Preventing Forwarding Loops
Unidirectional link detection

A unidirectional link occurs when traffic is transmitted between neighbors in one direction only. Unidirectional links can cause Spanning Tree topology loops. UDLD allows devices to detect when a unidirectional link exists, and also to shut down the affected interface.

UDLD is a Layer 2 protocol that works with the Layer 1 mechanisms to determine the physical status of a link. If one fiber strand in a pair is disconnected, autonegotiation would not allow the link to become active or stay up. If both fiber strands are operant from a Layer 1 perspective, UDLD determines if traffic is flowing bi-directionally between the correct neighbors.

The switch periodically transmits UDLD packets on an interface with UDLD enabled. If the packets are not echoed back within a specific time frame, the link is flagged as unidirectional and the interface is shut down. Devices on both ends of the link must support UDLD for the protocol to successfully identify and disable unidirectional links.

The table describes the default status for the UDLD globally and on an interface basis.