Cisco's documentation says "If both fibers are working normally at Layer 1, then UDLD at Layer 2 determines whether those fibers are connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation, because autonegotiation is a Layer 1 feature."
layer 1 determines if there exists a proper physical connection between the 2 communicating devices........right? if there is some problem with either the TX. or RX. fibers,then that will be detected at the physical layer itself......right? then what does udld do? Cisco's documentation says "Layer 2 determines whether those fibers are connected correctly and whether traffic is flowing bidirectionally between the 'correct' neighbors. " what do you mean by correct neighbors