Cisco Catalyst 4500 Series, Software Configuration Manual

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C H A P T E R
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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
33
Configuring UDLD
This chapter describes how to configure UniDirectional Link Detection (UDLD) Ethernet on the switch,
and includes the following major sections:
•
About UDLD, page 33-1
• Default UDLD Configuration, page 33-3
•
Configuring UDLD on the Switch, page 33-4
• Displaying UDLD Link Status, page 33-9
Note For complete syntax and usage information for the switch commands used in this chapter, see the
Cisco IOS Command Reference Guides for the Catalyst 4500 Series Switch .
If a command is not in the Cisco Catalyst 4500 Series Switch Command Reference , you can locate it in
the Cisco IOS Master Command List, All Releases .
About UDLD
UDLD is a Layer 2 protocol that initializes devices connected through fiber-optic or twisted-pair
Ethernet cables. This protocol monitors a physical connection (such as wrong cabling) to detect
unidirectional links to avoid spanning-tree topology loops or silent drop traffic.
All connected devices must support UDLD for the protocol to successfully identify the unidirectional
links. When UDLD detects a unidirectional link, it can administratively shut down the affected port and
send you a warning message.
With UDLD, the time to detect a unidirectional link can vary from a few seconds to several minutes
depending on how the timers are configured. Link status messages are exchanged every couple of
seconds.
Starting with Cisco IOS Release 12.2(54)SG, the enhancement Fast UDLD was added, which supports
timers in the few-hundred milliseconds range, which enables subsecond unidirectional link detection.
With Fast UDLD, the time to detect a unidirectional link can vary from less than one second to a few
seconds (the detection time also depends on how the timers are configured). Link status messages are
exchanged every couple of hundred milliseconds.