CHAPTER
16-1
Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
24R9746
16
Configuring UDLD
This chapter describes how to configure the UniDirectional Link Detect ion (UDLD) protocol on your
Cisco Systems Intelligent Gigabit Ethernet Switch Module.
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 16-1
•Configuring UDLD, page 16-4
•Displaying UDLD Status, page 16-7

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 administratively shuts down the
affected port and alerts 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 aggressive. In normal mode, UDLD
can detect unidirectional links due to misconnected interfaces on fiber-optic connections. In aggressive
mode, UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair
links and to misconnected interfaces on fiber-optic links.
In normal and aggressive modes, UDLD works with the Layer 1 mechanisms to determine the physical
status of a link. At Layer 1, autonegotiation takes care of physical signa ling and fault detection. UDLD
performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors an d
shutting down misconnected interfaces. When you enable both autonegotiation and UDLD, the Layer 1
and Layer 2 detections work together to prevent physical and logical unidirectional conne ctions and the
malfunctioning of other protocols.