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  • 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.

From

Cover of Cisco LAN Switching

Note

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.)