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.
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.)
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