TCN BPDUs are sent in three cases:
• When a port is put in the Forwarding state and the bridge has at least one Designated Port (this is a part of change detection).
• When a port is transitioned from the Forwarding or Learning states back to the Blocking state (change detection).
• When a TCN BPDU is received on Designated Port, it is forwarded out the bridge's Root Port (propagation).
Several observations can be made about TCN BPDUs:
• TCN BPDUs are only sent out Root Ports.
• TCN BPDUs are the only BPDUs sent out Root Ports (Configuration BPDUs are only sent out Designated Ports, not Root Ports).
• TCN BPDUs are received by Designated Ports.
• TCN BPDUs flow upstream toward the Root Bridge.
• TCN BPDUs use a reliable mechanism to reach the Root Bridge. When a bridge sends a TCN BPDU, it continues repeating the BPDU every Hello Time seconds until the upstream bridge acknowledges receipt with a Topology Change Acknowledgement flag in a Configuration BPDU. TCN BPDUs are not periodic in the same sense as Configuration BPDUs. Other than the retransmission of already generated TCN BPDUs discussed in the previous bullet and used as a reliability mechanism, completely new TCN BPDUs are not sent until the next topology change occurs (this could be hours, days, or weeks later).
• TCN BPDUs are acknowledged even if the normal Configuration BPDU processing discussed earlier has stopped (because the flow of Configuration BPDUs from the Root Bridge has stopped flowing).
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