Spanning Tree Topology Change Notification (TCN)

Hello Americo

Once the root bridge is aware of a change in the topology of the network, it sets the TC flag on the BPDUs it sends out, which are then relayed to all the bridges in the network. When a bridge receives a BPDU with the TC flag bit set, it reduces its bridging-table aging time from 300 seconds to 15 seconds. This ensures a relatively quick flush of stale information.

I hope this has been helpful!