STP blocking port location in square topology (with attachment)

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Feb 25th, 2008
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Can someone explain me in detail how the STP decides on the location of the blocking port in a square/circle of switches?

I have attached a real life example. Why is it blocking on the indicated port on that segment and not on the segment between the top left switches for example?

Many thanks,

Pascal Coyles

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lamav Mon, 02/25/2008 - 08:22
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pascal, you didnt have to create a new thread to post the attachment. You could have done it on the original one.

Let me re-post what I wrote on the original thread, but I would suggest you abandon this one or it will confuse everyone..


In a square looped topology -- meaning, L2 connection exist for each side of the square, one of the inter switch links will have to be blocked to prevent a loop.

Now, as BJ rightly pointed out, the bridge with th elowest bridge ID will be elected as the root bridge for that VLAN, assuming you are running the ubiquitous pvst+. You can also rig that election and force any switch to be the root bridge, if you wanted.

Now, once the root bridge for a particular vlan is (s)elected, the root bridge will designate all the ports that face the other switches as designated ports, which SEND BPDUs to the root ports of the other switches. Root ports RECEIVE BPDUs.

Did I answer your question?


pascal.coyles Mon, 02/25/2008 - 08:37
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Hi Victor,

Yes, that part I understand. But how come interface 0/2 on the righthand switch is blocked? Why doesn't STP decides to block the interface 0/2 on the lefthand switch?

Are all switches sending BPDUs with their cummulative Root Path Cost and then the lefthand switch decides to block the interface 0/2?


lamav Mon, 02/25/2008 - 08:45
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Pascal, Please DELETE this thread and go back to the orignal one. I added more comments. We will be playing tennis otherwise... :-)



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