%UDLD-4-UDLD_PORT_DISABLED: UDLD disabled interface Gi1/0/49, unidirectional link detected
In plain English: UDLD is a watchdog that checks a link works in BOTH directions.
Platform: Cisco IOS / IOS XE. Values shown are the documented defaults — confirm them on your platform with the check commands below.
What it means
UDLD is a watchdog that checks a link works in BOTH directions. It found traffic flowing only one way — for example, a fiber strand that is broken, or a transmit fiber plugged into the wrong port — and shut the port down. A one-way link is dangerous: the side that can still transmit keeps forwarding into a black hole, and spanning tree can be fooled into opening a loop.
Likely causes, in order
One strand of a fiber pair is damaged, dirty, or unplugged.
Transmit and receive fibers were swapped or cross-patched wrongly during rack work.
A failing optic that receives fine but no longer transmits (or the reverse).
Check, in order
show udld neighborsWhat this switch hears from the far end, port by port — the failed port's entry will be missing or one-sided.
show interfaces Gi1/0/49Confirm the port state and error counters before touching fiber.
The fix
Inspect and clean the fiber, reseat both connectors, and swap the strand pair if transmit/receive are crossed.
Replace the patch lead, then the optic, if the one-way behavior survives.
Re-enable with shut / no shut (or the udld reset command) and confirm show udld neighbors shows a bidirectional entry.
Did this fix it for you? One tap — anonymous, nothing to type, and nothing you send is ever published.
Thanks — your tap helps the next engineer. A run of "still broken" reports sends this page back to the top of our re-check list.