STP BPDU traffic

[fsoffia]

hallo

when in a switched LAN the STP protocol has converged and the root-bridge has been determined, is the root-bridge the only that generate BPDUs or all switches generate BPDUs too ?

[aashish.c]

Hi,

Root switch generates the BPDUs and other switches recieve the BPDUs and forward them adding path cost upto them.

only, Before the election process of STP, all the switches generate BPDUs and send it to each other to elect the root bridge.

Kindly update for further queries...

aashish C

[fsoffia]

If there is a topology change all the switches restart generating BPDUs ?

If I add a new switch it restarts the election-process for all the switches or only the last and the existing root-bridge ?

thankyou

[milan.kulik]

Hi,

If there is a topology change all the switches restart generating BPDUs ?

No, see

http://www.cisco.com/warp/public/473/17.pdf

for a detailed TCN explanation.

If I add a new switch it restarts the election-process for all the switches or only the last and the existing root-bridge ?

No, there is no "root election" for all switches at the same time.

The new switch "thinks" it is the root until it receives a BPDU with a "better" bridge ID. It stops sending BPDUs then and behaves the way all non-root switches do.

The only exception is the case the new switch has really better bridge ID than the current root. The new switch neighbours detect a new root and start to forward (modified) BPDUs received from the new switch instead of the BPDUs originated by the current root. They send them to their neighbours, etc., and the new root info spreads to the whole network finally.

See

http://www.cisco.com/warp/public/473/5.pdf

and

http://www.cisco.com/warp/public/473/18.pdf

for better understanding.

Regards,

Milan