Hello Sathish
The aging time is the time it takes for a switch to age out, or remove, a MAC address from its MAC address table if it doesn’t hear from the device with that MAC address within the specified aging time.
When the aging time is reduced to 15 seconds (like when a TCN is received), the switch will quickly remove MAC addresses that it doesn’t hear from within 15 seconds. This can be useful for quickly flushing out the MAC address table.
When the aging time is set back to 300 seconds, it will take up to 300 seconds (or 5 minutes) for the switch to remove a MAC address if it doesn’t hear from the device. The key here however is to understand how the switch back to 300 seconds takes place.
The switch to 300 seconds doesn’t immediately change the aging time for all MAC addresses in its table. MAC addresses that were learned when the aging time was 15 seconds will still age out after 15 seconds. MAC addresses learned after the aging time was changed back to 300 seconds will age out after 300 seconds.
This is why it appears to take 1 minute to return to the default age of 300 seconds. The switch is waiting for the MAC addresses learned during the 15-second aging time to age out before it starts applying the 300-second aging time to new MAC addresses. Does that make sense?
I hope this has been helpful!
Laz