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CHAPTER
19-1
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
19
Configuring UDLD
This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol on your
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 19-1
• Configuring UDLD, page 19-3
• Displaying UDLD Status, page 19-6
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.
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, Layer 1 and Layer 2
detections work together to prevent physical and logical unidirectional connections and the
malfunctioning of other protocols.
A unidirectional link occurs whenever traffic sent by the local device is received by the neighbor but
traffic from the neighbor is not received by the local device. If one of the fiber-optic strands in a pair is
disconnected, as long as autonegotiation is active, the link does not stay up. In this case, the logical link
is undetermined, and UDLD does not take any action. If both fiber-optic strands are working normally
from a Layer 1 perspective, UDLD at Layer 2 determines whether those fiber-optic strands are
connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This
check cannot be performed by autonegotiation because autonegotiation operates at Layer 1.