(STP) Topology Changes

[markd51]

I have noticed that our network experiences intermittent connectivity issues. Upon investigation, I found that the Spanning Tree Protocol (STP) is frequently recalculating the network topology. What steps should I take to diagnose and resolve these frequent STP topology changes on our Cisco switch network?

[sidshas03 · Accepted solution]

To fix frequent STP topology changes, you should start by using the `show spanning-tree` command to identify any problematic ports. Then, check the physical connections to ensure there are no faults and that everything is stable. It's also important to verify the port configurations, making sure that trunk links are set up correctly and that end device ports have `spanning-tree portfast` enabled.

Next, you should adjust the STP settings by setting the proper bridge priorities and reviewing timers like `forward delay` and `max age`. Additionally, enabling BPDU Guard and Root Guard on the relevant ports can help prevent unauthorized topology changes. By following these steps, you'll be able to stabilize the STP topology and reduce network disruptions.

[markd51]

When I run the `show spanning-tree` command, I'm seeing some issues and errors. I've noticed that certain ports are frequently transitioning states and causing topology changes. Can you help me identify and resolve these issues?

[sidshas03]

If you're seeing issues with `show spanning-tree` and frequent port changes, first check physical connections and ensure trunk links are set with the correct VLANs. Enable `spanning-tree portfast` on end device ports, set proper bridge priorities, and review the `forward delay` and `max age` timers. Also, enable BPDU Guard and Root Guard on the relevant ports. These steps should help stabilize your STP topology.

[markd51]

Works, thank you

[MHM Cisco World]

show spanning tree interface detail <<- the interface that appear in log message 

MHM

[Martin L]

Easy solution switch to Rapid STP or MST;  Orginal 2003-4 STP is old and outdated; there is no point or reason to still running legacy STP in todays' networks

Regards, ML
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[Joseph W. Doherty]

I suspect @Martin L is presuming you're running Cisco's default PVST, but if your are, I very much endorse his suggestion to move to one of the rapid STP variants.  Of the two, rapid-PVST is a very, very simple change, and it will even continue to work with the non-rapid STP during a migration.  MST migration, though, is more involved.

However, even with the rapid variants, "port fast" on edge ports is still important, as also might be some of Cisco's STP enhancements (as mentioned by @sidshas03, although rapid variants automatically include some of them).

Regarding @sidshas03 mention of adjusting some STP parameters, such as "forward delay" and "max age", generally it's recommended not to change those unless you really, really understand both STP and your L2 topology.  If they really need to be adjusted, again, moving to rapid STP variant is often considered the better choice, instead.

Lastly, if you have L2 really depending on STP for redundancy, even better would be to consider migration to newer technology that negates the need for STP for that purpose (although it's normally kept enabled to help avoid accidental L2 loop creation - even for this, a rapid variant would be a better choice).