Exact MAC learning process of a switch

[chandra_rc16]

When a switch sees a frame whose source MAC is already in its CAM table on the same port, will it refresh the timer to 0 seconds

Or it will again just store the MAC on the same port again?       

Paraphrasing my question:

If it is refreshing the timer to 0--> It means that whenever a frame is received on a switch, it will compare the source MAC to its CAM table, if the MAC is present then it will just reset the timer on that port to 0.

If it is saving the MAC on the same port again--> It means that the switch doen't compare the source MAC.. whenever a frame is received it just saves the source mac in the frame and assign it to the port.

Can someone please eloborate this process?

Regards,
Chandu       

[Jon Marshall · Accepted solution]

Chandu

When a switch first learns of a mac address on a port it enters it into the mac address table (CAM table) and sets the counter to zero. Every second it increments the counter by one. If the switch receives another frame with the same mac address on the same port it resets the counter to zero.

If the counter gets to 300 seconds (the default aging time on most switches) then the switch flushes the entry from the table.

It is also important to note that there may be change in the topology of the network and the switch will receive an STP TCN (Topology Change Notification) which instructs the switch to flush the entries from the mac address table. It has to do this because it may be that after the network has reconverged that different links are now active and the mac addresses that were associated with one port may now be reachable via another.

Jon

[chandra_rc16]

Perfect- Thanks a lot Jon.

Regards,
Chandu