Hello muzaffar bhai,
Normally if you have STP loops on your network, there are some standard symptoms:
1) packet drops from a partcular switch or network ... intermittent loss of connectivity
2) High CPU/link/port utilizations...
3) Syslog messages that indicate packet looping on the network (for ex HSRP duplicate IP messages)
4) syslog messages that show MAC address flapping messages ....
I think 3rd and 4th point can clearly tell u that there is a L2 loop on your network, in many practical scenarios...
For troubleshooting L2 loops, you have to know your network in & out... You can probably see the syslogs and try isolating the trunk one by one.. You need to be really sure as to which network is causing this issue !! Best practice is to manually do a "show spanning-tree vlan no" on each edge switch and track the STP root.. you need to also force the root to the core switch and not leave the values to default...
In addition to this, you can check the port utilizations, trunk utilization, CPU util and come to a conclusion on the BAD network..
Refer this doc.. this has more troubleshooting tips for STP loops:
http://cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080136673.shtml
Hope this helps.. all the best.. rate replies if found useful..
Raj
