MSTP Topology Changes

need some help with Multiple Spanning Tree Protocol...We are seeing some syslog entries that reflect MSTP Topology Changes on our root switch but I am at a loss as to how to track down what is changing. I did some research and topology changes seem to be tied to non-edge switchports changing to a forwarding state whoch triggers a TCN flag to be sent out by the root to all switches in the CIST. We haven't seen any equipment go down, so i'm at a loss as to how to track down what changed....

Any ideas?

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networker050184

Can you post the actual log message you are getting?

vinbuck

May 25 8:19:27 core-switch mstp: MSTP instance 2 topology changes
May 25 8:19:29 core-switch mstp: MSTP instance 1 topology changes

Redacted a little bit for the web. Also need to mention these aren't Cisco, they are ZyXEL switches but they run the IEEE MSTP standard.

We deployed MSTP about 3 months ago on one section of our core network and we haven't see any major issues. I'm really trying to understand what all can trigger a topology change so I can get an idea of how to track down what is triggering it. It may just be normal network behavior since I work for a service provider and we have a fairly large and complex multi-state network.

SteveO86

You might want to document your Layer 2 topology as far as the Root bridge and what the ports are being used to get to the root bridge.

Then when you see syslog messages stating topology changes have occurred run through the switches and compare the topology maps.

Are you able to implement some type of rootguard to ensure your spanning tree topology does not change?

I'm not familiar ZyXel switches and I don't know the size of your network so I don't know how viable this option is to you.

networker050184

Do you have all of your devices logging to a server? If so I'd start looking through logs right about the same time this shows on the root to see what else is going on out there.

vinbuck

I think I finally figured it out....when I built the configs for MSTP, almost all the switches were 12 port switches except one that was 28 port switch. I only enabled MSTP for the first 12 ports on that switch which miraculously covered enough ports for MSTP to function the way we designed it, but it also enabled MSTP on some customer facing switches which is probably where many of those messages are coming from.