1. © 2009, Cisco Systems, Inc. All rights reserved.
Module 4
Layer 2 Switching
Fundamentals
By Dr. Percy DIAS

2. 2© 2009, Cisco Systems, Inc. All rights reserved.
Communication Between Devices in a
LAN Network
• Unicast – Communication where a frame is
sent from one host addressed to a specific
destination. In this case, you have just one
sender and one receiver. Unicast
transmission is still the predominant form of
transmission on LANs and within the Internet.

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Communication Between Devices in a
LAN Network
• Broadcast – Communication where a frame
is sent from one address to all other devices.
In this case, you have just one sender, but the
information is sent to all connected receivers
by using a universal broadcast address.
• Multicast – Communication where a
destination addresses is a specific group of
devices, or clients.

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Switch Modes

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Switching Frames
1. A frame is received.
2. If the destination is a broadcast, forward on all
ports except the port in which the frame was
received.
3. If the destination is a Unicast and the address is
not in the address table, forward on all ports
except the port in which the frame was received.
4. If the destination is a Unicast and the address is in
the address table, and if the associated interface is
not the interface in which the frame arrived,
forward the frame out the one correct port.
5. Otherwise, filter (do not forward) the frame.

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Data Flow Through a Network

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Switching Methods
• Store-and-forward
– Switch copies the entire frame into its onboard
buffers before any forwarding takes place.
– Latency is greater with larger frames.
– Check the entire frame for errors, which allows
more error detection.
– Can forward between ports running at different
speeds (Asymmetric Switching).

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Frame Transmission Modes

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Switching Methods
• Cut-through
– The lowest level of latency.
– Immediately forwards a packet after reading the
source and destination address.
• Fragment-free
– Switching waits until the first 64 bytes of the
frame is received before forwarding.
– Filters out collision fragments.

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Collisions and Collision Domain
• Collision is a situation that can occur when 2
bits propagate at the same time on the same
network.
• Collision domains are the connected physical
network segments where collisions can occur.

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Collisions
• Hubs operate at Layer 1, simply repeating all
received signals out all other ports

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Collisions and Collision Domain

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Collision Domains with a Switch

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Collision Domains and Full Duplex
• Each port of a switch creates its own collision domain
• A frame in one collision domain does not cause
collisions in another collision domain
• The switch’s buffering process prevents the collisions
• When full duplex is enabled:
- NICs/Interfaces can send and receive at same time
- CSMA/CD is no longer needed as collisions can’t
occur
- Full duplex cannot be used if a hub is plugged into
the port

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Collision Domain Segmentation
• Breaking up or increasing the number of
collision domains with layer 2 and 3
devices is known as segmentation

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Collision Domain Segmentation

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Collision Domain Segmentation

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Layer 2 Broadcasts
• When a node needs to communicate with all
hosts on the network, it sends a broadcast
frame with a destination MAC address
0xFFFFFFFFFFFF.
• Layer 2 devices must flood all broadcast.
• The accumulation of broadcast traffic from
each device in the network is referred to as
broadcast radiation.

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Broadcasts in a Bridged Environment

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Broadcast Domain Segmentation

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Broadcast Domains
• A broadcast domain is a grouping of collision
domains that are connected by Layer 2
devices.
• Broadcasts are forwarded by Layer 2 devices
and if excessive, can reduce the efficiency of
the entire LAN.
• Broadcasts have to be controlled at Layer 3,
as Layer 2 and Layer 1 devices have no way
of controlling them.

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Collision Domains and Broadcast
Domains

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What is a Network Segment?

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Cisco Academy 3 References
Slide 4: CCNA1 8.1.1
Slide 5: CCNA3 4.2.5
Slide 7-9: CCNA1 8.1.5
Slide 6,11,13,
15-21,23:
CCNA1 8.2.1 - 8.2.7

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Cisco Academy 4 Exploration
Reference
Slide 4: LAN Switching and Wireless 2.1.1
Slide 5: LAN Switching and Wireless 2.1.1
Slide 7-9: LAN Switching and Wireless 2.2.1-2.2.4
Slide 10-14,15-20: LAN Switching and Wireless 2.1.2