Verify that the neighbor hears us
A port can report an up link even if only one direction carries frames. UDLD peers exchange messages identifying themselves and echoing their neighbor’s identity. A port looks for its own identity in the peer’s message as evidence that the peer heard it.
In normal mode, loss of information alone can leave a port in an undetermined state. Aggressive mode also retries when a known bidirectional neighbor disappears. If those attempts fail, it can disable the port. UDLD checks a single link. Spanning tree separately selects a loop-free topology across links.
Try it in a lab
- Enable UDLD on both ends of a supported test link and confirm bidirectional status. Use a test setup that can interrupt one direction while keeping carrier present.
- Compare physical link status with UDLD status after the fault. In aggressive mode, confirm whether the port is disabled after the retries fail, and check that traffic takes a working redundant path if one exists.