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.
