Restricted Topology Change Notification (TCN)

Restricted Topology Change Notification (TCN) is a Spanning Tree protocol feature that allows or disallows TCN propagation on specified ports. When Restricted TCN is disabled, TCN propagation is allowed. The port propagates received TCNs and topology changes to other ports. Restricted TCN is disabled by default. When Restricted TCN is enabled, the port does not propagate received TCNs and topology changes to other ports. Enable Restricted TCN to prevent unnecessary address flushing in the core region of the network caused by activation of bridges external to the core network.

A possible reason for not allowing TCN propagation is when bridges are not under the full control of the administrator or because MAC operational state for the attached or downstream LANs transitions frequently, causing disruption throughout the network.

Rapid Spanning Tree responds to TCNs by selectively flushing the filter database. Persistent TCNs are disruptive, causing persistent address flushing, which in turn causes increased flooding in the network. Restricted TCN is a useful tool when it is not possible to remove the source of the TCNs. Be cautioned that when enabled, temporary loss of connectivity can occur after changes in a Spanning Tree‘s active topology, due to persistent, incorrectly learned, station location information. This would be the case where the part of the topology that is the source of the unwanted TCNs is redundantly connected to other parts of the network.