Ports enabled for UDLD exchange health-check packets once every five seconds (the link-keepalive interval). If a
port does not receive a health-check packet from the port at the other end of the link within the keepalive interval,
the port waits for four more intervals. If the port still does not receive a health-check packet after waiting for five
intervals, the port concludes that the link has failed and blocks the UDLD-enabled port.
When a port is blocked by UDLD, the event is recorded in the switch log or via an SNMP trap (if configured); and
other port blocking protocols, like spanning tree or meshing, will not use the bad link to load balance packets. The
port will remain blocked until the link is unplugged, disabled, or fixed. The port can also be unblocked by disabling
UDLD on the port.
Configuring UDLD
When configuring UDLD, keep the following considerations in mind:
• UDLD is configured on a per-port basis and must be enabled at both ends of the link. See the note below for a
list of switches that support UDLD.
• To configure UDLD on a trunk group, you must configure the feature on each port of the group individually.
Configuring UDLD on a trunk group's primary port enables the feature on that port only.
• Dynamic trunking is not supported. If you want to configure a trunk group that contains ports on which UDLD is
enabled, you must remove the UDLD configuration from the ports. After you create the trunk group, you can
re-add the UDLD configuration.
Configuring uni-directional link detection (UDLD) (CLI)
For detailed information about UDLD, see Uni-directional link detection (UDLD) on page 78.
Syntax:
[no] interface <port-list> link-keepalive
Enables UDLD on a port or range of ports.
To disable this feature, enter the no form of the command.
Default: UDLD disabled
Syntax:
link-keepalive interval <interval>
Determines the time interval to send UDLD control packets. The interval parameter specifies how often the ports
send a UDLD packet. You can specify from 10 to 100, in 100-ms increments, where 10 is 1 second, 11 is 1.1
seconds, and so on.
Default: 50 (5 seconds)
Syntax:
link-keepalive retries <num>
Determines the maximum number of retries to send UDLD control packets. The num parameter specifies the
maximum number of times the port will try the health check. You can specify a value from 3 to 10.
Default: 5
Syntax:
[no] interface <port-list> link-keepalive vlan <vid>
Chapter 3 Port Status and Configuration 79
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