Looping in Layer 2 switch 2960

[prince.p]

Hi friend,

I want to know to to find network looping in Layer 2 Cisco 2960 series switch

We are using two Cisco 2960 series switch in this we connected firewall dmz interface in one of the switch interface.i want to how how to find looping in our Cisco switch.can any one please helpe to find

[Mark Malone]

Another way to find a loop is check the STP command below this will help you find loops in l2

if you constantly see changes when you run this command you need to trace back through the ports to the original culprit to see where the fault lies , so as below if it was port 2/19 the STP change kept coming from , CDP that port jump onto the next switch it shows for that port and run the same command again , if there is a loop the timers keep resetting themselves and you can follow it through layer 2 until you get to the device or port tahst the culprit usually at the access layer

xxxxxxxxx#sh spanning-tree detail | i ieee|occurr|from|is exec
 VLAN0001 is executing the ieee compatible Spanning Tree protocol
  Number of topology changes 122 last change occurred 2w4d ago
          from GigabitEthernet2/19