Generating BPDUs in RSTP

[Mitrixsen]

Hello, everyone

My resources mention that in RSTP, all switches generate BPDUs which is different from a classic STP operation where only the root bridge does so.

However, in Cisco CML, I only see the root bridge generating the BPDUs and the switches just relay them on to the downstream switches.

P1 - P2

P2 - P3

It's the same BPDU.

David

[MHM Cisco World · Accepted solution]

Hi David 

It not same 

The non root SW modify 

Bridge ID, cost, port ID.. Etc. 

Legacy stp only root generate bpdu and non root modify some field and resend to other non root SW. 

New rapid all SW generated bpdu

I try before to know different between root bpdu generate and non root bpdu generate and there is no feild in bpdu show that. 

MHM

[Martin L · Accepted solution]

We had several discussions about how Cisco implements BPDUs; whether is copy, modify, or new BPDU; What is the saying .... "Those who know; know...."  Without a word from actual Cisco engineers, I think BPDU is copied, modified and pass along.  

arakeshr has created nice picture of comparison of both STP modes; get picture and look at your capture of BPDU; in particular look at Flags in RSTP

Regards, ML
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[Giuseppe Larosa]

Hello @Mitrixsen ,

they are not the same BPDU as noted by other the total cost to root bridge and the bridge ID and port ID are updated .

the Bridge assurance concept is what converts Rapid STP or other variant of modern STP in a sort of hello exchanged at both ends of a link . Bridge assurance is a smarter tool then STP loop guard to avoid some issues with STP.

To make an example bridge assurance should be enabled by default on Nexus switches.

Hope to help

Giuseppe

[Martin L]

found this picture of packet/frame; get it and zoom in to see differences