STP - TCNs

[Netmart]

Hello,

I am wondering whether a TCN is also sent if a forwarding will be manually admin shut.

To my understanding a TCN is sent when a STP engaged port is changing to discarding, blocking or listening state.

Following situation: in this case port will be admin shut, meaning no transitioning is happening between STP port states. Thus no TCN is shut. However, if a switch has two links to root: one in forwarding, and in blocking: 

• The forwarding port will be admin shut, it is expected that the blocking port will transition to forwarding.
• Doing so, it triggers a TCN for the blocking port to become a forwarding one.

Please confirm/correct/advise on my STP understanding.

Thanks.

[MHM Cisco World]

Yes 

When port up/down it send tcn 

By admin or l1/l2 issue.

MHM

[M02@rt37 · Accepted solution]

Hello @Netmart 

You see TCN both when the forwarding port is shut down and when the alternate port comes up forwarding. Shut a blocked port alone would not trigger any TCN.

So, the rule should be, TCN is generated whenever a port that is part of the active forwarding topology changes state, regardless of whether that change is cause by an admin shutdonw or by a link issue...

[Netmart]

Thank you M02@rt37 for confirming.

Your statement is also confirmed in:

TC is triggered by the change of a port’s STP state to or from the STPforwardingstate. After TC, even if the particular destination MAC address has aged out, flooding not continue for long. The address is relearned by the first packet that comes from the host whose MAC address has been aged out. The issue can arise when TC occur repeatedly, with short intervals. The switches are constantly fast aging their forwarding tables, so flooding can be nearly constant.

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/28943-170.html

Th

[srimal99]

In addtion, you can refer to link for below explain in diagram which will help to understand better 

https://www.networkacademy.io/ccna/spanning-tree/stp-topology-changes