what are the scenarios in which a switch generates a TCN BPDU other than when a port goes down or come back up?

(root bridge)         (Non Root)              (Non Root)

In the topology above, when switch-1 sends a best BPDU to switch-2, it updates its Root bridge as switch-1. Does this also trigger switch-2 to send a TCN to switch-1 with the agreement BPDU it sends back in response to switch-1's best BPDU? Also, what kind of BPDU does it send to Switch-3 in terms of BPDU falgs?

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    Commented Feb 19, 2018 at 20:40

1 Answer 1


Let me break down as the following:

  1. STP is used when you have redundant paths in order to avoid an endless loop which can harm your network. Based on your topology A<->B<->C this is a straight topology tree hence the STP might not be entirely necessary.
  2. There are two types of BPDU's: Configuration and Topology Change Notification. Do not confuse both. Config Bpdu is indeed used during initial setup to determine the root bridge and the best path cost to it, additionally depending on the port role a state will be assumed, ie: forwarding, blocking. TCN will be generated if somewhere on the topology an event (i.e: switch down, link down) caused the path to be affected.

    3.The switch you call root bridge will only be elected so if it has the lowest bridge id (lowest priority, else, lowest priority plus lowest bridge address) through the configuration bpdu. This will occur all the way to your last switch so that they are in synch on who is the root.

    4.For a detailed explanation on TC's feel free to check this


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