Chapter 30 Configuring Udld; Understanding How Udld Works - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Configuring UDLD
This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol on the
Catalyst 6000 family switches.
For complete syntax and usage information for the commands used in this chapter, refer to the
Note
Catalyst 6000 Family Command Reference publication.
This chapter consists of these sections:
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Understanding How UDLD Works

The UDLD protocol allows devices connected through 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. 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
tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting
down misconnected ports. 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 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. 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 traffic is flowing bidirectionally between the right neighbors. This
check cannot be performed by autonegotiation, because autonegotiation is a Layer 1 mechanism.
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Understanding How UDLD Works, page 30-1
Default UDLD Configuration, page 30-2
Configuring UDLD, page 30-3
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
C H A P T E R
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