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Routing & Switching · Advanced

Topology Change Notification (TCN)

An 802.1D BPDU sent toward the root when a port changes state, causing switches to shorten MAC aging to forward delay.

Definition

The root acknowledges and sets the topology change flag in configuration BPDUs for max age plus forward delay, prompting all switches to age out MAC entries quickly. RSTP replaces TCNs with TC flags that flush MAC entries immediately. Frequent TCNs from host ports are prevented with PortFast.

Example

show spanning-tree detail shows the number of topology changes and their source.

Exam tip

Edge (PortFast) ports do not generate topology changes when they flap.

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