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.