STP topology change process

There are two types of BPDUs, TCNs and configuration BPDUs. When a topology change occurs on an STP-enabled 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 Topology Change (TC) flag will be set informing all switches that a change has occurred.

For more information about the contents of a BPDU, take a look at STP contents of a BPDU.