CHAPTE R
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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Configuring UDLD
This chapter describes how to configure the UniDirectional Link Det ecti on (UD LD) prot ocol o n th e
switch. Unless otherwise noted, the term switch refers to a standalone switch and to a switch stack.
Note For complete syntax and usage information for the commands used in this chapter, see the command
reference for this release.
This chapter consists of these sections:
Understanding UDLD, page 28-1
Configuring UDLD, page 28-3
Displaying UDLD Status, page28-7

Understanding UDLD

UDLD is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet
cables to monitor the physical configuration of the cables and detect when a unidirectional link exists.
All connected devices must support UDLD for the protocol to successfully identify and disable
unidirectional links. When UDLD detects a unidirectional link, it disables the affected port and aler ts
you. Unidirectional links can cause a variety of problems, including spanning-tree topology loops.

Modes of Operation

UDLD supports two modes of operation: normal (the default) and aggr e ssive. In norm al mo de, U DL D
can detect unidirectional links due to misconnected ports on fiber-optic con nections. In aggressive mode,
UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links
and to misconnected ports on fiber-optic links.
In normal and aggressive modes, UDLD works with the Layer 1 mechanisms to learn the physical status
of a link. At Layer 1, autonegotiation takes care of physical s ig na ling a nd fau lt det ecti on. UD LD
performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors an d
shutting down misconnected ports. When you enable both autonegotiation and UDLD, the Layer 1 and
Layer 2 detections work together to prevent physical and logical unidirectional connections and the
malfunctioning of other protocols.
A unidirectional link occurs whenever traffic sent by a local device is recei v ed by its neighbor b ut traf f ic
from the neighbor is not received by the local device.