UDLD is designed to detect which condition on a network link?

• A. LLDP
• B. CDP
• C. STP
• D. UDLD

Answer: (D)

UDLD is geared toward confirming that a link is truly bidirectional. It sends and monitors frames exchanged between the two devices on a link to verify that both ends can transmit and receive. If one side stops receiving the neighbor’s frames, the link is considered unidirectional. To prevent traffic from looping or causing a split-brain situation, UDLD can automatically shut down the port in aggressive mode (while in normal mode it logs the condition). This specific focus on detecting unidirectional faults sets UDLD apart from discovery protocols like LLDP and CDP, which merely learn about neighbors, and from STP, which manages topology to prevent loops but doesn’t diagnose a unidirectional link.