UDLD (Unidirectional Link Detection) detects one-way links – typically on fiber.

Where it’s used

Fiber uplinks between switches. Normal mode detects; aggressive mode err-disables the port on a unidirectional link – fast fault detection for critical uplinks.

Traps ⚠️

• Only works on fiber
• Aggressive = the port gets shut

CLI

Global:

udld enable
udld aggressive

Per interface:

interface Gi0/1
 udld port
! or:
 udld port aggressive

Reset err-disabled ports:

udld reset