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Alexey Stytsenko

RSTP alternate port behavior

Good day, folks.

In Petr Lapukhov document "understanding-stp-and-rstp-convergence" in section where considered convergense process  in case of root bridge failure

(see pic.1 and notice that bridge X and bridge Y have best priorities in corresponded side and will be elected as temporary bridge, btw X has better BID than Y)


                                             Pic. 1

there is a possible pace of developments:

If the bridge “A” hears about “X” before it hears about “Y”, it will invalidate

cached information about “R” and reach an agreement with “X” about new

root bridge. At the moment when inferior information from “Y” reaches “A”,

the latter will properly synchronize with “Y”, and a new root bridge will be

elected. The extra synchronization time required to wipe out old root

bridge “R” information is no longer required, as this information is never

propagated to “Y”.

I cant get why "A" should invalidate cached information about “R”, if it has ALTN port and didnt hear on it any iferior information from "Y"? In my understanding when “A” hears about “X” it should make re-root operation (cos it still thinks that "R" alive on it ALTN port) - change it ROOT port to previoulsy ALTN and start sync process downside to "X" bridge propagading that it have path to "R".

Only after inferior information from “Y” reaches “A”, it will try to propagate this information again downside to "X" bridge. "X" will reject this and start anounce himself as the root and propagation wave will bounce back to "Y". After this topology will become converged.

So the question - who is right here? I bet on Petr Lapukhov but want clarifications. I cant get why bridge can accept inferior information still having ALTN port to previous root bridge.


Hi Alexey,

I just read this document and this is what I understand to happen.

Bridge A invalidates the information about R because X has declared itself root.

Bridge A will no longer receive information from R because R has failed, therefore it can not be considered the root bridge.

Due to the time it takes for RSTP information to propagate through the clouds, bridge A only hears root election information from X, so it believes X is root.

It takes time for the root information from Y to propagate to A, and by the time A receives the root election information from Y, it already believes X is root.

Does that answer your question?



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