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Module 4 Layer 2 Switching

Fundamentals

By Dr. Percy DIAS

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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