question regarding STP

Unanswered Question
Feb 19th, 2007


If i have two switches connected by two parallel links then on the non root switch one port will be root port and the other port will be blocked. So what happens if the blocked port stops receiving bpdus from the root. Will there be any change in the state of the blocked port?

In general if ablocked port stops receiving bupdus from its uplink switch what happens

I have this problem too.
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Tsasbrink Tue, 02/20/2007 - 02:57

In my believe it will usualy go to forwarding.

If you mean that the one rx to tx media fails it will cause a so called uni directional loop witch can be prevented with cisco's udld protocol.

Rolf Fischer Tue, 02/20/2007 - 04:20

If a blocked Port stops receiving BPDUs it will normally (after not-recieving 10 BPDU -> 20 Seconds) change into Listening-Learning-Forwarding.

If no BPDUs are received there should be no loop. Unidirectional links are often the reason for such behavior.

Do you know this flash-animation:

It's pretty helpful for understanding STP.


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