This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol in
Release 12.1(2)E and later releases on the Catalyst 6500 series switches.
For complete syntax and usage information for the commands used in this chapter, refer to the
Catalyst 6500 Series Switch Cisco IOS Command Reference publication.
This chapter consists of these sections:
Understanding How UDLD Works
These sections describe how UDLD works:
The UDLD protocol allows devices connected through fiber-optic or copper (for example, Category 5
cabling) Ethernet cables connected to LAN ports to monitor the physical configuration of the cables and
detect when a unidirectional link exists. When a unidirectional link is detected, UDLD shuts down the
affected LAN port and alerts the user. Unidirectional links can cause a variety of problems, including
spanning tree topology loops.
UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine 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 LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2
detections work together to prevent physical and logical unidirectional connections and the
malfunctioning of other protocols.
Understanding How UDLD Works, page 32-1
Default UDLD Configuration, page 32-3
Configuring UDLD, page 32-3
UDLD Overview, page 32-1
UDLD Aggressive Mode, page 32-2
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
C H A P T E R