Whereas Configuration BPDUs are the general workhorse of the STP algorithm, TCN BPDUs perform a very specific role by assisting in network recovery after changes in the active topology. When a non-Root Bridge detects a change in the active topology, a TCN BPDU is propagated upstream through the network until the Root Bridge is reached. The Root Bridge then tells every bridge in the network to shorten their bridge table aging periods from 300 seconds to the interval specified by Forward Delay. In other words, TCN BPDUs are used to tell the Root Bridge that the topology has changed so that Configuration BPDUs can be used to tell every other bridge of the event.
TCN BPDUs are sent in three cases. It is useful to group these into two categories, change detection and propagation:
• Change detection— Occurs in the event that a bridge port is put into the Forwarding state and the bridge has at least one Designated Port. Change detection also occurs when a port in the Forwarding or Learning states transitions to the Blocking state.
• Propagation— Occurs in the event that a non-Root Bridge receives a TCN (from a downstream bridge) on a Designated Port.
The first two conditions categorized under change detection constitute a change in the active topology that needs to be reflected in bridging tables throughout the network. The last condition is used to propagate TCN BPDUs up through the branches of the Spanning Tree until they reach the Root Bridge.
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