Udld Commands; Detecting Unidirectional Links On A Device Port - Dell Networking 2024 Reference Manual

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UDLD Commands

The UDLD feature detects unidirectional links on physical ports. A

unidirectional link is a forwarding anomaly in a Layer 2 communication

channel in which a bi-directional link stops passing traffic in one direction.

UDLD must be enabled on the both sides of the link in order to detect a

unidirectional link. The UDLD protocol operates by exchanging packets

containing information about neighboring devices.

UDLD enabled devices send announcements to the multicast destination

address 01-00-0c-cc-cc-cc. UDLD packets are transmitted using SNAP

encapsulation, with OUI value 0x00000c (Cisco) and protocol ID 0x0111.

UDLD is supported on individual physical ports that are members of port

channel interface. If any of the aggregated links becomes unidirectional,

UDLD detects it and disables the individual link, but not the entire port

channel. This improves fault tolerance of port-channel.

UDLD PDUs act as network control packets. They are unaffected by

Spanning Tree state. Thus, they are transmitted and received regardless of

Spanning Tree state.

For the successful operation of UDLD, it is required that its neighbors are

UDLD-capable and UDLD is enabled on the corresponding ports. All ports

should also be configured to use the same mode of UDLD, either normal or

aggressive mode.

A device detects unidirectional links on its port via UDLD. Every UDLD-

capable device distributes service information over the network via a layer 2

broadcast frame. This service frame contains information about sender

(source device) and all discovered neighbors. Every sender expects to receive

an UDLD echo frame. If an echo frame is received, but does not contain

information about the sender itself, it implies that the sender's frames have

not reached the neighbors. This can happen when the link is able to receive

traffic but cannot send traffic. In other words, a UDLD-capable device can

UDLD Commands

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