Management and Configuration Guide K/KA/KB.15.15

Uni-directional link detection (UDLD)
Uni-directional link detection (UDLD) monitors a link between two HP switches and blocks the ports
on both ends of the link if the link fails at any point between the two devices. This feature is
particularly useful for detecting failures in fiber links and trunks. Figure 35 (page 106) shows an
example.
Figure 35 UDLD
In this example, each HP switch load balances traffic across two ports in a trunk group. Without
the UDLD feature, a link failure on a link that is not directly attached to one of the HP switches
remains undetected. As a result, each switch continue to send traffic on the ports connected to the
failed link. When UDLD is enabled on the trunk ports on each HP switch, the switches detect the
failed link, block the ports connected to the failed link, and use the remaining ports in the trunk
group to forward the traffic.
Similarly, UDLD is effective for monitoring fiber optic links that use two uni-direction fibers to transmit
and receive packets. Without UDLD, if a fiber breaks in one direction, a fiber port may assume
the link is still good (because the other direction is operating normally) and continue to send traffic
on the connected ports. UDLD-enabled ports; however, will prevent traffic from being sent across
a bad link by blocking the ports in the event that either the individual transmitter or receiver for
that connection fails.
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.
106 Port Status and Configuration