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Webcast-Catalyst9k
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Question Regarding Topology Change Notification

In STP when a link between two switches goes down then which switch will start sending TCN BPDUS's. Assume that the root bridge is not connected directly to either of the two switches.

I have read documentaion on hoe TCN works and all that stuff. But I havent got information on my question. Thanks

3 REPLIES

Re: Question Regarding Topology Change Notification

Both switches will send TCNs on it's root port towards the root bridge. Every intermediate switch should acknowledge the TCN BPDU and forward it out it's root port. The process will continue till the TCN makes it to the root bridge.

Hope this helps!

New Member

Re: Question Regarding Topology Change Notification

Hi Sundar

Thanks for your reply. If one of the switch has only a root port and a blocked as below then what happens

1. port 1 went down [this is the root port]

2. port 2 is in blocking down. [before the root port went down]

Will this switch send any BPDU's on the blocked port. My question is

1. If the switches root port goes down and it doesnot has any othre designated ports except blocked ports then what happens.

Re: Question Regarding Topology Change Notification

Hi,

The upstream switch (designated bridge) to which the root port is connected will initiate the TCN bpdu towards the root bridge when the topology change occurred. Moreover, the blocked port state should change to root port and TCNs will get flooded out the newly designated root port as well.

HTH

Sundar

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