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Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
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Chapter 30 Configuring UDLD
Default UDLD Configuration
The switch periodically transmits UDLD messages (packets) to neighbor devices on ports with UDLD
enabled. If the messages are echoed back to the sender within a specific time frame and they are lacking
a specific acknowledgment (echo), the link is flagged as unidirectional and the port is shut down.
Devices on both ends of the link must support UDLD in order for the protocol to successfully identify
and disable unidirectional links.
Note
With supervisor engine software release 5.4(3) and later releases, you can specify the message
interval between UDLD messages. Previously, the message interval was fixed at 60 seconds. With a
configurable message interval, UDLD reacts much faster to link failures.
Note
By default, UDLD is locally disabled on copper ports to avoid sending unnecessary control traffic
on this type of media since it is often used for access ports.
Figure 30-1
shows an example of a unidirectional link condition. Each switch can send packets to a
neighbor switch but is not able to receive packets from the same switch that it is sending packets to. UDLD
detects and disables these one-way connections.
Figure 30-1 Unidirectional Link
Default UDLD Configuration
Table 30-1
shows the default UDLD configuration.
Switch X
Switch Y
Switch Z
Tx
Rx
Tx
Rx
Rx
Tx
144608
Table 30-1 UDLD Default Configuration
Feature
Default Value
UDLD global enable state
Globally disabled
UDLD per-port enable state for fiber-optic media Enabled on all Ethernet fiber-optic ports
UDLD per-port enable state for twisted-pair
(copper) media
Disabled on all Ethernet 10/100 and
1000BASE-TX ports
UDLD message interval
15 seconds
UDLD aggressive mode
Disabled
