Switches

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In this lecture, we will learn about Collision Domain and Microsegmentation Switches – a layer two device

◦ MAC address Table◦ Frame Filtering◦ Frame Forwarding◦ Frame Buffering◦ Full Duplexing

Collision domain in switches

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A collision domain is a logical area in a computer network in which data packets can collide with one another. A collision occurs when two or more devices attempt to send a signal along the same transmission channel (e.g., the same section of copper wire or optical fiber) simultaneously.

Disadvantages of data collisions are:◦ Decreased network efficiency ◦ Packet Loss. ◦ Slow Performance of the network. ◦ More bandwidth utilization

Network devices are used to reduce the size of network segment or Collision domain.◦ Layer 3 device: Router◦ Layer 2 device: Switch and bridge

Collision Domain

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Layer 2 Multiport device ◦E.g., 12 ports, 24 ports, 48

ports Switches are specified by

number of ports and bandwidth supported◦10/100 Mbps ports connecting computers in a

network.

Port

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Packet Tracer file

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How hub works

Switches reduce the size of collision domain Microsegmentation is the dividing of a network

through the use of network switches such that only two nodes coexist within each collision domain.

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hub hub hub hub

switch switch

router

Collision Domains Collision Domains

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Switches associate MAC addresses with port numbers and store them in its memory in a table called MAC Address Table

Whenever a frame enters a switch, it will add Source Address (1111) against port number (2) through which it came in is in it’s table.

This table for catalyst switches is called CAM (Content Addressable Memory) table

The CAM table times out every 5 minutes

Source Address TablePort Source MAC Add. 2 1111 5 22227 3333

switch

1111

2222

3333

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Source Address TablePort Source MAC Add. Port Source MAC

Add.2 1111

Switches output a frame on a specific destination port .

Switch examines the MAC address table for the Destination MAC address.

If it finds a match, it filters the frame by only sending it out on that port.

◦ If no match is found, it floods it out on all the ports.

11113333

switch

1111

2222

3333

Sample MAC addresses

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PC 3333 wants to send data back to 1111.

The switch now has an entry for the Source Address (3333) ◦ It does NOT add it.

It checks the Destination Address, it has it in its table

As both MAC addresses are in the switch’s table, information exchanged between 1111 and 3333 can be filtered out the appropriate port (port 8) and not Flooded

switch

1111

2222

3333

MAC Address TablePort Source MAC Address. 2 1111 8 3333

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1111

2222

3333

11113333

22223333

when two devices (1111 and 2222) send to same destination (3333)?

The switch buffers the frames and sends them out on port number 7 one at a time.

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Full-duplex allows simultaneous (send and receive at the same time) communication between a pair of devices.

Both ends of the link must be in full-duplex mode. Most switches operate at either full-duplex but can operate in half-

duplex.

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For every frame that enters a switch… Learning Stage (Building/Updating of MAC table)

◦ Examines Source MAC Address: If Source MAC Address is in the MAC table, update 5 minute timer If Source MAC Address is NOT in the MAC table, add Source MAC Address

and incoming port number to MAC table Forwarding Stage (Flood or Filter)

◦ Examines Destination MAC Address: If Destination MAC Address is in the MAC table, forward the frame only out

that port (Filter), If Destination MAC Address is NOT in the MAC table, forward the frame only

out all ports except incoming port (Flood)

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Switches have multiple collision domains. To communicate with all collision domains,

broadcast and multicast frames are used at Layer 2. Every NIC must recognize this address Since broadcasts generate excess traffic in the

network, broadcasts must be controlled. Devices such as router which operate at Layer 3

control broadcasts.

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Five new terms that you learnt in this session

Find information about switches available in the lab◦ Cisco 2950 switches◦ Cisco 3560 switches

Compare their features Hardware options possible

Memory –RAM/Flash IOS images supported Applications of switches Find the differences between Layer 2 /Layer 3 Switches

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