Dell N1100-ON, Reference Manual

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Layer 2 Switching Commands
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UDLD Commands
Dell EMC Networking N1100-ON/N1500/N2000/N2100-
ON/N3000/N3100-ON/N4000 Series Switches
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.
Detecting Unidirectional Links on a Device Port
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