Chapter 23 Configuring Udld; Understanding How Udld Works - Cisco WS-C2948G-GE-TX Configuration Manual

Configuring UDLD
This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol on the
Catalyst enterprise LAN switches.
Note For complete syntax and usage information for the commands that are used in this chapter, refer to the
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Command
Reference.
This chapter consists of these sections:
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Understanding How UDLD Works

The UDLD protocol allows devices that are connected through fiber-optic or copper Ethernet cables (for
example, Category 5 cabling) 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. Unidirectional links can cause a variety of problems, including spanning-tree
topology loops.
UDLD is a Layer 2 protocol that works with Layer 1 mechanisms, such as autonegotiation, to determine
the physical status of a link. At Layer 1, autonegotiation handles physical signaling and fault detection.
UDLD also performs tasks that autonegotiation cannot perform, such as detecting the identities of
neighbors and shutting down misconnected ports. When both autonegotiation and UDLD are enabled,
Layer 1 and Layer 2 detection features can work together to prevent physical and logical unidirectional
connections and malfunctioning of other protocols.
A unidirectional link occurs whenever traffic that is transmitted by the local device over a link is received
by the neighbor, but traffic that is transmitted from the neighbor is not received by the local device. For
example, 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 situation, the logical link is undetermined, and UDLD does not take any actions.
If both fibers are working normally at Layer 1, then UDLD at Layer 2 determines whether those fibers
are connected correctly and whether traffic is flowing bidirectionally between the correct neighbors.
This check cannot be performed by autonegotiation, because autonegotiation is a Layer 1 feature.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Understanding How UDLD Works, page 23-1
UDLD Software and Hardware Requirements, page 23-2
Default UDLD Configuration, page 23-2
Configuring UDLD on the Switch, page 23-3
C H A P T E R
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