Note For complete syntax and usage information for the commands used in this chapter, refer to the Command Reference publication for your switch. When a unidirectional link is detected, UDLD shuts down the affected port and alerts the user.
Unidirectional links can cause a variety of problems, including spanning-tree topology loops.
UDLD is a Layer 2 protocol that works with Layer 1 mechanisms such as autonegotiation to determine the physical status of a link. At Layer 1, autonegotiation handles physical signaling and fault detection. UDLD also performs tasks that autonegotiation cannot perform such as detecting the identities of neighbors and shutting down misconnected ports. When both autonegotiation and UDLD are enabled, Layer 1 and Layer 2 detection features can work together to prevent physical and logical unidirectional connections and malfunctioning of other protocols. A unidirectional link occurs whenever traffic transmitted by the local device over a link is received by the neighbor but traffic transmitted from the neighbor is not received by the local device.