Maintaining and Configuring STP
Identifying topology changes

The steps that occur in a topology change are as follows:

Step 1 Switch D notices that a change to a link has occurred.
Step 2 Switch D sends a TCN BPDU out the root port destined ultimately for the root bridge. The switch will send out the TCN BPDU until the designated switch responds with a topology change acknowledgement.
Step 3 Switch B, the designated switch, sends out a topology change acknowledgement to the originating switch D. The designated switch also sends a TCN BPDU out the root port destined for either the designated switch or the root bridge. (This is a propagation TCN.)
Step 4 When the root bridge receives the topology change message, the root bridge changes the Flag portion of outbound BPDUs to indicate that a topology change is occurring. The root bridge sets the topology change in the configuration for a period of time equal to the sum of the forward delay and max_age parameters, which is approximately 50 seconds.
Step 5 A switch receiving the topology change configuration message from the root bridge uses the forward delay timer to age out entries in the MAC address table. This time specification allows the switch to age out MAC address, switch port, and VLAN mapping entries faster than the normal five-minute default. The bridge continues this process until it no longer receives topology change configuration messages from the root bridge.
Step 6 The backup link, if there is one, is enabled and the address table is repopulated.