[c-nsp] "The TCN only has an impact on the aging time"

[Phil Bedard]

With 802.1d, when the switch receives a TCN from the root it will  
drop the mac-address aging time to 15 seconds.
The root switch will send PDUs with the TCN flag set for max age 
+forwarding delay= 35 seconds.

I'm fairly certain it only ages out the mac table entries associated  
with the STP instance that received
the TCN.  Using per-vlan spanning tree.

If you are using Portfast on the access port, it doesn't generate a TCN.

All of this only applies to 802.1d, if you are using 802.1w (RSTP) it  
uses a different mechanism for topology notification
and just flushes the mac table instead of lowering the aging time.

Phil

On Jul 14, 2007, at 9:41 AM, Kamlesh Sharma wrote:

>  As stated in the sentence
> "The TCN only has an impact on the aging time"
>
> When there is a TC-N received by the switch. It will change it's  
> aging time
> to 15 sec or max_age + forwarding delay = 35.
>
> My confusion is :
> it will change it's aging time for whole mac forwarding table
> or
>
> Case 1 - TCN recieved on access port for VLAN 20
> age out all mac address learned on that access VLAN 20
>
> Case 2 - TCN received on trunk port for VLAN 2 and VLAN 3
>
> age out for all mac address learned on the trunk for VLAN 2 and VLAN 3
>
> Case 3 - TCN received on trunk port for all allowed VLAN
>
> age out for whole MAC forwarding table.
>
> Thanks
>
> -- 
> Thanks
> Kamlesh Sharma
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