UDLD - Few Words

Unidirectional link failure occurs when a system can transmit or receive on a link but cannot communicate in the other direction. This problem is not discovered by the normal data link layer error detection mechanisms of the router. As a result, unidirectional link failures can lead to Layer 2 loops. While both UDLD and Loop Guard can prevent Layer 2 loops as a result of unidirectional link failures, only Loop Guard can also guard against STP failures caused by problems in software. UDLD is limited to just the detection of unidirectional link failures.