How Root bridge notifies it Topology change ?

[arunsenthil.ece]

Hi Friends ..

when there is any topology change in non-root bridge , it will generate the TCN bpdu and it will send to all bridges , once root bridge sends the TC bpdu it will flush out the mac table in 15 seconds. What happens when there is a topology change in the root bridge itself ? Will it directly starts the TC bpdu or it will also start the TCN bpdu ?

[Wassim Aouadi]

My personal guess is that there will be a STP recalculation and not simple TCN frame exchange. If a link on a designated port fails, then that port will no longer be in a forwarding state. And given the fact that a root bridge has Designated Ports which are in a forwarding state, then logically an election might take place.