Cisco Systems 3750E manual Configuring Udld, Understanding Udld, Modes of Operation, 29-1

C H A P T E R 29

Configuring UDLD

This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol on the Catalyst 3750-E or 3560-E switch. Unless otherwise noted, the term switch refers to a Catalyst 3750-E or 3560-E standalone switch and to a Catalyst 3750-E 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 29-1

•Configuring UDLD, page 29-3

•Displaying UDLD Status, page 29-7

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 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 ports 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 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 signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and 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.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 29-1