network, sending and receiving data
D. to provide the means by which signals are transmitted from one networked device to another

4. TCP is best for which two applications? (Choose two.) (Source: Understanding the TCP/IP Transport
Layer)
A. email
B. voice streaming
C. downloading
D. video streaming
5. Match each IPv4 address type with its description. (Source: Understanding the TCP/IP Internet
Layer)
A. loopback address used to let the system send a message to itself for testing
B. local broadcast address used to address all devices in local network
C. all zeros address only used as source address until valid IP is obtained
D. directed broadcast address special address for each network that allows
communication to all of the hosts in that network
6. Which network does a host with an IP address of 10.44.45.46/23 belong to? (Source: IP Addressing
and Subnets)
A. 10.44.45.45
B. 10.44.45.0
C. 10.44.44.0
D. 10.44.0.0
7. Match the numerical addresses with their descriptions for network 192.168.4.0/24. (Source:
Understanding IP Addressing and Subnets)
A. network address 192.168.4.254
B. first host address 192.168.4.1
C. last host address 192.168.5.0
D. next network address 192.168.4.255
E. broadcast address 192.168.4.0
8. Match the types of routes with their descriptions. (Source: Exploring the Functions of Routing)
A. default routes This entry comes from having router interfaces that are
directly attached to network segments.
B. dynamic routes They are used when no explicit path to a destination is
found in the routing table.
C. static routes The system administrator manually enters them into the
configuration of a router.
D. directly connected
networks
The router learns about them automatically through a
routing protocol.

9. Which two statements about Cisco Discovery Protocol are correct? (Choose two.) (Source:
Configuring a Cisco Router)
A. It was developed by the IETF.
B. It gathers information about directly connected Cisco switches, routers, and other Cisco devices.
C. If two neighbor devices have misconfigured IP addresses, they will still learn about each other
through this protocol.
D. The original name of the protocol was Connected Discovery Protocol.
10. Which two statements correctly describe MAC addresses? (Choose two.) (Source: Exploring the
Packet-Delivery Process)
A. Ethernet uses MAC addresses.
B. They are used for addressing at the TCP/IP internet layer.
C. A MAC address is a 32-bit binary value.
D. The MAC address identifies end devices in a LAN.
11. Which two statements correctly describe ARP? (Choose two.) (Source: Exploring the Packet-
Delivery Process)
A. resolves IP addresses to MAC addresses
B. resolves MAC addresses to IP addresses
C. has an expiration period for entries in the ARP table, usually 300 seconds for PCs
D. is turned off by default on Cisco switches
12. In which three situations would you use static routing? (Choose three.) (Source: Enabling Static
Routing)
A. when the network is large
B. when the network is expected to scale
C. in a small network that requires simple routing
D. in a hub-and-spoke network topology
E. when you want to quickly create an ad hoc route
13. Which command syntax correctly configures a default route? (Source: Enabling Static Routing)
A. ip default-route 172.16.0.1
B. ip default-route 0.0.0.0 0.0.0.0 172.16.0.1
C. ip route 0.0.0.0 0.0.0.0 172.16.0.1
D. ip route 172.16.0.1
14. Which two statements correctly identify ACLs? (Choose two.) (Source: Managing Traffic Using
ACLs)
A. have an implicit deny all at the bottom of each ACL
B. are consulted in a bottom-to-top order
C. use wildcard masks
D. can be used only for filtering

15. Which command syntax correctly describes an ACL that permits all hosts from subnet 10.1.0.0/16?
(Source: Managing Traffic Using ACLs)
A. access-list 10.1.1.0 0.0.255.255
B. access-list 1 permit 10.1.0.0 255.255.0.0
C. access-list 1 deny 10.1.0.0 0.0.255.255
D. access-list 1 permit 10.1.0.0 0.0.255.255
16. Which two statements correctly describe NAT? (Choose two.) (Source: Enabling Internet
Connectivity)
A. The total number of internal addresses that NAT can translate into one external address is 128.
B. PAT uses unique source port numbers on the inside global IPv4 address to distinguish between
translations.
C. Static, dynamic, and automatic are three NAT types.
D. Private users can access the Internet by sharing one or more public IP addresses.

Answer Key
1. D
2. A
3. A
4. A, C
5. A. local broadcast address used to address all devices in local network
B. directed broadcast address special address for each network that allows communication
to all of the hosts in that network
C. loopback address used to let the system send a message to itself for testing
D. all zeros address only used as source address until valid IP is obtained
6. C
7. A. network address 192.168.4.0
B. broadcast address 192.168.4.255
C. first host address 192.168.4.1
D. last host address 192.168.4.254
E. next network address 192.168.5.0
8. A. directly connected networks This entry comes from having router interfaces that are
directly attached to network segments.
B. static routes The system administrator manually enters them into the
configuration of a router.
C. dynamic routes The router learns about them automatically through a
routing protocol.
D. default routes They are used when no explicit path to a destination is
found in the routing table.
9. B, C
10. A, D
11. A, C
12. C, D, E
13. C
14. A, C

15. D
16. B, D

Module 3
Managing Network Device
Security
This module describes the steps that are required to secure local and remote access to network devices. It
provides general recommendations for improving device hardening. It describes how network devices can
control traffic on the network. It explains ACLs and their use for traffic filtering and shows configuration
examples.
Objectives
Implement a basic security configuration
Implement basic steps to harden network devices
Implement standard, extended, numbered, and named ACLs to filter traffic

Lesson 1
Securing Administrative
Access
Overview
After physical access has been enabled, you must ensure that access to the switch via the console port and
the vty ports is secure. You must also filter access to network devices from remote or internal locations.
Objectives
List the actions that are required to secure a network device
Secure access to privileged EXEC mode
Secure console access to a network device
Secure remote access to a network device
Enable remote access connectivity on a switch
Limit remote access with an ACL
Explain why external authentication should be used
Configure a login banner

Network Device Security Overview
This topic describes common threats to network device security.
Network Device Security Overview
Network devices are vulnerable to these
common threats:
Remote access threats
Unauthorized remote access
Local access and physical threats
Damage to equipment
Password recovery
Device theft
Environmental threats
Extreme temperature
High humidity
Electrical threats
Insufficient power supply voltage
Voltage spikes
Maintenance threats
Improper handling
Poor cabling
Inadequate labeling
Common threats to network device security and mitigation strategies can be summarized as follows:
Remote access threats: Unauthorized remote access is a threat when security is weak in remote access
configuration. Mitigation techniques for this type of threat include configuring strong authentication
and encryption for remote access policy and rules, configuration of login banners, use of ACLs, and
VPN access.
Local access and physical threats: Threats include physical damage to network device hardware,
password recovery that is allowed by weak physical security policies, and device theft. Mitigation
techniques for this type of threat include locking the wiring closet and allowing access only by
authorized personnel and blocking physical access through a dropped ceiling, raised floor, window,
duct work, or other possible point of entry. Use electronic access control, and log all entry attempts.
Monitor facilities with security cameras.
Environmental threats: Temperature extremes (heat or cold) or humidity extremes (too wet or too
dry) can present a threat. Mitigation techniques for this type of threat include creating the proper
operating environment through temperature control, humidity control, positive air flow, remote
environmental alarms, and recording and monitoring.
Electrical threats: Voltage spikes, insufficient supply voltage (brownouts), unconditioned power
(noise), and total power loss are potential electrical threats. Mitigation techniques for this type of threat
include limiting potential electrical supply problems by installing UPS systems and generator sets,
following a preventative maintenance plan, installing redundant power supplies, and using remote
alarms and monitoring.
Maintenance threats: These threats include improper handling of important electronic components,
lack of critical spare parts, poor cabling, and inadequate labeling. Mitigation techniques for this type of
threat include using neat cable runs, labeling critical cables and components, stocking critical spares,
and controlling access to console ports.

Securing Access to Privileged EXEC Mode
This topic describes how to configure access to privileged EXEC mode.
Configuring enable password:
Switch(config)#enable password C1sco123
Configuring enable secret password:
Switch(config)#enable secret sanfran
Verification of configured passwords:
Switch#show running-config | include enable
enable secret 5 $1$WPHF$uWo4ucV0/vA1/abu6LlWQ1
enable password C1sco123
(Cont.)
Encrypting plaintext passwords:
Switch(config)#service password-encryption
Switch(config)#exit
Switch#show running-config | include enable
enable secret 5 $1$vWZa$2sYQLDv4R4xMtU5NFDrbX.
enable password 7 04785A150C2E1D1C5A
You can secure a switch by using passwords to restrict access. Using passwords and assigning privilege
levels is a way to provide terminal access control in a network and is a form of management-plane
hardening. Passwords can be established on individual lines, such as the console, and to privileged EXEC
mode. Passwords are case-sensitive.

Note The passwords that are shown in the figure are for instructional purposes only. Passwords that are used
in an actual implementation should meet the requirements for strong passwords.
The enable password global command restricts access to privileged EXEC mode. You can assign an
encrypted form of the enable password, called the enable secret password, by entering the enable secret
password command with the desired password at the global configuration mode prompt. When the enable
secret password is configured, it is used instead of the enable password rather than in addition to it.
You can also add a further layer of security, which is particularly useful for passwords that cross the
network or are stored on a TFTP server. Cisco provides a feature that allows the use of encrypted
passwords. To set password encryption, enter the service password-encryption command in global
configuration mode.
Passwords that are displayed or set after you configure the service password-encryption command will be
encrypted. Service password encryption uses type-7 encryption, which is not very secure. There are several
tools and web pages available that convert an encrypted password into a plaintext string.
On the other hand, the enable secret command uses MD5-type encryption that, to this point, has not been
broken. It is recommended that you use always use the enable secret secret password instead of the enable
password command.
Note Although the enable password command can be encrypted with the service password-encryption
command, the encryption is weak and has been broken with cracker programs available online.
However, the enable secret password uses MD5-type encryption that, to this point, has not been broken.
It is recommended that you use the enable secret secret password, not the enable password
command.

Securing Console Access
This topic describes how to secure console access.
Securing Console Access
Console password:
Switch(config)#line console 0
Switch(config-line)#password C1sco123
Switch(config-line)#login
EXEC timeout:
Switch(config-line)#exec-timeout 5
Use the line console 0 command followed by the password and login subcommands to require login and
establish a login password on a console terminal or vty port. By default, logging in is not enabled on
console or vty ports.
Note Enter the service password-encryption command in global configuration mode to encrypt the console
password. Although this encryption is weak and can be easily decrypted, it is still better than a cleartext
password. At least you are protected against exposing the password to casual observers.
The exec-timeout command prevents users from remaining connected to a console port when they leave a
station. In the example, when no user input is detected on the console for 5 minutes, the user that is
connected to the console port is automatically disconnected.
Note The EXEC timeout will be left at 60 in the lab for lab purposes.

Securing Remote Access
This topic describes how to secure remote access.
Securing Remote Access
Virtual terminal password:
Switch(config)#line vty 0 15
Switch(config-line)#login
Switch(config-line)#password CiScO
EXEC timeout:
Switch(config-line)#exec-timeout 5
Securing Remote Access (Cont.)
Configuring SSH:
Switch(config)#hostname SwitchX
SwitchX(config)#ip domain-name cisco.com
SwitchX(config)#username user1 secret C1sco123
SwitchX(config)#crypto key generate rsa modulus 1024
The name for the keys will be: SwitchX.cisco.com
% The key modulus size is 1024 bits
% Generating 1024 bit RSA keys, keys will be non-exportable...
[OK] (elapsed time was 1 seconds)
SwitchX(config)#line vty 0 15
SwitchX(config-line)#login local
SwitchX(config-line)#transport input ssh
SwitchX(config-line)#exit
SwitchX(config)#ip ssh version 2
The line vty 0 15 command, followed by the login and password subcommands, requires login and
establishes a login password on incoming Telnet sessions.
The login local command can be used to enable password checking on a per-user basis using the username
and secret password that are specified with the username global configuration command. The username
command establishes username authentication with encrypted passwords.

The exec-timeout command prevents users from remaining connected to a vty port when they leave a
station. In the example, when no user input is detected on a vty line for 5 minutes, the vty session is
automatically disconnected.
To configure SSH on a Cisco switch or router, you need to complete the following steps:
Use the hostname command to configure the host name of the device so that it is not Switch (on a
Cisco switch) or Router (on a Cisco router).
Configure the DNS domain with the ip domain name command. The domain name is required to be
able to generate certificate keys.
Generate RSA keys to be used in authentication with the crypto key generate rsa command.
Configure the user credentials to be used for authentication. By specifying the login local command for
vty lines, you are essentially telling the network device to use locally defined credentials for
authentication. Configure locally defined credentials using the username username secret password
command.
(Optional) You can also limit access to a device to users that use SSH and block Telnet with the
transport input ssh vty mode command. If you want to support login banners and enhanced security
encryption algorithms, force SSH version 2 on your device with the ssh version 2 command in global
configuration mode.
Verify that SSH is enabled:
Switch#show ip ssh
SSH Enabled - version 2.0
Authentication timeout: 120 secs; Authentication retries: 3
Check the SSH connection to the device:
Switch#show ssh
Connection Version Encryption State Username
0 1.5 3DES Session started cisco

You can establish an SSH connection to the SSH-enabled device (a switch, in this example) using an SSH
client on your PC, such as PuTTY. When you establish a connection for the first time from a specific
computer, you are presented with a security alert window that indicates that the server host key is not
cached in the PuTTY cache. By adding a key to the cache, you will avoid seeing this security alert window
every time that you establish an SSH connection from this computer.
To display the version and configuration data for SSH on the device that you configured as an SSH server,
use the show ip ssh command. In the example, SSH version 2 is enabled.
To check the SSH connection to the device, use the show ssh command.

Enabling Remote Access Connectivity
This topic describes how to enable remote access connectivity by configuring the IP address of a default
gateway for a switch.
Enabling Remote Access Connectivity
Configure the IP address of a default gateway on a switch
SwitchX(config)#ip default-gateway 10.1.1.1
Switch X Default Gateway
10.1.1.1
To configure the switch so that it can be accessed remotely, a default gateway is needed. To configure a
default gateway for the switch, use the ip default-gateway command. Enter the IP address of the next-hop
router interface (10.1.1.1) that is directly connected to the switch where a default gateway is being
configured.
Once the default gateway is configured, the switch sends packets for all unknown IP destinations to the
configured gateway, which enables switch connectivity with distant networks.

Limiting Remote Access with ACLs
This topic describes how to limit remote access to vty lines.
Limiting Remote Access with ACLs
PC1
Branch
10.1.1.100/24
PC2
10.1.2.100/24
Permit remote access from the 10.1.1.0/24
network but deny everybody else
X
Use an ACL to permit Telnet access from 10.1.1.0 /24 but deny everybody else:
Router(config)#access-list 1 deny any log
Apply the ACL on vty lines:
Router(config)#line vty 0 15
Router(config-line)#access-class 1 in
You can limit access to vty lines to specific IP addresses or subnets in order to control remote
administration of network devices. Remote administration is commonly run over a Telnet or SSH
connection, where the SSH connection is an encrypted communication channel between the administrator
workstation and the device.
There are commonly two steps that you must complete to limit remote access with ACLs:
Configure an ACL: The example shows an ACL that is being configured with two lines. The first line
permits Telnet access from network addresses in the 10.1.1.0 /24 subnet. The second line is not
mandatory because there is an implicit deny statement at the end of every ACL, but creating an explicit
deny statement and appending the log keyword allows you to monitor attempts to access the device by
unauthorized sources.
Apply the ACL to the lines: The access-class command applies the ACL on vty lines. Using the in
keyword after the name of the ACL tells the router to limit vty connections coming into the network
device.

External Authentication Options
This topic describes external authentication options for remote administration of network devices.
External Authentication Options
External authentication may be preferable to local
authentication:
A local authentication database can be an administrative burden.
External authentication provides scalability.
You can use RADIUS or TACACS+.
Console Network Device Authentication
Server
Query AAA Server
Request Authentication
In a small network, local authentication is often used. When you have more than a few user accounts in a
local device database, managing those user accounts becomes more complex. For example, if you have 100
network devices, adding one user account means that you have to add this user account on all 100 devices
in the network. Also, when you add one network device to the network, you have to add all user accounts to
the local device database to enable all users to access this device.
AAA is a security architecture for distributed systems that enables control over which users are allowed
access to certain services and how much of the resources they have used.
The two most popular options for external authentication of users are RADIUS and TACACS+:
RADIUS is an open standard with low use of CPU resources and memory. It is used by a range of
network devices, such as switches, routers, and wireless devices.
TACACS+ is a security mechanism that enables modular AAA services. It uses a TACACS+ daemon
running on a security server.
Note The details about AAA, RADIUS, and TACACS+ are out of the scope of this course.

Configuring the Login Banner
This topic describes how to configure a banner message that appears when a user tries to log into a Cisco
IOS network device.
Configuring the Login Banner
Configure a login banner:
Switch(config)#banner login "Access for authorized users only. Please enter
your username and password."
A user connecting to the device sees this message:
Access for authorized users only. Please enter your username and password.
User Access Verification
Username:
You can define a customized login banner to be displayed before the username and password login prompts.
To configure a login banner, use the banner login command in global configuration mode. Enclose the
banner text in quotation marks or use a delimiter that is different from any character appearing in the banner
string.
Note Use caution when you create the text that is used in the login banner. Words like “welcome” may imply
that access is not restricted and may allow hackers some legal defense of their actions.
To define and enable an MOTD banner, use the banner motd command in global configuration mode. To
delete the MOTD banner, use the no form of this command.
This MOTD banner is displayed to all terminals that are connected and is useful for sending messages that
affect all users (such as impending system shutdowns).

Summary
Summary
Security threats to network devices include remote access threats,
physical and local access threats, environmental threats, electrical
threats, and maintenance threats.
You can secure a network device by using passwords to restrict access.
You can secure console access to a network device by using console
passwords and by using an EXEC timeout setting to prevent access
from connected terminals.
You can secure a network device for Telnet and SSH access by using
vty passwords to restrict access and an EXEC timeout setting to prevent
access from connected terminals.
You can secure a network device for Telnet and SSH access by using
an ACL to limit the users who can access the device.
If you want a scalable option instead of a local authentication database,
use the RADIUS or TACACS+ external authentication service.
Use the banner command to configure a login or MOTD banner.

Lesson 2
Implementing Device
Hardening
Overview
This lesson first describes the need for securing unused ports. It presents port security as a solution to the
problem of maintaining control of utilized ports. The need to disable unused services is illustrated, and
configuration examples show how to disable them. The last part of this lesson explains why correct system
time is important and what can happen if the system time is not correct. NTP is introduced with a
configuration example.
Objectives
Secure unused ports
Describe port security
Configure port security
Verify port security
Disable unused services
Describe NTP
Configure basic NTP
Verify NTP

Securing Unused Ports
This topic describes the need to secure unused ports.
Securing Unused Ports
Unsecured ports can create a security vulnerability.
A device that is plugged into an unused port is added to the network.
Unused ports can be secured by disabling interfaces (ports).
Unused ports on a switch can be a security risk. A hacker can plug a switch into an unused port and become
part of the network. Therefore, unsecured ports can create a security hole.
A simple method that many administrators use to help secure their network from unauthorized access is to
disable all unused ports on a network switch.
Disabling an Interface (Port)
To shut down multiple ports, use the interface range
command and use the shutdown command.
SwitchX(config)#interface range FastEthernet0/1 – 3
SwitchX(config-if-range)#shutdown
SwitchX#show running-config
<output omitted>
!
shutdown
!
shutdown
!
shutdown
<output omitted>
The Fa0/1, Fa0/2, and Fa0/3 interfaces are disabled in the example.

For example, imagine that the Cisco switch has 24 ports. If there are three Fast Ethernet connections in use,
practicing good security demands that you disable the 21 unused ports.
It is simple to disable multiple ports on a switch. Navigate to each unused port and issue the Cisco IOS
shutdown command. An alternate way to shut down multiple ports is to use the interface range command.
If a port needs to be activated, you can manually enter the no shutdown command on this interface.
The process of enabling and disabling ports can become a tedious task, but the enhanced security on your
network is well worth the effort.

Port Security
This topic describes port security, which is a mechanism to protect ports in use on switches.
Port Security
How do you secure used ports?
How do you prevent users from connecting unauthorized host devices to
the network?
Example scenario:
A classroom with PCs is connected to the network.
How would you prevent students from unplugging classroom PCs and
connecting their own notebooks to the network?
Having control over used ports on the switch is as important as securing unused ports. While unused ports
on a switch can easily be secured by putting them into the shutdown state, securing ports in use presents a
different challenge. When securing used ports on the switch, you need a control over which kind of devices
are connected to the switch and have access to the network.
In the example scenario of a classroom environment, you will normally find PCs that are connected to the
network to enable students access to online exercises. To prevent students from disconnecting classroom
PCs and connecting their own notebooks, you will need to implement additional security mechanisms. Port
security is one possible solution of how to implement control over connected devices on Cisco switches.

Port Security (Cont.)
Port security restricts port access
by the MAC address.
Dynamic (limited number of
addresses)
Static (static configuration of
addresses)
Combination (static plus dynamic)
Sticky learning
0010.f6b3.d000
Unauthorized
MAC Address —
Access Denied
Port security, a feature that is supported on Cisco Catalyst switches, restricts access to a switch port to a
specific set or number of MAC addresses. The switch can learn these addresses dynamically, or you can
configure them statically. A port that is configured with port security accepts frames only from the
addresses that it has learned or that you have configured.
There are several implementations of port security:
Dynamic: You specify how many MAC addresses are permitted to use a port at one time. Use the
dynamic approach when you care only about how many rather than which specific MAC addresses are
permitted. Depending on how you configure the switch, these dynamically learned addresses age out
after a certain period, and new addresses are learned, up to the maximum that you have defined.
Static: You statically configure the specific MAC addresses that are permitted to use a port. Source
MAC addresses that you do not specifically permit are not allowed to source frames to the port.
A combination of static and dynamic learning: You can choose to specify some of the permitted
MAC addresses and let the switch learn the rest of the permitted MAC addresses. For example, if the
number of MAC addresses is limited to four, you can statically configure two MAC addresses. The
switch dynamically learns the next two MAC addresses that it receives on this port. Port access is
limited to these four addresses, two static and two dynamically learned. The two statically configured
addresses do not age out, but the two dynamically learned addresses can age out, depending on the
switch configuration.
“Sticky learning”: When this feature is configured on an interface, the interface converts dynamically
learned addresses to “sticky secure” addresses. This feature adds the dynamically learned addresses to
the running configuration as if they were statically configured using the switchport port-security
mac-address command. Sticky-learned addresses do not age out.
Imagine five individuals whose laptops are allowed to connect to a specific switch port when they visit an
area of the building. You want to restrict switch port access to the MAC addresses of those five laptops and
allow no addresses to be learned dynamically on this port.
The table describes the process that can achieve the desired results.

Step Action Notes
1 Port security is configured to allow only five
connections on this port, and one entry is
configured for each of the five allowed MAC
addresses.
This step populates the MAC address table with five
entries for this port and allows no additional entries to
be learned dynamically.
2 Allowed frames are processed. When frames arrive on the switch port, their source
MAC address is checked against the MAC address
table. If the source MAC address matches an entry in
the table for this port, the frames are forwarded to the
switch to be processed.
3 New addresses are not allowed to create new
MAC address table entries.
When frames with an unauthorized MAC address
arrive on the port, the switch determines that the
address is not in the current MAC address table and
does not create a dynamic entry for this new MAC
address.
4 The switch takes action in response to
unauthorized frames.
The switch disallows access to the port and takes one
of these configuration-dependent actions: (a) the
entire switch port can be shut down; (b) access can be
denied for only this MAC address, and a log error
message is generated; and (c) access can be denied
for this MAC address, but no log message is
generated.

Port Security Configuration
This topic describes how to configure port security.
Configuring Port Security
To configure port security on the Fa0/5 port to limit and identify
the MAC addresses of stations that are allowed to access the
port, do as follows:
1 Enable port security
2 Set the MAC address limit
3 Specify the allowable MAC addresses (optional)
4 Define the violation action
SwitchX(config)#interface FastEthernet0/5
SwitchX(config-if)#switchport mode access
SwitchX(config-if)#switchport port-security
SwitchX(config-if)#switchport port-security maximum 1
SwitchX(config-if)#switchport port-security mac-address sticky
SwitchX(config-if)#switchport port-security violation shutdown
Port security limits the number of valid MAC addresses that are allowed on a port. When you assign secure
MAC addresses to a secure port, the port does not forward packets with source addresses outside the group
of defined addresses.
If you limit the number of secure MAC addresses to one and assign a single MAC address to this port, only
the workstation with this particular secure MAC address can successfully connect to this switch port.
If a port is configured as a secure port and the maximum number of secure MAC addresses is reached, a
security violation occurs when the MAC address of a workstation that is attempting to access the port is
different from any of the identified secure MAC addresses.
The figure shows how to enable sticky port security on Fast Ethernet port 0/5 of switch SwitchX.
Note Before port security can be activated, the port mode must be set to “access” or "trunk" using the
switchport mode access|trunk command.
Use the switchport port-security interface command without keywords to enable port security on an
interface. Use the switchport port-security interface command with keywords to configure a secure MAC
address, a maximum number of secure MAC addresses, or the violation mode. Use the no form of this
command to disable port security or to set the parameters to their default state.
You can configure the maximum number of secure MAC addresses. In this figure, you can see the Cisco
IOS command syntax that is used to set the maximum number of MAC addresses to one (switchport port-
security maximum 1).
You can add secure addresses to the address table after setting the maximum number of secure MAC
addresses that are allowed on a port in these ways:
Manually configure all of the addresses (switchport port-security mac-address 0008.eeee.eeee).

Allow the port to dynamically configure all of the addresses (switchport port-security mac-address
sticky).
Configure a number of MAC addresses and allow the rest of the addresses to be dynamically
configured.
You can configure an interface to convert the dynamic MAC addresses to sticky secure MAC addresses and
add them to the running configuration by enabling sticky learning. To enable sticky learning, enter the
switchport port-security mac-address sticky interface configuration command. When you enter this
command, the interface converts all of the dynamic secure MAC addresses, including the MAC addresses
that were dynamically learned before sticky learning was enabled, to sticky secure MAC addresses.
The violation mode is set to shutdown (switchport port-security violation shutdown). This violation
mode is also a default setting. If any frames are detected from an address that is not allowed, the interface is
error-disabled, a log entry is made, an SNMP trap is sent, and manual intervention or error-disable recovery
must be used to make the interface usable.
Note Two other violation modes are protect and restrict. For both violation modes, frames from the address
that is not allowed are dropped, but the interface is not put into the error-disable state.
Security violation situations are as follows:
The maximum number of secure MAC addresses has been added to the address table. A station whose
MAC address is not in the address attempts to access the interface.
An address that is learned or configured on one secure interface is seen on another secure interface in
the same VLAN.
Note Port security is disabled by default.

Port Security Verification
This topic describes how to verify port security operation.
Port Security Verification
SwitchX#show port-security interface FastEthernet 0/5
Displays the port security settings that are defined for an interface
Port Security : Enabled
Port Status : Secure-up
Violation Mode : Shutdown
Aging Time : 0 mins
Aging Type : Absolute
SecureStatic Address Aging : Disabled
Maximum MAC Addresses : 1
Total MAC Addresses : 1
Configured MAC Addresses : 0
Sticky MAC Addresses : 0
Last Source Address:Vlan : fc99.47e5.2598:1
Security Violation Count : 0
Displays the port security settings that are defined for the FastEthernet 0/5 interface
After you have configured port security for your switch, verify that it has been configured correctly. You
must check each interface to verify that you have set the port security correctly. You must also verify that
you have configured static MAC addresses correctly. Use the show port-security interface privileged
EXEC command to display the port security settings that are defined for an interface.
The output displays this information (from the top down):
Whether the port security feature is enabled
The violation mode
The maximum allowed number of secure MAC addresses for each interface
The number of secure MAC addresses on the interface
The number of security violations that have occurred

Port Security Verification (Cont.)
Port Security : Enabled
Port Status : Secure-shutdown
Violation Mode : Shutdown
Aging Time : 0 mins
Aging Type : Absolute
SecureStatic Address Aging : Disabled
Maximum MAC Addresses : 1
Total MAC Addresses : 1
Configured MAC Addresses : 0
Sticky MAC Addresses : 1
Last Source Address:Vlan : 001a.2fe7.3089:1
Security Violation Count : 1
Displays the port security violation for the FastEthernet 0/5 interface
SwitchX#show interface status
Port Name Status Vlan Duplex Speed Type
Fa0/1 connected 1 a-full a-100 10/100BaseTX
Fa0/2 notconnect 1 auto auto 10/100BaseTX
Fa0/5 err-disabled 1 auto auto 10/100BaseTX
<output omitted>
Verifies the status of the interface
When MAC addresses are assigned to a secure port, the port does not forward frames with source MAC
addresses outside the group of defined addresses. When a port that is configured with port security receives
a frame, the source MAC address of the frame is compared to the list of secure source addresses that were
manually configured or autoconfigured (learned) on the port. If a MAC address of a device that is attached
to the port differs from the list of secure addresses, the port either shuts down until it is administratively re-
enabled (default mode) or drops incoming frames from the insecure host (the restrict option). The behavior
of the port depends on how it is configured to respond to a security violation.
The output in the figure shows that a security violation has occurred, and the port is in the secure-shutdown
state.

Because the port security violation mode is set to shutdown, the port with the security violation (source
MAC addresses outside the group of defined addresses) goes to the error-disabled state. You receive this
notification on the switch:
Sep 20 06:44:54.966: %PM-4-ERR_DISABLE: psecure-violation error detected on Fa0/5,
putting Fa0/5 in err-disable state
Sep 20 06:44:54.966: %PORT_SECURITY-2-PSECURE_VIOLATION: Security violation occurred,
caused by MAC
address 000c.292b.4c75 on port FastEthernet0/5.
Sep 20 06:44:55.973: %LINEPROTO-5-PPDOWN: Line protocol on Interface FastEthernet0/5,
changed state
to down
Sep 20 06:44:56.971: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
To verify the status of the interface, use the show interface status command.
To make the interface operational again, you need to disable the interface administratively and then enable
it again:
SwitchX(config)#interface FastEthernet 0/5
SwitchX(config-if)#shutdown
Sep 20 06:57:28.532: %LINK-5-CHANGED: Interface FastEthernet0/5, changed state to
administratively down
SwitchX(config-if)#no shutdown
Sep 20 06:57:48.186: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to up
Sep 20 06:57:49.193: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5,
changed state
to up
SwitchX#show port-security address
Secure Mac Address Table
-------------------------------------------------------------------
Vlan Mac Address Type Ports Remaining Age (mins)
---- ----------- ---- ----- -------------
1 0008.dddd.eeee SecureConfigured Fa0/5 -
-------------------------------------------------------------------
Total Addresses in System (excluding one mac per port) : 0
Max Addresses limit in System (excluding one mac per port) : 1024
Displays the secure MAC addresses for all ports
SwitchX#show port-security
Secure Port MaxSecureAddr CurrentAddr SecurityViolation Security Action
(Count) (Count) (Count)
-------------------------------------------------------------------
Fa0/5 1 1 0 Shutdown
-------------------------------------------------------------------
Total Addresses in System (excluding one mac per port) : 0
Max Addresses limit in System (excluding one mac per port) : 1024
Displays the port security settings for the switch
Use the show port-security address command to display the secure MAC addresses for all ports. Use the
show port-security command without keywords to display the port security settings for the switch.

Disabling Unused Services
This topic describes disabling unused services to increase network security.
Disabling Unused Services
Some services on Cisco devices may not be needed and
therefore can be disabled, providing these benefits:
Helps preserve system resources
Eliminates the potential for security exploits on the disabled services
Router#show control-plane host open-ports
Active internet connections (servers and established)
Prot Local Address Foreign Address Service State
tcp *:22 *:0 SSH-Server LISTEN
tcp *:23 *:0 Telnet LISTEN
udp *:49 172.26.150.206:0 TACACS service LISTEN
udp *:67 *:0 DHCPD Receive LISTEN
Displays the UDP or TCP ports that the router is listening to
To facilitate deployment, Cisco routers and switches start with a list of services that are turned on and
considered to be appropriate for most network environments. However, because not all networks have the
same requirements, some of these services may not be needed. Disabling these unnecessary services has
two benefits: it helps preserve system resources, and it eliminates the potential for security exploits on the
unneeded services.
The general best practice is to identify open ports. Use the show control-plane host open-ports command
to see which UDP or TCP ports the router is listening to and to determine which services need to be
disabled.
Note As an alternative, Cisco IOS Software provides the AutoSecure function that helps disable these
unnecessary services while enabling other security services.
In the example, services that are enabled on the router are SSH, Telnet, TACACS, and DHCP.

Disabling Unused Services (Cont.)
The following are some general best practices:
The finger, identification, TCP, and UDP small servers should remain
disabled on all routers and switches.
You should disable Cisco Discovery Protocol on interfaces where the
service may represent a risk.
It is strongly recommend that you turn off the HTTP service running on
the router (HTTPS can stay on).
Unless they are explicitly needed, ensure that finger, identification (identd), and TCP and UPD small
servers remain disabled on all routers and switches. In Cisco IOS Software Release 15.0 and later, all of
these services are disabled by default.
Disable Cisco Discovery Protocol on interfaces where the service may represent a risk. Examples are
external interfaces, such as those at the Internet edge, and data-only ports at the campus and branch access.
Cisco Discovery Protocol is enabled by default in Cisco IOS Software Release 15.0 and later.
Cisco routers can be accessed via a web page, but it is strongly recommend that you turn off the HTTP
service that is running on the router. HTTP service is disabled by default in Cisco IOS Software Release
15.0 and later.

Disabling Unused Services (Cont.)
There are two options to disable Cisco Discovery Protocol:
Disable it globally (on all interfaces)
Router(config)#no cdp run
Disable it on a specific interface
Router(config)#interface FastEthernet0/24
Router(config-if)#no cdp enable
It is recommended that you disable the HTTP service
Router(config)#no ip http server
If you prefer not to use the Cisco Discovery Protocol device discovery capability, you can disable it with
the no cdp run global configuration command. To re-enable Cisco Discovery Protocol after disabling it,
use the cdp run command in global configuration mode.
Cisco Discovery Protocol is enabled by default on all supported interfaces to send and receive Cisco
Discovery Protocol information. Cisco Discovery Protocol is not on by default on Frame Relay interfaces.
You can disable Cisco Discovery Protocol on an interface that supports it with the no cdp enable interface
configuration command. To re-enable Cisco Discovery Protocol on an interface after disabling it, use the
cdp enable command in interface configuration mode.
It is strongly recommended that you turn off the HTTP service that is running on the router. You can use the
no ip http server global configuration command to disable it. To re-enable the HTTP service after
disabling it, use the ip http server command in global configuration mode.

Network Time Protocol
This topic describes NTP.
Network Time Protocol
Correct time within networks is important:
Correct time allows the tracking of events in the network in the correct
order.
Clock synchronization is critical for the correct interpretation of events
within syslog data.
Clock synchronization is critical for digital certificates.
Networks use NTP to synchronize the clocks of various devices across a network. Clock synchronization
within a network is critical for digital certificates and for correct interpretation of events within syslog data.
A secure method of providing clocking for the network is for network administrators to implement their
own private network master clocks, synchronized to UTC, using satellite or radio. However, if network
administrators do not wish to implement their own master clocks because of cost or other reasons, other
clock sources are available on the Internet.
Network Time Protocol (Cont.)
NTP provides time synchronization between network devices.
NTP can get the correct time from an internal or external time source:
Local master clock
Master clock on the Internet
GPS or atomic clock
A router can act as an NTP server and client. Other devices (NTP
clients) synchronize time with the router (NTP server).

NTP is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency
data networks. NTP allows routers on your network to synchronize their time settings with an NTP server.
A group of NTP clients that obtain time and date information from a single source will have more
consistent time settings.
You can configure a router as an NTP server, to which other devices (NTP clients) synchronize their time
settings.

Configuring NTP
This topic describes how to configure NTP on Cisco devices.
Configuring NTP
Configure the Branch router as the NTP client, which will synchronize its time
with the NTP server.
Branch(config)#ntp server 209.165.201.15
Configure the SW1 switch as the NTP client, which will synchronize its time with
the Branch router.
SW1(config)#ntp server 10.1.1.1
209.165.201.15
10.1.1.0/24
Branch
SW1
NTP Server
.1
NTP Client NTP Client
The figure shows an example configuration scenario. Both router Branch and switch SW1 are configured as
NTP clients using the ntp server ip-address global configuration command. The IP address of the NTP
server is configured.
A Cisco IOS device acting as an NTP client will also respond to received time requests. This enables switch
SW1 to sync directly with router Branch and optimize traffic flows. Alternatively, you could configure
switch SW1 to sync with an external NTP server as well.
Cisco IOS devices can also act as NTP servers. To configure Cisco IOS Software as an NTP master clock to
which peers synchronize themselves, use the ntp master command in global configuration mode:
ntp master [stratum]
Note Use this command with caution. Valid time sources can be easily overridden using this command,
especially if a low stratum number is configured. Configuring multiple devices in the same network with
the ntp master command can cause instability in keeping time if the devices do not agree on the time.
The stratum value is a number from 1 to 15. The lowest stratum value indicates a higher NTP priority. It
also indicates the NTP stratum number that the system will claim.

Verifying NTP
This topic describes how to verify NTP on Cisco devices.
Verifying NTP
Branch#show ntp associations
address ref clock st when poll reach delay offset disp
*~209.165.201.15 127.127.1.1 1 17 64 1 0.856 0.050 187.57
* sys.peer, #selected, + candidate, - outlyer, x falseticker, ~ configured
Displays the status of NTP associations
Branch#show ntp status
Clock is synchronized, stratum 2, reference is 209.165.201.15
nominal freq is 119.2092 Hz, actual freq is 119.2092 Hz, precision is 2**17
reference time is D40ADC27.E644C776 (13:18:31.899 UTC Mon Sep 24 2012)
clock offset is 6.0716 msec, root delay is 1.47 msec
root dispersion is 15.41 msec, peer dispersion is 3.62 msec
loopfilter state is 'CTRL' (Normal Controlled Loop), drift is 0.000000091 s/s
system poll interval is 64, last update was 344 sec ago.
Displays the status of NTP
To display the status of NTP associations, use the show ntp associations command in privileged EXEC
mode.
Significant fields that are shown in the display follow:
*: Peer that is synchronized to this peer
~: Peer that is is statically configured
address: Address of the peer
st: Stratum setting for the peer
Note It may take several minutes for an NTP client to be synchronized with the NTP server.
To display the status of NTP, use the show ntp status command in user EXEC mode.
Significant fields that are shown in the display follow:
synchronized: System synchronized to an NTP peer
stratum: NTP stratum of this system
reference: Address of the peer to which a clock is synchronized

Summary
Summary
Secure unused ports by disabling interfaces.
The port security feature restricts a switch port to a specific set or
number of MAC addresses.
Before port security can be activated, the port mode must be set to
static switchport mode.
Use the show port-security interface command to display the port
security settings that are defined for an interface.
Some services on Cisco devices may not be needed and therefore can
be disabled.
NTP provides time synchronization between network devices.
A Cisco router can act as an authoritative NTP server.
Use the show ntp associations command to display the status of NTP
associations.

Lesson 3
Implementing Traffic Filtering
with ACLs
Overview
Once you understand how ACLs operate, you can implement them for an important network security
mechanism: traffic filtering. Standard ACLs provide only limited traffic filtering. Extended ACLs can
provide more precise traffic-filtering capabilities.
This lesson also describes access-list configuration mode. This mode enables you to define named ACLs
that are identified with descriptive names instead of numbers. Further, the lesson shows how to verify that
ACLs are functioning properly and discusses some common configuration errors.
Objectives
Describe traffic filtering with ACLs
Explain how inbound and outbound ACLs operate
Apply an ACL to an interface
Describe the need for extended ACLs
Configure and verify numbered, extended IPv4 ACLs
Configure and edit named IPv4 ACLs
Describe ACL configuration guidelines
Monitor and verify ACLs
Identify and resolve common ACL configuration errors

Using ACLs to Filter Network Traffic
This topic describes how to use ACLs to filter network traffic.
Using ACLs to Filter Network Traffic
How can you restrict Internet access for PC2?
Internet
PC1 PC2
Branch
The figure introduces a common task for network administrators: a need to implement network traffic
filtering to allow, limit, or restrict access to a network resource. A common mechanism that is used for
traffic filtering is ACLs, which enable you to control access based on Layer 3 packet-header information.
In the scenario in the figure, traffic filtering can be implemented on the router either inbound on an interface
that is connected to the LAN or outbound on an interface that is connected to the Internet. By using a simple
standard ACL, you can prevent packets from PC2 entering or leaving the interface. You should not forget to
explicitly allow traffic for other devices in the LAN, such as PC1.

ACL Operation
This topic describes how ACLs filter traffic.
ACL Operation
ACL operation outbound
Inbound
Interface
Packets
Outbound
Interface
Packet
Discard
Bucket
Test
ACL
Statements
Packet
Packet
ACL?
Choose
Outbound
Interface
Permit?
Routing
Table
Entry?
Y
Y
N
N Y
N
Discard Packet
Notify Sender
S0
S0
E0
When ACLs are used for traffic filtering, they can operate inbound or outbound. The direction determines at
which point packets are tested against the ACL as they pass through the router.
Outbound ACLs: Incoming packets are routed to the outbound interface and then are processed
through the outbound ACL. If packets match a permit statement, they are forwarded through the
interface. If packets match a deny statement or if there is no match, they are discarded.
Inbound ACLs: Incoming packets are processed by the ACL before they are routed to an outbound
interface. An inbound ACL is efficient because it saves the overhead of routing lookups if the filtering
tests deny the packet and it is discarded. If the tests permit the packet, it is processed for routing.

Applying ACLs to Interfaces
This topic describes how to apply ACLs to interfaces.
Applying ACLs to Interfaces
Applies ACL 1 on the interface as an outbound filter:
Branch(config-if)#ip access-group 1 out
Applies ACL 2 on the interface as an inbound filter:
Branch(config-if)#ip access-group 2 in
Important: Only one ACL per protocol, per direction, and per interface is allowed.
After an ACL is configured, it is linked to an interface using the ip access-group command. Only one ACL
per protocol, per direction, and per interface is allowed. The figure shows examples of this command,
showing how to apply the ACL as an inbound and outbound filter.
Note To remove an ACL from an interface, enter the no ip access-group command on the interface, then
enter the global no access-list command to remove the entire ACL if needed.
The table provides an example of the steps that are required to configure and apply a numbered standard
ACL on a router.

Numbered Standard ACL Configuration Procedure
Step Action Notes
1 Use the access-list global configuration
command to create an entry in a standard
IPv4 ACL.
Branch(config)#access-list 1 permit
10.1.1.0 0.0.0.255
The example ACL statement matches any address that
starts with 10.1.1.x.
2 Use the interface configuration command
to select an interface in which to apply the
ACL.
Branch(config)#interface
GigabitEthernet 0/0
After you enter the interface command, the CLI prompt
changes from (config)#to (config-if)#.
3 Use the ip access-group interface
configuration command to activate the
existing ACL on the interface.
This example activates the standard IPv4 ACL 1 on the
interface as an inbound filter.
Applying ACLs to Interfaces (Cont.)
Example:
Deny Internet access for a specific host (10.1.1.101).
Allow all other LAN hosts to access the Internet.
Internet
PC1 PC2
Gi0/0
Gi0/1
10.1.1.100 10.1.1.101
Branch(config)#access-list 1 deny 10.1.1.101
Branch(config)#access-list 1 permit 10.1.1.0 0.0.0.255
Branch(config)#interface GigabitEthernet 0/1
Branch(config-if)#ip access-group 1 out
The figure shows a scenario in which ACL 1 is applied in the outbound direction on router Branch to
provide traffic filtering. ACL 1 includes a deny statement that matches traffic from a specific host with IP
address 10.1.1.101. The second line in the ACL permits traffic from hosts within network 10.1.1.0 /24. It is
important to specify a permit statement because ACLs end with an implicit deny all statement.
Note Alternatively, the ACL could be applied in the inbound direction on interface GigabitEthernet 0/0. This
solution would not only prevent host PC2 from accessing the Internet but also would deny all
communication between PC2 and the router.

The Need for Extended ACLs
Standard ACLs cannot fulfill all traffic-filtering requirements. This topic describes extended ACLs and
illustrates why you need them.
The Need for Extended ACLs
How can you prevent PC2 from accessing only a specific server on the
Internet?
How can you allow other users only web access?
Internet
PC1 PC2
Gi0/0
Gi0/1
10.1.1.100 10.1.1.101
Server
209.165.202.197
Standard ACLs provide only limited options for network traffic filtering. It is impossible to meet the
requirement that is shown in the figure using just standard ACLs. For more precise traffic filtering, use
extended ACLs.
To prevent PC2 from communicating with a specific server, use an extended ACL matching the PC2 IP
address as the source address and the server IP address as the destination address.

The Need for Extended ACLs (Cont.)
Testing packets with extended IPv4 ACLs
Source
Address
Protocol
Port
Number
Destination
Address
Segment
(For Example,
TCP Header)
Data
Packet
(IP Header)
Frame
Header
(for example,
HDLC)
Deny Permit
Use ACL
Statements
100–199 or 2000–2699
to Test the
Packet
An Example from a TCP/IP Packet
Extended ACLs provide a greater range of control. In addition to verifying packet source addresses,
extended ACLs also check destination addresses, protocols, and port numbers, as shown in the figure. They
provide more criteria on which to base the ACL. For example, an extended ACL can simultaneously allow
email traffic from a network to a specific destination and deny file transfers and web browsing for a specific
host.
The ability to filter on protocol and port number allows you to build very specific extended ACLs. Using
the appropriate port number, you can specify an application by configuring either the port number or the
name of a well-known port.
Extended ACLs are numbered from 100 to 199 and from 2000 to 2699, providing a total of 800 possible
extended ACLs. Named, extended ACLs are also supported.

Configuring Numbered, Extended IPv4 ACLs
This topic describes how to configure numbered, extended IPv4 ACLs.
Configuring Numbered Extended IPv4 ACLs
Branch(config)#access-list 110 deny ip host 10.1.1.101 host 209.165.202.197
Branch(config)#access-list 110 permit tcp 10.1.1.0 0.0.0.255 any eq 80
The number 110 is chosen to define an ACL as an extended ACL.
The first statement matches IP traffic between two specific hosts and denies it.
The second statement matches HTTP TCP traffic from network 10.1.1.0 /24.
The operator eq (equal) is used to match TCP port 80.
The implicit deny statement is present at the end of the ACL
An extended ACL is activated on the interface in the same way as a standard
ACL.
The figure shows an example of configuring an extended ACL that specifies source and destination IP
addresses and TCP port numbers to match specific traffic.
The table explains the command syntax that is presented in the figure.
access-list Command Parameters
access-list-number Number of an ACL. Use a decimal number from 100 to 199 or
from 2000 to 2699 for an extended ACL.
permit | deny Either permits or denies traffic if conditions are matched
protocol Name of an Internet protocol. You can specify either the name or
number of the protocol. The most commonly used keywords are
ip, tcp, udp, and icmp.
source source-wildcard Number of the network and corresponding wildcard that define the
source network from which the packet is being sent
destination destination-wildcard Number of the network and corresponding wildcard that define the
destination network to which the packet is being sent
operator Optional parameter that compares source and destination ports
when TCP or UDP is specified as a protocol Possible operands
include lt (less than), gt (greater than), eq (equal), neq (not
equal), and range (inclusive range).
port Optional decimal number or name of a TCP or UDP port

To configure and apply an extended ACL on a router, follow the configuration procedure that is described
in the table.
Numbered, Extended ACL Configuration Procedure
Step Action Notes
1 Define an extended IPv4 ACL. Use the access-
list global configuration command.
Branch(config)#access-list 110 deny ip host
10.1.1.101 host 209.165.202.197
Branch(config)#access-list 110 permit tcp
10.1.1.0 0.0.0.255 any eq 80
Use the show access-lists command to display the
contents of the ACL.
In the example, ACL 110 denies IP traffic between
specific hosts 10.1.1.101 and 209.165.202.197. The
second statement allows TCP traffic destined to port 80
between hosts on network 10.1.1.0 /24 and any
destination.
All other traffic is denied by the implicit deny all at the
end of the ACL.
2 Select the interface to be configured. Use the
interface global configuration command.
Branch(config)#interface GigabitEthernet 0/0
After you enter the interface command, the CLI prompt
changes from (config)#to (config-if)#.
3 Link the extended IPv4 ACL to an interface.
Use the ip access-group interface
configuration command.
RouterX(config-if)#ip access-group 110 in
Use the show ip interfaces command to verify that an
IP ACL is applied to the interface.

Configuring Named ACLs
Designating ACLs by a descriptive name instead of a number provides a better way to describe the intention
of the ACL. This topic describes named ACLs and shows how to configure them.
Configuring Named ACLs
The ACL configuration mode is used to configure a named
ACL.
Branch(config)#ip access-list extended WEB_ONLY
Branch(config-ext-nacl)#permit tcp 10.1.1.0 0.0.0.255 any eq www
Branch(config-ext-nacl)#20 permit tcp 10.1.1.0 0.0.0.255 any eq 443
The alphanumeric name string (WEB_ONLY in the example) must be unique.
If sequence numbers are not configured, they are generated automatically, starting at 10
and incrementing by 10.
The no 10 command removes the specific test that is numbered with 10 from the named
ACL.
Branch(config-if)#ip access-group WEB_ONLY in
Named ACLs are activated on an interface with the same command as
numbered ACLs.
Naming an ACL makes it easier to understand its function. For example, an ACL to deny FTP could be
called NO_FTP. When you identify your ACL with a name instead of a number, the configuration mode
and command syntax are slightly different.
Access-list configuration mode is used to define named ACLs. To enter this mode, use the command ip
access-list.
Note Numbered ACLs can also be defined using access-list configuration mode. You just specify an ACL
number instead of a unique name.
In the figure, the command output shows the commands that are used to configure an extended ACL that is
named WEB_ONLY on the Branch router. The ACL permits traffic from hosts on the 10.1.1.0/24 network
that are destined to HTTP and HTTPS ports.
Capitalizing ACL names is not required, but it makes them stand out when you view the running
configuration output.

Configuring Named ACLs (Cont.)
Edit an ACL in the access-list configuration mode to deny web
access for host 10.1.1.25:
Branch#show access-lists
Extended IP access list WEB_ONLY
10 permit tcp 10.1.1.0 0.0.0.255 any eq www
20 permit tcp 10.1.1.0 0.0.0.255 any eq 443
Branch#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch(config)#ip access-list extended WEB_ONLY
Branch(config-ext-nacl)#5 deny ip host 10.1.1.25 any
Branch(config-ext-nacl)#end
Extended IP access list WEB_ONLY
5 deny ip host 10.1.1.25 any
10 permit tcp 10.1.1.0 0.0.0.255 any eq www
20 permit tcp 10.1.1.0 0.0.0.255 any eq 443
Named IP ACLs allow you to add, modify, or delete individual entries in a specific ACL. You can use
sequence numbers to insert statements anywhere in the named ACL.
When statements are added to the ACL, the default increment is 10. The figure shows an additional entry
that is numbered 5 in the WEB_ONLY ACL, which is inserted in front of line 10.
Note that reload will change the sequence numbers in the ACL. The sequence numbers will be 10, 20, and
30 instead of 5, 10, and 20 after reload. Use the access-list resequence command to renumber the ACL
entries in an ACL without having to reload.
For more details about the ip access-list and related commands, see the Cisco IOS Security Command
Reference at: http://www.cisco.com/en/US/docs/ios/security/command/reference/sec_i1.html#wp1079735.

ACL Configuration Guidelines
This topic describes recommended configuration guidelines for ACLs.
ACL Configuration Guidelines
These guidelines are recommended:
The type of ACL, standard or extended, determines what is filtered.
Only one ACL per interface, per protocol, and per direction is allowed.
The most specific statement should be at the top of an ACL. The most
general statement should be at the bottom of an ACL.
The last ACL test is always an implicit “deny everything else” statement,
so every list needs at least one permit statement.
When placing an ACL in a network, follow these guidelines:
Place extended ACLs close to the source.
Place standard ACLs close to the destination.
An ACL can filter traffic going through the router or traffic to and from the
router, depending on how it is applied.
Well-designed and well-implemented ACLs add an important security component to your network. Follow
these general principles to ensure that the ACLs that you create have the intended results:
Based on the test conditions, choose a standard or an extended ACL.
Only one ACL per protocol, per direction, and per interface is allowed. Multiple ACLs are permitted
per interface, but each must be for a different protocol or different direction.
Your ACL should be organized to allow processing from the top down. The more specific references to
a network or subnet should appear before more general conditions. Place conditions that occur more
frequently before conditions that occur less frequently.
Your ACL always contains an implicit deny any statement at the end.
Unless you add an explicit permit any statement, the ACL by default denies all traffic that fails to
match any of the ACL lines.
Every ACL should have at least one permit statement. Otherwise, all traffic is denied.
You should typically place extended ACLs as close as possible to the source of the traffic that you want
to deny. Because standard ACLs do not specify destination addresses, you must put the standard ACL
as close as possible to the destination of the traffic that you want to deny so the source can reach
intermediary networks.
Depending on how you apply the ACL, the ACL filters traffic either going through the router or going
to and from the router, such as traffic to or from vty lines.

Monitoring ACLs
This topic describes how to monitor and verify ACLs.
Monitoring ACLs
Displays the content of ACLs
Standard IP access list SALES
10 deny 10.1.1.0, wildcard bits 0.0.0.255
20 permit 10.3.3.1
30 permit 10.4.4.1
40 permit 10.5.5.1
Extended IP access list ENG
10 permit tcp host 10.22.22.1 any eq telnet (25 matches)
20 permit tcp host 10.33.33.1 any eq ftp
30 permit tcp host 10.44.44.1 any eq ftp-data
When you finish the ACL configuration, use the show access-lists command to display the contents of all
ACLs. By entering the ACL name or number as an option for this command, you can display a specific
ACL.
The output in the figure shows counters that describe how many packets were matched by a specific ACL
statement. This information is useful for analyzing an ACL operation.
Monitoring ACLs (Cont.)
Branch#show ip interface GigabitEthernet 0/0
Internet address is 10.1.1.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound access list is WEB_ONLY
Proxy ARP is enabled
Local Proxy ARP is disabled
<text omitted>
Shows whether an ACL is applied to an interface

Troubleshooting Common ACL Errors
This topic illustrates common ACL mistakes.
Scenario 1
Troubleshooting Common ACL Errors:
Scenario 1
Host 10.10.1.1 has no connectivity with 10.1.1.2.
Branch#show access-lists 10
10.160.22.11
192.168.1.0/24
10.10.1.1
10.10.1.0/24
172.16.0.0/16
S 0/0/0
10.160.22.33
S 0/0/0
10.1.1.101
10.1.1.2
10.1.1.1
ACL Inbound
Branch
Analyze the output of the show access-lists command to troubleshoot this problem:
Problem: Host 10.10.1.1 has no connectivity with 10.1.1.2.
One of the most common errors involves the "deny all traffic" statement that is implied at the end of
every ACL, also referred to as the "implicit deny any" statement. If a packet does not match any of the
ACL entries, it is automatically blocked. The implied denial of all traffic that does not match an ACL
entry is the default behavior of ACLs and cannot be changed. Therefore, in the figure, host 10.10.1.1 is
not able to communicate with host 10.1.1.2.
Solution: The ACL requires a new statement to allow traffic from the 10.10.1.0 network.

Scenario 2
Scenario 2
Host 10.10.1.1 has no connectivity with 10.1.1.2.
10 deny 10.10.1.0, wildcard bits 0.0.0.255
20 permit 10.10.1.1
30 permit any
10.160.22.11
192.168.1.0/24
10.10.1.1
10.10.1.0/24
172.16.0.0/16
S 0/0/0
10.160.22.33
S 0/0/0
10.1.1.101
10.1.1.2
10.1.1.1
ACL Inbound
Branch
Analyze the output from the show access-lists command to troubleshoot this problem:
Problem: Host 10.10.1.1 has no connectivity with 10.1.1.2.
Solution: Host 10.10.1.1 has no connectivity with 10.1.1.2 because of the order of the rules in ACL 10.
Because the router processes ACLs from the top down, statement 10 would deny host 10.10.1.1, and
statement 20 would not be processed. Statements 10 and 20 should be reversed.
Note Later Cisco IOS Software versions display an alert if you use the wrong order for ACL statements.

Scenario 3
Scenario 3
Users from the 192.168.1.0 network cannot open a TFTP
session to 10.1.1.2.
Extended IP access list 120
10 deny tcp 172.16.0.0 0.0.255.255 any eq telnet
20 deny tcp 192.168.1.0 0.0.0.255 any eq smtp
30 permit tcp any any
10.160.22.11
192.168.1.0/24
10.10.1.1
10.10.1.0/24
172.16.0.0/16
S 0/0/0
10.160.22.33
S 0/0/0
10.1.1.101
10.1.1.2
10.1.1.1
ACL Inbound
Branch
Problem: Users from the 192.168.1.0 network cannot use TFTP to connect to 10.1.1.2.
Solution: The 192.168.1.0 network cannot use TFTP to connect to 10.1.1.2 because TFTP uses the
UDP transport protocol. Statement 30 in ACL 120 allows all other TCP traffic. Because TFTP uses
UDP, it is implicitly denied. Statement 30 should be permit ip any any.

Scenario 4
Scenario 4
Users from the 172.16.0.0 network can use Telnet to connect
to 10.1.1.2, but this connection should not be allowed.
10 deny tcp any eq telnet any
30 permit ip any any
10.160.22.11
192.168.1.0/24
10.10.1.1
10.10.1.0/24
172.16.0.0/16
S 0/0/0
10.160.22.33
S 0/0/0
10.1.1.101
10.1.1.2
10.1.1.1
ACL Inbound
Branch
Problem: The 172.16.0.0 network can use Telnet to connect to 10.1.1.2, but this connection should not
be allowed.
Solution: The 172.16.0.0 network can use Telnet to connect to 10.1.1.2 because the Telnet port number
in statement 10 of ACL 130 is listed in the wrong position. Statement 10 currently denies any source
with a Telnet port number that is trying to establish a connection to any IP address. If you want to deny
Telnet inbound on S0/0/0, you should deny the destination port number that is equal to the Telnet port.
For example, configure deny tcp any any eq telnet or deny tcp any any eq 23.

Scenario 5
Scenario 5
Host 10.10.1.1 can use Telnet to connect to 10.1.1.2, but this
connection should not be allowed.
10 deny tcp host 10.160.22.11 any eq telnet
10.160.22.11
192.168.1.0/24
10.10.1.1
10.10.1.0/24
172.16.0.0/16
S 0/0/0
10.160.22.33
S 0/0/0
10.1.1.101
10.1.1.2
10.1.1.1
ACL Inbound
Branch
Problem: Host 10.10.1.1 can use Telnet to connect to 10.1.1.2, but this connection should not be
allowed.
Solution: Host 10.10.1.1 can use Telnet to connect to 10.1.1.2 because there are no rules that deny host
10.10.1.1 or its network as the source. Statement 10 of ACL 140 denies the router interface from which
traffic would depart. But as these packets depart from the router, they have a source address of
10.10.1.1 and not the address of the router interface.

Scenario 6
Scenario 6
Host 10.1.1.2 can use Telnet to connect to 10.10.1.1, but this
connection should not be allowed.
10.160.22.11
192.168.1.0/24
10.10.1.1
10.10.1.0/24
172.16.0.0/16
S 0/0/0
10.160.22.33
S 0/0/0
10.1.1.101
10.1.1.2
10.1.1.1
ACL Inbound
Branch
10 deny tcp host 10.1.1.2 any eq telnet
Branch#show running-config interface Serial 0/0/0
interface Serial0/0/0
ip address 10.160.22.33 255.255.255.0
ip access-group 150 in
<output omitted>
Analyze the output from the show access-lists command and information about the ACL to troubleshoot
this problem:
Problem: Host 10.1.1.2 can use Telnet to connect to 10.10.1.1, but this connection should not be
allowed.
Solution: Host 10.1.1.2 can use Telnet to connect to 10.10.1.1 because of the direction in which ACL
150 is applied to the S0/0/0 interface. Statement 10 denies the source address of 10.1.1.2, but this
address would be the source only if the traffic were outbound on S0, not inbound.

Scenario 7
Scenario 7
Host 10.10.1.1 can use Telnet to connect into the Branch
router IP address, but this connection should not be allowed.
10 deny tcp any host 10.160.22.33 eq telnet
10.160.22.11
192.168.1.0/24
10.10.1.1
10.10.1.0/24
172.16.0.0/16
S 0/0/0
10.160.22.33
S 0/0/0
10.1.1.101
10.1.1.2
10.1.1.1
ACL Inbound
Branch
Problem: Host 10.10.1.1 can connect into router B using Telnet, but this connection should not be
allowed.
Solution: Host 10.10.1.1 can connect into the Branch router using Telnet because using Telnet to
connect into the router is different from using Telnet to connect through the router to another device.
Statement 10 of ACL 160 denies Telnet access to the address that is assigned to the S0/0/0 interface of
router B. Host 10.10.1.1 can still use Telnet to connect into the Branch router simply by using a
different interface address (for example 10.1.1.1). When you want to block Telnet traffic into and out
of the router itself, use the access-class command to apply ACLs to the vty lines.

Summary
Summary
ACLs that are used for traffic filtering can operate in the inbound or
outbound direction.
One ACL per protocol, per direction, per interface is supported.
Extended ACLs are used to filter traffic based on source and destination
IP addresses, protocol, and port numbers.
Numbered, extended ACLs use numbers from 100 to 199 and from
2000 to 2699.
Summary (Cont.)
Access-list configuration mode allows adding, modifying, and deleting
individual statements from an ACL.
Place more specific statements at the top of an ACL and more general
ones at the bottom.
Use the show access-lists verification command to troubleshoot
common ACL configuration errors.

Module Summary
Module Summary
You should secure network devices by using passwords to restrict
console, SSH, and Telnet access.
Device hardening includes disabling unused ports, disabling unneeded
services, configuring the port security feature, and configuring NTP.
ACLs can be used for traffic filtering.

Module Self-Check
Questions
1. Which three types of access can be protected with a password? (Choose three.) (Source: Securing
Administrative Access)
A. console access
B. vty access
C. user-level access
D. privileged EXEC-level access
E. configuration mode access
2. Which message is customized text that is displayed before the username and password login
prompts? (Source: Securing Administrative Access)
A. message of the day
B. login banner
C. access warning
D. user banner
E. warning message
3. Which Cisco IOS feature can be used to control access to the vty ports? (Source: Securing
Administrative Access)
A. shutdown
B. port security
C. ACL
D. firewall

4. Which Cisco IOS command can be used to increase the security of unused switch ports? (Source:
Implementing Device Hardening)
A. shutdown
B. port security
C. mac-secure
D. firewall
5. Which command disables Cisco Discovery Protocol on a device as a whole? (Source: Implementing
Device Hardening)
A. no run cdp
B. no cdp run
C. no cdp enable
D. no cdp execute
6. What does the command no cdp enable do? (Source: Implementing Device Hardening)
A. disables Cisco Discovery Protocol on a specific interface
B. enables Cisco Discovery Protocol on the device as a whole
C. enables Cisco Discovery Protocol on an individual interface
D. enables Cisco Discovery Protocol on a specific type of interface
7. Which protocol is used on a device to synchronize its time with a time server? (Source:
Implementing Device Hardening)
A. Cisco Discovery Protocol
B. NTP
C. STP
D. FTP
8. What does a Cisco router do with a packet when the packet matches an outbound ACL permit
statement on the interface? (Source: Implementing Traffic Filtering with ACLs)
A. discards the packet
B. returns the packet to its originator
C. sends the packet to the output buffer
D. holds the packet for further processing
9. Which phrase is the term for the final default statement at the end of every ACL? (Source:
Implementing Traffic Filtering with ACLs)
A. implicit deny any
B. implicit deny host
C. implicit permit any
D. implicit permit host

10. A system administrator wants to configure an IPv4 standard ACL that is attached to the interface on a
Cisco router to allow only packets from hosts on subnet 10.1.1.0/24 to enter an interface on a router.
Which ACL configuration accomplishes this goal? (Source: Implementing Traffic Filtering with
ACLs)
A. access-list 1 permit 10.1.1.0
B. access-list 1 permit 10.1.1.0 host
C. access-list 99 permit 10.1.1.0 0.0.0.255
D. access-list 100 permit 10.1.1.0 0.0.0.255

Answer Key
1. A, B, D
2. B
3. C
4. A
5. B
6. A
7. B
8. C
9. A
10. C

Glossary
Term Definition
AAA authentication, authorization, and accounting. Pronounced "triple a."
ACL access control list. A list kept by routers to control access to or from
the router for a number of services (for example, to prevent packets
with a certain IP address from leaving a particular interface on the
router).
AfriNIC African Network Information Center.
API application program interface. The means by which an application
program talks to communications software. Standardized APIs allow
application programs to be developed independently of the underlying
method of communication. A set of standard software interrupts, calls,
and data formats that computer application programs use to initiate
contact with other devices (for example, network services, mainframe
communications programs, or other program-to-program
communications). Typically, APIs make it easier for software
developers to create the links that an application needs to
communicate with the operating system or with the network.
APNIC Asia Pacific Network Information Center. Nonprofit Internet registry
organization for the Asia Pacific region. The other Internet registries
are currently IANA, RIPE NCC, and InterNIC.
ARIN American Registry for Internet Numbers. A nonprofit organization
established for the purpose of administrating and registrating IP
numbers to the geographical areas currently managed by Network
Solutions (InterNIC). Those areas include, but are not limited to, North
America, South America, South Africa, and the Caribbean.
ARP Address Resolution Protocol. Internet protocol that is used to map an
IP address to a MAC address. Defined in RFC 826.
BIA burned-in MAC address.
CAM content-addressable memory.
CIDR classless interdomain routing. Technique supported by BGP4 and
based on route aggregation. CIDR allows routers to group routes
together to reduce the quantity of routing information carried by the
core routers. With CIDR, several IP networks appear to networks
outside the group as a single, larger entity. With CIDR, IP addresses
and their subnet masks are written as four octets, separated by
periods, followed by a forward slash and a two-digit number that
represents the subnet mask.
CoS class of service. An indication of how an upper-layer protocol requires
a lower-layer protocol to treat its messages. In SNA subarea routing,
CoS definitions are used by subarea nodes to determine the optimal
route to establish a given session. A CoS definition comprises a
virtual route number and a transmission priority field. Also called ToS.
© 2013 Cisco Systems, Inc. Student Guide G-1

Term Definition
CPE customer premises equipment. Terminating equipment, such as
terminals, telephones, and modems, supplied by the telephone
company, installed at customer sites, and connected to the telephone
company network. Can also refer to any telephone equipment
residing on the customer site.
CRC cyclic redundancy check. Error-checking technique in which the frame
recipient calculates a remainder by dividing frame contents by a prime
binary divisor and compares the calculated remainder to a value
stored in the frame by the sending node.
CSMA/CD Carrier Sense Multiple Access with Collision Detection. Media-access
mechanism wherein devices ready to transmit data first check the
channel for a carrier. If no carrier is sensed for a specific period of
time, a device can transmit. If two devices transmit at once, a collision
occurs and is detected by all colliding devices. This collision
subsequently delays retransmissions from those devices for some
random length of time. Ethernet and IEEE 802.3 use CSMA/CD
access.
DHCP Dynamic Host Configuration Protocol. Provides a mechanism for
allocating IP addresses dynamically so that addresses can be reused
when hosts no longer need them.
DNS Domain Name System. System used on the Internet for translating
names of network nodes into addresses.
DSL digital subscriber line. Public network technology that delivers high
bandwidth over conventional copper wiring at limited distances. There
are four types of DSL: ADSL, HDSL, SDSL, and VDSL. All are
provisioned via modem pairs, with one modem located at a central
office and the other at the customer site. Because most DSL
technologies do not use the whole bandwidth of the twisted pair, there
is room remaining for a voice channel.
DUAL Diffusing Update Algorithm. Convergence algorithm used in EIGRP
that provides loop-free operation at every instant throughout a route
computation. Allows routers involved in a topology change to
synchronize at the same time, while not involving routers that are
unaffected by the change.
EBCDIC extended binary coded decimal interchange code. Any of a number of
coded character sets developed by IBM consisting of 8-bit coded
characters. Older IBM systems and telex machines use this character
code.
EIGRP Enhanced Interior Gateway Routing Protocol. Advanced version of
IGRP developed by Cisco. Provides superior convergence properties
and operating efficiency, and combines the advantages of link-state
protocols with those of distance vector protocols.
EMI electromagnetic interference. Interference by electromagnetic signals
that can cause reduced data integrity and increased error rates on
transmission channels.
FC fiber-optic connector.
G-2 Interconnecting Cisco Networking Devices, Part 1 © 2013 Cisco Systems, Inc.

Term Definition
FCS frame check sequence. Extra characters added to a frame for error
control purposes. Used in HDLC, Frame Relay, and other data link
layer protocols.
FTP File Transfer Protocol. Application protocol, part of the TCP/IP
protocol stack, used for transferring files between network nodes.
FTP is defined in RFC 959.
IANA Internet Assigned Numbers Authority. Organization operated under
the auspices of the ISOC as a part of the IAB. IANA delegates
authority for IP address-space allocation and domain-name
assignment to the InterNIC and other organizations. IANA also
maintains a database of assigned protocol identifiers used in the
TCP/IP stack, including autonomous system numbers.
ICMP Internet Control Message Protocol. Network layer Internet protocol
that reports errors and provides other information relevant to IP
packet processing. Documented in RFC 792.
IEEE 802.11 A set of standards for implementing wireless local area network
(WLAN) computer communication in the 2.4-, 3.6-, and 5-GHz
frequency bands.
IGMP Internet Group Management Protocol. Used by IP hosts to report their
multicast group memberships to an adjacent multicast router.
IHL Internet Header Length. Name of an IP header field.
InterNIC Organization that serves the Internet community by supplying user
assistance, documentation, training, registration service for Internet
domain names, and other services. Formerly called NIC.
IP Internet Protocol. Network layer protocol in the TCP/IP stack offering
a connectionless internetwork service. IP provides features for
addressing, type-of-service specification, fragmentation and
reassembly, and security. Defined in RFC 791.
IPsec IP Security. A framework of open standards that provides data
confidentiality, data integrity, and data authentication between
participating peers. IPsec provides these security services at the IP
layer. IPsec uses IKE to handle the negotiation of protocols and
algorithms based on local policy and to generate the encryption and
authentication keys to be used by IPsec. IPsec can protect one or
more data flows between a pair of hosts, between a pair of security
gateways, or between a security gateway and a host.
IPv4 IP version 4
IPv6 IP version 6. Replacement for the current version of IP (version 4).
IPv6 includes support for flow ID in the packet header, which can be
used to identify flows. Formerly called IPng (next generation).
LACNIC Latin American and Caribbean Network Information Center.
LC local connector.
LIR local Internet registry

Term Definition
LLC Logical Link Control. The higher of the two data link layer sublayers
defined by the IEEE. The LLC sublayer handles error control, flow
control, framing, and MAC-sublayer addressing. The most prevalent
LLC protocol is IEEE 802.2, which includes both connectionless and
connection-oriented variants.
LLDP Link Layer Discovery Protocol
MD5 Message Digest 5. A one-way hashing algorithm that produces a 128-
bit hash. Both MD5 and Secure Hash Algorithm (SHA) are variations
on MD4 and are designed to strengthen the security of the MD4
hashing algorithm. Cisco uses hashes for authentication within the
IPsec framework. Also used for message authentication in SNMP v.2.
MD5 verifies the integrity of the communication, authenticates the
origin, and checks for timeliness.
MMF multimode fiber. Optical fiber supporting the propagation of multiple
frequencies of light.
MOTD banner message-of-the-day banner
MSB most significant bit. Bit n–1 in an n bit binary number, the bit with the
greatest weight (2^(n–1)). The first or leftmost bit when the number is
written in the usual way.
MTBF mean time between failures.
MTU maximum transmission unit. Maximum packet size, in bytes, that a
particular interface can handle.
NAT Network Address Translation. Mechanism for reducing the need for
globally unique IP addresses. NAT allows an organization with
addresses that are not globally unique to connect to the Internet by
translating those addresses into globally routable address space. Also
known as Network Address Translator.
NCC Network Coordination Centre.
NFS Network File System. As commonly used, a distributed file system
protocol suite developed by Sun Microsystems that allows remote file
access across a network. In actuality, NFS is simply one protocol in
the suite. NFS protocols include NFS, RPC, XDR, and others. These
protocols are part of a larger architecture that Sun refers to as ONC.
NIC network interface card. Board that provides network communication
capabilities to and from a computer system. Also called an adapter.
NTP Network Time Protocol. Protocol built on top of TCP that ensures
accurate local timekeeping with reference to radio and atomic clocks
located on the Internet. This protocol is capable of synchronizing
distributed clocks within milliseconds over long time periods.
OSI Open Systems Interconnection. International standardization program
created by ISO and ITU-T to develop standards for data networking
that facilitate multivendor equipment interoperability.

Term Definition
OSPF Open Shortest Path First. Link-state, hierarchical IGP routing
algorithm proposed as a successor to RIP in the Internet community.
OSPF features include least-cost routing, multipath routing, and load
balancing. OSPF was derived from an early version of the IS-IS
protocol.
OUI Organizational Unique Identifier. Three octets that are assigned by
the IEEE in a block of 48-bit LAN addresses.
PAT port address translation. Translation method that allows the user to
conserve addresses in the global address pool by allowing source
ports in TCP connections or UDP conversations to be translated.
Different local addresses then map to the same global address, with
port translation providing the necessary uniqueness. When translation
is required, the new port number is picked out of the same range as
the original following the convention of Berkeley Standard Distribution
(SD).
PDU protocol data unit. OSI term for packet.
POST power-on self test. Set of hardware diagnostics that runs on a
hardware device when this device is powered on.
QoS quality of service. Measure of performance for a transmission system
that reflects its transmission quality and service availability.
RFI radio frequency interference. Radio frequencies that create noise that
interferes with information being transmitted across unshielded
copper cable.
RIP Routing Information Protocol. IGP supplied with UNIX BSD systems.
The most common IGP in the Internet. RIP uses hop count as a
routing metric.
RIPE Reseaux IP Europeennes. Group formed to coordinate and promote
TCP/IP-based networks in Europe.
RIR regional Internet registry
RSA Acronym stands for Rivest, Shamir, and Adleman, the inventors of the
technique. Public-key cryptographic system that can be used for
encryption and authentication.
RTT round-trip time. Time required for a network communication to travel
from the source to the destination and back. RTT includes the time
required for the destination to process the message from the source
and to generate a reply. RTT is used by some routing algorithms to
aid in calculating optimal routes.
SC subscriber connector.
SCP Service Control Point. An element of an SS7-based Intelligent
Network that performs various service functions, such as number
translation, call setup and teardown, and so on.

Term Definition
SMF single-mode fiber. Fiber-optic cabling with a narrow core that allows
light to enter only at a single angle. Such cabling has higher
bandwidth than multimode fiber, but requires a light source with a
narrow spectral width (for example, a laser). Also called monomode
fiber.
SNMP Simple Network Management Protocol. Network management
protocol used almost exclusively in TCP/IP networks. SNMP provides
a means to monitor and control network devices, and to manage
configurations, statistics collection, performance, and security.
SPF shortest path first. Routing algorithm that iterates on length of path to
determine a shortest-path spanning tree. Commonly used in link-state
routing algorithms. Sometimes called Dijkstra's algorithm.
SSH Secure Shell Protocol. Protocol that provides a secure remote
connection to a router through a TCP application.
SSL Secure Socket Layer. Encryption technology for the Web used to
provide secure transactions, such as the transmission of credit card
numbers for e-commerce.
ST straight tip.
TCP/IP Transmission Control Protocol/Internet Protocol. Common name for
the suite of protocols developed by the U.S. DoD in the 1970s to
support the construction of worldwide internetworks. TCP and IP are
the two best-known protocols in the suite.
TLS Transport Layer Security. A future IETF protocol to replace SSL.
ULP upper-layer protocol. Protocol that operates at a higher layer in the
OSI reference model, relative to other layers. ULP is sometimes used
to refer to the next-highest protocol (relative to a particular protocol) in
a protocol stack.
UPS uninterruptible power supply.
UTC Coordinated Universal Time. Time zone at zero degrees longitude.
Formerly called Greenwich Mean Time (GMT) and Zulu time.
UTP unshielded twisted-pair. Four-pair wire medium used in a variety of
networks. UTP does not require the fixed spacing between
connections that is necessary with coaxial-type connections. Five
types of UTP cabling are commonly used: Category 1, Category 2,
Category 3, Category 4, and Category 5.
VLSM variable-length subnet mask. Capability to specify a different subnet
mask for the same network number on different subnets. VLSM can
help optimize available address space.
3G mobile network third generation mobile network. Refers generically to a category of
next-generation mobile networks, such as UMTS and IMT-2000.
4G the fourth generation of mobile phone mobile communications
standards.

Interconnecting_Cisco_Networking_Devices.pdf

More Related Content

Similar to Interconnecting_Cisco_Networking_Devices.pdf (20)

Recently uploaded (20)

Interconnecting_Cisco_Networking_Devices.pdf