We have about 30 switches (multiple vendors) in a network which consists of a main ring and several branches. A couple of the branches may have loops. Some of the switches are smart switches with RSTP enabled, most of the switches are either smart switches with RSTP disabled or dumb switches.
When the diameter of the RSTP-enabled switches is around 10, we're getting convergence times after a topology change of under a minute. As we increase the diameter the convergence time scales up dramatically, hitting 15-20 minutes as the diameter approaches 13 or 14. This looks like it might be a case of the network falling back to STP and the diameters being too large for the default STP settings on the switches. The same setup is being used for a different network and it rapidly converges in seconds.
Is there a way to isolate which switch might be causing the network to fall back to STP? Some of the switches provide more status details about the RSTP state (e.g. whether the port is RSTP/STP), but most of the switches provide very little diagnostic info.