Ok I've just read the rstp config guide. Can someone now clarify my understanding is correct please?
RSTP port roles:
- alternate
- backup
- edge
- designated
- root
RSTP port states:
- discarding
- learning
- forwarding
In the event where a cable has just been connected between two switches (this would be the entire topology by the way), the ports will start in the discarding port state, but not explicity have a port role (is that correct?). They are just considered non-edge ports because they don't have portfast configured. Ports in this state/hyperthetical role then begin the sync process. In the discarding state, ports send BPDU's to the well known destination mac address 01:80:C2:00:00:00, this is how each port can learn such things as what the bridge-id is from it's neighbor (even though it will not learn the actual mac address of that neighbor) on that link. When a BPDU is received, the the switch asks itself "does the bridge-id in the bpdu that was received have a lower numerical value than my bridge-id?". There are two possible outcomes:
- No (the bridge-id was not lower) therefore I should be the desginated switch. Therefore it sends a a configuration bpdu with the proposal flag set to the other switch, proposing itself as the designated switch
- Yes (the bridge-id was lower) therefore it cannot be the designated switch on this segment. It stays in the discarding state as a non-edge port (is this correct?)
Does this sound correct? A lot of guides seem to skip exactly what happens at the start of the convergence. They seem to just say "a switch elects itself as designated and sends a proposal". So I've tried to make some assumptions on precisely how that event occurd.