topology change event trigger:
STP: occurs when a port moves:
– from FWD to BLK
– from BLK to FWD
RSTP: occurs when a nonedge port moves from BLK to FWD
topology change detection time:
STP: after MaxAge and no BPDU received (~20sec)–> TCN sent towards Root Bridge.
RSTP: 3 Hellos missed in a row from a neighbor (~6sec)–> Neighbor seen as “down” and MAC addresses learned on that port are flushed.