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Huawei Certification
HCIA-
Routing&Switching
ENTRY
Huawei Networking Technology and Device
Huawei Technologies Co.,Ltd.

Copyright © Huawei Technologies Co., Ltd. 2019.
All rights reserved.
Huawei owns all copyrights, except for references to other parties. No part of this
document may be reproduced or transmitted in any form or by any means without
prior written consent of Huawei Technologies Co., Ltd.
Trademarks and Permissions
and other Huawei trademarks are trademarks of Huawei Technologies Co., Ltd.
All other trademarks and trade names mentioned in this document are the property of
their respective holders.
Notice
The information in this manual is subject to change without notice. Every effort has
been made in the preparation of this manual to ensure accuracy of the contents, but all
statements, information, and recommendations in this manual do not constitute the
warranty of any kind, express or implied.
Huawei Certification
HCIA-Routing&Switching Huawei Networking Technology
and Device
Entry
Version 2.5

Huawei Certification System
Relying on its strong technical and professional training and certification system
and in accordance with customers of different ICT technology levels, Huawei
certification is committed to providing customers with authentic, professional
certification, and addresses the need for the development of quality engineers that
are capable of supporting Enterprise networks in the face of an ever changing ICT
industry. The Huawei certification portfolio for routing and switching (R&S) is
comprised of three levels to support and validate the growth and value of customer
skills and knowledge in routing and switching technologies.
The Huawei Certified Network Associate (HCIA) certification level validates the skills
and knowledge of IP network engineers to implement and support small to
medium-sized enterprise networks. The HCIA certification provides a rich
foundation of skills and knowledge for the establishment of such enterprise
networks, along with the capability to implement services and features within
existing enterprise networks, to effectively support true industry operations.
HCIA certification covers fundamentals skills for TCP/IP, routing, switching and
related IP network technologies, together with Huawei data communications
products, and skills for versatile routing platform (VRP) operation and
management.
The Huawei Certified Network Professional (HCIP-R&S) certification is aimed at
enterprise network engineers involved in design and maintenance, as well as
professionals who wish to develop an in depth knowledge of routing, switching,
network efficiency and optimization technologies. HCIP-R&S consists of three units
including Implementing Enterprise Routing and Switching Network (IERS),
Improving Enterprise Network Performance (IENP), and Implementing Enterprise
Network Engineering Project (IEEP), which includes advanced IPv4 routing and
switching technology principles, network security, high availability and QoS, as well
as application of the covered technologies in Huawei products.
The Huawei Certified Internet Expert (HCIE-R&S) certification is designed to imbue
engineers with a variety of IP network technologies and proficiency in maintenance,
for the diagnosis and troubleshooting of Huawei products, to equip engineers with
in-depth competency in the planning, design and optimization of large-scale IP
networks.

CONTENTS
Introduction to Transmission Media .................................................................. 1
Ethernet Framing.................................................................................................14
IP Addressing .......................................................................................................34
Internet Control Message Protocol ..................................................................61
Address Resolution Protocol ............................................................................76
Transport Layer Protocols..................................................................................91
Data Forwarding Scenario................................................................................107
VRP Foundation .................................................................................................124
Navigating the CLI.............................................................................................139
File System Navigation and Management.....................................................157
VRP Operating System Image Management.................................................176
Establishing a Single Switched Network........................................................190
Spanning Tree Protocol ....................................................................................202

Rapid Spanning Tree Protocol.........................................................................234
Basic Knowledge of IP Routing .......................................................................260
IP Static Routes..................................................................................................274
Link State Routing with OSPF..........................................................................290
DHCP Protocol Principles ...............................................................................319
FTP Protocol Principles.....................................................................................336
Telnet Protocol Principles ................................................................................347

HCIA-Routing_&amp;_Switching_V2.5_Entry__Training_Materials.pdf

 A network can be understood to be the capability of two or more entities to
communicate over a given medium. The development of any network relies on this
same principle for establishing communication. Commonly the entities within a
network that are responsible for the transmission and reception of communication
are known as end stations, while the means by which communication is enabled is
understood to be the medium. Within an enterprise network, the medium exists in
a variety of forms from a physical cable to radio waves.

 The coaxial cable represents a more historic form of transmission medium that
may today be limited in usage within the enterprise network. As a transmission
medium, the coaxial cable comprises generally of two standards, the 10Base2 and
10Base5 forms, that are known as Thinnet or Thinwire, and Thicknet or Thickwire
respectively.
 The standards both support a transmission capacity of 10Mbps transmitted as
baseband signals for respective distances of 185 and 500 meters. In today’s
enterprise networks, the transmission capacity is extremely limited to be of any
significant application. The Bayonet Neill-Concelman (BNC) connector is the
common form of connector used for thin 10Base2 coaxial cables, while a type N
connector was applied to the thicker 10Base5 transmission medium.

 Ethernet cabling has become the standard for many enterprise networks providing
a transmission medium that supports a much higher transmission capacity. The
medium supports a four copper wire pair contained within a sheath which may or
may not be shielded against external electrical interference. The transmission
capacity is determined mainly based on the category of cable with category 5
(CAT5) supporting Fast Ethernet transmission capacity of up to 100Mbps, while a
higher Gigabit Ethernet transmission capacity is supported from Category 5
extended (CAT5e) standards and higher.
 The transmission over Ethernet as a physical medium is also susceptible to
attenuation, causing the transmission range to be limited to 100 meters. The RJ-45
connector is used to provide connectivity with wire pair cabling requiring specific
pin ordering within the RJ-45 connector, to ensure correct transmission and
reception by end stations over the transmission medium.

 Optical media uses light as a means of signal transmission as opposed to electrical
signals found within both Ethernet and coaxial media types. The optical fiber
medium supports a range of standards of 10Mbps, 100Mbps, 1Gbps and also
10Gbps (10GBASE) transmission. Single or multi-mode fiber defines the use of an
optical transmission medium for propagating of light, where single mode refers to
a single mode of optical transmission being propagated, and is used commonly for
high speed transmission over long distances.
 Multi mode supports propagation of multiple modes of optical transmission that
are susceptible to attenuation as a result of dispersion of light along the optical
medium, and therefore is not capable of supporting transmission over longer
distances. This mode is often applied to local area networks which encompass a
much smaller transmission range. There are an extensive number of fiber
connector standards with some of the more common forms being recognized as
the ST connector, LC connector and SC, or snap connector.

 Serial represents a standard initially developed over 50 years ago to support
reliable transmission between devices, during which time many evolutions of the
standard have taken place. The serial connection is designed to support the
transmission of data as a serial stream of bits. The common standard implemented
is referred to as (Recommended Standard) RS-232 but it is limited somewhat by
both distance and speed. Original RS-232 standards define that communication
speeds supported be no greater that 20Kbps, based on a cable length of 50ft (15
meters), however transmission speeds for serial is unlikely to be lower than 115
Kbps. The general behavior for serial means that as the length of the cable
increases, the supported bit rate will decrease, with an approximation that a cable
of around 150 meters, or 10 times the original standards, the supported bit rate
will be halved.
 Other serial standards have the capability to achieve much greater transmission
ranges, such as is the case with the RS-422 and RS-485 standards that span
distances of up to 4900ft (1200 meters) and are often supported by V.35
connectors that were made obsolete during the late 1980’s but are still often
found and maintained today in support of technologies such as Frame Relay and
ATM, where implemented. RS-232 itself does not define connector standards,
however two common forms of connector that support the RS-232 standard
include the DB-9 and DB-25 connectors. Newer serial standards have been
developed to replace much of the existing RS-232 serial technology, including
both FireWire and the universal serial bus (USB) standards, that latter of which is
becoming common place in many newer products and devices.

 In order to enable communication over physical links, signals must be transmitted
between the transmitting and receiving stations. This signal will vary depending on
the medium that is being used, as in the case of optical and wireless transmission.
The main purpose of the signal is to ensure that synchronization (or clocking)
between the sender and receiver over a physical medium is maintained, as well as
support transmission of the data signal in a form that can be interpreted by both
the sender and receiver.
 A waveform is commonly recognized as a property of line encoding where the
voltage is translated into a binary representation of 0 and 1 values that can be
translated by the receiving station. Various line coding standards exist, with 10Base
Ethernet standards supporting a line encoding standard known as Manchester
encoding. Fast Ethernet with a frequency range of 100MHz invokes a higher
frequency than can be supported when using Manchester encoding.
 An alternative form of line encoding is therefore used known as NZRI, which in
itself contains variations dependant on the physical media, thus supporting MLT-3
for 100Base-TX and 100Base-FX together with extended line encoding known as
4B/5B encoding to deal with potential clocking issues. 100Base-T4 for example
uses another form known as 8B/6T extended line encoding. Gigabit Ethernet
supports 8B/10B line encoding with the exception of 1000Base-T which relies on a
complex block encoding referred to as 4D-PAM5.

 Ethernet represents what is understood to be a multi-access network, in which two
or more end stations share a common transmission medium for the forwarding of
data. The shared network is however susceptible to transmission collisions where
data is forwarded by end stations simultaneously over a common medium. A
segment where such occurrences are possible is referred to as a shared collision
domain.
 End stations within such a collision domain rely on contention for the transmission
of data to an intended destination. This contentious behavior requires each end
station monitor for incoming data on the segment before making any attempt to
transmit, in a process referred to as Carrier Sense Multiple-Access Collision
Detection (CSMA/CD). However, even after taking such precautions the potential
for the occurrence of collisions as a result of simultaneous transmission by two
end stations remains highly probable.

 Transmission modes are defined in the form of half and full duplex, to determine
the behavior involved with the transmission of data over the physical medium.
 Half duplex refers to the communication of two or more devices over a shared
physical medium in which a collision domain exists, and with it CSMA/CD is
required to detect for such collisions. This begins with the station listening for
reception of traffic on its own interface, and where it is quiet for a given period,
will proceed to transmit its data. If a collision were to occur, transmission would
cease, followed by initiation of a backoff algorithm to prevent further
transmissions until a random value timer expires, following which retransmission
can be reattempted.
 Full duplex defines the simultaneous bidirectional communication over dedicated
point-to-point wire pairs, ensuring that there is no potential for collisions to occur,
and thus there is no requirement for CSMA/CD.

 Gigabit Ethernet transmission is supported by CAT 5e cabling and higher, and also
any form of 1000Base Fiber Optic cabling or greater.
 A collision domain is a network segment for which the same physical medium is
used for bi-directional communication. Data simultaneously transmitted between
hosts on the same shared network medium is susceptible to a collision of signals
before those signals reach the intended destination. This generally results in
malformed signals either larger or smaller than the acceptable size for
transmission (64 bytes – 1500 bytes), also know as runts and giants, being received
by the recipient.
 CSMA/CD is a mechanism for detecting and minimizing the possibility of collision
events that are likely to occur in a shared network. CSMA requires that the
transmitting host first listen for signals on the shared medium prior to
transmission. In the event that no transmissions are detected, transmission can
proceed. In the unfortunate circumstance that signals are transmitted
simultaneously and a collision occurs, collision detection processes are applied to
cease transmission for a locally generated period of time, to allow collision events
to clear and to avoid further collisions from occurring between transmitting hosts.

 Communication over networks relies on the application of rules that govern how
data is transmitted and processed in a manner that is understood by both the
sending and receiving entities. As a result, multiple standards have been
developed over the course of time with some standards becoming widely adopted.
There exists however a clear distinction between the standards that manage
physical data flow and the standards responsible for logical forwarding and
delivery of traffic.
 The IEEE 802 standards represent a universal standard for managing the physical
transmission of data across the physical network and comprises of standards
including the Ethernet standard 802.3 for physical transmission over local area
networks.
 Alternative standards exist for transmission over wide area networks operating
over serial based media, including Ethernet, PPP and HDLC. TCP/IP has been widely
adopted as the protocol suite defining the upper layer standards, regulating the
rules (protocols) and behavior involved in managing the logical forwarding and
delivery between end stations.

 The TCP/IP reference model primarily concerns with the core principles of the
protocol suite, which can be understood as the logical transmission and delivery of
traffic between end stations. As such the TCP/IP protocol reference model
provides a four layer representation of the network, summarizing physical
forwarding behavior under the network interface layer, since lower layer operation
is not the concern of the TCP/IP protocol suite.
 Primary focus remains on the network (or Internet) layer which deals with how
traffic is logically forwarded between networks, and the transport (sometimes
referred to as host-to-host) layer that manages the end-to-end delivery of traffic,
ensuring reliability of transportation between the source and destination end
stations. The application layer represents an interface through a variety of
protocols that enable services to be applied to end user application processes.

 Although the TCP/IP reference model is primarily supported as the standard model
based on TCP/IP protocol suite, the focus of the TCP/IP reference model does not
clearly separate and distinguish the functionality when referring lower layer
physical transmission.
 In light of this, the open systems interconnection, or OSI reference model is often
recognized as the model for reference to IEEE 802 standards due to the clear
distinction and representation of the behavior of lower layers which closely
matches the LAN/MAN reference model standards that are defined as part of the
documented IEEE 802-1990 standards for local and metropolitan area networks. In
addition, the model that is generally in reference to the ISO protocol suite,
provides an extended breakdown of upper layer processing.

 As upper layer application data is determined for transmission over a network
from an end system, a series of processes and instructions must be applied to the
data before transmission can be successfully achieved. This process of appending
and pre-pending instructions to data is referred to as encapsulation and for which
each layer of the reference model is designed to represent.
 As instructions are applied to the data, the general size of the data increases. The
additional instructions represent overhead to the existing data and are recognized
as instructions to the layer at which the instructions were applied. To other layers,
the encapsulated instructions are not distinguished from the original data. The
final appending of instructions is performed as part of the lower layer protocol
standards (such as the IEEE 802.3 Ethernet standard) before being carried as an
encoded signal over a physical medium.

 As part of the IEEE 802.3 Ethernet standard, data is encapsulated with instructions
in the form of a header and a trailer before it can be propagated over physical
media on which Ethernet is supported. Each stage of encapsulation is referred to
by a protocol data unit or PDU, which at the data link layer is known as a frame.
 Ethernet frames contain instructions that govern how and whether data can be
transmitted over the medium between two or more points. Ethernet frames come
in two general formats, the selection of which is highly dependant on the
protocols that have been defined prior to the framing encapsulation.

 Two frame formats are recognized as standard for Ethernet based networks. The
DIX version 2 frame type standard was originally developed during the early
1980’s, where today it is recognized as the Ethernet II frame type. Ethernet II was
eventually accepted and integrated into the IEEE 802 standards, highlighted as
part of section 3.2.6 of the IEEE 802.3x-1997 standards documentation. The IEEE
802.3 Ethernet standard was originally developed in 1983, with key differences
between the frame formats including a change to the type field that is designed to
identify the protocol to which the data should be forwarded to once the frame
instructions have been processed. In the IEEE 802.3 Ethernet format, this is
represented as a length field which relies on an extended set of instructions
referred to as 802.2 LLC to identify the forwarding protocol.
 Ethernet II and IEEE 802.3 associate with upper layer protocols that are
distinguished by a type value range, where protocols supporting a value less than
or equal to 1500 (or 05DC in Hexadecimal) will employ the IEEE 802.3 Ethernet
frame type at the data link layer. Protocols represented by a type value greater
than or equal to 1536 (or 0600 in Hexadecimal) will employ the Ethernet II
standard, and which represents the majority of all frames within Ethernet based
networks.
 Other fields found within the frame include the destination and source MAC
address fields that identify the sender and the intended recipient(s), as well as the
frame check sequence field that is used to confirm the integrity of the frame
during transmission.

 The Ethernet II frame references a hexadecimal type value which identifies the
upper layer protocol. One common example of this is the Internet Protocol (IP)
which is represented by a hexadecimal value of 0x0800. Since this value for IP
represents a value greater than 0x0600, it is determined that the Ethernet II frame
type should be applied during encapsulation. Another common protocol that relies
on the Ethernet II frame type at the data link layer is ARP, and is represented by
the hexadecimal value of 0x0806.

 For the IEEE 802.3 frame type, the type field is contained as part of the SNAP
extension header and is not so commonly applied the protocols in today’s
networks, partially due to the requirement for additional instructions which results
in additional overhead per frame. Some older protocols that have existed for many
years but that are still applied in support of Ethernet networks are likely to apply
the IEEE 802.3 frame type. One clear example of this is found in the case of the
Spanning Tree Protocol (STP) that is represented by a value of 0x03 within the type
field of the SNAP header.

 Ethernet based networks achieve communication between two end stations on a
local area network using Media Access Control (MAC) addressing that allows end
systems within a multi access network to be distinguished. The MAC address is a
physical address that is burned into the network interface card to which the
physical medium is connected. This same MAC address is retrieved and used as the
destination MAC address of the intended receiver by the sender, before the frame
is transferred to the physical layer for forwarding over the connected medium.

 Each MAC address is a 48 bit value commonly represented in a hexadecimal (base
16) format and comprised of two parts that attempt to ensure that every MAC
address is globally unique. This is achieved by the defining of an organizationally
unique identifier that is vendor specific, based on which it is possible to trace the
origin of a product back to its vendor based on the first 24 bits of the MAC
address. The remaining 24 bits of the MAC address is a value that is incrementally
and uniquely assigned to each product (e.g. a Network Interface Card or similar
product supporting port interfaces for which a MAC is required).

 The transmission of frames within a local network is achieved using one of three
forwarding methods, the first of these is unicast and refers to the transmission
from a single source location to a single destination. Each host interface is
represented by a unique MAC address, containing an organizationally unique
identifier, for which the 8th bit of the most significant octet (or first byte) in the
MAC address field identifies the type of address. This 8th bit is always set to 0
where the MAC address is a host MAC address, and signifies that any frame
containing this MAC address in the destination MAC address field is intended for a
single destination only.
 Where hosts exist within a shared collision domain, all connected hosts will receive
the unicast transmission but the frame will be generally ignored by all hosts where
the MAC address in the destination MAC field of the frame does not match the
MAC value of the receiving host on a given interface, leaving only the intended
host to accept and process the received data. Unicast transmissions are only
forwarded from a single physical interface to the intended destination, even in
cases where multiple interfaces may exist.

 Broadcast transmission represents a forwarding method that allows frames to be
flooded from a single source received by all destinations within a local area
network. In order to allow traffic to be broadcasted to all hosts within a local area
network, the destination MAC address field of the frame is populated with a value
that is defined in hexadecimal as FF:FF:FF:FF:FF:FF, and which specifies that all
recipients of a frame with this address defined should accept receipt of this frame
and process the frame header and trailer.
 Broadcasts are used by protocols to facilitate a number of important network
processes including discovery and maintenance of network operation, however
also generate excessive traffic that often causes interrupts to end systems and
utilization of bandwidth that tend to reduce the overall performance of the
network.

 A more efficient alternative to broadcast that has begun to replace the use of
broadcasts in many newer technologies is the multicast frame type. Multicast
forwarding can be understood as a form of selective broadcast that allows select
hosts to listen for a specific multicast MAC address in addition to the unicast MAC
address that is associated with the host, and process any frames containing the
multicast MAC address in the destination MAC field of the frame.
 Since there is no relative distinction between unicast MAC addresses and multicast
MAC address formats, the multicast address is differentiated using the 8th bit of
the first octet. Where this bit value represents a value of 1, it identifies that the
address is part of the multicast MAC address range, as opposed to unicast MAC
addresses where this value is always 0.
 In a local area network, the true capability of multicast behavior at the data link
layer is limited since forwarding remains similar to that of a broadcast frame in
which interrupts are still prevalent throughout the network. The only clear
difference with broadcast technology is in the selective processing by receiving
end stations. As networks expand to support multiple local area networks, the true
capability of multicast technology as an efficient means of transmission becomes
more apparent.

 As traffic is prepared to be forwarded over the physical network, it is necessary for
hosts in shared collision domains to determine whether any traffic is currently
occupying the transmission medium. Transmission media such as in the case of
10Base2 provides a shared medium over which CSMA/CD must be applied to
ensure collisions are handled should they occur. If the transmission of a frame is
detected on the link, the host will delay the forwarding of its own frames until such
time as the line becomes available, following which the host will begin to forward
frames from the physical interface towards the intended destination.
 Where two hosts are connected over a medium capable of supporting full duplex
transmission as in the case of media such as 10BaseT, it is considered not possible
for transmitted frames to suffer collisions since transmission and receipt of frames
occurs over separate wires and therefore there is no requirement for CSMA/CD to
be implemented.

 Once a frame is forwarded from the physical interface of the host, it is carried over
the medium to its intended destination. In the case of a shared network, the frame
may be received by multiple hosts who will assess whether the frame is intended
for their interface by analyzing the destination MAC address in the frame header. If
the destination MAC address and the MAC address of the host are not the same,
or the destination MAC address is not a MAC broadcast or multicast address to
which the host is listening for, the frame will be ignored and discarded.
 For the intended destination, the frame will be received and processed, initially by
confirming that the frame is intended for the hosts physical interface. The host
must also confirm that the integrity of the frame has been maintained during
transmission by taking the value of the frame check sequence (FCS) field and
comparing this value with a value determined by the receiving host. If the values
do not match, the frame will be considered as corrupted and will be subsequently
discarded.
 For valid frames, the host will then need to determine the next stage of processing
by analyzing the type field of the frame header and identify the protocol to which
this frame is intended. In this example the frame type field contains a hexadecimal
value of 0x0800 that identifies that the data taken from the frame should be
forwarded to the Internet Protocol, prior to which, the frame header and trailer are
discarded.

 Data link layer frames contain a Type field that references the next protocol to
which data contained within the frame should be forwarded. Common examples
of forwarding protocols include IP (0x0800) and ARP (0x0806).
 The destination MAC address contained within the frame header is analyzed by the
receiving end station and compared to the MAC address associated with the
interface on which the frame was received. If the destination MAC address and
interface MAC address do not match, the frame is discarded.

 Prior to discarding the frame header and trailer, it is necessary for the next set of
instructions to be processed to be determined from the frame header. As
highlighted, this is identified by determining the field value in the type field, which
in this instance represents a frame that is destined for the IP protocol following
completion of the frame process.
 The key function of the frame is to determine whether the intended physical
destination has been reached, that the integrity of the frame has remained intact.
The focus of this section will identify how data is processed following the
discarding of the frame headers and propagation of the remaining data to the
Internet Protocol.

 The IP header is used to support two key operations, routing and fragmentation.
Routing is the mechanism that allows traffic from a given network to be forwarded
to other networks, since the data link layer represents a single network for which
network boundaries exist. Fragmentation refers to the breaking down of data into
manageable blocks that can be transmitted over the network.
 The IP header is carried as part of the data and represents an overhead of at least
20 bytes that references how traffic can be forwarded between networks, where
the intended destination exists within a network different from the network on
which the data was originally transmitted. The version field identifies the version of
IP that is currently being supported, in this case the version is known as version
four or IPv4. The DS field was originally referred to as the type of service field
however now operates as a field for supporting differentiated services, primarily
used as a mechanism for applying quality of service (QoS) for network traffic
optimization, and is considered to be outside of the scope of this training.
 The source and destination IP addressing are logical addresses assigned to hosts
and used to reference the sender and the intended receiver at the network layer.
IP addressing allows for assessment as to whether an intended destination exists
within the same network or a different network as a means of aiding the routing
process between networks in order to reach destinations beyond the local area
network.

 Each IPv4 address represents a 32 bit value that is often displayed in a dotted
decimal format but for detailed understanding of the underlying behavior is also
represented in a binary (Base 2) format. IP addresses act as identifiers for end
systems as well as other devices within the network, as a means of allowing such
devices to be reachable both locally and by sources that are located remotely,
beyond the boundaries of the current network.
 The IP address consists of two fields of information that are used to clearly specify
the network to which an IP address belongs as well as a host identifier within the
network range, that is for the most part unique within the given network.

 Each network range contains two important addresses that are excluded from the
assignable network range to hosts or other devices. The first of these excluded
addresses is the network address that represents a given network as opposed to a
specific host within the network. The network address is identifiable by referring to
the host field of the network address, in which the binary values within this range
are all set to 0, for which it should also be noted that an all 0 binary value may not
always represent a 0 value in the dotted decimal notation.
 The second excluded address is the broadcast address that is used by the network
layer to refer to any transmission that is expected to be sent to all destinations
within a given network. The broadcast address is represented within the host field
of the IP address where the binary values within this range are all set to 1. Host
addresses make up the range that exists between the network and broadcast
addresses.

 The use of binary, decimal and hexadecimal notations are commonly applied
throughout IP networks to represent addressing schemes, protocols and
parameters, and therefore knowledge of the fundamental construction of these
base forms is important to understanding the behavior and application of values
within IP networks.
 Each numbering system is represented by a different base value that highlights the
number of values used as part of the base notations range. In the case of binary,
only two values are ever used, 0 and 1, which in combination can provide for an
increasing number of values, often represented as 2 to the power of x, where x
denotes the number of binary values. Hexadecimal represents a base 16 notation
with values ranging from 0 to F, (0-9 and A-F) where A represents the next value
following 9 and F thus represents a value equivalent to 15 in decimal, or 1111 in
binary.

 A byte is understood to contain 8 bits and acts as a common notation within IP
networks, thus a byte represents a bit value of 256, ranging from 0 through to 255.
This information is clearly represented through translation of decimal notation to
binary, and application of the base power to each binary value, to achieve the 256
bit value range. A translation of the numbering system for binary can be seen
given in the example to allow familiarization with the numbering patterns
associated with binary. The example also clearly demonstrates how broadcast
address values in decimal, binary and hexadecimal are represented to allow for
broadcasts to be achieved in both IP and MAC addressing at the network and data
link layers.

 The combination of 32 bits within an IP address correlates to four octets or bytes
for which each can represent a value range of 256, giving a theoretical number of
4’294’967’296 possible IP addresses, however in truth only a fraction of the
total number of addresses are able to be assigned to hosts. Each bit within a byte
represents a base power and as such each octet can represent a specific network
class, with each network class being based on either a single octet or a
combination of octets. Three octets have been used as part of this example to
represent the network with the fourth octet representing the host range that is
supported by the network.

 The number of octets supported by a network address is determined by address
classes that break down the address scope of IPv4. Classes A, B and C are
assignable address ranges, each of which supports a varied number of networks,
and a number of hosts that are assignable to a given network. Class A for instance
consist of 126 potential networks, each of which can support 224, or 16’777’216
potential host addresses, bearing in mind that network and broadcast addresses of
a class range are not assignable to hosts.
 In truth, a single Ethernet network could never support such a large number of
hosts since Ethernet does not scale well, due in part to broadcasts that generate
excessive network traffic within a single local area network. Class C address ranges
allow for a much more balanced network that scales well to Ethernet networks,
supplying just over 2 million potential networks, with each network capable of
supporting around 256 addresses, of which 254 are assignable to hosts.
 Class D is a range reserved for multicast, to allow hosts to listen for a specific
address within this range, and should the destination address of a packet contain a
multicast address for which the host is listening, the packet shall be processed in
the same way as a packet destined for the hosts assigned IP address. Each class is
easily distinguishable in binary by observing the bit value within the first octet,
where a class A address for instance will always begin with a 0 for the high order
bit, whereas in a Class B the first two high order bits are always set as 1 and 0,
allowing all classes to be easily determined in binary.

 Within IPv4, specific addresses and address ranges have been reserved for special
purposes. Private address ranges exist within the class A, B and C address ranges
to prolong the rapid decline in the number of available IP addresses. The number
of actual end systems and devices that require IP addressing in the world today
exceeds the 4’294’967’296 addresses of the 32 bit IPv4 address range, and
therefore a solution to this escalating problem was to allocate private address
ranges that could be assigned to private networks, to allow for conservation of
public network addresses that facilitate communication over public network
infrastructures such as the Internet.
 Private networks have become common throughout the enterprise network but
hosts are unable to interact with the public network, meaning that address ranges
can be reused in many disparate enterprise networks. Traffic bound for public
networks however must undergo a translation of addresses before data can reach
the intended destination.
 Other special addresses include a diagnostic range denoted by the 127.0.0.0
network address, as well as the first and last addresses within the IPv4 address
range, for which 0.0.0.0 represents any network and for which its application shall
be introduced in more detail along with principles of routing. The address
255.255.255.255 represents a broadcast address for the IPv4 (0.0.0.0) network,
however the scope of any broadcast in IP is restricted to the boundaries of the
local area network from which the broadcast is generated.

 In order for a host to forward traffic to a destination, it is necessary for a host to
have knowledge of the destination network. A host is naturally aware of the
network to which it belongs but is not generally aware of other networks, even
when those networks may be considered part of the same physical network. As
such hosts will not forward data intended for a given destination until the host
learns of the network and thus with it the interface via which the destination can
be reached.
 For a host to forward traffic to another host, it must firstly determine whether the
destination is part of the same IP network. This is achieved through comparison of
the destination network to the source network (host IP address) from which the
data is originating. Where the network ranges match, the packet can be forwarded
to the lower layers where Ethernet framing presides, for processing. In the case
where the intended destination network varies from the originating network, the
host is expected to have knowledge of the intended network and the interface via
which a packet/frame should be forwarded before the packet can be processed by
the lower layers. Without this information, the host will proceed to drop the packet
before it even reaches the data link layer.

 The identification of a unique network segment is governed by the
implementation of a mask value that is used to distinguish the number of bits that
represent the network segment, for which the remaining bits are understood as
representing the number of hosts supported within a given network segment. A
network administrator can divide a network address into sub-networks so that
broadcast packets are transmitted within the boundaries of a single subnet. The
subnet mask consists of a string of continuous and unbroken 1 values followed by
a similar unbroken string of 0 values. The 1 values correspond to the network ID
field whereas the 0 values correspond to the host ID field.

 For each class of network address, a corresponding subnet mask is applied to
specify the default size of the network segment. Any network considered to be
part of the class A address range is fixed with a default subnet mask pertaining to
8 leftmost bits which comprise of the first octet of the IP address, with the
remaining three octets remaining available for host ID assignment.
 In a similar manner, the class B network reflects a default subnet mask of 16 bits,
allowing a greater number of networks within the class B range at the cost of the
number of hosts that can be assigned per default network. The class C network
defaults to a 24 bit mask that provides a large number of potential networks but
limits greatly the number of hosts that can be assigned within the default network.
The default networks provide a common boundary to address ranges, however in
the case of class A and class B address ranges, do not provide a practical scale for
address allocation for Ethernet based networks.

 Application of the subnet mask to a given IP address enables identification of the
network to which the host belongs. The subnet mask will also identify the
broadcast address for the network as well as the number of hosts that can be
supported as part of the network range. Such information provides the basis for
effective network address planning. In the example given, a host has been
identified with the address of 192.168.1.7 as part of a network with a 24 bit default
(class C) subnet mask applied. In distinguishing which part of the IP address
constitutes the network and host segments, the default network address can be
determined for the segment.
 This is understood as the address where all host bit values are set to 0, in this case
generating a default network address of 192.168.1.0. Where the host values are
represented by a continuous string of 1 values, the broadcast address for the
network can be determined. Where the last octet contains a string of 1 values, it
represents a decimal value of 255, for which a broadcast address of 192.168.1.255
can be derived.
 Possible host addresses are calculated based on a formula of 2n where n
represents the number of host bits defined by the subnet mask. In this instance n
represents a value of 8 host bits, where 28 gives a resulting value of 256. The
number of usable host addresses however requires that the network and
broadcast addresses be deducted from this result to give a number of valid host
addresses of 254.

 The case scenario provides a common class B address range to which it is
necessary to determine the network to which the specified host belongs, along
with the broadcast address and the number of valid hosts that are supported by
the given network. Applying the same principles as with the class C address range,
it is possible for the network address of the host to be determined, along with the
range of hosts within the given network.

 One of the main constraints of the default subnet mask occurs when multiple
network address ranges are applied to a given enterprise in order to generate
logical boundaries between the hosts within the physical enterprise network. The
application of a basic addressing scheme may require a limited number of hosts to
be associated with a given network, for which multiple networks are applied to
provide the logical segmentation of the network. In doing so however, a great deal
of address space remains unused, displaying the inefficiency of default subnet
mask application.

 As a means of resolving the limitations of default subnet masks, the concept of
variable length subnet masks are introduced, which enable a default subnet mask
to be broken down into multiple sub-networks, which may be of a fixed length
(a.k.a. fixed length subnet masks or FLSM) or of a variable length known commonly
by the term VLSM. The implementation of such subnet masks consists of taking a
default class based network and dividing the network through manipulation of the
subnet mask.
 In the example given, a simple variation has been made to the default class C
network which by default is governed by a 24 bit mask. The variation comes in the
form of a borrowed bit from the host ID which has been applied as part of the
network address. Where the deviation of bits occurs in comparison to the default
network, the additional bits represent what is known as the subnet ID.
 In this case a single bit has been taken to represent the sub-network for which two
sub-networks can be derived, since a single bit value can only represent two states
of either 1 or 0. Where the bit is set to 0 it represents a value of 0, where it is set to
1 it represents a value of 128. In setting the host bits to 0, the sub-network
address can be found for each sub-network, by setting the host bits to 1, the
broadcast address for each sub-network is identifiable. The number of supported
hosts in this case represents a value of 27 minus the sub-network address and
broadcast address for each sub-network, resulting in each sub-network supporting
a total of 126 valid host addresses.

 In relation to problem of address limitations in which default networks resulted in
excessive address wastage, the concept of variable length subnet masks can be
applied to reduce the address wastage and provide a more effective addressing
scheme to the enterprise network.
 A single default class C address range has been defined, for which variable length
subnet masks are required to accommodate each of the logical networks within a
single default address range. Effective subnet mask assignment requires that the
number of host bits necessary to accommodate the required number of hosts be
determined, for which the remaining host bits can be applied as part of the subnet
ID, that represents the variation in the network ID from the default network
address.

 Classless inter-domain routing was initially introduced as a solution to handle
problems that were occurring as a result of the rapid growth of what is now known
as the Internet. The primary concerns were to the imminent exhaustion of the class
B address space that was commonly adopted by mid-sized organizations as the
most suited address range, where class C was inadequate and where class A was
too vast, and management of the 65534 host addresses could be achieved
through VLSM. Additionally, the continued growth meant that gateway devices
such as routers were beginning to struggle to keep up with the growing number of
networks that such devices were expected to handle. The solution given involves
transitioning to a classless addressing system in which classful boundaries were
replaced with address prefixes.
 This notation works on the principle that classful address ranges such as that of
class C can be understood to have a 24 bit prefix that represents the subnet or
major network boundary, and for which it is possible to summarize multiple
network prefixes into a single larger network address prefix that represents the
same networks but as a single address prefix. This has helped to alleviate the
number of routes that are contained particularly within large scale routing devices
that operate on a global scale, and has provided a more effective means of
address management. The result of CIDR has had far reaching effects and is
understood to have effectively slowed the overall exhaustion rate of the IPv4
address space.

 The forwarding of packets requires that the packet first determine a forwarding
path to a given network, and the interface via which a packet should be forwarded
from, before being encapsulated as a frame and forwarded from the physical
interface. In the case where the intended network is different from the originating
network, the packet must be forwarded to a gateway via which the packet is able
to reach it’s intended destination.
 In all networks, the gateway is a device that is capable of handling packets and
making decisions as to how packets should be routed, in order to reach their
intended destination. The device in question however must be aware of a route to
the intended destination IP network before the routing of packets can take place.
Where networks are divided by a physical gateway, the interface IP address (in the
same network or sub-network) via which that gateway can be reached is
considered to be the gateway address.
 In the case of hosts that belong to different networks that are not divided by a
physical gateway, it is the responsibility of the host to function as the gateway, for
which the host must firstly be aware of the route for the network to which packets
are to be forwarded, and should specify the hosts own interface IP address as the
gateway IP address, via which the intended destination network can be reached.

 The data of forwarded packets exists in many formats and consists of varying sizes,
often the size of data to be transmitted exceeds the size that is supported for
transmission. Where this occurs it is necessary for the data block to be broken
down into smaller blocks of data before transmission can occur. The process of
breaking down this data into manageable blocks is known as fragmentation.
 The identification, flags and fragment offset fields are used to manage reassembly
of fragments of data once they are received at their final intended destination.
Identification distinguishes between data blocks of traffic flows which may
originate from the same host or different hosts. The flags field determines which
of a number of fragments represents the last fragment at which time initiation of a
timer is started prior to reassembly, and to notify that reassembly of the packet
should commence.
 Finally the fragment offset labels the bit value for each fragment as part of a
number of fragments, the first fragment is set with a value of 0 and subsequent
fragments specify the value of first bit following the previous fragment, for
example where the initial fragment contains data bits 0 through to 1259, the
following fragment will be assigned an offset value of 1260.

 As packets are forwarded between networks, it is possible for packets to fall into
loops where routes to IP networks have not been correctly defined within devices
responsible for the routing of traffic between multiple networks. This can result in
packets becoming lost within a cycle of packet forwarding that does not allow a
packet to reach its intended destination. Where this occurs, congestion on the
network will ensue as more and more packets intended for the same destination
become subject to the same fate, until such time as the network becomes flooded
with erroneous packets.
 In order to prevent such congestion occurring in the event of such loops, a time to
live (TTL) field is defined as part of the IP header, that decrements by a value of 1
each time a packet traverses a layer 3 device in order to reach a given network.
The starting TTL value may vary depending on the originating source, however
should the TTL value decrement to a value of 0, the packet will be discarded and
an (ICMP) error message is returned to the source, based on the source IP address
that can be found in the IP header of the wandering packet.

 Upon verification that the packet has reached its intended destination, the network
layer must determine the next set of instructions that are to be processed. This is
determined by analyzing the protocol field of the IP header. As with the type field
of the frame header, a hexadecimal value is used to specify the next set of
instructions to be processed.
 It should be understood that the protocol field may refer to protocols at either the
network layer, such as in the case of the Internet Control Message Protocol (ICMP),
but may also refer to upper layer protocols such as the Transmission Control
Protocol (06/0x06) or User Datagram Protocol (17/0x11), both of which exist as
part of the transport layer within both the TCP/IP and OSI reference models.

 The IP subnet mask is a 32 bit value that describes the logical division between the
bit values of an IP address. The IP address is as such divided into two parts for
which bit values represent either a network or sub-network, and the host within a
given network or sub-network.
 IP packets that are unable to reach the intended network are susceptible to being
indefinitely forwarded between networks in an attempt to discover their ultimate
destination. The Time To Live (TTL) feature is used to ensure that a lifetime is
applied to all IP packets, so as to ensure that in the event that an IP packet is
unable to reach it’s destination, it will eventually be terminated. The TTL value
may vary depending on the original source.
 Gateways represent points of access between IP networks to which traffic can be
redirected, or routed in the event that the intended destination network varies
from the network on which the packet originated.

 The Internet Control Message Protocol is an integral part of IP designed to
facilitate the transmission of notification messages between gateways and source
hosts where requests for diagnostic information, support of routing, and as a
means of reporting errors in datagram processing are needed. The purpose of
these control messages is to provide feedback about problems in the
communication environment, and does not guarantee that a datagram will be
delivered, or that a control message will be returned.

 ICMP Redirect messages represent a common scenario where ICMP is used as a
means of facilitating routing functions. In the example, a packet is forwarded to
the gateway by host A based on the gateway address of host A. The gateway
identifies that the packet received is destined to be forwarded to the address of
the next gateway which happens to be part of the same network as the host that
originated the packet, highlighting a non-optimal forwarding behavior between
the host and the gateways.
 In order to resolve this, a redirect message is sent to the host. The redirect
message advises the host to send its traffic for the intended destination directly to
the gateway to with which the destination network is associated, since this
represents a shorter path to the destination. The gateway proceeds however to
forward the data of the original packet to its intended destination.

 ICMP echo messages represent a means of diagnosis for determining primarily
connectivity between a given source and destination, but also provides additional
information such as the round trip time for transmission as a diagnostic for
measuring delay. Data that is received in the echo message is returned as a
separate echo reply message.

 ICMP provides various error reporting messages that often determine reachability
issues and generate specific error reports that allow a clearer understanding from
the perspective of the host as to why transmission to the intended destination
failed.
 Typical examples include cases where loops may have occurred in the network,
and consequentially caused the time to live parameter in the IP header to expire,
resulting in a “ttl exceeded in transit” error message being generated. Other
examples include an intended destination being unreachable, which could relate to
a more specific issue of the intended network not being known by the receiving
gateway, or that the intended host within the destination network not being
discovered. In all events an ICMP message is generated with a destination based
on the source IP address found in the IP header, to ensure the message notifies
the sending host.

 ICMP messages are sent using the basic IP header, which functions together as an
integral part of the ICMP message, such is the case with the TTL parameter that is
used to provide support for determining whether a destination is reachable. The
format of the ICMP message relies on two fields for message identification in the
form of a type/code format, where the type field provides a general description of
the message type, and the code and a more specific parameter for the message
type.
 A checksum provides a means of validating the integrity of the ICMP message. An
additional 32 bits are included to provide variable parameters, often unused and
thus set as 0 when the ICMP message is sent, however in cases such as an ICMP
redirect, the field contains the gateway IP address to which a host should redirect
packets. The parameter field in the case of echo requests will contain an identifier
and a sequence number, used to help the source associate sent echo requests with
received echo replies, especially in the event multiple requests are forwarded to a
given destination.
 As a final means of tracing data to a specific process, the ICMP message may carry
the IP header and a portion of the data that contains upper layer information that
enables the source to identify the process for which an error occurred, such as
cases where the ICMP TTL expires in transit.

 A wide number of ICMP type values exist to define clearly the different
applications of the ICMP control protocol. In some cases the code field is not
required to provide a more specific entry to the type field, as is found with echo
requests that have a type field of 8 and the corresponding reply, which is
generated and sent as a separate ICMP message to the source address of the
sender, and defined using a type field of 0.
 Alternatively, certain type fields define a very general type for which the variance is
understood through the code field, as in the case of the type 3 parameter. A type
field of 3 specifies that a given destination is unreachable, while the code field
reflects the specific absence of either the network, host, protocol, port (TCP/UDP),
ability to perform fragmentation (code 4), or source route (code 5) in which a
packet, for which a forwarding path through the network is strictly or partially
defined, fails to reach its destination.

 The application of ICMP can be understood through the use of tools such as Ping.
The Ping application may be used as a tool in order to determine whether a
destination is reachable as well as collect other related information. The
parameters of the Ping application allow an end user to specify the behavior of the
end system in generating ICMP messages, with consideration of the size of the
ICMP datagram, the number of ICMP messages generated by the host, and also
the duration in which it is expected a reply is received before a timeout occurs.
This is important where a large delay occurs since a timeout may be reported by
the Ping application before the ICMP message has had the opportunity to return
to the source.

 The general output of an ICMP response to a Ping generated ICMP request details
the destination to which the datagram was sent and the size of the datagram
generated. In addition the sequence number of the sequence field that is carried
as part of the echo reply (type 0) is displayed along with the TTL value that is taken
from the IP header, as well as the round trip time which again is carried as part of
the IP options field in the IP header.

 Another common application to ICMP is traceroute, which provides a means of
measuring the forwarding path and delay on a hop-by-hop basis between multiple
networks, through association with the TTL value within the IP header.
 For a given destination, the reachability to each hop along the path is measured by
initially defining a TTL value in the IP header of 1, causing the TTL value to expire
before the receiving gateway is able to propagate the ICMP message any further,
thus generating a TTL expired in transit message together with timestamp
information, allowing for a hop-by-hop assessment of the path taken through the
network by the datagram to the destination, and a measurement of the round trip
time. This provides an effective means of identifying the point of any packet loss
or delay that may be incurred in the network and also aids in the discovery of
routing loops.

 The implementation of traceroute in Huawei ARG3 series routers adopts the use of
the UDP transport layer protocol to define a service port as the destination. Each
hop sends three probe packets, for which the TTL value is initially set to a value of
1 and incremented after every three packets. In addition, a UDP destination port of
33434 is specified for the first packet and incremented for every successive probe
packet sent. A hop-by-hop result is generated, allowing for the path to be
determined, as well as for any general delay that may occur to be discovered.
 This is achieved by measuring the duration between when the ICMP message was
sent and when the corresponding TTL expired in transit ICMP error is received.
When receiving a packet, the ultimate destination is unable to discover the port
specified in the packet, and thus returns an ICMP Type 3, Code 3 (Port
Unreachable) packet, and after three attempts the traceroute test ends. The test
result of each probe is displayed by the source, in accordance with the path taken
from the source to the destination. If a fault occurs when the trace route
command is used, the following information may be displayed:
 !H: The host is unreachable.
 !N: The network is unreachable.
 !: The port is unreachable.
 !P: The protocol type is incorrect.
 !F: The packet is incorrectly fragmented.
 !S: The source route is incorrect.

 The Ping application uses the echo request message of type 8 to attempt to
discover the destination. A separate echo reply message, defined by a type field of
0, is returned to the original source based on the source IP address in the IP
header field.
 In the event that the TTL value of an IP datagram reaches 0 before the datagram is
able to reach the intended destination, the gateway device receiving the datagram
will proceed to discard it and return an ICMP message to the source to notify that
the datagram in question was unable to reach the intended destination. The
specific reason will be defined by the code value to reflect for example whether
the failure was due to a failure to discover the host, a port on the host or whether
the service for a given protocol was not supported etc.

 As data is encapsulated, the IP protocol at the network layer is able to specify the
target IP address to which the data is ultimately destined, as well as the interface
via which the data is to be transmitted, however before transmission can occur, the
source must be aware of the target Ethernet (MAC) address to which data should
be transmitted. The Address Resolution Protocol (ARP) represents a critical part of
the TCP/IP protocol suite that enables discovery of MAC forwarding addresses to
facilitate IP reachability. The Ethernet next hop must be discovered before data
encapsulation can be completed.

 The ARP packet is generated as part of the physical target address discovery
process. Initial discovery will contain partial information since the destination
hardware address or MAC address is to be discovered. The hardware type refers to
Ethernet with the protocol type referring to IP, defining the technologies
associated with the ARP discovery. The hardware and protocol length identifies the
address length for both the Ethernet MAC address and the IP address, and is
defined in bytes.
 The operation code specifies one of two states, where the ARP discovery is set as
REQUEST for which reception of the ARP transmission by the destination will
identify that a response should be generated. The response will generate REPLY for
which no further operation is necessary by the receiving host of this packet, and
following which the ARP packet will be discarded. The source hardware address
refers to the MAC address of the sender on the physical segment to which ARP is
generated. The source protocol address refers to the IP address of the sender.
 The destination hardware address specifies the physical (Ethernet) address to
which data can be forwarded by the Ethernet protocol standards, however this
information is not present in an ARP request, instead replaced by a value of 0. The
destination protocol address identifies the intended IP destination for which
reachability over Ethernet is to be established.

 The network layer represents a logical path between a source and a destination.
Reaching an intended IP destination relies on firstly being able to establish a
physical path to the intended destination, and in order to do that, an association
must be made between the intended IP destination and the physical next hop
interface to which traffic can be forwarded.
 For a given destination the host will determine the IP address to which data is to
be forwarded, however before encapsulation of the data can commence, the host
must determine whether a physical forwarding path is known. If the forwarding
path is known encapsulation to the destination can proceed, however quite often
the destination is not known and ARP must be implemented before data
encapsulation can be performed.

 The ARP cache (pronounced as [kash]) is a table for association of host destination
IP addresses and associated physical (MAC) addresses. Any host that is engaged in
communication with a local or remote destination will first need to learn of the
destination MAC via which communication can be established.
 Learned addresses will populate the ARP cache table and remain active for a fixed
period of time, during which the intended destination can be discovered without
the need for addition ARP discovery processes. Following a fixed period, the ARP
cache table will remove ARP entries to maintain the ARP cache table’s integrity,
since any change in the physical location of a destination host may result in the
sending host inadvertently addressing data to a destination at which the
destination host no longer resides.
 The ARP cache lookup is the first operation that an end system will perform before
determining whether it is necessary to generate an ARP request. For destinations
beyond the boundaries of the hosts own network, an ARP cache lookup is
performed to discover the physical destination address of the gateway, via which
the intended destination network can be reached.

 Where an ARP cache entry is unable to be determined, the ARP request process is
performed. This process involves generation of an ARP request packet, and
population of the fields with the source and destination protocol addresses, as well
as the source hardware address. The destination hardware address is unknown. As
such the destination hardware address is populated with a value equivalent to 0.
The ARP request is encapsulated in an Ethernet frame header and trailer as part of
the forwarding process. The source MAC address of the frame header is set as the
source address of the sending host.
 The host is currently unaware of the location of the destination and therefore must
send the ARP request as a broadcast to all destinations within the same local
network boundary. This means that a broadcast address is used as the destination
MAC address. Once the frame is populated, it is forwarded to the physical layer
where it is propagated along the physical medium to which the host is connected.
The broadcasted ARP packet will be flooded throughout the network to all
destinations including any gateway that may be present, however the gateway will
prevent this broadcast from being forwarded to any network beyond the current
network.

 If the intended network destination exists, the frame will arrive at the physical
interface of the destination at which point lower layer processing will ensue. ARP
broadcasts mean that all destinations within the network boundary will receive the
flooded frame, but will cease to process the ARP request, since the destination
protocol address does not match to the IP address of those destinations.
 Where the destination IP address does match to the receiving host, the ARP packet
will be processed. The receiving host will firstly process the frame header and then
process the ARP request. The destination host will use the information from the
source hardware address field in the ARP header to populate its own ARP cache
table, thus allowing for a unicast frame to be generated for any frame forwarding
that may be required, to the source from which the ARP request was received.

 The destination will determine that the ARP packet received is an ARP request and
will proceed to generate an ARP reply that will be returned to the source, based on
the information found in the ARP header. A separate ARP packet is generated for
the reply, for which the source and destination protocol address fields will be
populated. However, the destination protocol address in the ARP request packet
now represents the source protocol address in the ARP reply packet, and similarly
the source protocol address of the ARP request becomes the destination protocol
address in the ARP reply.
 The destination hardware address field is populated with the MAC of the source,
discovered as a result of receiving the ARP request. For the required destination
hardware address of the ARP request, it is included as the source hardware
address of the ARP reply, and the operation code is set to reply, to inform the
destination of the purpose of the received ARP packet, following which the
destination is able to discard the ARP packet without any further communication.
The ARP reply is encapsulated in the Ethernet frame header and trailer, with the
destination MAC address of the Ethernet frame containing the MAC entry in the
ARP cache table, allowing the frame to be forwarded as a unicast frame back to
the host that originated the ARP request.

 Upon receiving the ARP reply, the originating host will validate that the intended
destination is correct based on the frame header, identify that the packet header is
ARP from the type field and discard the frame headers. The ARP reply will then be
processed, with the source hardware address of the ARP reply being used to
populate the ARP cache table of the originating host (Host A).
 Following the processing of the ARP reply, the packet is discarded and the
destination MAC information is used to facilitate the encapsulation process of the
initial application or protocol that originally requested discovery of the destination
at the data link layer.

 The ARP protocol is also applied to other cases such as where transparent subnet
gateways are to be implemented to facilitate communication across data-link
networks, where hosts are considered to be part of the same subnetwork. This is
referred to as Proxy ARP since the gateway operates as a proxy for the two data-
link networks. When an ARP request is generated for a destination that is
considered to be part of the same subnet, the request will eventually be received
by the gateway. The gateway is able to determine that the intended destination
exists beyond the data-link network on which the ARP request was generated.
 Since ARP requests cannot be forwarded beyond the boundaries of the broadcast
domain, the gateway will proceed to generate its own ARP request to determine
the reachability to the intended destination, using its own protocol and hardware
addresses as the source addresses for the generated ARP request. If the intended
destination exists, an ARP reply shall be received by the gateway for which the
destinations source hardware address will be used to populate the ARP cache table
of the gateway.
 The gateway upon confirming the reachability to the intended destination will then
generate an ARP reply to the original source (Host A) using the hardware address
of the interface on which the ARP reply was forwarded. The gateway will as a result
operate as an agent between the two data-link networks to facilitate data link
layer communication, with both hosts forwarding traffic intended for destinations
in different data-link networks to the relevant physical address of the “Proxy”
gateway.

 In the event that new hardware is introduced to a network, it is imperative that the
host determine whether or not the protocol address to which it has been assigned
is unique within the network, so as to prevent duplicate address conflicts. An ARP
request is generated as a means of determining whether the protocol address is
unique, by setting the destination address in the ARP request to be equal to the
host’s own IP address.
 The ARP request is flooded throughout the network to all link layer destinations by
setting the destination MAC as broadcast, to ensure all end stations and gateways
receive the flooded frame. All destinations will process the frame, and should any
destination discover that the destination IP address within the ARP request match
the address of a receiving end station or gateway, an ARP reply will be generated
and returned to the host that generated the ARP request.
 Through this method the originating host is able to identify duplication of the IP
address within the network, and flag an IP address conflict so to request that a
unique address be assigned. This means of generating a request based on the
hosts own IP address defines the basic principles of gratuitous ARP.

 The host is required to initially determine whether it is already aware of a link layer
forwarding address within its own ARP cache (MAC address table). If an entry is
discovered the end system is capable of creating the frame for forwarding without
the assistance of the address resolution protocol. If an entry cannot be found
however, the ARP process will initiate, and an ARP request will be broadcasted on
the local network.
 Gratuitous ARP messages are commonly generated at the point in which an IP
address is configured or changed for a device connected to the network, and at
any time that a device is physically connected to the network. In both cases the
gratuitous ARP process must ensure that the IP address that is used remains
unique.

 TCP is a connection-oriented, end-to-end protocol that exists as part of the
transport layer of the TCP/IP protocol stack, in order to support applications that
span over multi-network environments. The transmission control protocol provides
a means of reliable inter-process communication between pairs of processes in
host computers that are attached to distinct but interconnected computer
communication networks. TCP relies on lower layer protocols to provide the
reachability between process supporting hosts, over which a reliable connection
service is established between pairs of processes. The connection-oriented
behavior of TCP involves prior exchanges between the source and destination,
through which a connection is established before transport layer segments are
communicated.

 As a means for allowing for many processes within a single host to use TCP
communication facilities simultaneously, TCP provides a set of logical ports within
each host. The port value together with the network layer address is referred to as
a socket, for which a pair of sockets provide a unique identifier for each
connection, in particular where a socket is used simultaneously in multiple
connections. That is to say, a process may need to distinguish among several
communication streams between itself and another process (or processes), for
which each process may have a number of ports through which it communicates
with the port or ports of other processes.
 Certain processes may own ports and these processes may initiate connections on
the ports that they own. These ports are understood as IANA assigned system
ports or well-known ports and exist in the port value range of 0 – 1023. A range of
IANA assigned user or registered ports also exist in the range of 1024 – 49151,
with dynamic ports, also known as private or ephemeral ports in the range of
49152 – 65535, which are not restricted to any specific application. Hosts will
generally be assigned a user port value for which a socket is generated to a given
application.
 Common examples of TCP based applications for which well-known port numbers
have been assigned include FTP, HTTP, TELNET, and SMTP, which often will work
alongside other well-known mail protocols such as POP3 (port 110) and IMAP4
(port 143).

 The TCP header allows TCP based applications to establish connection-oriented
data streams that are delivered reliability, and to which flow control is applied. A
source port number is generated where a host intends to establish a connection
with a TCP based application, for which the destination port will relate to a well-
known/registered port to which a well-known/registered application is associated.
 Code bits represent functions in TCP, and include an urgent bit (URG) used
together the urgent pointer field for user directed urgent data notifications,
acknowledgment of received octets in association with the acknowledgement field
(ACK), the push function for data forwarding (PSH), connection reset operations
(RST), synchronization of sequence numbers (SYN) and indication that no more
data is to be received from the sender (FIN). Additional code bits were introduced
in the form of ECN-Echo (ECE) and Congestion Window Reduced (CWR) flags, as a
means of supporting congestion notification for delay sensitive TCP applications.
 The explicit congestion notification (ECN) nonce sum (NS) was introduced as a
follow-up alteration to eliminate the potential abuse of ECN where devices along
the transmission path may remove ECN congestion marks. The Options field
contains parameters that may be included as part of the TCP header, often used
during the initial connection establishment, as in the case of the maximum
segment size (MSS) value that may be used to define the size of the segment that
the receiver should use. TCP header size must be a sum of 32 bits, and where this
is not the case, padding of 0 values will be performed.

 When two processes wish to communicate, each TCP must first establish a
connection (initialize the synchronization of communication on each side). When
communication is complete, the connection is terminated or closed to free the
resources for other uses. Since connections must be established between
unreliable hosts and over the unreliable Internet domain, a handshake mechanism
with clock-based sequence numbers is used to avoid erroneous initialization of
connections.
 A connection progresses through a series of states during establishment. The
LISTEN state represents a TCP waiting for a connection request from any remote
TCP and port. SYN-SENT occurs after sending a connection request and before a
matching request is received. The SYN-RECEIVED state occurs while waiting for a
confirming connection request acknowledgment, after having both received and
sent a connection request. The ESTABLISHED state occurs following the handshake
at which time an open connection is created, and data received can be delivered to
the user.
 The TCP three-way handshake mechanism begins with an initial sequence number
being generated by the initiating TCP as part of the synchronization (SYN) process.
The initial TCP segment is then set with the SYN code bit, and transmitted to the
intended IP destination TCP to achieve a SYN-SENT state. As part of the
acknowledgement process, the peering TCP will generate an initial sequence
number of its own to synchronize the TCP flow in the other direction. This peering
TCP will transmit this sequence number, as well as an acknowledgement number
that equals the received sequence number incremented by one, together with set
SYN and ACK code bits in the TCP header to achieve a SYN-RECEIVED state.
 The final step of the connection handshake involves the initial TCP acknowledging
the sequence number of the peering TCP by setting the acknowledgement number
to equal the received sequence number plus one, together with the ACK bit in the
TCP header, allowing an ESTABLISHED state to be reached.

 Since the TCP transmission is sent as a data stream, every octet can be sequenced,
and therefore each octet can be acknowledged. The acknowledgement number is
used to achieve this by responding to the sender as confirmation of receipt of data,
thus providing data transport reliability. The acknowledgement process however is
cumulative, meaning that a string of octets can be acknowledged by a single
acknowledgement by reporting to the source the sequence number that
immediately follows the sequence number that was successfully received.
 In the example a number of bytes (octets) are transmitted together before TCP
acknowledgement is given. Should an octet fail to be transmitted to the
destination, the sequence of octets transmitted will only be acknowledged to the
point at which the loss occurred. The resulting acknowledgement will reflect the
octet that was not received in order to reinitiate transmission from the point in the
data stream at which the octet was lost.
 The ability to cumulate multiple octets together before an acknowledgement
enables TCP to operate much more efficiently, however a balance is necessary to
ensure that the number of octets sent before an acknowledgement is required is
not too extreme, for if an octet fails to be received, the entire stream of octets
from the point of the loss must be retransmitted.

 The TCP window field provides a means of flow control that governs the amount
of data sent by the sender. This is achieved by returning a "window" with every
TCP segment for which the ACK field is set, indicating a range of acceptable
sequence numbers beyond the last segment successfully received. The window
indicates the permitted number of octets that the sender may transmit before
receiving further permission.
 In the example, TCP transmission from host A to server A contains the current
window size for host A. The window size for server A is determined as part of the
handshake, which based on the transmission can be assumed as 2048. Once data
equivalent to the window size has been received, an acknowledgement will be
returned, relative to the number of bytes received, plus one. Following this, host A
will proceed to transmit the next batch of data.
 A TCP window size of 0 will effectively deny processing of segments, with
exception to segments where the ACK, RST and URG code bits are set for incoming
segments. Where a window size of 0 exists, the sender must still periodically check
the window size status of receiving TCP to ensure any change in the window size is
effectively reported, the period for retransmission is generally two minutes. When
a sender sends periodic segments, the receiving TCP must still acknowledge with a
sequence number announcement of the current window size of 0.

 As part of the TCP connection termination process, a number of states are defined
through which TCP will transition. These states include FIN-WAIT-1 that represents
waiting for a connection termination (FIN) request from the remote TCP, or an
acknowledgment of a connection termination request that was previously sent.
The FIN-WAIT-2 represents waiting for a connection termination request from the
remote TCP following which will generally transition to a TIME-WAIT state. A
CLOSE-WAIT state indicates waiting for a locally defined connection termination
request, typically when a server’s application is in the process of closing.
 The LAST-ACK state represents waiting for an acknowledgment of the connection
termination request previously sent to the remote TCP (which includes an
acknowledgment of its connection termination request). Finally, a TIME-WAIT state
occurs and waits for enough time to pass to ensure that the remote TCP received
the acknowledgment of its connection termination request. This period is
managed by the Max Segment Lifetime (MSL) timer that defines a waiting period
of 2 minutes. Following a wait period equal to two times the MSL, the TCP
connection is considered closed/terminated.

 The User Datagram Protocol, or UDP, represents an alternative to TCP and applied
where TCP is found to act as an inefficient transport mechanism, primarily in the
case of highly delay sensitive traffic. Where TCP is considered a segment, the UDP
is recognized as a datagram form of Protocol Data Unit (PDU), for which a
datagram can be understood to be a self-contained, independent entity of data
carrying sufficient information to be routed from the source to the destination end
system without reliance on earlier exchanges between this source and destination
end systems and the transporting network, as defined in RFC 1594. In effect this
means that UDP traffic does not require the establishment of a connection prior to
the sending of data.
 The simplified structure and operation of UDP makes it ideal for application
programs to send messages to other programs, with a minimum of protocol
mechanism such in the case of acknowledgements and windowing for example, as
found in TCP segments. In balance however, UDP does not guarantee delivery of
data transmission, nor protection from datagram duplication.

 The UDP header provides a minimalistic approach to the transport layer,
implementing only a basic construct to help identify the destination port to which
the UDP based traffic is destined, as well as a length field and a checksum value
that ensures the integrity of the UDP header. In addition the minimal overhead
acts as an ideal means for enabling more data to be carried per packet, favoring
real time traffic such as voice and video communications where TCP provides a 20
byte overhead and mechanisms that influence delays, such as in the case of
acknowledgements, however the lack of such fields means that datagram delivery
is not guaranteed.

 Since UDP datagram transmission is not sent as a data stream, transmission of
data is susceptible to datagram duplication. In addition, the lack of sequence
numbers within UDP means that delivery of data transmission over various paths is
likely to be received at the destination in an incorrect, non-sequenced order.
 Where stream data is transported over UDP such as in the case of voice and video
applications, additional protocol mechanisms may be applied to enhance the
capability of UDP, as in the case of the real time transport protocol (RTP) which
helps to support the inability of UDP by providing a sequencing mechanism using
timestamps to maintain the order of such audio/video data streams, effectively
supporting partial connection oriented behavior over a connectionless transport
protocol.

 The general UDP forwarding behavior is highly beneficial to delay sensitive traffic
such as voice and video. It should be understood that where a connection-
oriented transport protocol is concerned, lost data would require replication
following a delay period, during which time an acknowledgement by the sender is
expected. Should the acknowledgement not be received, the data shall be
retransmitted.
 For delay sensitive data streams, this would result in incomprehensible audio and
video transmissions due to both delay and duplication, as a result of
retransmission from the point where acknowledgements are generated. In such
cases, minimal loss of the data stream is preferable over retransmission, and as
such UDP is selected as the transport mechanism, in support of delay sensitive
traffic.

 The acknowledgement field in the TCP header confirms receipt of the segment
received by the TCP process at the destination. The sequence number in the TCP
header of the received IP datagram is taken and incremented by 1. This value
becomes the acknowledgement number in the returned TCP header and is used to
confirm receipt of all data, before being forwarded along with the ACK code bit set
to 1, to the original sender.
 The three-way handshake involves SYN and ACK code bits in order to establish
and confirm the connection between the two end systems, between which
transmission of datagrams is to occur.

 Data forwarding can be collectively defined as either local or remote for which the
forwarding process relies on the application of the protocol stack in order to
achieve end-to-end transmission. End systems may be part of the same network,
or located in different networks, however the general forwarding principle to
enable transmission between hosts follows a clear set of protocols that have been
introduced as part of the unit. How these protocols work together shall be
reinforced, as well as building the relationship between the upper layer TCP/IP
protocols and the lower link layer based Ethernet protocol standards.

 An end system that intends to forward data to a given destination must initially
determine whether or not it is possible to reach the intended destination. In order
to achieve this, the end system must go through a process of path discovery. An
end system should be understood to be capable of supporting operation at all
layers since its primary function is as a host to applications. In relation to this, it
must also be capable of supporting lower layer operations such as routing and link
layer forwarding (switching) in order to be capable of upper/application layer data
forwarding. The end system therefore contains a table that represents network
layer reachability to the network for which the upper layer data is destined.
 End systems will commonly be aware of the network to which they reside, but may
be without a forwarding path in cases where remote network discovery has not
been achieved. In the example given, host A is in possession of a path to the
destined network through the ‘any network’ address that was briefly introduced
as part of the IP Addressing section. The forwarding table identifies that traffic
should be forwarded to the gateway as a next-hop via the interface associated
with the logical address of 10.1.1.1.

 Following discovery of a feasible route towards the intended destination network,
a physical next-hop must also be discovered to facilitate frame forwarding. The
TCP/IP protocol suite is responsible for determining this before packet
encapsulation can proceed. The initial step involves determining whether a
physical path exists to the next-hop identified as part of the path discovery
process.
 This requires that the ARP cache table be consulted to identify whether an
association between the intended next-hop and the physical path is known. From
the example it can be seen that an entry to the next-hop gateway address is
present in the ARP cache table. Where an entry cannot be found, the Address
Resolution Protocol (ARP) must be initiated to perform the discovery and resolve
the physical path.

 When both the logical and physical path forwarding discovery is complete, it is
possible for encapsulation of data to be performed for successful transmission
over IP/Ethernet based networks. Upper layer processes in terms of encryption and
compression may be performed following which transport layer encapsulation will
occur, identifying the source and destination ports via which upper layer data
should be forwarded.
 In the case of TCP, sequence and acknowledgement fields will be populated, code
bits set as necessary with the ACK bit commonly applied. The window field will be
populated with the current supported window size, to which the host will notify of
the maximum data buffer that can be supported before data is acknowledged.
 Values representing the TCP fields are included as part of the checksum, which is
calculated using a ones compliment calculation process, to ensure TCP segment
integrity is maintained once the TCP header is received and processed at the
ultimate destination. In the case of basic TCP code operations, upper layer data
may not always be carried in the segment, as in the case of connection
synchronization, and acknowledgements to received data.

 Following transport layer encapsulation, it is generally required that instructions be
provided, detailing how transmission over one or more networks is to be achieved.
This involves listing the IP source as well as the ultimate destination for which the
packet is intended. IP packets are generally limited to a size of 1500 bytes by
Ethernet, inclusive of the network and transport layer headers as well as any upper
layer data. Initial packet size will be determined by Ethernet as the maximum
transmission unit, or MTU to which packets will conform, therefore fragmentation
will not occur at the source.
 In the case that the MTU changes along the forwarding path, only then will
fragmentation will be performed. The Time to Live field will be populated with a
set value depending on the system, in ARG3 series routers, this is set with an initial
value of 255. The protocol field is populated based on the protocol encapsulated
prior to IP. In this case the protocol in question is TCP for which the IP header will
populate the protocol field with a value of 0x06 as instruction for next header
processing. Source and destination IP addressing will reflect the originating source
and the ultimate destination.

 Link layer encapsulation relies on IEEE 802.3 Ethernet standards for physical
transmission of upper layer data over Ethernet networks. Encapsulation at the
lower layers is performed by initially determining the frame type that is used.
 Where the upper layer protocol is represented by a type value greater than 1536
(0x0600) as is the case with IP (0x0800), the Ethernet II frame type is adopted. The
type field of the Ethernet II frame header is populated with the type value of
0x0800 to reflect that the next protocol to be processed following frame
processing will be IP. The destination MAC address determines the next physical
hop, which in this case represents the network gateway.

 As part of the link layer operation, it is imperative to ensure that the transmission
medium is clear of signals in shared collision domain. The host will first listen for
any traffic on the network as part of CSMA/CD and should the line remain clear,
will prepare to transmit the data. It is necessary for the receiving physical interface
to be made aware of the incoming frame, so as to avoid loss of initial bit values
that would render initial frames as incomplete. Frames are therefore preceded by a
64 bit value indicating to the link layer destination of the frame’s imminent arrival.
 The initial 56 bits represent an alternating 1, 0 pattern is called the preamble, and
is followed immediately by an octet understood as the Start of Frame Delimiter
(SFD). The final two bits of the SFD deviate from an alternating pattern to a 1,1 bit
combination that notifies that the bits that follow represent the first bit values of
the destination MAC address and therefore the start of the frame header.

 As a frame is received by the link layer destination, it must go through a number of
checks to determine its integrity as well as validity. If the frame was transmitted
over a shared Ethernet network, other end stations may also receive an instance of
the frame transmitted, however since the frame destination MAC address is
different from the MAC address of the end station, the frame will be discarded.
 Frames received at the intended destination will perform error checking by
calculating the ones compliment value based on the current frame fields and
compare this against the value in the Frame Check Sequence (FCS) field. If the
values do not match, the frame will be discarded. Receiving intermediate and end
systems that receive valid frames will need to determine whether the frame is
intended for their physical interface by comparing the destination MAC address
with the MAC address of the interface (or device in some cases).
 If there is a match, the frame is processed and the type field is used to determine
the next header to be processed. Once the next header is determined, the frame
header and trailer are discarded.

 The packet is received by the network layer, and in particular IP, at which point the
IP header is processed. A checksum value exists at each layer of the protocol stack
to maintain the integrity at all layers for all protocols. The destination IP is used to
determine whether the packet has reached its ultimate destination. The gateway
however determines that this is not the case since the destination IP and the IP
belonging to the gateway do not match.
 The gateway must therefore determine the course of action to take with regards to
routing the packet to an alternate interface, and forward the packet towards the
network for which it is intended. The gateway must firstly however ensure that the
TTL value has not reached 0, and that the size of the packet does not exceed the
maximum transmission unit value for the gateway. In the event that the packet is
larger than the MTU value of the gateway, fragmentation will generally commence.
 Once a packet’s destination has been located in the forwarding table of the
gateway, the packet will be encapsulated in a new frame header consisting of new
source and destination MAC addresses for the link layer segment, over which the
resulting frame is to be forwarded, before being once again transmitted to the
next physical hop. Where the next physical hop is not known, ARP will again be
used to resolve the MAC address.

 Frames received at the ultimate destination will initially determine whether the
frame has arrived at the intended location. The example shows two servers on a
shared Ethernet network over which both receive a copy of the frame.
 The frame is ultimately discarded by server B since the destination MAC value and
the interface MAC address of server B do not match. Server A however successfully
receives the frame and learns that the MAC fields are the same, the integrity of the
frame based on the FCS can also be understood to be correct. The frame will use
the type field to identify 0x0800 as the next header, following which the frame
header and trailer are discarded and the packet is received by IP.

 Upon reaching the ultimate destination, the IP packet header must facilitate a
number of processes. The first includes validating the integrity of the packet
header through the checksum field, again applying a ones compliment value
comparison based on a sum of the IP header fields. Where correct, the IP header
will be used to determine whether the destination IP matches the IP address of the
current end station, which in this case is true.
 If any fragmentation occurred during transmission between the source and the
destination, the packet must be reassembled at this point. The identification field
will collect the fragments belonging to a single data source together, the offset will
determine the order and the flags field will specify when the reassembly should
commence, since all fragments must be received firstly and a fragment with a flag
of 0 will be recognized as the last fragment to be received.
 A timer will then proceed during which time the reassembly must be completed,
should reassembly fail in this time period, all fragments will be discarded. The
protocol field will be used to identify the next header for processing and the
packet header will be discarded. It should be noted that the next header may not
always be a transport layer header, a clear example of where this can be
understood is in the case of ICMP, which is understood to also be a network layer
protocol with a protocol field value of 0x01.

 In the case where a packet header is discarded, the resulting segment or datagram
is passed to the transport layer for application-to-application based processing.
The header information is received in this case by TCP (0x06).
 In the example it can be understood that a TCP connection has already been
established and the segment represents an acknowledgement for the transmission
of HTTP traffic from the HTTP server to the acknowledging host. The host is
represented by the port 1027 as a means to distinguish between multiple HTTP
connections that may exist between the same source host and destination server.
In receiving this acknowledgement, the HTTP server will continue to forward to the
host within the boundaries of the window size of the host.

 Prior to the encapsulation and forwarding of data, a source must have knowledge
of the IP destination or an equivalent forwarding address such as a default address
to which data can be forwarded. Additionally it is necessary that the forwarding
address be associated with a physical next-hop to which the data can be
forwarded within the local network.
 Any frame that is received by a gateway or end system (host) to which it is not
intended, is subsequently dropped, following inspection of the destination MAC
address in the frame header.
 The delivery of data relies on the destination port number in the TCP and UDP
headers to identify the application to which the data is intended. Following
analysis of this value by the TCP or UDP protocol, the data is forwarded.
 The source port of the TCP header for the HTTP traffic distinguishes between the
different application sessions that are active. Return HTTP traffic from the HTTP
server is able to identify each individual web browser session based on this source
port number. For example, the source port of two separate requests for HTTP
traffic originating from IP source 10.1.1.1 may originate from source ports 1028
and 1035, however the destination port in both cases remains as port 80, the HTTP
server.

 The Ethernet network until now has been understood to be a collection of devices
communicating over shared media such as 10Base2, through which hosts are able
to communicate with neighboring hosts or end systems. It has been determined
that the Ethernet network is a contentious network, meaning that hosts must
compete for media access which becomes increasingly limited as more and more
devices are connected over this shared media; which causes additional limitations
in scalability and the increasing potential for collisions.
 As a result, the need for collision detection in the form of CSMA/CD is ever present
in such shared Ethernet networks. Following the adoption of switched media such
as that of 100BaseT, data transmission and reception became isolated within
channels (wire pairs), enabling the potential for collisions to occur to be eliminated.
This medium as a form of non-shared Ethernet provides only a means for point-to-
point communication, however used together with other devices such as hubs, a
shared Ethernet network is once again possible, along with the potential for
collisions.
 The switch was introduced as part of the evolution of the bridge, and is capable of
breaking down the shared collision domain into multiple collision domains. The
collision domains operate as a collection of point-to-point links for which the
threat of collisions is removed and link-layer traffic is isolated, to allow higher
transmission rates that optimize traffic flow within the Ethernet network.

 A broadcast domain is capable of being comprised of a single, or multiple collision
domains, and any broadcast transmission is contained within the boundary of a
broadcast domain. The edge of a broadcast domain’s boundary is typically
defined by a gateway that acts as the medium, via which other networks are
reachable, and will restrict the forwarding of any broadcast traffic beyond the
interface on which the broadcast is received.
 Routers are synonymous with the term gateway for which the two are often used
interchangeably. A single IP network can generally be understood to make up a
broadcast domain, which refers to the scope of a link-layer segment. Routers are
generally responsible for routing Internet datagrams (IP packets) to a given
destination based on the knowledge of a forwarding address for the destination
network, found within an internally managed forwarding table.

 The Versatile Routing Platform (VRP) represents the foundation of many of Huawei
products including routers and switches. Its design has been through many
evolutions to enable continuous improvement of data management and
forwarding. The architectural design has resulted in ever enhanced modularity that
allows for greater overall performance. The configuration, management and
monitoring of devices using VRP is based on a standardized and hierarchical
command line system for which a familiarity should be developed to support
navigation and operation of Huawei products managed using VRP software.

 A familiarity with the versions of VRP network operating system (NOS) aids in
ensuring that the version currently being used is up to date and supports certain
features that may be required in an enterprise network. The general trend for most
Huawei devices is to operate using VRP version 5.x currently, where x may vary
depending on the product and VRP release. VRP version 8 is a recent revision of
VRP built with a highly refined architecture for the next generation of technologies
and constructed around the need for greater efficiency, but is not present in all
Huawei products.

 AR series enterprise routers (AR) include the AR150, AR200, AR1200, AR2200, and
AR3200. They are the next-generation of Huawei products, and provide routing,
switching, wireless, voice, and security functionality. The AR series are positioned
between the enterprise network and a public network, functioning as an ingress
and egress gateway for data transmitted between the two networks. Deployment
of various network services over the AR series routers reduces operation &
maintenance (O&M) costs as well as costs associated with establishing an
enterprise network. AR series routers of different specifications can be used as
gateways based on the user capacity of an enterprise.
 The Sx7 Series Ethernet Switch provides data transport functionality, and has been
developed by Huawei to meet the requirements for reliable access and high-
quality transmission of multiple services on the enterprise network. This series of
switch is positioned for access or aggregation layer operation in the enterprise
network, and provides a large switching capacity, high port density, and cost-
effective packet forwarding capabilities.
 Management of the ARG3 series routers and Sx7 series of switch can be achieved
through establishing a connection to the console interface, and in the case of the
AR2200, a connection is also possible to be established via a Mini USB interface.

 A console cable is used to debug or maintain a locally established device such as a
router or switch, and will interface with the console port of such devices. The
console interface of the S5700 series switch and the AR2200 router is an RJ-45
type connection, while the interface to which a host connection is made,
represents an RS-232 form of serial connector. Often such serial connectors are no
longer present on newer devices that can be used for establishing connectivity,
such as laptop computers, and therefore an RS-232 to USB conversion is
performed. For most desktop devices however, an RS-232 based console
connection can be established to a COM port on the host device.

 Console establishment is set up through one of a number of available terminal
emulation programs. Windows users often apply the HyperTerminal application as
shown in the example to interface with the VRP operating system. Following
specification of the COM port that is to be used to establish the connection, port
settings must be defined.
 The example defines the port settings that should be applied, for which the restore
default button will automatically reassign should any change have been made to
these settings. Once the OK button is pressed, a session will be established with
the VRP of the device. If the device is operating using factory default settings, the
user will be prompted for a password, which will be assigned as the default login
password for future connection attempts.

 The Huawei AR2200 router, additionally supports the means for terminal
connectivity via a USB connection. A type B mini USB interface exists on the front
panel of the AR2200 series router through which hosts are able to establish a USB
based connection as a serial alternative to that of RS-232.

 A slight variation in the setup process requires that the mini USB firstly establish
drivers to allow USB functionality. The mini USB driver can be obtained by visiting
http://support.huawei.com/enterprise, and under the path Support > Software >
Enterprise Networking > Router > Access Router > AR > AR2200, choose the
relevant VRP version & patch path option, and download the file labeled
AR&SRG_MiniUSB_driver.zip. It should be noted that the mini USB driver supports
only Windows XP, Windows Vista, and Windows 7 operating systems.
 When upgrading the device software or installing a patch, the MD5 hash value can
be checked to confirm software validity. In order to prevent the software from
being modified or replaced, you are advised to perform this operation.
 Installing requires the user to firstly double-click the driver installation file on the
PC and click Next. Secondly select I accept the terms in the license agreement and
click Next. Click the Change button to change the driver directory if required, and
click Next. Click Install and decompress the driver. When the system finishes
decompressing the driver, click Finish.
 Users should then find the DISK1 folder in the specified driver directory, and
double-click the file setup.exe. Following the opening of a second installation
window click Next. Users should again select I accept the terms in the license
agreement and click Next to install the driver. Once complete, click Finish to finish
installing the driver. Right-click My Computer, and choose Manage > Device
Manager > Ports(COM&LPT). The system should display the TUSB3410 Device
indicating the driver that has been installed.

 As with the RS-232 console connection, the Mini USB serial connection requires
establishment to terminal emulation software to enable interaction with the VRP
command line.
 Use the terminal emulation software to log in to the device through the mini USB
port, (for which the Windows HyperTerminal is used as an example). On the host
PC, start the HyperTerminal application, for which the location may vary for each
version of Windows, and create a connection by providing a suitable terminal
connection name and click OK. Select the relevant connection (COM) port and
then set the communication parameters for the serial port of the PC. These
parameters should match the default values that are set when pressing the Restore
Defaults button.
 After pressing Enter, the console information is displayed requesting a login
password. Enter a relevant password and confirmation password, and the system
will save the password.

 Any broadcast that is generated by an end system within a local network will be
forwarded to all destinations. Once a frame is broadcast to a router or device
acting as a gateway for the network, the frame will be analyzed and should it be
determined that the destination is for a locally defined host other than the
gateway, the frame will be dropped. This as such defines the boundary of any
broadcast domain.
 VRP version 5 is supported by a large number of current Huawei products, while
high end products may often be supported by VRP version 8.

 The startup/boot process is the initial phase of operation for any administrator or
engineer accessing Huawei based products operating with VRP. The boot screen
informs of the system startup operation procedures as well as the version of the
VRP image that is that currently implemented on the device, along with the
storage location from where it is loaded. Following the initial startup procedure, an
option for auto-configuration of the initial system settings prompts for a response,
for which the administrator can choose whether to follow the configuration steps,
or manually configure the basic system parameters. The auto-configuration
process can be terminated by selecting the yes option at the given prompt.

 The hierarchical command structure of VRP defines a number of command views
that govern the commands for which users are able to perform operations. The
command line interface has multiple command views, of which common views
have been introduced in the example. Each command is registered to run in one or
more command views, and such commands can run only after entering the
appropriate command view. The initial command view of VRP is the User View,
which operates as an observation command view for observing parameter statuses
and general statistical information. For application of changes to system
parameters, users must enter the System View. A number of sub command levels
can also be found, in the form of the interface and protocol views for example,
where sub system level tasks can be performed.
 The command line views can be determined based on the parenthesis, and
information contained within these parenthesis. The presence of chevrons
identifies that the user is currently in the User View, whereas square brackets show
that a transition to the System View has occurred.

 The example demonstrates a selection of common system defined shortcut keys
that are widely used to simplify the navigation process within the VRP command
line interface. Additional commands are as follows:
 CTRL+B moves the cursor back one character.
 CTRL+D deletes the character where the cursor is located.
 CTRL+E moves the cursor to the end of the current line.
 CTRL+F moves the cursor forward one character.
 CTRL+H deletes the character on the left side of the cursor.
 CTRL+N displays the next command in the historical command buffer.
 CTRL+P displays the previous command in the historical command buffer.
 CTRL+W deletes the word on the left side of the cursor.
 CTRL+X deletes all the characters on the left side of the cursor.
 CTRL+Y deletes all the characters on the right side of the cursor.
 ESC+B moves the cursor one word back.
 ESC+D deletes the word on the right side of the cursor.
 ESC+F moves the cursor forward one word.

 Additional key functions can be used to perform similar operations, the backspace
operation has the same behavior as using CTRL+H to delete a character to the left
of the cursor. The left (←) and right (→) cursor keys can be used to perform the
same operation as the CTRL+B and CTRL+F shortcut key functions. The down
cursor key (↓) functions the same as Ctrl+N, and the up cursor key (↑) acts as an
alternative to the CTRL+P operation.
 Additionally, the command line functions support a means of auto completion
where a command word is unique. The example demonstrates how the command
word interface can be auto completed by partial completion of the word to such a
point that the command is unique, followed by the tab key which will provide auto
completion of the command word. Where the command word is not unique, the
tab function will cycle through the possible completion options each time the tab
key is pressed.

 There are two forms of help feature that can be found within the VRP, these come
in the form of partial help and complete help functions. In entering a character
string followed directly by a question mark (?), VRP will implement the partial help
function to display all commands that begin with this character string. An example
of this is demonstrated. In the case of the full help feature, a question mark (?) can
be placed on the command line at any view to display all possible command
names, along with descriptions for all commands pertaining to that view.
Additionally the full help feature supports entry of a command followed by a
question mark (?) that is separated by a space. All keywords associated with this
command, as well as simple descriptions, are then displayed.

 For the majority of industries, it is likely that multiple devices will exist, each of
which needs to be managed. As such, one of the first important tasks of device
commissioning involves setting device names to uniquely identify each device in
the network. The system name parameter on AR2200 series router is configured as
Huawei by default, for the S5720 series of switch the default system name is
HUAWEI. The implementation of the system name takes effect immediately after
configuration is complete.

 The system clock reflects the system timestamp, and is able to be configured to
comply with the rules of any given region. The system clock must be correctly set
to ensure synchronization with other devices and is calculated using the formula:
Coordinated Universal Time (UTC) + Time zone offset + Daylight saving time offset.
The clock datetime command is used to set the system clock following the
HH:MM:SS YYYY-MM-DD formula. It should be noted however that if the time
zone has not been configured or is set to 0, the date and time set are considered
to be UTC, therefore it is recommended that the clock timezone be set firstly
before configuring the system time and date.
 The setting of the local timezone is achieved using the clock timezone command
and is implemented based on the time-zone-name { add | minus } offset formula,
where the add value indicates that the time of time-zone-name is equal to the UTC
time plus the time offset and minus indicates the time of time-zone-name is equal
to the UTC time minus the time offset.
 Certain regions require that the daylight saving time be implemented to maintain
clock synchronization with any change in the clock timezone during specific
periods of the year. VRP is able to support daylight saving features for both fixed
dates and dates which are determined based on a set of predetermined rules. For
example, daylight saving in the United Kingom occurs on the last Sunday of March
and the last Sunday of October, therefore rules can be applied to ensure that
changes occur based on such fluctuating dates.

 The header command provides a means for displaying notifications during the
connection to a device. The login header indicates a header that is displayed when
the terminal connection is activated, and the user is being authenticated by the
device. The shell header indicates a header that is displayed when the session is set
up, after the user logs in to the device. The header information can be applied
either as a text string or retrieved from a specified file. Where a text string is used,
a start and end character must be defined as a marker to identify the information
string, where in the example the “character defines the information string. The
string represents a value in the range of 1 to 2000 characters, including spaces.
The information based header command follows the format of header { login |
shell } information text where information represents the information string,
including start and end markers.
 In the case of a file based header, the format header { login | shell } file file-name is
applied, where file-name represents the directory and file from which the
information string can be retrieved.

 The system structures access to command functions hierarchically to protect
system security. The system administrator sets user access levels that grant specific
users access to specific command levels. The command level of a user is a value
ranging from 0 to 3, whilst the user access level is a value ranging from 0 to 15.
Level 0 defines a visit level for which access to commands that run network
diagnostic tools, (such as ping and traceroute), as well as commands such as telnet
client connections, and select display commands.
 The Monitoring level is defined at a user level of 1 for which command levels 0 and
1 can be applied, allowing for the majority of display commands to be used, with
exception to display commands showing the current and saved configuration. A
user level of 2 represents the Configuration level for which command levels up to 2
can be defined, enabling access to commands that configure network services
provided directly to users, including routing and network layer commands. The
final level is the Management level which represents a user level of 3 through to 15
and a command level of up to 3, enabling access to commands that control basic
system operations and provide support for services.
 These commands include file system, FTP, TFTP, configuration file switching, power
supply control, backup board control, user management, level setting, system
internal parameter setting, and debugging commands for fault diagnosis. The
given example demonstrates how a command privilege can be changed, where in
this case, the save command found under the user view requires a command level
of 3 before the command can be used.

 Each user interface is represented by a user interface view or command line view
provided by the system. The command line view is used to configure and manage
all the physical and logical interfaces in asynchronous mode. Users wishing to
interface with a device will be required to specify certain parameters in order to
allow a user interface to become accessible. Two common forms of user interface
implemented are the console interface (CON) and the virtual teletype terminal
(VTY) interface.
 The console port is an asynchronous serial port provided by the main control
board of the device, and uses a relative number of 0. VTY is a logical terminal line
that allows a connection to be set up when a device uses telnet services to connect
to a terminal for local or remote access to a device. A maximum of 15 users can
use the VTY logical user interface to log in to the device by extending the range
from 0 – 4 achieved by applying the user-interface maximum-vty 15 command. If
the set maximum number of login users is 0, no users are allowed to log in to the
router through telnet or SSH. The display user-interface command can be used to
display relevant information regarding the user interface.

 For both the console and VTY terminal interfaces, certain attributes can be applied
to modify the behavior as a means of extending features and improving security. A
user allows a connection to remain idle for a given period of time presents a
security risk to the system. The system will wait for a timeout period before
automatically terminating the connection. This idle timeout period on the user
interface is set to 10 minutes by default.
 Where it may be necessary to increase or reduce the number of lines displayed on
the screen of a terminal when using the display command for example, the screen-
length command can be applied. This by default is set to 24 however is capable of
being increased to a maximum of 512 lines. A screen-length of 0 however is not
recommended since no output will be displayed.
 For each command that is used, a record is stored in the history command buffer
which can be retrieved through navigation using the (↑) or CTRL+P and the (↓) or
Ctrl+N key functions. The number of recorded commands in the history command
buffer can be increased using the history-command max-size command to define
up to 256 stored commands. The number of commands stored by default is 10.

 Access to user terminal interfaces provides a clear point of entry for unauthorized
users to access a device and implement configuration changes. As such the
capability to restrict access and limit what actions can be performed is necessary
as a means of device security. The configuration of user privilege and
authentication are two means by which terminal security can be improved. User
privilege allows a user level to be defined which restricts the capability of the user
to a specific command range. The user level can be any value in the range of 0 –
15, where values represent a visit level (0), monitoring level (1), configuration level
(2), and management level (3) respectfully.
 Authentication restricts a user capability to access a terminal interface by
requesting the user be authenticated using a password or a combination of
username and password before access via the user interface is granted. In the case
of VTY connections, all users must be authenticated before access is possible. For
all user interfaces, three possible authentication modes exist, in the form of AAA,
password authentication and non-authentication. AAA provides user
authentication with high security for which a user name and password must be
entered for login. Password authentication requires that only the login password is
needed therefore a single password can be applied to all users. The use of non-
authentication removes any authentication applied to a user interface. It should be
noted that the console interface by default uses the non-authentication mode.
 It is generally recommended that for each user that is granted telnet access, the
user be identified through usernames and passwords to allow for distinction of
individual users. Each user should also be granted privilege levels, based on each
users role and responsibility.

 In order to run IP services on an interface, an IP address must be configured for
the interface. Generally, an interface needs only the primary IP address. In special
cases, it is possible for a secondary IP address to be configured for the interface.
For example, when an interface of a router such as the AR2200 connects to a
physical network, and hosts on this physical network belong to two network
segments.
 In order to allow the AR2200 to communicate with all the hosts on the physical
network, configure a primary IP address and a secondary IP address for the
interface. The interface has only one primary IP address. If a new primary IP
address is configured on an interface that already has a primary IP address, the
new IP address overrides the original one. The IP address can be configured for an
interface using the command ip address <ip-address > { mask | mask-length }
where mask represents the 32 bit subnet mask e.g. 255.255.255.0, and mask-
length represents the alternative mask-length value e.g. 24, both of which can be
used interchangeably.
 The loopback interface represents a logical interface that is applied to represent a
network or IP host address, and is often used as a form of management interface
in support of a number of protocols through which communication is made to the
IP address of the loopback interface, as opposed to the IP address of the physical
interface on which data is being received.

 The console interface is capable of supporting only a single user at any given time;
this is represented by the console 0 user interface view.
 The loopback interface represents a logical interface that is not present in a router
until it is created. Once created, the loopback interface is considered up. On ARG3
devices, the loopback interfaces can however be shut down.

 The file system manages files and directories on the storage devices. It can create,
delete, modify, or rename a file or directory, or display the contents of a file.
 The file system has two functions: managing storage devices and managing the
files that are stored on those devices. A number of directories are defined within
which files are stored in a logical hierarchy. These files and directories can be
managed through a number of functions which allow the changing or displaying
of directories, displaying files within such directories or sub-directories, and the
creation or deletion of directories.
 Common examples of file system commands for general navigation include the cd
command used to change the current directory, pwd to view the current directory
and dir to display the contents of a directory as shown in the example. Access to
the file system is achieved from the User View.

 Making changes to the existing file system directories generally relates to the
capability to create and delete existing directories within the file system. Two
common commands that are used in this case. The mkdir directory command is
used to create a folder in a specified directory on a designated storage device,
where directory refers to the name given to the directory and for which the
directory name can be a string of 1 to 64 characters. In order to delete a folder
within the file system, the rmdir directory command is used, with directory again
referring to the name of the directory. It should be noted that a directory can only
be deleted if there are no files contained within that directory.

 Making changes to the files within a file system includes copying, moving,
renaming, compressing, deleting, undeleting, deleting files in the recycle bin,
running files in batch and configuring prompt modes. Creating a duplicate of an
existing file can be done using the copy source-filename destination-filename
command, where if the destination-filename is the same as that of an existing file
(source-filename), the system will display a message indicating that the existing file
will be replaced. A target file name cannot be the same as that of a startup file,
otherwise the system displays a message indicating that the operation is invalid
and that the file is a startup file.
 The move source-filename destination-filename command can be used to move
files to another directory. After the move command has been successfully
executed, the original file is cut and moved to the defined destination file. It
should be noted however that the move command can only move files in the same
storage device.

 For the removal of files within a file system, the delete function can be applied
using the command delete [ /unreserved ] [ /force ] { filename | device-name }.
Generally files that are deleted are directed to a recycle bin from where files can
recovered using the undelete { filename | device-name } command, however
should the /unreserved command be used, the file will be permanently deleted.
The system will generally display a message asking for confirmation of file deletion,
however if the /force parameter is included, no prompt will be given. The filename
parameter refers to the file which is to be deleted, while the device-name
parameter defines the storage location.
 Where a file is directed to the recycle bin, it is not permanently deleted and can be
easily recovered. In order to ensure that such files in the recycle bin are deleted
permanently, the reset recycle-bin [ filename ]command can be applied, where the
filename parameter can be used to define a specific file for permanent deletion.

 When powered on, the device retrieves configuration files from a default save path
to initialize itself, which is then stored within the RAM of the device. If
configuration files do not exist in the default save path, the router uses default
initialization parameters.
 The current-configuration file indicates the configurations in effect on the device
when it is actually running. When the configuration is saved, the current
configuration is stored in a saved-configuration file within the storage location of
the device. If the device loaded the current-configuration file based on default
initialization parameters, a saved-configuration file will not exist in the storage
location of the default save path, but will be generated once the current
configuration is saved.

 Using the display current-configuration command, device parameters that take
effect can be queried. If default values of certain parameters are being used, these
parameters are not displayed. The current-configuration command includes a
number of parameters that allow for filtering of the command list during the used
of the display function. The display current-configuration | begin {regular-
expression} is an example of how the current-configuration can be used to display
active parameters that begin with a specific keyword or expression. An alternative
to this command is the display current-configuration | include {regular-expression}
which allows parameters that include a specific keyword or expression within the
current-configuration file.
 The display saved-configuration [ last | time ] shows the output of the stored
configuration file used at startup to generate the current-configuration. Where the
last parameter is used it displays the configuration file used in the current startup.
The configuration file is displayed only when it is configured for the current startup.
The time parameter will display the time when the configuration was last saved.

 Using the save [configuration-file] command will save the current configuration
information to a default storage path. The configuration-file parameter allows the
current configuration information to be saved to a specified file. Running the save
command with the configuration-file parameter does not affect the current startup
configuration file of the system. When configuration-file is the same as the
configuration file stored in the default storage path of the system, the function of
this command is the same as that of the save command.
 The example demonstrates the use of the save command to save the current-
configuration, which by default will be stored to the default vrpcfg.zip file in the
default storage location of the device.

 The currently used save configuration file can be discovered through the use of
the display startup command. In addition the display startup command can be
used to query the name of the current system software file, name of the next
system software file, name of the backup system software file, names of the four
currently used (if used) system software files, and names of the next four system
software files. The four system software files are the aforementioned configuration
file, voice file, patch file, and license file.

 Following discovery of the startup saved-configuration file, it may be necessary to
define a new configuration file to be loaded at the next startup. If a specific
configuration file is not specified, the default configuration file will be loaded at
the next startup.
 The filename extension of the configuration file must be .cfg or .zip, and the file
must be stored in the root directory of a storage device. When the router is
powered on, it reads the configuration file from the flash memory by default to
initialize. The data in this configuration file is the initial configuration. If no
configuration file is saved in the flash memory, the router uses default parameters
to initiate.
 Through the use of the startup saved-configuration [configuration-file] where the
configuration-file parameter is the configuration file to be used at startup, it is
possible to define a new configuration file to initialize at the next system startup.

 When the compare configuration [configuration-file] [current-line-number save-
line-number] command is used, the system performs a line by line comparison of
the saved configuration with the current configuration starting from the first line. If
the current-line-number save-line-number parameters are specified, the system
skips the non-relevant configuration before the compared lines and continues to
find differences between the configuration files.
 The system will then proceed to output the configuration differences between the
saved configuration and the current configuration files. The comparison output
information is restricted to 150 characters by default. If the comparison requires
less than 150 characters, all variations until the end of two files are displayed.

 The reset saved-configuration command is used in order to delete a device startup
configuration file from the storage device. When performed, the system compares
the configuration files used in the current startup and the next startup when
deleting the configuration file from the router.
 If the two configuration files are the same, they are deleted at the same time after
this command is executed. The default configuration file is used when the router is
started next time. If the two configuration files are different, the configuration file
used in the current startup is deleted after this command is executed.
 If no configuration file is configured for the device current startup, the system
displays a message indicating that the configuration file does not exist after this
command is executed. Once the reset saved-configuration command is used, a
prompt will be given to confirm the action, for which the user is expected to
confirm, as shown in the example.

 The storage devices are product dependant, and include flash memory, SD cards,
or USB flash drives. The AR2200E router for example has a built-in flash memory
and a built-in SD card (in slot sd1). The router provides two reserved USB slots
(usb0 and usb1) and an SD card slot (sd0). For the S5700 it includes a built-in flash
memory with a capacity that varies dependant on the model, with 64MB
supported in the S5700C-HI, S5700-LI, S5700S-LI and S5710-EI models, and 32 MB
for all others. The details regarding the Huawei product storage devices can be
detailed by using the display version command as shown.

 Formatting a storage device is likely to result in the loss of all files on the storage
device, and the files cannot be restored, therefore extra care should be taken when
performing any format command and should be avoided unless absolutely
necessary. The format [storage-device] command is used along with the storage-
device parameter to define the storage location which is required to be formatted.

 When the terminal device displays that the system has failed, the fixdisk command
can be used to attempt to fix the abnormal file system in the storage device,
however it does not provide any guarantee as to whether the file system can be
restored successfully. Since the command is used to rectify problems, if no
problem has occurred in the system it is not recommended that this command be
run. It should also be noted that this command does not rectify device-level
problems.

 The file system attribute d represents that the entry is a directory in the file system.
It should be noted that this directory can only be deleted once any files contained
within the directory have been deleted. The remaining rwx values refer to whether
the directory (or file) can be read, written to, and/or executed.
 A configuration may be saved under a separate name from the default vrpcfg.zip
file name and stored within the storage device of the router or switch. If this file is
required to be used as the active configuration file in the system, the command
startup saved-configuration <configuration-file-name> should be used where the
configuration-file-name refers to the file name and file extension.

 The VRP platform is constantly updated to maintain alignment with changes in
technology and support new advancements to the hardware. The VRP image is
generally defined by a VRP version and a product version number. Huawei ARG3
and Sx7 series products generally align with VRP version 5 to which different
product versions are associated.
 As the product version increases, so do the features that are supported by the
version. The product version format includes a product code Vxxx , Rxxx denotes a
major version release and Cxx a minor version release. If a service pack is used to
patch the VRP product version, an SPC value may also be included in the VRP
product version number. Typical examples of the VRP version upgrades for the
AR2200E include:
 Version 5.90 (AR2200 V200R001C00)
 Version 5.110 (AR2200 V200R002C00)
 Version 5.160 (AR2200 V200R007C00)

 File transfer refers to the means by which files are sent to or retrieved from a
remote server or storage location. Within the IP network this application can be
implemented for a wide range of purposes. As part of effective practice, it is
common for important files be duplicated and backed up within a remote storage
location to prevent any loss that would affect critical systems operations. This
includes files such as the VRP image of products which (should the existing image
suffer loss through use of the format command or other forms of error), can be
retrieved remotely and used to recover system operations. Similar principles apply
for important configuration files and maintaining records of activity within devices
stored in log files, which may be stored long term within the remote server.

 FTP is a standard application protocol based on the TCP/IP protocol suite and used
to transfer files between local clients and remote servers. FTP uses two TCP
connections to copy a file from one system to another. The TCP connections are
usually established in client-server mode, one for control (the server port number
is 21) and the other for data transmission (the sever port number is 20). FTP as a
file transfer protocol is used to control connections by issuing commands from the
client (RTA) to the server and transmits replies from the server to the client,
minimizing the transmission delay. In terms of data transmission, FTP transmits
data between the client and server, maximizing the throughput.
 Trivial File Transfer Protocol (TFTP) is a simple file transfer protocol over which a
router can function as a TFTP client to access files on a TFTP server. Unlike FTP,
TFTP has no complex interactive access interface and authentication control.
Implementation of TFTP is based on the User Datagram Protocol (UDP). The client
initiates the TFTP transfer. To download files, the client sends a read request
packet to the TFTP server, receives packets from the server, and returns an
acknowledgement to the server. To upload files, the client sends a write request
packet to the TFTP server, sends packets to the server, and receives
acknowledgement from the server.

 The example demonstrates how connection between an FTP server and client is
established in order to retrieve a VRP image that can be used as part of the system
upgrade process. Prior to any transfer of data, it is necessary to establish the
underlying connectivity over which files can be transferred. This begins by
providing suitable IP addressing for the client and the server. Where the devices
are directly connected, interfaces can be applied that belong to the same network.
Where devices belong to networks located over a large geographic area, devices
must establish relevant IP addressing within their given networks and be able to
discover a relevant network path over IP via which client/server connectivity can be
established.

 A user must determine for any system upgrade as to whether there is adequate
storage space in which to store the file that is to be retrieved. The file system
commands can be used to determine the current status of the file system,
including which files are currently present within the file storage location of the
device and also the amount of space currently available. Where the storage space
is not adequate for file transfer, certain files can be deleted or uploaded to the FTP
server in the event that they may still be required for future use.
 The example demonstrates the use of the delete file system command to remove
the existing image file. It should be noted that the system image, while deleted will
not impact the current operation of the device as long as the device remains
operational, therefore the device should not be powered off or restarted before a
new VRP image file is restored within the storage location of the device, and set to
be used during the next system startup.

 The retrieving of files from an FTP server requires that a connection be established
firstly before any file transfer can take place. Within the client device, the ftp
service is initiated using the ftp <ip address> where the IP address relates to the
address of the FTP server to which the client wishes to connect. FTP connections
will be established using TCP, and requires authentication in the form of a
username and password which is defined by the FTP server. Once authentication
has been successfully achieved, the client will have established access to the FTP
server and will be able to use a variety of commands to view existing files stored
within the local current directory of the server.
 Prior to file transmission, the user may be required to set the file type for which
two formats exist, ASCII and Binary. ASCII mode is used for text, in which data is
converted from the sender's character representation to "8-bit ASCII" before
transmission, and then to the receiver's character representation. Binary mode on
the other hand requires that the sender send each file byte for byte. This mode is
often used to transfer image files and program files, and should be applied when
sending or retrieving any VRP image file. In the example, the get vrp.cc command
has been issued in order to retrieve the new VRP image located within the remote
server.

 In the event that the client wishes to retrieve a VRP image from a TFTP server, a
connection to the server need not first be established. Instead the client must
define the path to the server within the command line, along with the operation
that is to be performed. It should also be noted that the AR2200E & S5720 models
serve as the TFTP client only and transfer files only in binary format. As can be
seen from the example, the get command is applied for retrieval of the VRP image
file from the TFTP server following the defining of the destination address of the
TFTP server.

 The transfer of the VRP image file to the client once successfully achieved, requires
that the image be enabled as the startup system software during the next system
startup process. In order to change the system software version, the startup
system-software command must be run and include the system software file to be
used in the next startup. A system software file must use .cc as the file name
extension, and the system software file used in the next startup cannot be that
used in the current startup.
 Additionally, the storage directory of a system software file must be the root
directory, otherwise the file will fail to run. The display startup command should be
used to verify that the change to the startup system software has been performed
successfully. The output for the startup system software should show the existing
VRP image, while the next startup system software should display the transferred
VRP image that is now present within the root directory of the device.

 Confirmation of the startup system software allows for the safe initiation of the
system software during the next system boot. In order to apply the changes and
allow for the new system software to take effect, the device must be restarted. The
reboot command can be used in order to initiate the system restart. During the
reboot process, a prompt will be displayed requesting confirmation regarding
whether the configuration file for the next system startup be saved.
 In some cases, the saved-configuration file may be erased by the user in order to
allow for a fresh configuration to be implemented. Should this have occurred, the
user is expected define a response of ‘no’ at the ‘Continue?’ prompt. If the
user chooses ‘yes’ at this point, the current-configuration will be rewritten to
the saved-configuration file and applied once again during the next startup. If the
user is unaware of the changes for which the save prompt is providing a warning,
it is recommended that the user select ‘no’ or ‘n’ and perform a comparison
of the saved and current configuration to verify the changes. For the reboot
prompt, a response of ‘yes’ or ‘y’ is required to complete the reboot process.

 A client device must have the capability to reach the FTP server over IP, requiring
an IP address be configured on the interface via which the FTP server can be
reached. This will allow a path to be validated to the FTP server at the network
layer if one exists.
 The user can run the configuration command display startup to validate that
current startup system software (VRP) is active, identified by the .cc extension.

 As the enterprise network expands, multiple users need to be established as part
of a multi-access network. The evolution of network technologies has seen a shift
away from shared local networks, to networks which support multiple collision
domains and support the use of 100BaseT forms of media that isolated the
transmission and reception of data over separate wire pairs, thus eliminating the
potential for collisions to occur and allowing higher full duplex transmission rates.
The establishment of a switch brings the capability for increased port density to
enable the connection of a greater number of end system devices within a single
local area network. Each end system or host within a local area network is required
to be connected as part of the same IP network in order for communication to be
facilitated at the network layer. The IP address however is only relevant to the host
systems since switch devices operate within the scope of the link layer and
therefore rely on MAC addressing for frame forwarding.

 As a link layer device, each switch relies on a MAC based table that provides
association between a destination MAC address and the port interface via which a
frame should be forwarded. This is commonly referred to as the MAC address
table.
 The initiation of a switch begins with the switch having no knowledge of end
systems and how frames received from end systems should be forwarded. It is
necessary that the switch build entries within the MAC address table to determine
the path that each frame received should take in order to reach a given destination,
so as to limit broadcast traffic within the local network. These path entries are
populated in the MAC address table as a result of frames received from end
systems. In the example, Host A has forwarded a frame to Switch A, which
currently has no entries within its MAC address table.

 The frame that is forwarded from Host A contains a broadcast MAC address entry
in the destination address field of the frame header. The source address field
contains the MAC address of the peering device, in this case Host A. This source
MAC address is used by the switch in order to populate the MAC address table, by
associating the MAC entry in the source address field with the switch port interface
upon which the frame was received. The example demonstrates how the MAC
address is associated with the port interface to allow any returning traffic to this
MAC destination to be forwarded directly via the associated interface.

 The general behavior of an ARP request involves the frame being flooded to all
intended destinations primarily due to the MAC broadcast (FF-FF-FF-FF-FF-FF) that
represents the current destination. The switch is therefore responsible for
forwarding this frame out of every port interface with exception to the port
interface on which the frame was received, in an attempt to locate the intended IP
destination as listed within the ARP header for which an ARP reply can be
generated. As demonstrated in the example, individual frames are flooded from
the switch via port interfaces G0/0/2 and G0/0/3 towards hosts B and host C
respectively.

 As a result of the ARP request header, the receiving host is able to determine that
the ARP header is intended for the IP destination of 10.1.1.3, along with the local
source address (MAC) from which the frame originated, and use this information
to generate a unicast reply. The information regarding Host A is associated with
the IP address of 10.1.1.3 and stored within the MAC address table of Host C. In
doing so, the generation of broadcast traffic is minimized, thereby reducing the
number of interrupts to local destinations as well as reduction of the number of
frames propagating the local network.
 Once the frame is received from Host C by Switch A, the switch will populate the
MAC address table with the source MAC address of the frame received, and
associate it with the port interface on which the frame was received. The switch
then uses the MAC address table to perform a lookup, in order to discover the
forwarding interface, based on the destination MAC address of the frame. In this
case the MAC address of the frame refers to Host A, for which an entry now exists
via interface G0/0/1, allowing the frame to be forwarded to the known destination.

 Early Ethernet systems operated based on a 10Mbps half-duplex mode and
applied mechanisms such as CSMA/CD to ensure system stability. The transition to
a twisted pair medium gave rise to the emergence of full-duplex Ethernet, which
greatly improved Ethernet performance and meant two forms of duplex could be
negotiated. The auto-negotiation technology allows newer Ethernet systems to be
compatible with earlier Ethernet systems.
 In auto-negotiation mode, interfaces on both ends of a link negotiate their
operating parameters, including the duplex mode, rate, and flow control. If the
negotiation succeeds, the two interfaces work with the same operating parameters.
In some cases however it is necessary to manually define the negotiation
parameters, such as where Gigabit Ethernet interfaces that are working in auto-
negotiation mode are connected via a 100 Mbps network cable. In such cases,
negotiation between the interfaces will fail.
 Due to different product models, HUAWEI switches may not support the change
port duplex mode, see the product manual.

 In the event that the configuration parameters for negotiation are changed from
using auto negotiation, the defined parameters should be checked using the
display interface <interface> command to verify that the negotiated parameters
allow for the link layer interface negotiation to be successful. This is verified by the
line protocol current state being displayed as UP. The displayed information
reflects the current parameter settings for an interface.

 When a host or other end system is connected to a switch port interface, a
gratuitous ARP is generated that is designed to ensure that IP addresses remain
unique within a network segment. The gratuitous ARP message however also
provides the switch with information regarding the MAC address of the host,
which is then included in the MAC address table and associated with the port
interface on which the host is connected.
 If the physical connection of a host connected to a switch port interface is
removed, the switch will discover the physical link is down and remove the MAC
entry from the MAC address table. Once the medium is connected to another port
interface, the port will detect that the physical link is active and a gratuitous ARP
will be generated by the host, allowing the switch to discover and populate the
MAC address table with the MAC address of the connected host.

 Enterprise growth results in the commissioning of multiple switches in order to
support the interconnectivity of end systems and services required for daily
operations. The interconnection of multiple switches however brings additional
challenges that need to be addressed. Switches may be established as single
point-to-point links via which end systems are able to forward frames to
destinations located via other switches within the broadcast domain. The failure
however of any point-to-point switch link results in the immediate isolation of the
downstream switch and all end systems to which the link is connected. In order to
resolve this issue, redundancy is highly recommended within any switching
network.
 Redundant links are therefore generally used on an Ethernet switching network to
provide link backup and enhance network reliability. The use of redundant links,
however, may produce loops that cause the communication quality to drastically
deteriorate, and major interruptions to the communication service to occur.

 One of the initial effects of redundant switching loops comes in the form of
broadcast storms. This occurs when an end system attempts to discover a
destination for which neither itself nor the switches along the switching path are
aware of. A broadcast is therefore generated by the end system which is flooded
by the receiving switch.
 The flooding effect means that the frame is forwarded via all interfaces with
exception to the interface on which the frame was received. In the example, Host A
generates a frame, which is received by Switch B which is subsequently forwarded
out of all other interfaces. An instance of the frame is received by the connected
switches A and C, which in turn flood the frame out of all other interfaces. The
continued flooding effect results in both Switch A and Switch C flooding instances
of the frame from one switch to the other, which in turn is flooded back to Switch
B, and thus the cycle continues. In addition, the repeated flooding effect results in
multiple instances of the frame being received by end stations, effectively causing
interrupts and extreme switch performance degradation.

 Switches must maintain records of the path via which a destination is reachable.
This is identified through association of the source MAC address of a frame with
the interface on which the frame was received. Only one instance of a MAC
address can be stored within the MAC address table of a switch, and where a
second instance of the MAC address is received, the more recent information takes
precedence.
 In the example, Switch B updates the MAC address table with the MAC address of
Host A and associates this source with interface G0/0/3, the port interface on
which the frame was received. As frames are uncontrollably flooded within the
switching network, a frame is again received with the same source MAC address as
Host A, however this time the frame is received on interface G0/0/2. Switch B must
therefore assume that the host that was originally reachable via interface G0/0/3 is
now reachable via G0/0/2, and will update the MAC address table accordingly. The
result of this process leads to MAC instability and continues to occur endlessly
between both the switch port interfaces connecting to Switch A and Switch C since
frames are flooded in both directions as part of the broadcast storm effect.

 The challenge for the switching network lies in the ability to maintain switching
redundancy to avoid isolation of end systems in the event of switch system or link
failure, and the capability to avoid the damaging effects of switching loops within
a switching topology which implements redundancy. The resulting solution for
many years has been to implement the spanning tree protocol (STP) in order to
prevent the effects of switching loops. Spanning tree works on the principle that
redundant links be logically disabled to provide a loop free topology, whilst being
able to dynamically enable secondary links in the event that a failure along the
primary switching path occurs, thereby fulfilling the requirement for network
redundancy within a loop free topology. The switching devices running STP
discover loops on the network by exchanging information with one another, and
block certain interfaces to cut off loops. STP has continued to be an important
protocol for the LAN for over 20 years.

 The removal of any potential for loops serves as the primary goal of spanning tree
for which an inverted tree type architecture is formed. At the base of this logical
tree is the root bridge/switch. The root bridge represents the logical center but not
necessarily the physical center of the STP-capable network. The designated root
bridge is capable of changing dynamically with the network topology, as in the
event where the existing root bridge fails to continue to operate as the root bridge.
Non-root bridges are considered to be downstream from the root bridge and
communication to non-root bridges flows from the root bridge towards all non-
root bridges. Only a single root bridge can exist in a converged STP-capable
network at any one time.

 Discovery of the root bridge for an STP network is a primary task performed in
order to form the spanning tree. The STP protocol operates on the basis of
election, through which the role of all switches is determined. A bridge ID is
defined as the means by which the root bridge is discovered. This comprises of
two parts, the first being a 16 bit bridge priority and the second, a 48 bit MAC
address.
 The device that is said to contain the highest priority (smallest bridge ID) is elected
as the root bridge for the network. The bridge ID comparison takes into account
initially the bridge priority, and where this priority value is unable to uniquely
identify a root bridge, the MAC address is used as a tie breaker. The bridge ID can
be manipulated through alteration to the bridge priority as a means of enabling a
given switch to be elected as the root bridge, often in support of an optimized
network design.

 The spanning tree topology relies on the communication of specific information to
determine the role and status of each switch in the network. A Bridge Protocol
Data Unit (BPDU) facilitates communication within a spanning tree network. Two
forms of BPDU are used within STP. A Configuration BPDU is initially created by
the root and propagated downstream to ensure all non-root bridges remain aware
of the status of the spanning tree topology and importantly, the root bridge. The
TCN BPDU is a second form of BPDU, which propagates information in the
upstream direction towards the root and shall be introduced in more detail as part
of the topology change process.
 Bridge Protocol Data Units are not directly forwarded by switches, instead the
information that is carried within a BPDU is often used to generate a switches own
BPDU for transmission. A Configuration BPDU carries a number of parameters that
are used by a bridge to determine primarily the presence of a root bridge and
ensure that the root bridge remains the bridge with the highest priority. Each LAN
segment is considered to have a designated switch that is responsible for the
propagation of BPDU downstream to non-designated switches.
 The Bridge ID field is used to determine the current designated switch from which
BPDU are expected to be received. The BPDU is generated and forwarded by the
root bridge based on a Hello timer, which is set to 2 seconds by default. As BPDU
are received by downstream switches, a new BPDU is generated with locally
defined parameters and forwarded to all non-designated switches for the LAN
segment.

 Another feature of the BPDU is the propagation of two parameters relating to path
cost. The root path cost (RPC) is used to measure the cost of the path to the root
bridge in order to determine the spanning tree shortest path, and thereby
generate a loop free topology. When the bridge is the root bridge, the root path
cost is 0.
 The path cost (PC) is a value associated with the root port, which is the port on a
downstream switch that connects to the LAN segment, on which a designated
switch or root bridge resides. This value is used to generate the root path cost for
the switch, by adding the path cost to the RPC value that is received from the
designated switch in a LAN segment, to define a new root path cost value. This
new root path cost value is carried in the BPDU of the designated switch and is
used to represent the path cost to the root.

 Huawei Sx7 series switches support a number of alternative path cost standards
that can be implemented based on enterprise requirements, such as where a
multi-vendor switching network may exist. The Huawei Sx7 series of switches use
the 802.1t path cost standard by default, providing a stronger metric accuracy for
path cost calculation.

 A converged spanning tree network defines that each interface be assigned a
specific port role. Port roles are used to define the behavior of port interfaces that
participate within an active spanning tree topology. For the spanning tree protocol,
three port roles of designated, root and alternate are defined.
 The designated port is associated with a root bridge or a designated bridge of a
LAN segment and defines the downstream path via which Configuration BPDU are
forwarded. The root bridge is responsible for the generation of configuration
BPDU to all downstream switches, and thus root bridge port interfaces always
adopt the designated port role.
 The root port identifies the port that offers the lowest cost path to the root, based
on the root path cost. The example demonstrates the case where two possible
paths exist back to the root, however only the port that offers the lowest root path
cost is assigned as the root port. Where two or more ports offer equal root path
costs, the decision of which port interface will be the root port is determined by
comparing the bridge ID in the configuration BPDU that is received on each port.
 Any port that is not assigned a designated or root port role is considered an
alternate port, and is able to receive BPDU from the designated switch for the LAN
segment for the purpose of monitoring the status of the redundant link, but will
not process the received BPDU. The IEEE 802.1D-1990 standard for STP originally
defined this port role as backup, however this was amended to become the
alternate port role within the IEEE 802.1D-1998 standards revision.

 The port ID represents a final means for determining port roles alongside the
bridge ID and root path cost mechanism. In scenarios where two or more ports
offer a root path cost back to the root that is equal and for which the upstream
switch is considered to have a bridge ID that is equal, primarily due to the
upstream switch being the same switch for both paths, the port ID must be
applied to determine the port roles.
 The port ID is tied to each port and comprises of a port priority and a port number
that associates with the port interface. The port priority is a value in the range of 0
to 240, assigned in increments of 16, and represented by a value of 128 by default.
Where both port interfaces offer an equal port priority value, the unique port
number is used to determine the port roles. The highest port identifier (the lowest
port number) represents the port assigned as the root port, with the remaining
port defaulting to an alternate port role.

 The root bridge is responsible for the generation of configuration BPDU based on
a BPDU interval that is defined by a Hello timer. This Hello timer by default
represents a period of 2 seconds. A converged spanning tree network must ensure
that in the event of a failure within the network, which switches within the STP
enabled network are made aware of the failure. A Max Age timer is associated with
each BDPU and represents life span of a BPDU from the point of conception by the
root bridge, and ultimately controls the validity period of a BDPU before it is
considered obsolete. This MAX Age timer by default represents a period of 20
seconds.
 Once a configuration BPDU is received from the root bridge, the downstream
switch is considered to take approximately 1 second to generate a new BPDU, and
propagate the generated BPDU downstream. In order to compensate for this time,
a message age (MSG Age) value is applied to each BPDU to represent the offset
between the MAX Age and the propagation delay, and for each switch this
message age value is incremented by 1.
 As BPDU are propagated from the root bridge to the downstream switches the
MAX Age timer is refreshed. The MAX Age timer counts down and expires when
the MAX Age value exceeds the value of the message age, to ensure that the
lifetime of a BPDU is limited to the MAX Age, as defined by the root bridge. In the
event that a BPDU is not received before the MAX Age timer expires, the switch
will consider the BPDU information currently held as obsolete and assume an STP
network failure has occurred.

 The spanning tree convergence process is an automated procedure that initiates at
the point of switch startup. All switches at startup assume the role of root bridge
within the switching network. The default behavior of a root bridge is to assign a
designated port role to all port interfaces to enable the forwarding of BPDU via all
connected port interfaces. As BPDU are received by peering switches, the bridge ID
will be compared to determine whether a better candidate for the role of root
bridge exists. In the event that the received BPDU contains an inferior bridge ID
with respect to the root ID, the receiving switch will continue to advertise its own
configuration BPDU to the neighboring switch.
 Where the BDPU is superior, the switch will acknowledge the presence of a better
candidate for the role of root bridge, by ceasing to propagate BPDU in the
direction from which the superior BPDU was received. The switch will also amend
the root ID field of its BPDU to advertise the bridge ID of the root bridge
candidate as the current new root bridge.

 An elected root bridge, once established will generate configuration BPDU to all
other non-root switches. The BPDU will carry a root path cost that will inform
downstream switches of the cost to the root, to allow for the shortest path to be
determined. The root path cost carried in the BPDU that is generated by the root
bridge always has a value of 0. The receiving downstream switches will then add
this cost to the path cost of the port interfaces on which the BPDU was received,
and from which a switch is able to identify the root port.
 In the case where equal root path costs exist on two or more LAN segments to the
same upstream switch, the port ID is used to discover the port roles. Where an
equal root path cost exists between two switches as in the given example, the
bridge ID is used to determine which switch represents the designated switch for
the LAN segment. Where the switch port is neither a root port nor designated port,
the port role is assigned as alternate.

 As part of the root bridge and port role establishment, each switch will progress
through a number of port state transitions. Any port that is administratively
disabled will be considered to be in the disabled state. Enabling of a port in the
disabled state will see a state transition to the blocking state ①.
 Any port considered to be in a blocking state is unable to forward any user traffic,
but is capable of receiving BPDU frames. Any BPDU received on a port interface in
the blocking state will not be used to populate the MAC address table of the
switch, but instead to determine whether a transition to the listening state is
necessary. The listening state enables communication of BPDU information,
following negotiation of the port role in STP ②, but maintains restriction on the
populating of the MAC address table with neighbor information.
 A transition to the blocking state from the listening or other states ③ may occur in
the event that the port is changed to an alternate port role. The transition between
listening to learning and learning to forwarding states ④ is greatly dependant on
the forward delay timer, which exists to ensure that any propagation of BDPU
information to all switches in the spanning tree topology is achievable before the
state transition occurs.
 The learning state maintains the restriction of user traffic forwarding to ensure
prevention of any switching loops however allows for the population of the MAC
address table throughout the spanning tree topology to ensure a stable switching
network. Following a forward delay period, the forwarding state is reached. The
disabled state is applicable at any time during the state transition period through
manual intervention (i.e. the shutdown command) ⑤.

 Events that cause a change in the established spanning tree topology may occur in
a variety of ways, for which the spanning tree protocol must react to quickly re-
establish a stable and loop free topology. The failure of the root bridge is a
primary example of where re-convergence is necessary. Non-root switches rely on
the intermittent pulse of BPDU from the root bridge to maintain their individual
roles as non-root switches in the STP topology. In the event that the root bridge
fails, the downstream switches will fail to receive a BPDU from the root bridge and
as such will also cease to propagate any BPDU downstream. The MAX Age timer is
typically reset to the set value (20 seconds by default) following the receipt of each
BPDU downstream.
 With the loss of any BPDU however, the MAX Age timer begins to count down the
lifetime for the current BPDU information of each non-root switch, based on the
(MAX Age – MSG Age) formula. At the point at which the MSG Age value is greater
than the MAX Age timer value, the BPDU information received from the root
becomes invalid, and the non-root switches begin to assume the role of root
bridge. Configuration BPDU are again forwarded out of all active interfaces in a bid
to discover a new root bridge. The failure of the root bridge invokes a recovery
duration of approximately 50 seconds due to the Max Age + 2x Forward Delay
convergence period.

 In the case of an indirect link failure, a switch loses connection with the root bridge
due to a failure of the port or media, or due possibly to manual disabling of the
interface acting as the root port. The switch itself will become immediately aware
of the failure, and since it only receives BPDU from the root in one direction, will
assume immediate loss of the root bridge, and assert its position as the new root
bridge.
 From the example, switch B begins to forward BPDU to switch C to notify of the
position of switch B as the new root bridge, however switch C continues to receive
BPDU from the original root bridge and therefore ignores any BPDU from switch B.
The alternate port will begin to age its state through the MAX Age timer, since the
interface no longer receives BPDU containing the root ID of the root bridge.
 Following the expiry of the MAX Age timer, switch C will change the port role of
the alternate port to that of a designated port and proceed to forward BPDU from
the root towards switch B, which will cause the switch to concede its assertion as
the root bridge and converge its port interface to the role of root port. This
represents a partial topology failure however due to the need to wait for a period
equivalent to MAX Age + 2x forward delay, full recovery of the STP topology
requires approximately 50 seconds.

 A final scenario involving spanning tree convergence recovery occurs where
multiple LAN segments are connected between two switch devices for which one is
currently the active link while the other provides an alternate path to the root.
Should an event occur that causes the switch that is receiving the BPDU to detect a
loss of connection on its root port, such as in the event that a root port failure
occurs, or a link failure occurs, for which the downstream switch is made
immediately aware, the switch can instantly transition the alternate port.
 This will begin the transition through the listening, learning and forwarding states
and achieve recovery within a 2x forward delay period. In the event of any failure,
where the link that provides a better path is reactivated, the spanning tree
topology must again re-converge in order to apply the optimal spanning tree
topology.

 In a converged spanning tree network, switches maintain filter databases, or MAC
address tables to manage the propagation of frames through the spanning tree
topology. The entries that provide an association between a MAC destination and
the forwarding port interface are stored for a finite period of 300 seconds (5
minutes) by default. A change in the spanning tree topology however means that
any existing MAC address table entries are likely to become invalid due to the
alteration in the switching path, and therefore must be renewed.
 The example demonstrates an existing spanning tree topology for which switch B
has entries that allow Host A to be reached via interface Gigabit Ethernet 0/0/3
and Host B via interface Gigabit Ethernet 0/0/2. A failure is simulated on switch C
for which the current root port has become inactive. This failure causes a
recalculation of the spanning tree topology to begin and predictably the activation
of the redundant link between switch C and switch B.
 Following the re-convergence however, it is found that frames from Host A to Host
B are failing to reach their destination. Since the MAC address table entries have
yet to expire based on the 300 second rule, frames reaching switch B that are
destined for Host B continue to be forwarded via port interface Gigabit Ethernet
0/0/2, and effectively become black holed as frames are forwarded towards the
inactive port interface of switch C.

 An additional mechanism must be introduced to handle the MAC entries timeout
period issue that results in invalid path entries being maintained following
spanning tree convergence. The process implemented is referred to as the
Topology Change Notification (TCN) process, and introduces a new form of BPDU
to the spanning tree protocol operation.
 This new BPDU is referred to as the TCN BPDU and is distinguished from the
original STP configuration BPDU through the setting of the BPDU type value to
128 (0x80). The function of the TCN BPDU is to inform the upstream root bridge of
any change in the current topology, thereby allowing the root to send a
notification within the configuration BPDU to all downstream switches, to reduce
the timeout period for MAC address table entries to the equivalent of the forward
delay timer, or 15 seconds by default.
 The flags field of the configuration BPDU contains two fields for Topology Change
(TC) and Topology Change Acknowledgement (TCA). Upon receiving a TCN BPDU,
the root bridge will generate a BPDU with both the TC and TCA bits set, to
respectively notify of the topology change and to inform the downstream switches
that the root bridge has received the TCN BPDU, and therefore transmission of the
TCN BPDU should cease.
 The TCA bit shall remain active for a period equal to the Hello timer (2 seconds),
following which configuration BPDU generated by the root bridge will maintain
only the TC bit for a duration of (MAX Age + forward delay), or 35 seconds by
default.

 The effect of the TCN BPDU on the topology change process ensures that the root
bridge is notified of any failure within the spanning tree topology, for which the
root bridge is able to generate the necessary flags to flush the current MAC
address table entries in each of the switches. The example demonstrates the
results of the topology change process and the impact on the MAC address table.
The entries pertaining to switch B have been flushed, and new updated entries
have been discovered for which it is determined that Host B is now reachable via
port interface Gigabit Ethernet 0/0/1.

 Huawei Sx7 series switches to which the S5700 series model belongs, is capable of
supporting three forms of spanning tree protocol. Using the stp mode command,
a user is able to define the mode of STP that should be applied to an individual
switch. The default STP mode for Sx7 series switches is MSTP, and therefore must
be reconfigured before STP can be used.

 As part of good switch design practice, it is recommended that the root bridge be
manually defined. The positioning of the root bridge ensures that the optimal path
flow of traffic within the enterprise network can be achieved through configuration
of the bridge priority value for the spanning tree protocol. The stp priority [priority]
command can be used to define the priority value, where priority refers to an
integer value between 0 and 61440, assigned in increments of 4096. This allows for
a total of 16 increments, with a default value of 32768. It is also possible to assign
the root bridge for the spanning tree through the stp root primary command.

 It has been understood that Huawei Sx7 series of switches support three forms of
path cost standard in order to provide compatibility where required, however
defaults to support the 802.1t path cost standard. The path cost standard can be
adjusted for a given switch using the stp pathcost-standard {dot1d-1998 | dot1t |
legacy } command, where dot1d-1998, dot1t and legacy refer to the path cost
standards described earlier in this section.
 In addition, the path cost of each interface can also be assigned manually to
support a means of detailed manipulation of the stp path cost. This method of
path cost manipulation should be used with great care however as the path cost
standards are designed to implement the optimal spanning tree topology for a
given switching network and manipulation of the stp cost may result in the
formation of a sub-optimal spanning tree topology.
 The command stp cost [cost] is used, for which the cost value should follow the
range defined by the path cost standard. If a Huawei legacy standard is used, the
path cost ranges from 1 to 200000. If the IEEE 802.1D standard is used, the path
cost ranges from 1 to 65535. If the IEEE 802.1t standard is used, the path cost
ranges from 1 to 200000000.

 If the root switch on a network is incorrectly configured or attacked, it may receive
a BPDU with a higher priority and thus the root switch becomes a non-root switch,
which causes a change of the network topology. As a result, traffic may be
switched from high-speed links to low-speed links, causing network congestion.
 To address this problem, the switch provides the root protection function. The root
protection function protects the role of the root switch by retaining the role of the
designated port. When the port receives a BPDU with a higher priority, the port
stops forwarding packets and turns to the listening state, but it still retains a
designated port role. If the port does not receive any BPDU with a higher priority
for a certain period, the port status is restored from the listening state.
 The configured root protection is valid only when the port is the designated port
and the port maintains the role. If a port is configured as an edge port, or if a
command known as loop protection is enabled on the port, root protection cannot
be enabled on the port.

 Using the display stp command, the current STP configuration can be determined.
A number of timers exist for managing the spanning tree convergence, including
the hello timer, max age timer, and forward delay, for which the values displayed
represent the default timer settings, and are recommended to be maintained.
 The current bridge ID can be identified for a given switch through the CIST Bridge
configuration, comprised of the bridge ID and MAC address of the switch.
Statistics provide information regarding whether the switch has experienced
topology changes, primarily through the TC or TCN received value along with the
last occurrence as shown in the time since last TC entry.

 For individual interfaces on a switch it is possible to display this information via the
display stp command to list all interfaces, or using the display stp interface
<interface> command to define a specific interface. The state of the interface
follows MSTP port states and therefore will display as either Discarding, Learning
or Forwarding. Other valid information such as the port role and cost for the port
are also displayed, along with any protection mechanisms applied.

 Following the failure of the root bridge for a spanning tree network, the next best
candidate will be elected as the root bridge. In the event that the original root
bridge becomes active once again in the network, the process of election for the
position of root bridge will occur once again. This effectively causes network
downtime in the switching network as convergence proceeds.
 The Root Path Cost is the cost associated with the path back to the root bridge,
whereas the Path Cost refers to the cost value defined for an interface on a switch,
which is added to the Root Path Cost, to define the Root Path Cost for the
downstream switch.

 STP ensures a loop-free network but has a slow network topology convergence
speed, leading to service deterioration. If the network topology changes frequently,
the connections on the STP capable network are frequently torn down, causing
regular service interruption.
 RSTP employs a proposal and agreement process which allows for immediate
negotiation of links to take place, effectively removing the time taken for
convergence based timers to expire before spanning tree convergence can occur.
The proposal and agreement process tends to follow a cascading effect from the
point of the root bridge through the switching network, as each downstream
switch begins to learn of the true root bridge and the path via which the root
bridge can be reached.

 Switches operating in RSTP mode implement two separate port roles for
redundancy. The alternate port represents a redundant path to the root bridge in
the event that the current path to the root bridge fails. The backup port role
represents a backup for the path for the LAN segment in the direction leading
away from the root bridge. It can be understood that a backup port represents a
method for providing redundancy to the designated port role in a similar way that
an alternate port provides a method of redundancy to the root port.
 The backup port role is capable of existing where a switch has two or more
connections to a shared media device such as that of a hub, or where a single
point-to-point link is used to generate a physical loopback connection between
ports on the same switch. In both instances however the principle of a backup port
existing where two or more ports on a single switch connect to a single LAN
segment still applies.

 In RSTP, a designated port on the network edge is called an edge port. An edge
port directly connects to a terminal and does not connect to any other switching
devices. An edge port does not receive configuration BPDU, so it does not
participate in the RSTP calculation.
 It can directly change from the Disabled state to the Forwarding state without any
delay, just like an STP-incapable port. If an edge port receives bogus configuration
BPDU from attackers, it is deprived of the edge port attributes and becomes a
common STP port. The STP calculation is implemented again, causing network
flapping.

 RSTP introduces a change in port states that are simplified from five to three types.
These port types are based on whether a port forwards user traffic and learns MAC
addresses. If a port neither forwards user traffic nor learns MAC addresses, the
port is in the Discarding state. The port is considered to be in a learning state
where a port does not forward user traffic but learns MAC addresses. Finally where
a port forwards user traffic and learns MAC addresses, the port is said to be in the
Forwarding state.

 The BPDU format employed in STP is also applied to RSTP with variance in some of
the general parameters. In order to distinguish STP configuration BPDU from
Rapid Spanning Tree BPDU, thus known as RST BPDU, the BPDU type is defined.
STP defines a configuration BPDU type of 0 (0x00) and a Topology Change
Notification BPDU (TCN BPDU) of 128 (0x80), RST BPDU are identified by the BPDU
type value 2 (0x02). Within the flags field of the RST BPDU, additional parameter
designations are assigned to the BPDU fields.
 The flags field within STP implemented only the use of the Topology Change (TC)
and Acknowledgement (TCA) parameters as part of the Topology Change process
while other fields were reserved. The RST BPDU has adopted these fields to
support new parameters. These include flags indicating the proposal and
agreement process employed by RSTP for rapid convergence, the defining of the
port role, and the port state.

 In STP, after the topology becomes stable, the root bridge sends configuration
BPDU at an interval set by the Hello timer. A non-root bridge does not send
configuration BPDU until it receives configuration BPDU sent from the upstream
device. This renders the STP calculation complicated and time-consuming. In RSTP,
after the topology becomes stable, a non-root bridge sends configuration BPDU at
Hello intervals, regardless of whether it has received the configuration BPDU sent
from the root bridge; such operations are implemented on each device
independently.

 The convergence of RSTP follows some of the basic principles of STP in
determining initially that all switches upon initialization assert the role of root
bridge, and as such assign each port interface with a designated port role. The
port state however is set to a discarding state until such time as the peering
switches are able to confirm the state of the link.

 Each switch proclaiming to be the root bridge will negotiate the port states for a
given LAN segment by generating an RST BPDU with the proposal bit set in the
flags field. When a port receives an RST BPDU from the upstream designated
bridge, the port compares the received RST BPDU with its own RST BPDU. If its
own RST BPDU is superior to the received one, the port discards the received RST
BPDU and immediately responds to the peering device with its own RST BPDU that
includes a set proposal bit.

 Since timers do not play a role in much of the RSTP topology convergence process
as found with STP, it is important that the potential for switching loops during port
role negotiation be restricted. This is managed by the implementation of a
synchronization process that determines that following the receipt of a superior
BPDU containing the proposal bit, the receiving switch must set all downstream
designated ports to discarding as part of the sync process.
 Where the downstream port is an alternate port or an edge port however, the
status of the port role remains unchanged. The example demonstrates the
temporary transition of the designated port on the downstream LAN segment to a
discarding state, and therefore blocking any frame forwarding during the
upstream proposal and agreement process.

 The confirmed transition of the downstream designated port to a discarding state
allows for an RST BPDU to be sent in response to the proposal sent by the
upstream switch. During this stage the port role of the interface has been
determined to be the root port and therefore the agreement flag and port role of
root are set in the flags field of the RST BPDU that is returned in response to the
proposal.

 During the final stage of the proposal and agreement process, the RST BPDU
containing the agreement bit is received by the upstream switch, allowing the
designated port to transition immediately from a discarding state to forwarding
state. Following this, the downstream LAN segment(s) will begin to negotiate the
port roles of the interfaces using the same proposal and agreement process.

 In STP, a device has to wait a Max Age period before determining a negotiation
failure. In RSTP, if a port does not receive configuration BPDUs sent from the
upstream device for three consecutive Hello intervals, the communication between
the local device and its peer fails, causing the proposal and agreement process to
be initialized in order to discover the port roles for the LAN segment.

 Topology changes affect RSTP similarly to the way STP is affected, however there
are some minor differences between the two. In the example, a failure of the link
has occurred on switch C. Switch A and switch C will detect the link failure
immediately and flush the address entries for ports connected to that link. An RST
BPDU will begin to negotiate the port states as part of the proposal and
agreement process, following which a Topology Change notification will occur,
together with the forwarding of the RST BPDU containing the agreement.
 This RST BPDU will have both the Agreement bit and also the TC bit set to 1, to
inform upstream switches of the need to flush their MAC entries on all port
interfaces except the port interface on which the RST BPDU containing the set TC
bit was received.
 The TC bit will be set in the periodically sent RST BPDU, and forwarded upstream
for a period equivalent to Hello Time+1 second, during which all relevant
interfaces will be flushed and shall proceed to re-populate MAC entries based on
the new RSTP topology. The red (darker) ‘x’ in the example highlights which
interfaces will be flushed as a result of the topology change.

 The implementation of STP within an RSTP based switching topology is possible,
however is not recommended since any limitation pertaining to STP becomes
apparent within the communication range of the STP enabled switch. A port
involved in the negotiation process for establishing its role within STP must wait
for a period of up to 50 seconds before convergence can be completed, as such
the benefits of RSTP are lost.

 The configuration of the spanning tree mode of Sx7 switches requires that the stp
mode command be used to set the mode to RSTP. In doing so the Sx7 series
switch will generate RST BPDU in relation to RSTP, as opposed to other spanning
tree implementations. This command is configured from the system-view and
should be applied to all switches participating in the rapid spanning tree topology.

 The display stp command will provide relative information regarding RSTP
configuration as many of the parameters follow the principle STP architecture. The
mode information will determine as to whether a switch is currently operating
using RSTP.

 An edge interface defines a port that does not participate in the spanning tree
topology. These interfaces are used by end systems to connect to the switching
network for the purpose of forwarding frames. Since such end systems do not
require to negotiate port interface status, it is preferable that the port be
transitioned directly to a forwarding state to allow frames to be forwarded over
this interface immediately.
 The stp edged-port enable command is used to switch a port to become an edge
port, as all ports are considered non-edge ports on a switch by default. In order to
disable the edge port the stp edged-port disable command is used. These
commands apply only to a single port interface on a given switch. It is important
to note that the edge port behavior is associated with RSTP as defined in the IEEE
802.1D-2004 standards documentation, however due to the VRP specific
application of the underlying RSTP state machine to STP (which also results in the
RSTP port states being present in STP), it is also possible to apply the RSTP edge
port settings to STP within Huawei Sx7 series products.

 In the event that multiple ports on a switch are to be configured as edge ports, the
stp edged-port default command is applied which enforces that all port interfaces
on a switch become edge ports. It is important to run the stp edged-port disable
command on the ports that need to participate in STP calculation between devices,
so as to avoid any possible loops that may be caused as a result of STP topology
calculations.

 The port that is directly connected to a user terminal such as a PC or a file server,
is understood to be configured as an edge port to ensure fast transition of the
port status. Usually, no BPDU are sent to edge ports, however if the switch is
attacked by pseudo BPDU, the switch sets edge ports as non-edge ports. After
these edge ports receive a BPDU the spanning tree topology is recalculated, and
as a result network flapping occurs.
 To defend against pseudo BPDU attacks, RSTP provides BPDU protection. After
BPDU protection is enabled, the switch shuts down the edge port that receives
BPDU and informs any active network management station (NMS). The edge ports
that are shut down by the switch can be manually started only by the network
administrator. The stp bpdu-protection command should be used to enable BPDU
protection and is configured globally within the system-view.

 The switch maintains the status of the root port and blocked ports by continually
receiving BPDU from the upstream switch. If the root switch cannot receive BPDU
from the upstream switch due to link congestion or unidirectional link failure, the
switch re-selects a root port. The previous root port then becomes a designated
port and the blocked ports change to the forwarding state. As a result, loops may
occur on the network.
 The switch provides loop protection to prevent network loops. After the loop
protection function is enabled, the root port is blocked if it cannot receive BPDU
from the upstream switch. The blocked port remains in the blocked state and does
not forward packets. This prevents loops on the network. If an interface is
configured as an edge interface or root protection is enabled on the interface,
loop protection cannot be enabled on the interface. The stp loop-protection
command should be applied to enable this feature in the interface-view.

 Validation of the RSTP configuration for a given interface is attained through the
display stp interface <interface> command. The associated information will
identify the port state of the interface as either Discarding, Learning or Forwarding.
Relevant information for the port interface including the port priority, port cost,
the port status as an edge port or supporting point-to-point etc, are defined.

 The sync is a stage in the convergence process that involves the blocking of
designated ports while RST BPDU are transmitted containing proposal and
agreement messages to converge the switch segment. The process is designed to
ensure that all interfaces are in agreement as to their port roles in order to ensure
that no switching loops will occur once the designated port to any downstream
switch is unblocked.

 An enterprise network generally can be understood as an instance of an
autonomous system. As defined within RFC 1030, an autonomous system or AS, as
it is also commonly known, is a connected group of one or more IP prefixes run by
one or more network operators which has a SINGLE and CLEARLY DEFINED routing
policy.
 The concept of autonomous systems originally considered the existence of a single
routing protocol, however as networks have evolved, it is possible to support
multiple routing protocols that interoperate through the injection of routes from
one protocol to another. A routing policy can be understood to be a set of rules
that determine how traffic is managed within an autonomous system, to which a
single, or multiple operator(s) must adhere to.

 The principles surrounding switching have dealt mainly with the forwarding of
traffic within the scope of a local area network and the gateway, which has until
now defined the boundary of the broadcast domain. Routers are the primary form
of network layer device used to define the gateway of each local area network and
enable IP network segmentation. Routers generally function as a means for routing
packets from one local network to the next, relying on IP addressing to define the
IP network to which packets are destined.

 The router is responsible for determining the forwarding path via which packets
are to be sent the route to a given destination. It is the responsibility of each
router to make decisions as to how the data is forwarded. Where a router has
multiple paths to a given destination, route decisions based on calculations are
made to determine the best next hop to the intended destination. The decisions
governing the route that should be taken can vary depending on the routing
protocol in use, ultimately relying on metrics of each protocol to make decisions in
relation to varying factors such as bandwidth and hop count.

 Routers forward packets based on routing tables and a forwarding information
base (FIB), and maintain at least one routing table and one FIB. Routers select
routes based on routing tables and forward packets based on the FIB. A router
uses a local routing table to store protocol routes and preferred routes. The router
then sends the preferred routes to the FIB to guide packet forwarding. The router
selects routes according to the priorities of protocols and costs stored in the
routing table. A routing table contains key data for each IP packet.
 The destination & mask are used in combination to identify the destination IP
address or the destination network segment where the destination host or router
resides.
 The protocol (Proto) field, indicates the protocol through which routes are learned.
The preference (Pre) specifies the preference value that is associated with the
protocol, and is used to decide which protocol is applied to the routing table
where two protocols offer similar routes. The router selects the route with the
highest preference (the smallest value) as the optimal route.
 A cost value represents the metric that is used to distinguish when multiple routes
to the same destination have the same preference, the route with the lowest cost
is selected as the optimal route.
 A next-hop value indicates the IP address of the next network layer device or
gateway that an IP packet passes through. In the example given a next-hop of
127.0.0.1 refers to the local interface of the device as being the next-hop.
 Finally the interface parameter indicates the outgoing interface through which an
IP packet is forwarded.

 A routing table may contain the routes originating from multiple protocols to a
given destination. Not all routing protocols are considered equal, and where the
longest match for multiple routes of differing routing protocols to the same
destination are equal, a decision must be made regarding which routing protocol
(including static routes) will take precedence.
 Only one routing protocol at any one time determines the optimal route to a
destination. To select the optimal route, each routing protocol (including the static
route) is configured with a preference (the smaller the value, the higher the
preference). When multiple routing information sources coexist, the route with the
highest preference is selected as the optimal route and added to the local routing
table.
 In the example, two protocols are defined that provide a means of discovery of the
10.1.1.0 network via two different paths. The path defined by the RIP protocol
appears to provide a more direct route to the intended destination, however due
to the preference value, the route defined by the OSPF protocol is preferred and
therefore installed in the routing table as the preferred route. A summary of the
default preference values of some common routing mechanisms are provided to
give an understanding of the default preference order.

 Where the route is unable to be distinguished by either a longest match value or
preference, the cost metric is taken as the decision maker in identifying the route
that should be installed in the routing table. Cost represents the length of a path
to a destination network.
 Each segment provides a cost metric value along a path that is combined to
identify the cost of the route. Another common factor is network bandwidth, on
which the cost mechanism is sometimes based. A link with a higher speed
(capacity) represents a lower cost value, allowing preference of one path over
another to be made, whilst links of equal speed are given a balanced cost for
efficient load balancing purposes. A lower metric always takes precedence and
therefore the metric of 50 as shown in the example, defines the optimal route to
the given destination for which an entry can be found in the routing table.

 By using the “preference” and the “cost”, IP Routing-table can be established.
 And the IP Routing-table can be devided into three kinds based on different
source.
 Direct routes
 Static routes
 Dynamic routes

 In order to allow packets to reach their intended destination, routers must make
specific decisions regarding the routes that are learned and which of those routes
are applied. A router is likely to learn about the path to a given network
destination via routing information that is advertised from neighboring routers,
alternatively it is possible for the statically applied routes to be manually
implemented through administrator intervention.
 Each entry in the FIB table contains the physical or logical interface through which
a packet is sent in order to reach the next router. An entry also indicates whether
the packet can be sent directly to a destination host in a directly connected
network. The router performs an "AND" operation on the destination address in
the packet and the network mask of each entry in the FIB table.
 The router then compares the result of the "AND" operation with the entries in the
FIB table to find a match. The router chooses the optimal route to forward packets
according to the best or "longest" match. In the example, two entries to the
network 10.1.1.0 exist with a next-hop of 20.1.1.2. Forwarding to the destination of
10.1.1.1 will result in the longest match principle being applied, for which the
network address 10.1.1.0/30 provides the longest match.

 The capability of a router to forward an IP packet to a given destination requires
that certain forwarding information be known. Any router wishing to forward an IP
packet must firstly be aware of a valid destination address to which the packet is
to be forwarded, this means that an entry must exist in the routing table that the
router is able to consult. This entry must also identify the interface via which IP
packets must be transmitted and the next-hop along the path, to which the packet
is expected to be received before consultation for the next forwarding decision is
performed.

 When router chooses the best route, first, put the routes with the smallest
preference value into IP routing-table; if the priority is equal, then compares metric
values to decide which routes to put into the routing table; finally, when looking
up the routing table, it chooses the route items to guide the data packet
forwarding according to the longest mask matching principle.
 The preference is typically used to denote the reliability of a route over routes that
may be considered less reliable. Vendors of routing equipment may however
assign different preference values for protocols that are supported within each
vendors own product. The preference values of some common routing protocols
supported by Huawei routing devices can be found within this section.

 A static route is a special route that is manually configured by a network
administrator. The disadvantage of static routes is that they cannot adapt to the
change in a network automatically, so network changes require manual
reconfiguration. Static routes are fit for networks with comparatively simple
structures. It is not advisable to configure and maintain static routes for a network
with a complex structure. Static routes do however reduce the effect of bandwidth
and CPU resource consumption that occurs when other protocols are
implemented.

 Static routes can be applied to networks that use both serial and Ethernet based
media, however in each situation the conditions of applying the static route vary in
which either the outbound interface or the next-hop IP address must be defined.
 The serial medium represents a form of point-to-point (P2P) interface for which
the outbound interface must be configured. For a P2P interface, the next-hop
address is specified after the outbound interface is specified. That is, the address
of the remote interface (interface on the peer device) connected to this interface is
the next-hop address.
 For example, the protocol used to encapsulate over the serial medium is the Point-
to-Point protocol (PPP). The remote IP address is obtained following PPP
negotiation, therefore it is necessary to specify only the outbound interface. The
example also defines a form of point-to-point Ethernet connection, however
Ethernet represents a broadcast technology in nature and therefore the principles
of point-to-point technology do not apply.

 In the case of broadcast interfaces such as Ethernet, the next-hop must be defined.
Where the Ethernet interface is specified as the outbound interface, multiple next
hops are likely to exist and the system will not be able to decide which next-hop is
to be used. In determining the next-hop, a router is able to identify the local
connection over which the packet should be received.
 In the example, packets intended for the destination of 192.168.2.0/24 should be
forwarded to the next-hop of 10.0.123.2 to ensure delivery. Alternatively reaching
the destination of 192.168.3.0 requires that the next-hop of 10.0.123.3 be defined.

 The configuration of the static route is achieved using the ip route-static ip-
address { mask | mask-length } interface-type interface-number [ nexthop-address ]
where the ip-address refers to the network or host destination address. The mask
field can be defined as either a mask value or based on the prefix number. In the
case of a broadcast medium such as Ethernet, the next-hop address is used. Where
a serial medium is used, the interface-type and interface-number are assigned (e.g.
serial 1/0/0) to the command to define the outgoing interface.

 Where equal cost paths exist between the source and destination networks, load
balancing can be implemented to allow traffic to be carried over both links. In
order to achieve this using static routes, both routes must meet the parameters for
an equal longest match, preference and metric value. The configuration of multiple
static routes, one for each next-hop or outbound interface in the case of serial
medium is required.
 The example demonstrates how two ip route-static commands are implemented,
each defining the same IP destination address and mask, but alternate next-hop
locations. This ensures that the longest match (/24) is equal, and naturally so is the
preference value, since both routes are static routes that carry a default preference
of 60. The cost of both paths is also equal allowing load balancing to occur.

 The routing table can be queried to verify the results by running the display ip
routing-table command after the static routes are configured. The static route is
displayed in the routing table, and results show two entries to the same
destination, with matching preference and metric values. The different next-hop
addresses and variation in the outbound interface identifies the two paths that are
taken, and confirms that load balancing has been achieved.

 The application of static routes allows for a number of ways that routes can be
manipulated to achieve routing requirements. It is possible for the preference of a
static route to be changed for the purpose of enabling the preference of one static
route over another, or where used with other protocols, to ensure the static route
is either preferred or preference is given to the alternative routing protocol.
 The default preference value of a static route is 60, therefore by adjusting this
preference value, a given static route can be treated with unequal preference over
any other route, including other static routes. In the example given, two static
routes exist over two physical LAN segments, while normally both static routes
would be considered equal, the second route has been given a lesser preference
(higher value) causing it to be removed from the routing table. The principle of a
floating static route means that the route with a lesser preference will be applied
to the routing table, should the primary route ever fail.

 In using the display ip routing-table command, it is possible for the results of the
change to the preference value that results in the floating static route, to be
observed. Normally two equal cost routes would be displayed in the routing table
defining the same destination, however having alternative next-hop values and
outbound interfaces. In this case however, only one instance can be seen,
containing the default static route preference value of 60. Since the second static
route now has a preference value of 100, it is not immediately included in the
routing table since it is no longer considered an optimal route.

 In the event that the primary static route should fail as a result of physical link
failure or through the disabling of an interface, the static route will no longer be
able to provide a route to the intended destination and therefore will be removed
from the routing table. The floating static route is likely to become the next best
option for reaching the intended destination, and will be added to the routing
table to allow packets to be transmitted over a second alternative path to the
intended destination, allowing continuity in light of any failure.
 When the physical connection for the original route is restored, the original static
route also will take over from the current floating static route, for which the route
will be restored in the routing table causing the floating static route to once again
await application.

 The default static route is a special form of static route that is applied to networks
in which the destination address is unknown, in order to allow a forwarding path
to be made available. This provides an effective means of routing traffic for an
unknown destination to a router or gateway that may have knowledge of the
forwarding path within an enterprise network.
 The default route relies on the “any network” address of 0.0.0.0 to match any
network to which a match could not be found in the routing table, and provides a
default forwarding path to which packets for all unknown network destinations
should be routed. In the example, a default static route has been implemented on
RTA, identifying that should packets for a network that is unknown be received,
such packets should be forwarded to the destination 10.0.12.2.
 In terms of routing table decision making, as a static route, the default route
maintains a preference of 60 by default, however operates as a last resort in terms
of the longest match rule in the route matching process.

 The configuration of the static route once configured will appear within the
routing table of the router. The display ip routing-table command is used to view
this detail. As a result, all routes in the example where not associated with any
other routes in the routing table will be forwarded to the next-hop destination of
10.0.12.2 via the interface Gigabit Ethernet 0/0/0.

 A floating static route can be implemented by adjusting the preference value of a
static route where two static routes support load balancing.
 A default static route can be implemented in the routing table by specifying the
‘any network’ address of 0.0.0.0 as the destination address along with a next-
hop address of the interface to which packets captured by this default static route
are to be forwarded.

 Open Shortest Path First or OSPF is regarded as a link state protocol that is
capable of quickly detecting topological changes within the autonomous system
and establish loop free routes in a short period of time, with minimum additional
communication overhead for negotiating topology changes between peering
routers. OSPF also deals with scalability issues that occur when communication
between an expanding number of routers becomes so extreme that it begins to
lead to instability within the autonomous system. This is managed through the use
of areas that limits the scope of router communication to an isolated group within
the autonomous system allowing small, medium and even large networks to be
supported by OSPF. The protocol is also able to work over other protocols such as
MPLS, a label switching protocol, to provide network scalability even over
geographically disperse locations. In terms of optimal path discovery, OSPF
provides rich route metrics that provides more accuracy than route metrics applied
to protocols such as RIP to ensure that routes are optimized, based on not only
distance but also link speed.

 The convergence of OSPF requires that each and every router actively running the
OSPF protocol have knowledge of the state of all interfaces and adjacencies
(relationship between the routers that they are connected to), in order to establish
the best path to every network. This is initially formed through the flooding of Link
State Advertisements (LSA) which are units of data that contain information such
as known networks and link states for each interface within a routing domain. Each
router will use the LSA received to build a link state database (LSDB) that provides
the foundation for establishing the shortest path tree to each network, the routes
from which are ultimately incorporated into the IP routing table.

 The router ID is a 32-bit value assigned to each router running the OSPF protocol.
This value uniquely identifies the router within an Autonomous System. The router
ID can be assigned manually, or it can be taken from a configured address. If a
logical (loopback) interface has been configured, the router ID will be based upon
the IP address of the highest configured logical interface, should multiple logical
interfaces exist.
 If no logical interfaces have been configured, the router will use the highest IP
address configured on a physical interface. Any router running OSPF can be
restarted using the graceful restart feature to renew the router ID should a new
router ID be configured. It is recommended that the router ID be configured
manually to avoid unexpected changes to the router ID in the event of interface
address changes.

 OSPF supports various network types, and in each case will apply a different
behavior in terms of how neighbor relationships are formed and how
communication is facilitated. Ethernet represents a form of broadcast network that
involves multiple routers connected to the same network segment. One of the
primary issues faced regards how communication occurs between the neighboring
routers in order to minimize OSPF routing overhead. If an Ethernet network is
established, the broadcast network type will be applied automatically in OSPF.

 Where two routers are established in a point-to-point topology, the applied
network type will vary depending on the medium and link layer technology
applied. As mentioned, the use of an Ethernet medium will result in the broadcast
network type for OSPF being assigned automatically. Where the physical medium
is serial, the network type is considered point-to-point. Common forms of
protocols that operate over serial media at the link layer include Point-to-Point
Protocol (PPP) and High-level Data Link Control (HDLC).

 OSPF may operate over multi access networks that do not support broadcasts.
Such networks include Frame Relay and ATM that commonly operate using hub
and spoke type topologies, which rely on the use of virtual circuits in order for
communication be to achieved. OSPF may specify two types of networks that can
be applied to links connected to such environments. The Non-Broadcast Multi
Access (NBMA) network type emulates a broadcast network and therefore requires
each peering interface be part of the same network segment. Unlike a broadcast
network, the NBMA forwards OSPF packets as a unicast, thereby requiring multiple
instances of the same packet be generated for each destination.
 Point-to-Multipoint may also be applied as the network type for each interface, in
which case a point-to-point type behavior is applied. This means that each peering
must be associated with different network segments. Designated Routers are
associated with broadcast networks, and therefore are implemented by NBMA
networks. Most importantly is the positioning of a DR which must be assigned on
the hub node of the hub and spoke architecture to ensure all nodes can
communicate with the DR.

 In order to address and optimize the communication of OSPF over broadcast
networks, OSPF implements a Designated Router (DR) that acts as a central point
of communication for all other routers associated with a broadcast network on at
least one interface. In a theoretical broadcast network that does not apply a DR, it
can be understood that the communication follows an n(n-1)/2 formula, where n
represents the number of router interfaces participating in OSPF. In the example
given, this would refer to 6 adjacencies between all routers. When the DR is
applied, all routers establish a relationship with the DR to which is responsible for
acting as a central point of communication for all neighboring routers in a
broadcast network.
 A Backup Designated Router (BDR) is a router that is elected to take over from the
DR should it fail. As such it is necessary that the BDR establish a link state database
as that of the DR to ensure synchronization. This means that all neighboring
routers must also communicate with the BDR in a broadcast network. With the
application of the DR and BDR, the number of associations is reduced from 6 to 5
since RTA and RTB need only communicate with the DR and BDR. This may appear
to have a minimal effect however where this is applied to a network containing for
example 10 routers, i.e. (10*9)/2 the resulting communication efficiency becomes
apparent.

 OSPF creates adjacencies between neighboring routers for the purpose of
exchanging routing information. Not every two neighboring routers will become
adjacent, particularly where one of the two routers establishing an adjacency is
considered to not be the DR or BDR. These routers are known as DROther and only
acknowledge the presence of the DROther but do not establish full
communication; this state is known as the neighbor state. DROther routers do
however form full adjacency with both DR and BDR routers to allow
synchronization of the link state database of the DR and BDR routers with each of
the DROther routers. This synchronization is achieved by establishing an adjacent
state with each DROther.
 An adjacency is bound to the network that the two routers have in common. If two
routers have multiple networks in common, they may have multiple adjacencies
between them.

 Each router participating in OSPF will transition through a number of link states to
achieve either a neighbor state or an adjacent state. All routers begin in the down
state upon initialization and go through a neighbor discovery process, which
involves firstly making a routers presence known within the OSPF network via a
Hello packet. In performing this action the router will transition to an init state.
 Once the router receives a response in the form of a Hello packet containing the
router ID of the router receiving the response, a 2-way state will be achieved and a
neighbor relationship formed. In the case of NBMA networks, an attempt state is
achieved when communication with the neighbor has become inactive and an
attempt is being made to re-establish communication through periodic sending of
Hello packets. Routers that do not achieve an adjacent relationship will remain in a
neighbor state with a 2-way state of communication.
 Routers such as DR and BDR will build an adjacent neighbor state with all other
neighboring routers, and therefore must exchange link state information in order
to establish a complete link state database. This requires that peering routers that
establish an adjacency first negotiate for exchange of link state information
(ExStart) before proceeding to exchange summary information regarding the
networks they are aware of. Neighbors may identify routes they are either not
aware of or do not have up to date information for, and therefore request
additional details for these routes as part of the loading state. A fully synchronized
relationship between neighbors is determined by the full state at which time both
peering routers can be considered adjacent.

 Neighbor discovery is achieved through the use of Hello packets that are
generated at intervals based on a Hello timer, which by default is every 10 seconds
for broadcast and point-to-point network types; whereas for NBMA and Point-to-
Multipoint network types the hello interval is 30 seconds. The hello packet
contains this interval period, along with a router priority field that allows neighbors
to determine the neighbor with the highest router ID for identification of the DR
and BDR in broadcast and NBMA networks.
 A period specifying how long a hello packet is valid before the neighbor is
considered lost must also be defined, and this is carried as the router dead interval
within the hello packet. This dead interval is set by default to be four times the
hello interval, thus being 40 seconds for broadcast and point-to-point networks,
and 120 seconds for NBMA and Point-to-Multipoint networks. Additionally, the
router ID of both the DR and BDR are carried, where applicable, based on the
network for which the hello packet is generated.

 Following neighbor discovery, the DR election may occur depending on the
network type of the network segment. Broadcast and NMBA networks will perform
DR election. The election of the DR relies on a priority that is assigned for each
interface that participates in the DR election process. This priority value is set as 1
by default and a higher priority represents a better DR candidate.
 If a priority of 0 is set, the router interface will no longer participate in the election
to become the DR or BDR. It may be that where point-to-point connections (using
Ethernet as the physical medium) are set to support a broadcast network type,
unnecessary DR election will occur, which generates excessive protocol traffic. It
therefore is recommended that the network type be configured as a point-to-
point network type.

 In order to make improve the efficiency of transition to a new Designated Router,
a Backup Designated Router is assigned for each broadcast and NBMA network.
The Backup Designated Router is also adjacent to all routers on the network, and
becomes the Designated Router when the previous Designated Router fails. If
there were no Backup Designated Router present, new adjacencies would have to
be formed between the new Designated Router and all other routers attached to
the network.
 Part of the adjacency forming process involves the synchronizing of link-state
databases, which can potentially take quite a long time. During this time, the
network would not be available for the transit of data. The Backup Designated
Router obviates the need to form these adjacencies, since they already exist. This
means the period of disruption in transit traffic lasts only as long as it takes to
flood the new LSAs (which announce the new Designated Router). The Backup
Designated Router is also elected by the Hello packet. Each Hello packet has a field
that specifies the Backup Designated Router for the network.

 In a link-state routing algorithm, it is very important for all routers' link-state
databases to stay synchronized. OSPF simplifies this by requiring only adjacent
routers remain synchronized. The synchronization process begins as soon as the
routers attempt to bring up the adjacency. Each router describes its database by
sending a sequence of Database Description packets to its neighbor. Each
Database Description packet describes a set of LSAs belonging to the router's
database.
 When the neighbor sees an LSA that is more recent than its own database copy, it
makes a note that this newer LSA should be requested. This sending and receiving
of Database Description packets is called the "Database Exchange Process". During
this process, the two routers form a master/slave relationship. Each Database
Description packet has a sequence number. Database Description packets sent by
the master are acknowledged by the slave through echoing of the sequence
number.

 During and after the Database Exchange Process, each router has a list of those
LSAs for which the neighbor has more up-to-date instances. The Link State
Request packet is used to request the pieces of the neighbor's database that are
more up-to-date. Multiple Link State Request packets may need to be used.
 Link State Update packets implement the flooding of LSAs. Each Link State Update
packet carries a collection of LSAs one hop further from their origin. Several LSAs
may be included in a single packet. On broadcast networks, the Link State Update
packets are multicast. The destination IP address specified for the Link State
Update Packet depends on the state of the interface. If the interface state is DR or
Backup, the address AllSPFRouters (224.0.0.5) should be used. Otherwise, the
address AllDRouters (224.0.0.6) should be used. On non-broadcast networks,
separate Link State Update packets must be sent, as unicast, to each adjacent
neighbor (i.e. those in a state of Exchange or greater). The destination IP addresses
for these packets are the neighbors' IP addresses.
 When the Database Description Process has completed and all Link State Requests
have been satisfied, the databases are deemed synchronized and the routers are
marked fully adjacent. At this time the adjacency is fully functional and is
advertised in the two routers' router-LSAs.

 OSPF calculates the cost of an interface based on bandwidth of the interface. The
calculation formula is: cost of the interface=reference value of
bandwidth/bandwidth. The reference value of bandwidth is configurable for which
the default is 100 Mbps. With the formula 100000000/Bandwidth, this gives a cost
metric of 1562 for a 64 kbit/s Serial port, 48 for an E1 (2.048 Mbit/s) interface and
a cost of 1 for Ethernet (100 Mbit/s) or higher.
 To be able to distinguish between higher speed interfaces it is imperative that the
cost metric be adjusted to match the speeds currently supported. The bandwidth-
reference commands allows the metric to be altered by changing the reference
value of the bandwidth in the cost formula. The higher the value, the more
accurate the metric. Where speeds of 10Gb are being supported, it is
recommended that the bandwidth-reference value be increased to ‘10000’ or
1010/bandwidth to provide metrics of 1, 10 and 100 for 10Gb, 1Gb and 100Mb
bandwidth links respectively.
 Alternatively the cost can be manually configured by using the ospf cost command
to define a cost value for a given interface. The cost value ranges from 1 to 65535
with a default cost value of 1.

 A router that has achieved a full state is considered to have received all link state
advertisements (LSA) and synchronized its link state database (LSDB) with that of
the adjacent neighbors. The link state information collected in the link state
database is then used to calculate the shortest path to each network. Each router
only relies on the information in the LSDB in order to independently calculate the
shortest path to each destination, as opposed to relying on select route
information from peers which is deemed to be the best route to a destination. The
calculation of the shortest path tree however means that each router must utilize
additional resources to achieve this operation.

 Smaller networks may involve a select number of routers which operate as part of
the OSPF domain. These routers are considered to be part of an area which is
represented by an identical link state database for all routers within the domain. As
a single area, OSPF can be assigned any area number, however for the sake of
future design implementation it is recommended that this area be assigned as area
0.

 The need to forward link state advertisements and subsequent calculation of the
shortest path based on the link state database becomes increasingly complex as
more and more routers become a part of the OSPF domain. As such, OSPF is
capable of supporting a hierarchical structure to limit the size of the link state
database, and the number of calculations that must be performed when
determining the shortest path to a given network.
 The implementation of multiple areas allows an OSPF domain to compartmentalize
the calculation process based on a link state database that is only identical for
each area, but provides the information to reach all destinations within the OSPF
domain. Certain routers known as area border routers (ABR) operate between
areas and contain multiple link state databases for each area that the ABR is
connected to. Area 0 must be configured where multi-area OSPF exists, and for
which all traffic sent between areas is generally required to traverse area 0, in
order to ensure routing loops do not occur.

 Establishing of OSPF within an AS domain requires that each router that is to
participate in OSPF first enable the OSPF process. This is achieved using the ospf
[process id] command, where the process ID can be assigned and represents the
process with which the router is associated. If routers are assigned different
process ID numbers, separate link state databases will be created based on each
individual process ID. Where no process ID is assigned, the default process ID of 1
will be used. The router ID can also be assigned using the command ospf [process
id] [router-id <router-id>], where <router-id> refers to the ID that is to be
assigned to the router, bearing in mind that a higher ID value represents the DR in
broadcast and NBMA networks.
 The parenthesis information reflects the ospf process and level at which ospf
parameters can be configured, including the area to which each link (or interface)
is associated. Networks that are to be advertised into a given area are determined
through the use of the network command. The mask is represented as a wildcard
mask for which a bit value of 0 represents the bits that are fixed (e.g. network id)
and where the bit values in the mask represent a value of 1, the address can
represent any value.

 Configuration of the neighbor relationship between OSPF peers is verified through
the display ospf peer command. The attributes associated with the peer
connection are listed to provide a clear explanation of the configuration. Important
attributes include the area in which the peer association is established, the state of
the peer establishment, the master/slave association for adjacency negotiation in
order to reach the full state, and also the DR and BDR assignments which
highlights that the link is associated with a broadcast network type.

 OSPF is capable of supporting authentication to ensure that routes are protected
from malicious actions that may result from manipulation or damage to the
existing OSPF topology and routes. OSPF allows for the use of simple
authentication as well as cryptographic authentication, which provides enhanced
protection against potential attacks.
 Authentication is assigned on a per interface basis with the command for simple
authentication of ospf authentication-mode { simple [ [ plain ] <plain-text> | cipher
<cipher-text >] | null } where plain applies a clear-text password, cipher a cipher-
text password to hide the original contents, and null to indicate a null
authentication.
 Cryptographic authentication is applied using the ospf authentication-mode { md5
| hmac-md5 } [ key-id { plain <plain-text >| [ cipher ] <cipher-text >} ] command.
MD5 represents a cryptographic algorithm for securing authentication over the
link, with its configuration demonstrated within the given example. The key
identifies a unique authentication key ID of the cipher authentication of the
interface. The key ID must be consistent with that of the peer.

 Where authentication is applied, it is possible to implement debugging on the
terminal to view the authentication process. Since the debugging may involve
many events, the debugging ospf packet command should be used to specify that
debugging should only be performed for OSPF specific packets. As a result the
authentication process can be viewed to validate that the authentication
configuration has been successfully implemented.

 It is often necessary to control the flow of routing information and limit the range
for which such routing protocols can extend. This is particularly the case where
connecting with external networks from whom knowledge of internal routes is to
be protected. In order to achieve this, the silent interface command can be applied
as a means to restrict all OSPF communication via the interface on which the
command is implemented.
 After an OSPF interface is set to be in the silent state, the interface can still
advertise its direct routes. Hello packets on the interface, however, will be blocked
and no neighbor relationship can be established on the interface. The command
silent-interface [interface-type interface-number] can be used to define a specific
interface that is to restrict OSPF operation, or alternatively the command silent-
interface all can be used to ensure that all interfaces under a specific process be
restricted from participating in OSPF.

 The implementation of the silent interface on a per interface basis means that the
specific interface should be observed to validate the successful application of the
silent interface command. Through the display ospf <process_id> interface
<interface> command, where the interface represents the interface to which the
silent interface command has been applied, it is possible to validate the
implementation of the silent interface.

 The dead interval is a timer value that is used to determine whether the
propagation of OSPF Hello packets has ceased. This value is equivalent to four
times the Hello interval, or 40 seconds by default on broadcast networks. In the
event that the dead interval counts down to zero, the OSPF neighbor relationship
will terminate.
 The DR and BDR use the multicast address 224.0.0.6 to listen for link state updates
when the OSPF network type is defined as broadcast.

 Enterprise networks are often comprised of multiple end systems that require IP
address assignment in order to connect with the network segment to which the
end system is attached. For small networks, a minimal number of end systems
attached to the network allows for simple management of the addressing for all
end systems.
 For medium and large-scale networks however, it becomes increasingly difficult to
manually configure IP addresses with increased probability of duplication of
addressing, as well as misconfiguration due to human error, and therefore the
necessity to implement a centralized management solution over the entire
network becomes ever more prominent. The Dynamic Host Configuration Protocol
(DHCP) is implemented as a management solution to allow dynamic allocation of
addresses for existing fixed and temporary end systems accessing the network
domain.
 In cases it is also possible that there may be more hosts than available IP
addresses on a network. Some hosts cannot be allocated a fixed IP address and
need to dynamically obtain IP addresses using the DHCP server. Only a few hosts
on a network require fixed IP addresses.

 DHCP supports three mechanisms for IP address allocation. The method of
automatic allocation involves DHCP assigning a permanent IP address to a client.
The use of dynamic allocation employs DHCP to assign an IP address to a client for
a limited period of time or at least until the client explicitly relinquishes the IP
address.
 The third mechanism is referred to as manual allocation, for which a client's IP
address is assigned by the network administrator, and DHCP is used only to handle
the assignment of the manually defined 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. Thus, 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 being permanently connected to a network, where IP addresses are
sufficiently scarce that addresses are able to be reclaimed when old clients are
retired. Manual allocation allows DHCP to be used to eliminate the error-prone
process of manually configuring hosts with IP addresses in environments where it
may be more desirable to meticulously manage IP address assignment.

 A DHCP server and a DHCP client communicate with each other by exchanging a
range of message types. Initial communication relies on the transmission of a
DHCP Discover message. This is broadcast by a DHCP client to locate a DHCP
server when the client attempts to connect to a network for the first time. A DHCP
Offer message is then sent by a DHCP server to respond to a DHCP Discover
message and carries configuration information.
 A DHCP Request message is sent after a DHCP client is initialized, in which it
broadcasts a DHCP Request message to respond to the DHCP Offer message sent
by a DHCP server. A request message is also sent after a DHCP client is restarted,
at which time it broadcasts a DHCP Request message to confirm the configuration,
such as the assigned IP address. A DHCP Request message is also sent after a
DHCP client obtains an IP address, in order to extend the IP address lease.
 A DHCP ACK message is sent by a DHCP server to acknowledge the DHCP Request
message from a DHCP client. After receiving a DHCP ACK message, the DHCP
client obtains the configuration parameters, including the IP address. Not all cases
however will result in the IP address being assigned to a client. The DHCP NAK
message is sent by a DHCP server to in order reject the DHCP Request message
from a DHCP client when the IP address assigned to the DHCP client expires, or in
the case that the DHCP client moves to another network.
 A DHCP Decline message is sent by a DHCP client, to notify the DHCP server that
the assigned IP address conflicts with another IP address. The DHCP client will then
apply to the DHCP server for another IP address.
 A DHCP Release message is sent by a DHCP client to release its IP address. After
receiving a DHCP Release message, the DHCP server assigns this IP address to
another DHCP client.
 A final message type is the DHCP Inform message, and is sent by a DHCP client to
obtain other network configuration information such as the gateway address and
DNS server address after the DHCP client has obtained an IP address.

 The AR2200 and S5700 series devices can both operate as a DHCP server to assign
IP addresses to online users. Address pools are used in order to define the
addresses that should be allocated to end systems. There are two general forms of
address pools which can be used to allocate addresses, the global address pool
and the interface address pool.
 The use of an interface address pool enables only end systems connected to the
same network segment as the interface to be allocated IP addresses from this pool.
The global address pool once configured allows all end systems associated with
the server to obtain IP addresses from this address pool, and is implemented using
the dhcp select global command to identify the global address pool. In the case of
the interface address pool, the dhcp select interface command identifies the
interface and network segment to which the interface address pool is associated.
 The interface address pool takes precedence over the global address pool. If an
address pool is configured on an interface, the clients connected to the interface
obtain IP addresses from the interface address pool even if a global address pool
is configured. On the S5700 switch, only logical VLANIF interfaces can be
configured with interface address pools.

 The acquisition of an IP address and other configuration information requires that
the client make contact with a DHCP server and retrieve through request the
addressing information to become part of the IP domain. This process begins with
the IP discovery process in which the DHCP client searches for a DHCP server. The
DHCP client broadcasts a DHCP Discover message and DHCP servers respond to
the Discover message.
 The discovery of one or multiple DHCP servers results in each DHCP server
offering an IP address to the DHCP client. After receiving the DHCP Discover
message, each DHCP server selects an unassigned IP address from the IP address
pool, and sends a DHCP Offer message with the assigned IP address and other
configuration information to the client.
 If multiple DHCP servers send DHCP Offer messages to the client, the client
accepts the first DHCP Offer message received. The client then broadcasts a DHCP
Request message with the selected IP address. After receiving the DHCP Request
message, the DHCP server that offers the IP address sends a DHCP ACK message
to the DHCP client. The DHCP ACK message contains the offered IP address and
other configuration information.
 Upon receiving the DHCP ACK message, the DHCP client broadcasts gratuitous
ARP packets to detect whether any host is using the IP address allocated by the
DHCP sever. If no response is received within a specified time, the DHCP client
uses this IP address. If a host is using this IP address, the DHCP client sends the
DHCP Decline packet to the DHCP server, reporting that the IP address cannot be
used, following which the DHCP client applies for another IP address.

 After obtaining an IP address, the DHCP client enters the binding state. Three
timers are set on the DHCP client to control lease update, lease rebinding, and
lease expiration. When assigning an IP address to a DHCP client, a DHCP server
specifies values for the timers.
 If the DHCP server does not set the values for the timers, the DHCP client uses the
default values. The default values define that when 50% of the lease period
remains, the release renewal process should begin, for which a DHCP client is
expected to renew its IP address lease. The DHCP client automatically sends a
DHCP Request message to the DHCP server that has allocated an IP address to the
DHCP client.
 If the IP address is valid, the DHCP server replies with a DHCP ACK message to
entitle the DHCP client a new lease, and then the client re-enters the binding state.
If the DHCP client receives a DHCP NAK message from the DHCP server, it enters
the initializing state.

 After the DHCP client sends a DHCP Request message to extend the lease, the
DHCP client remains in an updating state and waits for a response. If the DHCP
client does not receive a DHCP Reply message from the DHCP server after the
DHCP server rebinding timer expires which by default occurs when 12.5% of the
lease period remains, the DHCP client assumes that the original DHCP server is
unavailable and starts to broadcast a DHCP Request message, for which any DHCP
server on the network can reply with a DHCP ACK or NAK message.
 If the received message is a DHCP ACK message, the DHCP client returns to the
binding state and resets the lease renewal timer and server binding timer. If all of
the received messages are DHCP NAK messages, the DHCP client goes back to the
initializing state. At this time, the DHCP client must stop using this IP address
immediately and request a new IP address.

 The lease timer is the final timer in the expiration process, and if the DHCP client
does not receive a response before the lease expiration timer expires, the DHCP
client must stop using the current IP address immediately and return to the
initializing state. The DHCP client then sends a DHCP DISCOVER message to apply
for a new IP address, thus restarting the DHCP cycle.

 There are two forms of pool configuration that are supported in DHCP, these
include defining a global pool or an interface based pool. The dhcp select interface
command is used to associate an interface with the interface address pool in order
to provide configuration information to connected hosts. The example
demonstrates how interface Gigabit Ethernet 0/0/0 has been assigned as part of
an interface address pool.

 Each DHCP server will define one or multiple pools which may be associated
globally or with a given interface. For determining the pool attributes associated
with an interface, the display ip pool interface <interface> command is used. The
DHCP pool will contain information including the lease period for each IP address
that is leased, as well as the pool range that is supported. In the event that other
attributes are supported for DHCP related propagation to clients such as with the
IP gateway, subnet mask, and DNS server, these will also be displayed.

 The example demonstrates the DHCP configuration for a global address pool that
is assigned to the network 10.2.2.0. The dhcp enable command is the prerequisite
for configuring DHCP-related functions, and takes effect only after the dhcp
enable command is run. A DHCP server requires the ip pool command be
configured in the system view to create an IP address pool and set IP address pool
parameters, including a gateway address, the IP address lease period etc. The
configured DHCP server can then assign IP addresses in the IP address pool to
clients.
 A DHCP server and its client may reside on different network segments. To enable
the client to communicate with the DHCP server, the gateway-list command is
used to specify an egress gateway address for the global address pool of the
DHCP server. The DHCP server can then assign both an IP address and the
specified egress gateway address to the client. The address is configured in dotted
decimal notation for which a maximum of eight gateway addresses, separated by
spaces, can be configured.

 The information regarding a pool can be also observed through the used of the
display ip pool command. This command will provide an overview of the general
configuration parameters supported by a configured pool, including the gateway
and subnet mask for the pool, as well general statistics that allow an administrator
to monitor the current pool usage, to determine the number of addresses
allocated, along with other usage statistics.

 IP addresses that are used for server allocation such as any local DNS servers in
order to avoid address conflicts.
 The default lease period for DHCP assigned IP addresses is set at a period equal to
one day.

 The implementation of an FTP server within the enterprise network allows for
effective backup and retrieval of important system and user files, which may be
used to maintain the daily operation of an enterprise network. Typical examples of
how an FTP server may be applied include the backing up and retrieval of VRP
image and configuration files. This may also include the retrieval of log files from
the FTP server to monitor the FTP activity that has occurred.

 The transfer of FTP files relies on two TCP connections. The first of these is a
control connection which is set up between the FTP client and the FTP server. The
server enables common port 21 and then waits for a connection request from the
client. The client then sends a request for setting up a connection to the server. A
control connection always waits for communication between the client and the
server, transmits related commands from the client to the server, as well as
responses from the server to the client.
 The server uses TCP port 20 for data connections. Generally, the server can either
open or close a data connection actively. For files sent from the client to the server
in the form of streams, however, only the client can close a data connection. FTP
transfers each file in streams, using an End of File (EOF) indicator to identify the
end of a file. A new data connection is therefore required for each file or directory
list to be transferred. When a file is being transferred between the client and the
server, it indicates that a data connection is set up.

 There are two FTP transmission modes which are supported by Huawei, these are
ASCII mode and binary mode. ASCII mode is used for text, in which data is
converted from the sender's character representation to "8-bit ASCII" before
transmission. Put simply, ASCII characters are used to separate carriage returns
from line feeds. In binary mode the sender sends each file byte for byte. This mode
is often used to transfer image files and program files for which characters can be
transferred without format converting.

 Implementing the FTP service is achievable on both the AR2200 series router and
S5700 series switch, for which the service can be enabled through the ftp server
enable command. After the FTP server function is enabled, users can manage files
in FTP mode. The set default ftp-directory command sets the default working
directory for FTP users. Where no default FTP working directory is set, the user will
be unable to log into the router, and will instead be prompted with a message
notifying that the user has no authority to access any working directory.

 Access to the FTP service can be achieved by assigning individual user login to
manage access on a per user basis. AAA is used to configure local authentication
and authorization. Once the AAA view is entered, the local user can be created, by
defining a user account and password. The account is capable of associating with a
variety of services, which are specified using the service-type command, to allow
for the ftp service type to be supported by AAA.
 If the ftp directory of the user should vary from the default directory, the ftp-
directory command can be used to specify the directory for the user. If the number
of active connections possible with a local user account is to be limited, the
access-limit command can be applied. This can range from 1 to 800, or unlimited
where an access limit is not applied.
 The configuration of an idle timeout helps to prevent unauthorized access in the
event that a session window is left idle for a period of time by a user. The idle
timeout command is defined in terms of minutes and seconds, with an idle
timeout of 0 0 representing that no timeout period is applied. Finally the privilege
level defines the authorized level of the user in terms of the commands that can be
applied during ftp session establishment. This can be set for any level from 0
through to 15, with a greater value indicating a higher level of the user.

 Following configuration of the FTP service on the FTP server, it is possible for users
to establish a connection between the client and the server. Using the ftp
command on the client will establish a session through which the AAA
authentication will be used to validate the user using AAA password authentication.
If correctly authenticated, the client will be able to configure as well as
send/retrieve files to and from the FTP server.

 In order for the control connection and data connection of the FTP service to be
established successfully, TCP ports 20 and 21 must be enabled.
 When a user is considered to have no authority to access any working directory, a
default FTP directory needs to be defined. This is done by using the command set
default ftp-directory <directory location>, where the directory name may be, for
example, the system flash.

 The Telecommunication Network Protocol (Telnet) enables a terminal to log in
remotely to any device which is capable of operating as a telnet server, and
provides an interactive operational interface via which the user can perform
operations, in the same manner as is achieved locally via a console connection.
Remote hosts need not be connected directly to a hardware terminal, allowing
instead for users to take advantage of the ubiquitous capabilities of IP in order to
remotely manage devices from almost any location in the world.

 Telnet operates on a client/server model principle for which a telnet TCP
connection is established between a user port and the server telnet port, which by
default is assigned as port 23. The server listens on this well-known port for such
connections. A TCP connection is full duplex and identified by the source and
destination ports. The server can engage in many simultaneous connections
involving its well-known port and user ports that are assigned from a non well-
known port range.
 The telnet terminal drivers interpret the keystrokes of users and translates these to
a universal character standard, based on a network virtual terminal (NVT) which
operates as a form of virtual intermediary between systems, following which the
transmission via the TCP/IP connection to the server is performed. The server
decodes the NVT characters and passes the decoded characters to a pseudo
terminal driver which exists to allow the operating system to receive the decoded
characters.

 Access to the telnet service commonly involves authentication of the user before
access is granted. There are three main modes that are defined for telnet
authentication.

 Establishment to a device operating as a telnet server commonly uses a general
password authentication scheme which is used for all users connecting to the user
vty interface. Once IP connectivity is established through the implementation of a
suitable addressing scheme, the authentication-mode password command set is
implemented for the vty range, along with the password to be used.

 Following configuration of the remote device that is to operate as a telnet server,
the client is able to establish a telnet connection through the telnet command, and
receive the prompt for authentication. The authentication password should match
the password implemented on the telnet server as part of the prior password
authentication configuration. The user will be then able to establish a remote
connection via telnet to the remote device operating as a telnet server and
emulate the command interface on the local telnet client.

 If a user is unable to establish a telnet connection, the user should verify the
device supporting the telnet service is reachable. If the device can be reached, the
password should be verified. If the password is considered to be correct, the
number of users currently accessing the device via telnet should be checked. If it is
necessary to extend the number of users accessing the device through telnet, the
user-interface maximum-vty <0-15> command should be used, where 0-15
denotes the number of supported users.

HCIA-Routing_&amp;_Switching_V2.5_Entry__Training_Materials.pdf

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