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New Member

stp loop, Not able to trace source


I am new to cisco switches and learning about cisco switches now. we have a LAN with 6509 as core router and 2950s/3550s as access switches.

When I ran wireshark on my machine, I saw an stp loop repeating from a cisco device. I have noted down the MAC-address and tried in vain to find the same in our LAN. I am seeing packets like Address: "Spanning-tree-(for-bridges)_00" and "loop reply". I am not able to see any of the MAC addresses found in this loop conversation, on my LAN. I read that these loops are not good for the network. Where can I start to resolve this problem?

Thanks in advance for your advice.


Re: stp loop, Not able to trace source

There is likely no problem at all. ;-)

If you were really experiencing a loop, you would have other problems.

Best for you will be to start making a study of spanning tree (STP) and it's inner workings. Here is a good starting point:

Armed with this knowledge you can try to analyze the traffic that was observed by wireshark.



New Member

Re: stp loop, Not able to trace source

Hi - try enabling BPDUguard on your switches. If it goes into err-disable you know there's a switch there.. then go and investigate.


Re: stp loop, Not able to trace source

That sounds like normal spanning tree bpdu traffic. If you had to you can probably debug spanning tree and see if anything strange is going on but I would use this with extreme care as certain debug functions can drive the CPU high .