Topology changes Notification (TCN)

STP uses a Topology Change Notification (TCN) BPDU to alert the root bridge that a topology change to the spanning tree might need to occur. The Type field of the BPDU signifies the TCN BPDU: 0x80. TCN BPDUs improve convergence time when failures in the network occur—primarily because they help in a rapid updating of the MAC address tables.
The TCN process of 802.1D is as follows:
1. A bridge sends a TCN BPDU in two cases:
a. It takes a port into forwarding, and it has at least one designated port (DP).
b. A port goes from Forwarding/Learning to Blocking. TCNs are sent out the root port of nonroot devices; they are sent each hello interval until they are acknowledged by the upstream device.
2. Upstream bridges process TCN on DPs.
3. The upstream switch sets the Topology Change Acknowledgement (TCA) field of the next configuration BPDU received and sends this downstream. This causes the downstream switch to stop sending TCN BPDUs.
4. The upstream switch then sends the TCN further upstream.
5. This continues until the root bridge receives the TCN.
6. The root bridge then sets the TCA and Topology Change flags in the next configuration BPDU sent out downstream.
7. The root bridge sets the TC flag in all BPDUs sent for Forward Delay + Max Age. This instructs all switches to age MAC table address entries faster.