O'Reilly logo
  • Francis Chang thinks this is interesting:

1 A bridge originates a TCN BPDU in two conditions:

• It transitions a port into the Forwarding state and it has at least one Designated Port.

• It transitions a port from either the Forwarding or Learning states to the Blocking state.


Cover of Cisco LAN Switching


When a port that was forwarding is going down (blocking for instance).
When a port transitions to forwarding and the bridge has a designated port. (This means that the bridge is not standalone.)