Hi,
A loopback error occurs when the keepalive packet is looped back to the port that sent the keepalive.
The switch sends keepalives out all the interfaces by default. A device can loop the packets back to the source interface, which usually occurs because there is a logical loop in the network that the spanning tree has not blocked. The source interface receives the keepalive packet that it sent out, and the switch disables the interface (errdisable).
Error Msg will be like,
%PM-4-ERR_DISABLE: loopback error detected on Gi4/1, putting Gi4/1 in err-disable state.
The suggested workaround is to disable keepalives and upgrade to Cisco IOS Software Release 12.2SE or later.
I hope this will clear ur doubt,
Rate if it does,
Rgs
