What is UDLD?

UDLD (Unidirectional Link Detection) is a Layer 2 protocol that detects unidirectional links between devices, typically switches or routers. A unidirectional link occurs when traffic transmitted by one device is received by another, but traffic transmitted back from the receiving device is not received by the originating device. UDLD works by sending echo messages to a neighboring device and expecting to receive an echo back. If the echo is not received, the link is considered unidirectional. UDLD helps prevent spanning-tree loops and other network problems caused by these faulty links. It can operate in normal mode (detects unidirectional links) or aggressive mode (shuts down the port if a unidirectional link is detected). It is commonly used to increase network stability and prevent issues caused by faulty fiber optic or twisted-pair cable connections.