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Cisco CCNP SWITCH Exam Tutorial:

Topology Change Notification BPDUs

By Chris Bryant, CCIE #12933

During your CCNP studies, you're going to spend most of your time studying Configuration BPDUs.

The Topology Change Notification BPDU (TCN) is equally as important, but hopefully infrequent.

Configuration BPDUs are originated only by the root bridge, but a TCN BPDU will be generated by any switch in the network when one of two things happen:

A port has gone into Forwarding mode

A port has gone from Forwarding or Learning mode into Blocking mode

While the TCN BPDU is important, it doesn't give the other switches a lot of detail. The TCN doesn't say exactly what happened, just that something happened.

TCN BPDU Is Sent

As the TCN works its way toward the root bridge, each switch that receives the TCN will send an acknowledgement and forward the TCN.

TCN BPDU Is Forwarded

When the root bridge receives the TCN, the root will also respond with an acknowledgement, but this ack will take the form of a Configuration BPDU with the Topology Change bit set.

Topology Change Flag Is Set

This indicates to all receiving switches that the default aging time for their MAC tables should be changed from the default of 5 minutes to whatever the Forward Delay value is - by default, that's 15 seconds. (Another reason to be careful, if not downright hesitant, to start adjusting STP timers.)

TCNs And The Portfast Exception

Cisco switching veterans just know that Portfast has to get involved here somewhere! A Portfast-enabled port changing STP mode cannot result in the generation of a TCN BPDU, which makes perfect sense.

The most common usage of Portfast is when a single PC is connected directly to the switch port, and since such a port going into Forwarding mode (or leaving it) doesn't impact STP operation, there's no need to alert the entire network about it.

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