Recently, I was tasked with troubleshooting a weird problem where one of the interfaces was err-disabled. Fun Monday mornings :) and sifting through the logs I came up on this error message.
%PM-4-ERR_DISABLE: loopback error detected on Fa1/0/20, putting Fa1/0/20 in err-disable state
%ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on FastEthernet1/0/20.
Turns out that this was because of an interface somewhere in our network was configured incorrectly which then created a loop in our network. The STP topology in our network corrected/recovered itself within a few seconds and fa1/0/20 should have "err-disabled recovered", however "errdisable recovery cause loopback" was not configured.
Basically this error message states that the keepalives which are on by default are looping back to the source interface.
For example, if a keepalive was sent on fa1/0/20 and if that keepalive packet loops around the network and shows up at the source interface then that must mean that there is a loop in our network and the interface which sent out the keepalive will be err-disable.
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| Notice that the keepalive are set to 10sec by default on all interfaces. |
Workarounds:
- Fix the underlying misconfigured interface downstream and/or upstream.
- Disable keepalives on interfaces.
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