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Error : ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on gig0/2

Error code CSCea46385

Symptom:

An interface on a Catalyst switch is errordisabled after detecting a loopback. Mar 7 03:20:40: %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet0/2. The port is forced to linkdown.

Mar 7 03:20:42: %LINK-5-CHANGED: Interface GigabitEthernet0/2, changed state to administratively down

Mar 7 03:20:43: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/2, changed state to down

Conditions:

This might be seen on a Catalyst 2940, 2950, 2950-LRE, 2955, 2970, 3550, 3560 or 3750 switch running 12.1EA or 12.2SE based code.

Workaround:

Disable keepalives by using the no keepalive interface command. This will prevent the port from being errdisabled, but it does not resolve the root cause of the problem. Please see section below for more information.

Additional Information:

The problem occurs because the keepalive packet is looped back to the port that sent the keepalive. There is a loop in the network.

Although disabling the keepalive will prevent the interface from being errdisabled, it will not remove the loop.

The problem is aggravated if there are a large number of Topology Change Notifications on the network. When a switch receives a BPDU with the Topology Change bit set, the switch will fast age the MAC Address table.

When this happens, the number of flooded packets increases because the MAC Address table is empty.