Unidirectional Link Detection

Unidirectional Link Detection (UDLD), as in the below diagram, detects and disables unidirectional links. A unidirectional link occurs when traffic transmitted from the local switch is received by the neighbor, but traffic sent from the neighbor is not. Unidirectional links can cause a variety of problems, including spanning-tree loops. UDLD performs tasks that autonegotiation cannot perform.

To perform UDLD, packets are sent to neighbor devices on interfaces with UDLD enabled. Therefore, both sides of the link must support UDLD. By default, UDLD is locally disabled on copper interfaces and is locally enabled on all Ethernet fiber-optic interfaces. The Cisco IOScommand to enable UDLD on an interface is simply this:
udld enable