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UniDirectional Link Detection (UDLD)

Introduction

UDLD is a Layer 2 protocol that works with Layer 1 mechanisms to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports. When you enable both autonegotiation and UDLD, Layer 1 and 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.