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.
That is how RSTP is designed. It is meant to accept inferior BPDUs (unlike 802.1D which will discard them till max age expires and the BPDU ages out) and invalidate the previously cached information for that port.
Good day.
Let me qoute link you adduced:
The IEEE 802.1w committee decided to incorporate a similar mechanism into RSTP. When a bridge receives inferior information from its designated or root bridge, it immediately accepts it and replaces the one previously stored.
Thats treated to port level - it accept and replace BPDU for port but this dont mean that it will choose new root having alive ports to old one (and better one as well). Here quoutations from Understanding and Configuring Backbone Fast on Catalyst Switches:
A bridge stores the value of the BPDU on a port sent by the current designated bridge.
When S receives this new BPDU from B, it realizes it is inferior to the one it had stored for port P and ignores it.
S has now checked all its non-designated ports, and it still has connectivity to the root. It can then age out immediately the information stored on port P. P transitions to listening and starts to send BPDUs. At that stage, you have already saved max_age seconds, and the standard Spanning-Tree Algorithm (STA) applies then.
So when we talk that "RSTP accept inferior BPDUs" it meaned that RSTP have natively enabled behaviour to immideatly accept and replace inferior BPDU for port based on it assurance that every topology change will be explicitly syncronized all over the network. This doesn`t mean that it will accept Root in inferior bpdu as new root bridge. Indeed it will imediately responce with its own proposal with old root, if it still believe it alive. And in our case Bridge "A" must still believe that root "R" still accesible on it ALTN port leaded to bridge B, cos "it don`t heard about Y on it ALTN port yet".
What a matter having ALTN ports if any inferrior BPDU received on root port? If consider following picture,
By your logic "A" must accept BPDU coming from "X" and start propagate it to "Y" despite that roor "R" still alive:
