CHAPTER
20-1
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
20
Configuring UDLD
This chapter describes how to configure the UniDirectional Link Detection (UDLD) and Unidirectional
Ethernet on the Catalyst 4500 series switch. It also provides guidelines, procedures, and configuration
examples.
This chapter includes the following major sections:
•Overview of UDLD, page 20-1
•Default UDLD Configuration, page 20-2
•Configuring UDLD on the Switch, page 20-2
Note For complete syntax and usage information for the switch commands used in this chapter, refer to the
Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/index.htm.

Overview of UDLD

UDLD allows devices connected through fiber-optic or copper Ethernet cables (for example, Ca tegory 5
cabling) to monitor the physical configuration of the cables and detect when a unidirectional link exists.
A unidirectional link occurs whenever traffic transmitted by the local device over a link is received by
the neighbor but traffic transmitted from the neighbor is not received by the local device. When a
unidirectional link is detected, UDLD shuts down the affected interface and alerts the user.
Unidirectional links can cause a variety of problems, including spanning tree topology loops.
UDLD is a Layer2 protocol that 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. UD LD performs
tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting
down misconnected interfaces. When you enable both autonegotiation a nd UDLD, Layer 1 and Layer 2
detections work together to prevent physical and logical unidirectional connections and the
malfunctioning of other protocols.
If one of the fiber strands in a pair is disconnected, as long as autonegotiation i s active, the link does not
stay up. In this case, the logical link is undetermined, a nd UDLD does not take any action. If both fibers
are working normally from a Layer 1 perspective, then UDLD at Layer 2 determines whether or not those
fibers are connected correctly and whether or not traffic is flowing bidirectionally between the right
neighbors. This check cannot be performed by autonegotiation because autonegotiation opera tes at
Layer 1.