STP sending BPDU and loops

Answered Question
Jun 3rd, 2009


Few questions about STP:

1.Does access-ports take part in STP ? Does switch send BPDU on access port ? (let's assume tha portfast is disabled)

2. What will happen if i will connect SW1 and SW2 with two links and each port on each side of the link will be the access mode and same vlan number ?


I have this problem too.
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Correct Answer by Jon Marshall about 7 years 4 months ago

1) Yes access ports take part in STP even with portfast

2) One of the links will have to block.


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Correct Answer
Jon Marshall Wed, 06/03/2009 - 11:54

1) Yes access ports take part in STP even with portfast

2) One of the links will have to block.


Jon Marshall Wed, 06/03/2009 - 14:52


Portfast allows a port to transmission straight to forwarding ie. no listening/learning stages. But it does not actually disable STP on that port. Of course if the device connecting to a portfast port does create a loop then it is usually too late to stop it because you have gone straight to forwarding but i just wanted to emphasise that portfast does not disable STP as some people think.


lamav Wed, 06/03/2009 - 15:00

Hey, Jon:

Yes, I know you are 100% correct. I just wanted to see what your perspective on this was.

In fact, because STP is running, even on a PortFast port, you really should enable BPDU guard on it to avoid the case in which some knucklehead plugs a switch into the port. Once the BPDU is received, the PortFast port will transition to errdisable.

Jon Marshall Wed, 06/03/2009 - 15:07

Yep, I agree, BPDUGuard should be enabled on portfast ports as an extra safeguard.

And if you want to be really safe you should look at port security as well as BPDUGuard won't protect against hubs which don't send BPDUs.

But you know all this :-)


lamav Wed, 06/03/2009 - 15:10

Jon, I really didnt need any information from you...I confess, I was just British stud. LOL


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