Topology Change but no Root Bridge Change

[jeffm1]

Experiencing an issue with a switch having latency sporadically.  I am noticing that there are Topology Changes, however no Root bridge changes;

Region Config Change Count: 0
Time since Topology Change: 105 seconds
Topology Change Count: 103464
Count of Root Bridge Changes: 35

This will periodically reset, with the Topology Change time counter going back to 0s.  However, the Topology Change count, nor the Root Bridge changes. Obviously, the number of changes displayed are high (stack has been up ~250 days).

All ports have edge-port true and most have bpdu-filtering.  

The topology changes without root bridge changes are puzzling to me, but i'm not exactly sure how the switches account for this.  Is there any way to see in the logs what port is causing the topology change?

Thank you

[Ludovico_Steven · Accepted solution]

If you have bridge1-bridge2-bridge3 and say that bridge2 is the root bridge.

When bridge1 has a local port transitioning from Listening/Learning to Forwarding state, that counts as a Topology change on bridge1.

But it does not stop there; bridge 1 will have sent a TCN BPDU up towards the root bridge 2; now the root bridge will count a Topology change too.

And the root bridge will then set the TCN flag in the BPDUs it regularly emits.

So every other bridge in the spanning tree domain, like bridge3, will see that TCN flag and will also count a Topology change.

During the topology change, all bridges1-3 will fast age (flush) their MAC tables.