Implementing the Rapid Spanning Tree Protocol – RSTP
Identifying the RSTP topology change notification process

In 802.1D, any port state change generates a TCN. When an 802.1D bridge detects a topology change (TC), it sends TCNs toward the root bridge. The root bridge sets the TC flag on the outbound BPDUs that are relayed to switches down from the root. When a bridge receives a BPDU with the TC flag bit set, the bridge reduces its bridge-table aging time to forward delay seconds. This ensures a relatively quick flushing of the MAC address table.

In RSTP, only non-edge ports moving to the forwarding state cause a topology change. Loss of connectivity is not considered to be a topology change, and, under these conditions, a port moving to the blocking state does not generate a TC BDPU.

When an RSTP bridge detects a TC, it performs these actions.

The TCN is flooded across the entire network one switch at a time from the switch that is the source of the change rather than from the root bridge. The topology change propagation is now a one-step process. There is no need for each switch port to wait for the root bridge to be notified and then maintain the TC state for the value of the max_age plus forward delay seconds.

If the port consistently keeps receiving BPDUs that do not correspond to the current operating mode for two periods of hello time, the port switches to the mode indicated by the BPDUs.