UDLD (Unidirectional Link Detection)

UDLD (Unidirectional Link Detection) is a layer 2 Cisco propitiatory protocol that ensures that a unidirectional link is not present.

While copper / UTP (by design) does not suffer from unidrectional links (and hence is rarely used) - fiber does on the other hand - if the RX (or TX) fiber goes you are left with a unidirectional link.

UDLD sends heartbeats down the fiber in order to detect whether a unidrectional link is present.

Although UDLD can come in useful (with copper) when you have a single / non-aggregated port (not using LACP etc.) and there is a medium device between itself and the endpoint - e.g. media converter or WAN optimiser.

By default UDLD is not enabled - in order to turn it on we can issue:

int gi1/0/1
udld port

There is also an aggressive option - that forces UDLD to be enabled on the other end as well:

int gi1/0/1
udld port aggressive