UDLD (Uni-Directional Link Detection)

Uni-Directional Link Detection detects links where either the transmit or receive path has failed. Used to monitor fibre-optic or twisted pair cabling for unidirectional links.
UDLD used periodic hello packets and uses the concept of echos to ensure neighbors are receiving UDLD the UDLD hellos.

Supports 2 modes of operations,

  • Normal – Port is placed into undetermined state. Port is not shutdown.
  • Aggressive – Tries to get echo by sending hellow 8 times. Then places the interface into errdisable.


Disabled by default,

SW1(config) # udld enabled

Per Interface

(fibre and copper)

SW1(config-if)# udld port aggressive

UDLD Neighbors

SW1# show udld neigh


Rick Donato

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