Itn instructor ppt_chapter5_ethernet

© 2008 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 1
Chapter 5:
Ethernet
Introduction to Networks

5.0
5.1
5.2
5.3
5.4
Introduction
Ethernet Protocol
LAN Switches
Address Resolution
Summary
Protocol
2© 2013 Cisco and/or its affiliates. All rights reserved. Cisco Public

Upon completion of this section, you should be able to:
• Explain how the Ethernet sublayers are related to the frame fields.
• Describe the Ethernet MAC address.

Ethernet
• Most widely used LAN technology
• Operates in the data link layer and the physical layer
• Family of networking technologies that are defined in the IEEE 802.2
and 802.3 standards
• Supports data bandwidths of 10, 100, 1000, 10,000, 40,000, and
100,000 Mbps (100 Gbps)
Ethernet standards
• Define Layer 2 protocols and Layer 1 technologies
• Two separate sub layers of the data link layer to operate - Logical
control (LLC) and the MAC sublayers
link

Primary responsibilities:
• Data encapsulation
• Media access control

Ethernet II Frame Structure and Field Size

• Minimum Ethernet frame size is 64 bytes (Collision Frame or Runt)
Giant)• Maximum Ethernet frame size is 1518 bytes (Jumbo or Baby

• Layer 2 Ethernet MAC address
is a 48-bit binary value
expressed as 12 hexadecimal
digits.
• IEEE requires a vendor to
follow two simple rules:
Must use that vendor's assigned OUI
as the first three bytes.
All MAC addresses with the same
OUI must be assigned a unique
value in the last three bytes.

• The NIC views information to see if the destination MAC address in the
frame matches the device’s physical MAC address stored in RAM.
• If there is no match, the device discards the frame.
• If there is a match, the NIC passes the frame up the OSI layers, where
the de-encapsulation process takes place.

• Explain how a switch operates.
• Explain how a switch builds its MAC address table and forwards frames.
• Describe switch forwarding methods.
• Describe the types of port settings available for Layer 2 switches.

• An Ethernet Switch is a Layer 2 device.
• It uses MAC addresses to make forwarding decisions.
• The MAC address table is sometimes referred to as a content
addressable memory (CAM) table.

PC-D sends a frame back to PC-A and the switch learns PC-D’s MAC address.

Since the Switch MAC Address table contains PC-A’s MAC Address, it sends the
frame out only port 1.

PC-A sends another frame to PC-D. The switch’s table now contains PC-D’s
MAC address, so it sends the frame out only port 4.

• A switch can have multiple MAC addresses associated with a single port.
• This occurs when the switch is connected to another switch.
• See VIDEO DEMONSTRATION

• When a device has an IP address that is on a remote network, the
Ethernet frame cannot be sent directly to the destination device.
• The Ethernet frame is sent to the
which is the router.
MAC address of the default gateway,

Store-and-forward Cut-through
A store-end-forward switch receives
the entire frame,.. and computes the
CRC. � the CRv is valid, the switch
looks up the destination
address,
which determines the
outgoing
interface. The frame is then
forwarded out the correct port.
A cut-through switch forwards the
frame before it is entirely received. At
a minimum, the destination
address
of the frame must be read before
the
frame can be forwarded.

Fast-forward switching:
• Lowest level of latency immediately forwards
the destination address.
• Typical cut-through method of switching.
Fragment-free switching:
• Switch stores the first 64 bytes of the
frame before forwarding.
• Most network errors and collisions
occur during the first 64 bytes.
a packet after reading

Port-based memory In port-based memory buffering, frames are stored
queues that are linked to specific incoming and
outgoing ports.
Shared memory buffering deposits all frames into a
in
Shared memory
common memory buffer, which all the ports on the
switch share.

• Full-duplex – Both ends of the connection can send and receive
simultaneously.
• Half-duplex – Only one end of the connection can send at a time.

A common cause of performance issues on 10/100 Mb/s Ethernet links is
when one
duplex.
port on the link operates at half-duplex and the other on full-

MDIX auto detects the type of connection required and configures
theinterface
accordingly.

• Compare the roles of the MAC address and the IP address.
• Describe the purpose of ARP.
• Explain how ARP requests impact network and host performance.

There are two primary addresses assigned to a device on an Ethernet
LAN:
• Physical address (the MAC address) – Used for Ethernet NIC to
Ethernet NIC communications on the same
• Logical address (the IP address) –
network.
Used to send the packet from the original
source to the final destination.

ARP Table
Used to find the MAC address that is mapped to the destination IPv4
address.
If the destination IPv4 address is on the same network as the source
IPv4, the device will search the ARP table for the destination IPv4
address.
If the destination IPv4 address is on a different network, the device will
search for the IPv4 address of the default gateway.
•
•
•
If the device locates the IPv4 address, its corresponding
used as the destination MAC address in the frame.
If no entry is found, then an ARP request is sent.
MAC address is•
•

• Sent when a device needs a MAC address associated with an IPv4
address, and it does not have an entry in its ARP table.
• The ARP request message includes:
Target IPv4 address – This is the IPv4 address that requires a corresponding MAC
address.
Target MAC address – This is the unknown MAC address and will be empty in the ARP
request message.
• The ARP request is encapsulated in an Ethernet frame using the
following header information:
Destination MAC address – This is a broadcast address requiring all Ethernet NICs on
the LAN to accept and process the ARP request.
Source MAC address – This is the sender’s MAC address.
Type – ARP messages have a type field of 0x806.

• The device with the target IPv4 address in the ARP request will respond
with an ARP reply. The ARP reply message includes:
Sender’s IPv4 address – This is the IPv4 address of the sender, the device whose MAC
address was requested.
Sender’s MAC address – This is the MAC address of the sender, the MAC address
needed by the sender of the ARP request.
• The ARP reply is encapsulated in an Ethernet frame using the
header information:
Destination MAC address – This is the MAC address of the sender.
Source MAC address – This is the sender of the ARP reply’s MAC address.
Type – ARP messages have a type field of 0x806.
following

• When the destination IPv4 address is not on the same network as the
source IPv4 address, the source device needs to send the frame to its
default gateway.
• The source checks its ARP table for an entry with the IPv4 address of
the default gateway.
• If there is not an entry, it uses the ARP
address of the default gateway.
process to determine the MAC

• ARP cache timer removesARP entries that have not been used for a
specified period of time.
• Commands may also be used to manually remove all or some of the
entries in the ARP table.

All devices powered on at the same time
Shared Media (multiple access)
ARP broadcasts can flood
the local media.

Chapter Objectives:
• Explain
• Explain
• Explain
the operation of Ethernet.
how
how
a switch operates.
the address resolution protocol enables communication on
a network.

Itn instructor ppt_chapter5_ethernet

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