Abstract: UDLD is a layer protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables.It monitors a physical connection to detect unidirectional links to avoid spanning-tree topology loops or silent drop traffic. All connected devices must support UDLD for the protocol to successfully identify the unidirectional links. When UDLD detects a unidirectional link, it can administratively shut down the affected port and send you a warning message.

Key words: UDLD, SNAP, TLV