Uni-Directional Link Detection (UDLD) monitors a link between two Ruckus devices and brings the ports on both ends of the link down if the link goes down at any point between the two devices. This feature is useful for links that are individual ports and for trunk links. The following figure shows an example.
Normally, a Ruckus device load balances traffic across the ports in a trunk group. In this example, each Ruckus device load balances traffic across two ports. Without the UDLD feature, a link failure on a link that is not directly attached to one of the Ruckus devices is undetected by the Ruckus devices. As a result, the Ruckus devices continue to send traffic on the ports connected to the failed link.
When UDLD is enabled on the trunk ports on each Ruckus device, the devices detect the failed link, disable the ports connected to the failed link, and use the remaining ports in the trunk group to forward the traffic.
Ports enabled for UDLD exchange proprietary health-check packets once every second (the 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 two more intervals. If the port still does not receive a health-check packet after waiting for three intervals, the port concludes that the link has failed and takes the port down.