This read-only object reports the result of unidirectional link detection on a port: unknown(1), unidirectional(2), bidirectional(3), tx-rx-loop(4), or neighbor-mismatch(5). An admin monitors this to see the actual diagnosed condition of the link, useful for pinpointing cabling faults like a transmit/receive loopback or a link that only works in one direction. A value of tx-rx-loop(4) would tell the admin the port's transmit and receive fibers are likely looped back on themselves, a common fiber patching mistake.