Chapter 20 Configuring Udld; Understanding Udld; Modes Of Operation - Cisco Catalyst 3750 Software Configuration Manual

Metro switch

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Configuring UDLD

This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol on the

Catalyst 3750 Metro switch.

For complete syntax and usage information for the commands used in this chapter, refer to the command

reference for this release.

This chapter consists of these sections:

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 interface and alerts you. Unidirectional links can cause a variety of problems, including

spanning-tree topology loops.

UDLD supports two modes of operation: normal (the default) and aggressive. In normal mode, UDLD

can detect unidirectional links due to misconnected ports on fiber-optic connections. In aggressive mode,

UDLD also can detect unidirectional links due to one-way traffic on fiber-optic and on twisted-pair links

and to misconnected ports on fiber-optic links.

In normal and in aggressive mode, UDLD works with the Layer 1 mechanisms to learn the physical

status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD

performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and

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 connections and the

malfunctioning of other protocols.

Understanding UDLD, page 20-1

Configuring UDLD, page 20-4

Displaying UDLD Status, page 20-6

C H A P T E R

Catalyst 3750 Metro Switch Software Configuration Guide

Troubleshooting