STP - Aging time return to default

The aging time in a network switch is the duration it takes for the switch to remove a MAC address from its MAC address table if it doesn't receive any communication from the device associated with that MAC address within the specified time.

By default, the aging time is 300 seconds (5 minutes) on a Cisco switch. In the event that a topology change BPDU is received, this aging time is reduced to 15 seconds.

As a result, the switch will quickly remove MAC addresses that it doesn't hear from within 15 seconds. This rapid removal can be useful for quickly flushing out the MAC address table.

When the aging time is set back to its default, the switch does not immediately change the aging time for all MAC addresses in its table. MAC addresses learned during the 15-second aging period will still age out after 15 seconds. Only MAC addresses learned after the aging time is set back to 300 seconds will age out after 300 seconds.

This behavior explains why it may appear to take some time to return to the default aging time of 300 seconds. The switch is essentially waiting for the MAC addresses learned during the shorter aging period to expire before applying the longer aging time to new MAC addresses.

https://forum.networklessons.com/t/troubleshooting-spanning-tree/1185/37?u=lagapidis

https://networklessons.com/spanning-tree/spanning-tree-topology-change-notification-tcn