Switches

Overview

A solution to the issue of collisions was first provided by inserting Ethernet bridges between hubs to break up collision domains.

Ethernet bridges were quickly refined into the Ethernet switch appliances that underpin almost all modern office networks.

resolve problems of contention by moving from shared Ethernet to switched Ethernet
Like a hub, Ethernet switch provisions one port for each device
- supports more ports than a bridge
Unlike a hub, an Ethernet switch can decode each frame and identify the source and destination MAC addresses
can track which MAC source addresses are associated with each port

When it receives an incoming frame, the switch intelligently forwards it to the port that is a match for the destination MAC address
- means that each switch port is a separate collision domain
  - the negative effects of contention are eliminated
establishes a point to point full-duplex link between any two network nodes
- called microsegmentation
- can send and receive simultaneously
collisions can occur only if the port is operating in half-duplex mode
- would only be the case if a legacy network card or hub is attached
- even so, collisions affect only the microsegment between the switch port and the connected interface
  - does not slow down the whole network
traffic on all switch ports is in the same broadcast domain
- unless configured with VLAN

switch learns MAC address by reading source address when a frame is received on a port
address mapping for that port is cached in a MAC address table
- implemented as content addressable memory (CAM)
  - special type of memory optimized for searching rather than random access
- MAC address table is often referred to as CAM table
- entries remain in table for a period of time before being flushed out
  - ensures problems are not encountered when MAC address are changed
if MAC address cannot be found in MAC address table,
- switch acts like a hub and transmits frame to all ports except source
- called flooding
knowing MAC addresses associated to an interface is important for troubleshooting
- query MAC address table of a switch with show mac-address-table