CHAPTER
23-1
Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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23
Configuring UDLD
This chapter describes how to configure the UniDirectional Link Det ecti on (UD LD) prot ocol o n th e
Cisco ME 3400 Ethernet Access switch.
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 23-1
•Configuring UDLD, page 23-4
•Displaying UDLD Status, page 23-6

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 physic al statu s
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.