The switch reports this error message, and the port is forced to linkdown:
%ETHCNTR-3-LOOP_BACK_DETECTED : Keepalive packet loop-back detected on [chars]
Oct 2 10:40:13: %ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on GigabitEthernet0/1
Oct 2 10:40:13: %PM-4-ERR_DISABLE: loopback error detected on Gi0/1, putting Gi0/1 in err-disable state
Description
The problem occurs because the keepalive packet is looped back to the port that sent the keepalive. Keepalives are sent on the Catalyst switches in order to prevent loops in the network. Keepalives are enabled by default on all interfaces. You see this problem on the device that detects and breaks the loop, but not on the device that causes the loop.
Workaround
Issue the no keepalive interface command in order to disable keepalives. A disablement of the keepalive prevents errdisablement of the interface, but it does not remove the loop.
Permanent Fix
In Cisco IOS Software Release 12.2(x)SE-based releases and later, keepalives are not sent on fiber and uplink interfaces by default. Upgrading the IOS version to this or later images should prevent the above issue in the first place.
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