hi

[sajeer9745]

what is mac address table fields and how a switch uses mac table for switching decision?

[Deepak Kumar]

Hi, 

A MAC address table sometimes called a Content Addressable Memory (CAM) table and this table is used on Ethernet switches to determine where to forward traffic on a LAN.

Now suppose PC1 wants to send traffic to the server that has a MAC address of 00:00:00:00:00:01. It would encapsulate an Ethernet frame and send it off toward the switch. The first thing the switch would do when receiving the traffic is creating a new entry in its MAC address table for PC1's MAC address (PC1 -> Fa0/3). The switch would then perform a lookup on its MAC address table to determine whether it knows which port to send the traffic to; since no matching entries exist in the switch's tables, it would flood the frame out all of its interfaces (except the receiving port).

Because the frame was sent out to all of the switch's other ports, it would be received by the target server. Assuming that the server wants to respond to PC1, it would be sent a new frame back toward the switch. The switch would receive the frame and create a new entry in its MAC address table for the server's MAC address (Server -> Fa0/2). It would then perform a lookup of its MAC address table to determine whether it knows which port to send the server's traffic to; in this case, it does, so it sends the return traffic out only its Fa0/3 port (PC1), without flooding. Listing 1 shows what the MAC address table would look like at this point.

Example of mac address table:

Switch#show mac address-table
          Mac Address Table
-------------------------------------------
Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    00le.70ad.5c60    DYNAMIC     Fa0/1
   1    00lb.e7a4.ala0    DYNAMIC     Fa0/3

 Regards,

Deepak Kumar