UDLD detects unidirectional links — classic on fiber where TX works, RX is dead: the interface says up/up but STP breaks and blackholes/loops appear.

• Devices exchange UDLD hellos; no echo back ⇒ unidirectional.
• normal logs; aggressive err-disables the port — the production choice.

udld aggressive                          ! global
interface Gi1/0/1
 udld port aggressive
errdisable recovery cause udld
errdisable recovery interval 300

Verify: show udld. Exam model: UDLD = physical link truth, STP = logical loop logic; UDLD is fiber-only and doesn’t replace Loop Guard.