Hello Josh
First of all, there is no hard limit to the diameter of an RSTP topology. It’s not like if you have X bridges it will work, but if you have X+1 it will fail. The IEEE recommendation is 7 bridges. This means that they guarantee that the feature will function flawlessly with a diameter of 7 or fewer bridges. But this is an extremely conservative value, and it doesn’t mean it won’t work with more, with the appropriate precautions and tweaks.
There is however one parameter that may limit the actual topology by default, and that is the Message Age. This is a value found within BPDUs that starts off at 0 when it is sent from the root bridge, and is incremented by one every time it passes through a non-root bridge. Once the Message Age reaches the Max Age value, the BPDU is discarded. It works kind of like a TTL found in IP packets, to prevent BPDUs from being relayed forever in a topology.
By default, Max Age is 20, so by default, you cannot have a diameter of more than 20 switches in your topology. Max Age can be configured to up to 40, so theoretically, the maximum diameter you can have is 40 switches. But I believe that RSTP will break down well before that size.
Now with your particular case, it looks like it’s taking too long for BPDUs to reach the switch, and previously received BPDU info is becoming expired. This occurs when the Max Age has elapsed without any BPDUs arriving. (Note, this usage of Max Age is different than that of Message Age).
Some things to keep in mind:
- According to papers such as this one, the reconvergence of RSTP can surpass 10 seconds in the event that you have a diameter of 10 bridges. Note that performance degrades as diameter increases. So what is the limit? It depends upon what delays you can tolerate in your topology.
- According to IEEE, a ring topology is not recommended for RSTP, but a meshed topology is much more preferable. The protocol performs better in such scenarios.
From my understanding, you are using a ring topology, with 19 switches, which seems to be at the very edge of RSTPs operational capabilities. First, consider increasing Max Age, to simply make the topology operate, and secondly, to improve convergence, if possible, consider modifying the topology to a meshed one.
This makes sense when everything is working correctly, but the real test of the protocol is during a failure, how fast and if it reconverges.
I hope this has been helpful!
Laz