Cisco ICND1 Flashcard: Bridges, Switches on Collision Domains

HOW To deal with Carrier Sense Multiple Access/ Collision Detect (CSMA/ CD) in Ethernet.

Bridge
Bridges (I don't see these as often) will divide a single Ethernet LAN into multiple segments, to increase the number of collision domains and reduce network congestion.
Here are some facts about bridges:

Bridges operate at Layer 2 of OSI
Bridge is a "smart hub", can analyze incoming frames and forward/ drop based on address info
Bridges can buffer frames between two or more LAN segments
Bridges created more collision domains; this allows more than one device to transmit simultaneously without collisions
Bridges maintain MAC address tables, by examining the source MAC of an incoming frame

Here are some benefits:

Bridges can isolate a network problem to a specific segment
Filter frames within or between LAN segments
Extend a LAN to cover greater distances, joining multiple segments

Switch
For example, a Cisco Catalyst 2960-S, 24 ports.

Switches operate at Layer 2 of OSI
Forward, filter or flood frames based on the MAC table
Have many full-duplex ports to segment a LAN into small segments
Faster processing, can support different speeds

How does a switch build a MAC table?

The switch builds a MAC table that matches destination MAC address with the port used to connect to a node. The switch examines each incoming frame, the destination MAC address is compared to the list in the MAC table. The switch uses the MAC table to decide to forward, filter or flood frames.

What happens when switch examines a unicast frame?
Filter - when the switch determines that the destination MAC address of the frame resides on the same network segment as the source, it does not forward.
Forward - when the switch determines that the destination MAC address of the frame is not on the same network segment, it forwards the frame to the right segment
Flood - when the switch does not have an entry for the destination MAC address in the table, it will transmit the frame out all ports except the source port (the one which it received it from).

LAN Switch
These are more expensive, each port has it's own buffer and ASIC chip

1. Store and Forward switching, stores data in the buffer until the complete frame is received and error checking, while reading the destination address.

2. Cut-through switching, very low latency but subject to errors

Fast-forward switching: immediately forwards a packet after reading the destination address
Fragment-free switching: ensures enough bytes are read from the source to detect a collision before forwarding.

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