Loop-back detected: the switch heard its own voice come back
%ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet1/0/10. The port is forced to stay in errdisable state.
In plain English: The switch sends itself small keepalive frames on every port.
Platform: Cisco IOS / IOS XE. Values shown are the documented defaults — confirm them on your platform with the check commands below.
What it means
The switch sends itself small keepalive frames on every port. On this port, a keepalive came back — the wire leaving the port loops back into the same switch, or the far device is reflecting frames. The switch shuts the port because a loop like that, left open, copies traffic endlessly and can drown the whole LAN in a broadcast storm within seconds.
Likely causes, in order
A patch cable with both ends in the same switch — often two ports of one switch during rack cleanup.
Two wall jacks connected to each other by a stray cable in an office or conference room.
A far-end device — media converter, unmanaged switch, mis-wired panel — reflecting frames back.
A damaged cable or a bad patch-panel termination shorting the pairs.
Check, in order
show interfaces status err-disabledConfirm the port and the loopback reason, and whether a second port shows the same — loops usually involve two ports.
show mac address-table vlan 10One MAC address appearing on two ports, or moving rapidly between them, is the loop's footprint in the table.
Physically trace the cableThis fault lives in the physical world: follow the patch lead at both ends before changing any configuration.
The fix
Find the loop and remove it — unplug the stray cable or correct the panel patching.
Re-enable the port with shut / no shut and watch the MAC table settle: each address should sit on exactly one port.
If loops keep appearing in user areas, keep loop detection and BPDU Guard enabled on access ports and treat each event as a facilities ticket, not a nuisance alarm.
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Full case: this error is one stop in a bigger fault. Work the complete playbook: Port Keeps Going Err-Disabled — checks, ranked causes, the fix, and verification.