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CCNP (Pre 2020) CCNP Enterprise Core (350-401) Cisco Datalink Layer

UDLD – Unidirectional Link Detection

Unidirectional Link Detection allows for monitoring of physical media that separates its send and receive media, like fibre optic cables.

UDLD operates by transmitting a packet to the neighbour device that contains a system and port ID of the originating interface. The receiving interface gets this information and mirrors it back including its own system and port ID. This process continues to ensure that both send and receive parts of the link have communication between them.

Unidirectional Link Detection has two operating modes, normal and aggressive.

  • Aggressive mode sends eight additional packets in one second intervals when a UDLD packet is not acknowledged by the neighbouring switch. If none of those packets are acknowledged the port is changed into a error disabled state.
  • Normal mode does not send additional packets or disable the interface when a packet is not acknowledged. It changes the link status to undetermined and the port continues to be utilised regardless of what fault state it is in.

Unidirectional Link Detection can be enabled globally with the command udld enable with the optional additional suffix keyword of aggressive to enable aggressive mode. Enabling UDLD globally will switch on the UDLD mechanisms on all SFP ports.

Individual ports can have UDLD capabilities disabled in interface configuration mode with the command udld port disable

In order for UDLD to function correctly, both ends of the switch must have UDLD enabled to communicate packets correctly. The status of UDLD can be checked with the command show udld neighbors

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