stp questions

Two questions: By specification

1) Does the fact a switch receive a Configuration BPDU affect the it own schedule to send its BPDU, or each switch follows the Hello Timer period to send out Configuration BPDU?

2) Same question to the fact of receive a TCN BPDU


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Keep Asking
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Dear Keep,

I am a littly hazy on this however I am pretty sure that configuration BPDUs are not sent by any switch other than the root bridge UNLESS a BPDU has been received from the root bridge. In some sense each bridge forwards BPDUs received from the root and does not originate any itself. Of course the various parameters are adjusted to reflect the network as seen from the sending bridge.

Sadly --- bridge =3D switch. I would prefer to stick to the 802.1d terminology but we seem to be long past that now.

So - YES - the fact a switch receive a Configuration BPDU affects its own schedule to send its BPDU. No switch other than the root ever originates configuration BPDUs.

TCNs are sent towards the root from a bridge that detects a topology change. As I recall it, when the root receives the TCN it sets the TCN bit in all of its transmitted configuration BPDUs for some period.

I forget the details but the originator stops sending TCNs towards the root when its TCN is acknowledged. (Perhaps by the root, or perhaps the acknowledgement is hop by hop, let me know:). I have the idea that it is hop by hop but I am not sure.

If you work it out let us know.

STP is amazingly simple in some ways, however it is not all that easy to get a full understanding of it. Mucho respect to the designers. Radia Perlman - for it is she?


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