PC traffic err-disables switch port

Unanswered Question
Aug 27th, 2010


Is it normal that a PC - connected to an IP phone- sends BPDUs? I have one PC that often gets disconnected because the port is in err-disable state:

Port      Name               Status       Reason               Err-disabled Vlans
Fa1/0/2   --- Firas Yengui - err-disabled bpduguard

I should add that I have BPDU guard configured on all portfast interfaces.

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Overall Rating: 2.8 (4 ratings)
Lei Tian Fri, 08/27/2010 - 03:54


Does the switch actually receive bpdu from the port? turn on debug spanning bpdu? If it does then the switch is doing what it suppose to do.


Lei Tian

Nagaraja Thanthry Fri, 08/27/2010 - 05:48


Check the IP Phone. IP Phone will have an internal switch which will allow

the PC to be connected behind it. It could generate a BPDU that could cause

the issue. Also, to be sure who generated the BPDU, you can SPAN the port on

the switch end.

On a different note, have you configured the NIC card of the PC in question

as a trunk?



Wassim Aouadi Fri, 08/27/2010 - 06:33

Lei: debug spanning-tree bpdu generates a lot of traffic. Which pattern should I expect?

Nagaraja: You're talking about port monitoring, right? what about putting a sniffer on the port? is there any document that you would suggest to me?

Lei Tian Fri, 08/27/2010 - 06:44

Do you see bpdu receiving from port fa1/0/2? If yes, then follow Nagaraja's suggestion check why PC or phone sends out bpdu.


Lei Tian

Nagaraja Thanthry Fri, 08/27/2010 - 06:53


If you want to put a sniffer, you can do it on the host PC itself. But that

will not tell you if the IP Phone was generating the BPDU. So, configuring

port monitoring and sending that traffic to another PC would tell you who is

actually generating the BPDU (you can check it based on the MAC address).

Here is a guide to configure port monitoring on the Cisco Switches.



Also, another thing to note is if you have a PC with dual NIC cards

connected to two different ports of the switch that could create problems

too. I am not sure if that is the case here, but it is worth a check.




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