Hi John,
All steps that you described are correct, that’s how it works.
SwitchB doesn’t have a link to the root bridge so it claims to be the new root bridge. SwitchC which is still connected to the root doesn’t agree so it drops the inferior BPDU from SwitchB.
We want to get rid of the max age timer, that’s 20 seconds so instead we let SwitchC do a quick “check” to see if the root bridge is still reachable by sending a RLQ. When it receives a RLQ reply, it knows the root is still there and it can age out the interface immediately.
In case you don’t get a RLQ reply, we know the root bridge is gone and we’ll have to recompute STP.
Rene