Disable Ports to Break the Loop
If you can identify the part of the network that is affected most, begin to disable ports in
this area. Or, if possible, initially disable ports that should be blocking. Each time you
disable a port, check to see if you have restored connectivity in the network. By identifying
which disabled port stops the loop, you also identify the redundant path of this port. If this
port should have been blocking, you have probably found the link on which the failure
appeared.
Log STP Events
If you cannot precisely identify the source of the problem, or if the problem is transient,
enable the logging of STP events on the switches of the network that experiences the
failure. If you want to limit the number of devices to con?¬?gure, at least enable this logging
on devices that host blocked ports; the transition of a blocked port is what creates a loop.
Issue the privileged EXEC command debug spanning-tree events to enable STP debug
information. Issue the global con?¬?guration mode command logging buffered to capture
Troubleshooting Switched Networks 87
this debug information in the device buffers. You can also try to send the debug output to a
syslog device.
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