Unanswered Question
Feb 11th, 2008

am I right in saying all switches send bpdu's every 2 seconds? or is it just the root bridge that sends the initial bpdu and then each switch relays this every 2 seconds ?

I have this problem too.
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royalblues Mon, 02/11/2008 - 04:01

Once a root switch is decided, the configuration bpdu's are sent only form the root bridge and other switches just propogate them as they are received.


Kevin Dorrell Mon, 02/11/2008 - 05:27

Just one small addition to Narayan's posting. The other switches propagate the BPDUs, but they do also add their own information each BPDU, such as adding to the path cost.

Kevin Dorrell


carl_townshend Wed, 02/27/2008 - 05:14

what happens when a port goes up and down, does the switch send a tcn out to other switches, and do they propagate this to there neighbouring switches also ?

royalblues Wed, 02/27/2008 - 05:37

TCNs are directed towards the root switch and hence will be sent out of the root port.


Mohamed Sobair Wed, 02/27/2008 - 12:35


Normal BPDUs is sent out from the root bridge every 2 sec, BPDU format include lots of fields like Bridge-ID, COST, hellow, forward delay timer , maxage timer, port-id ...

If a designated switch doesnt hear from a root within 20 sec , the root declared dead.

The root-bridge Sends TCA flag to all designated switches on their RPs.

The TCN is sent out RPs from the designated Switch twoard the root-bridge, IF the upstream switch is not the root, it will also mark the TCN and send it out its root port toward the root-bridge.




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