Interview Questions : What is TCN ?

What is TCN ?

In normal case MAC address tables that age out after 300 seconds works perfectly fine in a stable network but not when the topology changes.

When a switch detects a change in the network (interface going down or into forwarding state) it will advertise this event to the whole switched network.When the switches receive this message they will reduce the aging time of the MAC address table from 300 seconds to 15 seconds (this is the forward delay timer). This message is called the TCN (Topology Change Notification).

In a normal situation a non-root switch will receive BPDUs on its root port but will never send any BPDUs to the root bridge. When a non-root switch detects a topology change it will generate a topology change notification and send it on its root port towards the root bridge.When a switch receives the topology change notification it will send a (TCA) topology change acknowledgement on its designated port towards the downstream switch. 

Once the topology change notification reaches the root bridge it will set the TC (topology change) bit in the BPDUs it will send. These BPDUs will be forwarded to all the other switches in our network so they can reduce their aging time of the MAC address table.Switches will receives these messages on both forwarding and blocked ports.


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