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Cisco IE 2000 Switch Software Configuration Guide
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Configuring UDLD
Finding Feature Information
Your software release may not support all the features documented in this chapter. For the latest feature
information and caveats, see the release notes for your platform and software release.
Use Cisco Feature Navigator to find information about platform support an d Cisco software image
support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on
Cisco.com is not required.

Prerequisites for UDLD

When configuring the mode (normal or aggressive), make sure that the same mode is configured on
both sides of the link.

Restrictions for UDLD

UDLD is not supported on ATM ports.
A UDLD-capable port cannot detect a unidirectional link if it is connected to a UDLD-incapable
port of another switch.
Loop guard works only on point-to-point links. We recommend that each end of the link has a
directly connected device that is running STP.

Information About UDLD

UDLD

UniDirectional Link Detection (UDLD) is a Layer 2 protocol that enables devices connected through
fiber-optic or twisted-pair Ethernet cables to monitor the physical configurat ion 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
disables the affected port and alerts you. Unidirectional links can cause a variety of problems, including
spanning-tree topology loops.