CHAPTER
23-1
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
23
Configuring UDLD
This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol on your
Catalyst 3550 switch.
Note 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:
• Understanding UDLD, page 23-1
• Configuring UDLD, page 23-4
• Displaying UDLD Status, page 23-7
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 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.
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