STP Loop

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Aug 27th, 2008
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I am playing with bpduguard/bpdu filter on a 2950 switch. I start off by first trying to create a loop. I enable portfast on FA0/1 and FA0/2 and plug a cable from fa0/1 to fa0/2 then do a show span. fa0/1 is FWD and fa0/2 is BLK.. Can anyone explain why STP is working if i am enableing portfast. No bpduguard of bpdufilter configs have been applied yet. This is a stand alone switch. Not connected to anything else.

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Giuseppe Larosa Wed, 08/27/2008 - 06:21
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Hello Mike,

even if you enable portfast STP BPDUs still flow out and are received on the two ports that are on the same switch.

So a designated port is elected.

Real troubles start when a port in portfast is connected to an hub and then to another port in another switch.

I would use the term bridging loop instead of STP loop becuase STP is there to avoid loops.

Hope to help


mikegrous Wed, 08/27/2008 - 06:46
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I am still semi confused. From all my readings portfast immediatelly puts the port into a FWD state. I understand bpdus will be sent but if portfast immediatelly puts it into FWD state wi still dont get why a port would be blocking.

This is my unerstanding... So since a DP is elected... than that port would end up blocking because u cant have 2 DPs in this scenario. Therefor the other is blocked since it hears bpdus from on that link from the same switch. Is this correct? Wed, 08/27/2008 - 06:23
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" To disable spanning tree on a bridged router interface is not the same as if you enable spanning tree PortFast on a switch port. The router does not block the port if bridge protocol data units (BPDUs) are received from a switch or if a bridge is mistakenly connected to the interface. Be careful to connect only workstations or other end hosts to interfaces with spanning tree disabled. Do not disable spanning tree on the port to which you connect a hub or switch."


I hope this helps.

Best regards.



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