STP Spanning Tree Details gives weird results

[afarrell53]

Hi I've been poking around my environment some. I noticed some spanning tree topology changes on our management vlan. I was able to trace the last TCN to two separate switches. Switch 1 has a direct trunk link back to the core and switch 2 is connected via trunk to switch 1 though a single fiber interface. Switch 2 has no other trunk interfaces. The show spanning-tree details on Switch 1 show the TCN came from switch 2 but the show spanning-tree details on switch 2 show the TCN coming from the switch 1. Has anyone else seen this behavior before or might have an idea what might be causing it. It seems weird to me that each switch is saying the TCN originated from the other.

Thanks

[David Ruess]

Hello,

I believe each switch sends a TCN that notice the change. Maybe they both sent a TCN and youre seeing the ones the other sent. Does the TCN have information in its data structure to point out which MAC address it came from?

-David

[Martin L]

First of all, are we talking about Original STP standard or Rapid version? Second, what/where was the change ?  in case of STP, notification must travel to Root switch and then Root informs all others (everyone else).  In case of Rapid, there is sync process happening at a point of change; Sw notifies about change to all its neighboring switches who then acknowledge and notify their neighbors. This is sort of no need to inform Root (unless change is on Root sw or Root is directly connected to change switch); eventually Root switch gets change.   Also, I think I noticed that not all switches in Rapid version will get a change, it;s depends on type of change and if affects me. 

Regards, ML
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[MHM Cisco World]

for your case I think there is nothing unusual, the TCN is initiate from SW1 and pass to SW2 and up to Root of your STP domain