32-2
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 32 Configuring UDLD
Default UDLD Configuration
The switch periodically transmits the UDLD messages (packets) to the neighbor devices on the ports that
have 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. The devices on both ends of the link must support UDLD in order for the protocol to successfully
identify and disable the 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 the copper ports to avoid sending unnecessary control traffic
on this type of media since it is often used for the access ports.
Figure 32-1
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 32-1
Unidirectional Link
Default UDLD Configuration
Table 32-1
shows the default UDLD configuration.
Switch X
Switch Y
Switch Z
Tx
Rx
Tx
Rx
Rx
Tx
144608
Table 32-1
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
