Spanning Tree Topology Change Notification (TCN)

[lagapidis (Lazarus Agapidis)]

Hello Marcus

Don’t think about the root bridge as “changing the BPDU type”. Think about it this way.

There are two types of BPDUs, TCNs and configuration BPDUs. When a topology change occurs on the network, the switch that detects it will send a TCN out of its root port. That TCN will be retransmitted by all bridges that receive it, via their root port. This ensures that the TCN will eventually reach the root bridge.

Once the root bridge receives the TCN, it will begin sending configuration BPDUs out of all of its ports. On those configuration BPDUs, the TC flag will indeed be set. More about the TC flag can be found here:

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! Laz

I hope this has been helpful!

Laz