Topology Changes in STP

Topology Changes in STP

The other type of STP BPDU that needs to be discussed is Topology Change Notification (TCN). TCNBPDU is generated when a bridge discovers a change in topology, usually because of a link failure, bridge failure, or a port transitioning to forwarding state. The TCN BPDU is set to 0x80 in the Type field and is subsequently forwarded on the root port toward the root bridge. The upstream bridge responds back with acknowledgment of the BPDU in the form of Topology Change Acknowledgment (TCA). The least significant bit is for TCN, and the most significant bit is for TCA in the Flag field.

Figure 1-18 shows the flow of topology change BPDUs. The bridge sends this message to its designated bridge. Remember, the designated bridge is a particular bridge's closest neighbor to the root (or the root, if it is directly connected). The designated bridge acknowledges the topology change back to the sending neighbor and sends the message to its designated bridge. This process repeats until the root bridge gets the message. The root learns about the topology changes in the network in this way.