Ethernet :
Framing and Addressing
802.2
802.2 frame
 802.3 Ethernet frame cannot by itself identify
the upper-layer (Network) protocol
 The IEEE defined the 802.2 LLC (Logical Link
Control) specifications to provide this function
and more.
 An 802.2 frame is an 802.3 frame with the LLC
information in the data field of the header so
we know what the upper-layer protocol is.
SNAP
SNAP (SubNetwork Access Protocol) frame
 The SNAP frame has its own protocol field to
identify the upper-layer protocol
 This is really a way to allow an Ethernet_II
Ether-Type field to be used in an 802.3 frame
 SNAP frame can be identified easily because
the DSAP and SSAP fields are always AA, and
the Command field is always 3.
Ethernet Layer 2: Framing
and Addressing
Ethernet Layer 2: Framing
and Addressing
Ethernet Layer 2: Framing
and Addressing
MAC Address
Types of Ethernet
Addresses
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Ethernet addresses, also frequently called MAC
addresses, are 6 bytes in length, typically listed in
hexadecimal form
There are three main types of Ethernet address
LAN Transmission Methods
Unicast Process
Unicast Process
•The source addresses the packet with the destination
address
•The packet is sent into the network
•The destination receives the packet
LAN Transmission Methods
Multicast Process
Multicast Process
•The source addresses the packet using a multicast
address
•The packet is sent into the network
•A copy is delivered to each destination that is included in
the multicast address
LAN Transmission Methods
Broadcast Process
Broadcast Process
•The source addresses the packet with the broadcast
address
•The packet is sent into the network
•The packet copies are delivered to all destinations
Ethernet Address Formats
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The IEEE intends for unicast addresses to be unique in the
universe by administering the assignment of MAC
addresses
The IEEE assigns each vendor a code to use as the first 3
bytes of its MAC addresses; that first half of the addresses
is called the Organizationally Unique Identifier (OUI)
The IEEE expects each manufacturer to use its OUI for the
first 3 bytes of the MAC assigned to any Ethernet product
created by that vendor
The vendor then assigns a unique value in the low-order 3
bytes for each Ethernet card that it manufactures—thereby
ensuring global uniqueness of MAC addresses
Ethernet Address Formats
I/G = individual/Group bit
U/L = Universal/Local bit
Broadcast Domains
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In an Ethernet LAN, a set of devices that receive a
broadcast sent by any one of the devices in the same
set is called a broadcast domain
A switch simply forwards all broadcasts out all
interfaces, except the interface on which it received
the frame
As a result, all the interfaces on an individual switch
are in the same broadcast domain
Also, if the switch connects to other switches and
hubs, the interfaces on those switches and hubs are
also in the same broadcast domain
Broadcast and Collision
Domains
Broadcast and Collision
Domains
Switch Operation
Switch Operation

There are three
primary operating
modes used to
handle frame
switching:

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Cut-through
Store-and-forward
Fragment Free
(modified cutthrough)
Switch Operation
Virtual LAN (VLAN)
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A VLAN is simply an administratively defined subset of
switch ports that are in the same broadcast domain
Ports can be grouped into different VLANs on a single
switch, and on multiple interconnected switches as well
By creating multiple VLANs, the switches create multiple
broadcast domains
By doing so, a broadcast sent by a device in one VLAN is
forwarded to the other devices in that same VLAN;
however, the broadcast is not forwarded to devices in the
other VLANs
Layer 2 switches forward frames between devices in the
same VLAN, but they do not forward frames between two
devices in different VLANs
Virtual LAN (VLAN)
Virtual LAN (VLAN)
Virtual LAN (VLAN)
Virtual LAN (VLAN)
VLAN Trunking
ISL encapsulation
IEEE 802.1Q tagged frame