Troubleshooting a suspect L2 loop

Hello,

I had an incident this week where a customer' server was misconfigured and
it was sending out BPDU which resulted in the core switches pegged the cpu
at 99%.
The log output showed multiple events involving duplicate ip addresses ,
specifically the ips of the neighbor core switch and show proc cpu sorted
showed
ARP Input at nearly 20%. I managed to fix the issue by shutting down all the
ports on one switch , bring the ports back online in groups and check if the
cpu levels
would increase. I found the ports involved in a L2 loop and configured
spanning-tree guard bpduguard. These ports immediately went into
err-disable and the cpu came down to about 3%. I managed to fix the problem
without a completely comprehending how L2 loop comes into existence, how to
classify the
type of L2 loop and the correct process to troubleshoot. Any assistance is
greatly appreciated.

Thanks
KS

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Received on Thu Aug 18 2011 - 21:46:54 ART