UDLD is used to detect what condition on a link?

• A. Unidirectional link failures
• B. Bidirectional link failures
• C. VLAN tagging errors
• D. Spanning tree loops

Answer: (A)

Unidirectional link failures are what UDLD watches for. UDLD (Unidirectional Link Detection) runs at the data link layer and exchanges periodic messages between connected devices to verify that traffic can travel in both directions on a link. If one side stops receiving the other’s messages or if the information in the messages doesn’t match what is expected, the link is put into a fault state. This prevents subtle issues where traffic can be sent in one direction but not acknowledged in the other, which could lead to network instability or loops.

VLAN tagging errors are about mismatches or misconfigurations in how frames are tagged on trunk links, which UDLD does not monitor. Spanning tree loops are handled by STP and related features; UDLD helps prevent problems that arise from a unilateral fault, but it doesn’t directly detect or diagnose a loop itself. A bidirectional link failure is simply a link that isn’t working in either direction, which is detected as a nonfunctional link rather than the specific asymmetric condition UDLD is designed to identify.