stp loop, Not able to trace source

[raghum]

Hello,

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.

[lgijssel]

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:

http://www.cisco.com/en/US/tech/tk389/tk621/tsd_technology_support_protocol_home.html

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

regards,

Leo

[james.hughes]

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

[glen.grant]

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 .