HI
What was connected into the port to cause this ?
its telling you it went errdisable to prevent a looped network causing even worse problems , so some kind of STP loop occurred
This will show if there was an STP change for that int or vlan
sh spanning-tree detail | i ieee|occurr|from|is exec
EDIT: Online DOC as its a known alert
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. .
%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.
