Spanning-tree understanding

[Vivek Ganapathi]

Hello,

Really been quite a while i stopped working much into switching side. I was just brushing up the concept & had a question on the attached topology. I have a task to run in backbonefast on this network.

Assuming that there is a failure on my primary path SW2-->SW1, how will the STP convergence happen in this scenario ?

Also, how will be the concept of backbonefast execute in this scenario.

Below is my understanding when backbonefast feature is enabled across all switches

1) As soon as the SW2 stops receiving the BPDU from the root bridge, it would invalidate the port it used to see as least cost path.

2) SW2 would attempt to find the alternative least path out & send across a superior BPDU (start assuming itself as root) to SW3. SW3 considers the BPDU to be inferior to the information it has stored in on that port. But as backbonefast is enabled, it needs to send in an RLQ request across to check if the alternate path is available to the root bridge.

Now the major question is, where will the SW3 switch send the RLQ over? Will it send it over the blocked port?

Please help me understanding it.

Thanks

Vivek

[Peter Paluch · Accepted solution]

Vivek,

This is perhaps an evading answer on my part, but the BackboneFast mechanism is quite nicely described in this document:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800c2548.shtml

Please note that if you are running RSTP, it natively incorporates a part of what Cisco calls the BackboneFast (it does not implement the root link query mechanism). In fact, the command spanning-tree backbonefast would be ignored if switch is running in RSTP. So if running RSTP, you do not need to activate anything in order to have BackboneFast-alike functionality.

Best regards,

Peter

[Vivek Ganapathi]

Hello Peter,

Just labbed it up. My doubt clarified now. Debugs helped me come up with a write up.

1) As the primary link between SW1 & SW2 breaks, SW2 sends in a superior BPDU stating itself as the root.

2) SW3 sends across the RLQ on it's blocked port to verify if the alternate path does exist to root bridge.

3) SW1 responds back to the RLQ positively.

Clearly noticed the saving of 20 sec Max Age timer. I wasn't running RSTP, just on standard STP with backbonefast.

Thanks for help !

Vivek.