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.

Cisco Systems, Inc.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 STP. Often, less capable devices (inexpensive unmanaged switches or hubs) are the culprit.

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