C H A P T E R
32-1
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
32
Configuring UDLD
This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol on the
Catalyst 6500 series switches.
Note
For complete syntax and usage information for the commands that are used in this chapter, refer to the
Catalyst 6500 Series Switch Command Reference
publication.
This chapter consists of these sections:
•
Understanding How UDLD Works, page 32-1
•
Default UDLD Configuration, page 32-2
•
Configuring UDLD on the Switch, page 32-3
Understanding How UDLD Works
The UDLD protocol allows devices that are connected through the fiber-optic or copper (for example,
Category 5 cabling) Ethernet cables 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 port
and alerts the user. The 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 mechanisms to determine the physical status of
a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs
the tasks that autonegotiation cannot perform such as detecting the identities of neighbors and shutting
down the misconnected ports. When you enable both autonegotiation and UDLD, the Layer 1 and
Layer 2 detections work together to prevent the physical and logical unidirectional connections and the
malfunctioning of other protocols.
A unidirectional link occurs whenever the traffic that is transmitted by the local device over a link is
received by the neighbor but the traffic that is transmitted from the neighbor is not received by the local
device. If one of the fiber 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 fibers are working normally from a Layer 1 perspective, then UDLD at Layer 2 determines whether
those fibers are connected correctly and whether the traffic is flowing bidirectionally between the right
neighbors. This check cannot be performed by autonegotiation, because autonegotiation is a Layer 1
mechanism.