CHAPT ER
28-1
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
28
Configuring UDLD
This chapter describes how to configure UniDirectional Link Detection (UDLD) Ethernet on a Catalyst
switch.
This chapter includes the following major sections:
About UDLD, page 28-1
Default UDLD Configuration, page 28-4
Configuring UDLD on the Switch, page 28-4
Displaying UDLD Link Status, page 28-9
Note For complete syntax and usage information for the switch commands used in this chapter, look at the
Cisco Catalyst 4500 Series Switch Command Reference and related publications at this location:
http://www.cisco.com/en/US/products/hw/switches/ps4324/index.html
If the command is not found in the Catalyst 4500 Command Reference, it is located in the larger Cisco
IOS library. Refer to the Cisco IOS Command Reference and related publications at this location:
http://www.cisco.com/en/US/products/ps6350/index.html

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.