In this case, the link between Cat-A and Cat-B fails. Cat-C:Port-1/2 receives no direct notification that anything has changed. All Cat-C notices is that Configuration BPDUs stop arriving on Port 1/2. After waiting for the number of seconds specified by the Max Age timer, Cat-C:Port-1/1 starts to take over as the Root Port. This reconvergence takes considerably longer: 50 seconds as opposed to 30 seconds.
The default Max Age value of 20 seconds is designed to take two factors into account:
• End-to-End BPDU Propagation Delay
• Message Age Overestimate
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