Query_Regarding_MAC-Address_Aging !

[Vaibhava Varma]

Dear All

I got stuck up with the concept of MAC-Address Aging in a Switched Network. Let's consider the below setup:

(10.0.10.1/24)PC1-----Access_Port--SW1--Trunk_Link----SW2--Trunk_Link---SW3---Access_Port----PC2(10.0.10.2/24)

Under normal conditions SW1,SW2 and SW3 learn the MAC of PC1 & PC2 and traffic flow happens. Now suppose the Access_Port of SW1 goes down then how long will it take forSW1, SW2 and SW3 to flush out the MAC-Address entry of PC1 from their MAC-Table.

My understanding was it would be the default aging time of 300 Seconds for SW2 & SW3 and immediate flushing from SW1 but when simulated this setup in test enviornment the MAC Flushing was almost instantaneous from all the Swithces.

Please help me to understand this behaviour. I know I have misunderstood the concept somewhere and missing some other key basic point.

Regards

Varma

[lrian]

what you're missing is that when the access port on sw1 goes down it generates a spanning tree topology change notice & the other switches go into fast aging mode.  enable portfast on the sw1 access port & it won't send a tcn when the port goes up/dn.  No tcn, no fast aging & the mac address stays on the other switches for however long the mac address aging time is - default 300 seconds

I haven't looked at this in ages but my recollection is that the fast aging time is 15 seconds.  Find out by disabling portfast, unplugging the pc so you get a tcn & do 'sh mac-add aging' - or maybe it's 'show mac-add aging vlan NN' - I don't remember which, and the switch will show you the current mac aging time

Regards

Lee