If you've spent any amount of time in Cisco networking with Catalyst switches, you've probably run across a port that is in err-disabled state. The most likely place to find this is in the output of show interfaces status. A switch port can end be error disabled if the software (IOS or CatOS) detects an error situation on the port. The port is effectively shut down until re-enabled manually or automatically if a recovery timer is specified for the error condition.
One such error condition results from the existence of a loopback on the port. The switch sends keepalive packets out all interfaces. If a keepalive packet is received on the same interface it was sent from, then a loop exists that has not been blocked by Spanning-Tree Protocol. If this occurs, these messages are generated (these specific messages are from a Cisco Catalyst 2940).
%ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet0/3.
%PM-4-ERR_DISABLE: loopback error detected on Fa0/3, putting Fa0/3 in err-disable state
To resolve the problem, you must investigate the network beyond the port that is complaining and discover what sort of logical loop might exist that was undetectable by <acronym title="Spanning-Tree Protocol">STP</acronym>. Often, less capable devices (inexpensive unmanaged switches or hubs) are the culprit.
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