2.1.b [ii] UDLD
Unidirectional Link Detection (UDLD) protocol can help to prevent forwarding loops and black holing of traffic in switched networks.
UDLD is a L2 protocol that works with the L1 mechanisms to determine the physical status of a link. At layer 1, auto-negotiation takes care of physical signaling and fault detection (FLP or fast link pulses are sent during auto-negotiation for copper Ethernet links). UDLD performs tasks that auto-negotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports. When you enable both auto-negotiation and UDLD, layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.
UDLD works by exchanging protocol packets between the neighboring devices. In order for UDLD to work, both devices on the link must support UDLD and have it enabled on respective ports. Each switch port configured for UDLD sends protocol packets that contain the port’s own device/ port ID, and the neighbor’s device/ port IDs seen by UDLD on that port. Neighboring ports should see their own device/ port ID (echo) in the packets received from the other side. If the port does not see its own device/ port ID in the incoming UDLD packets for a specific duration of time, the link is considered unidirectional.
It is recommended to keep Tdetection < Treconvergence by choosing an appropriate message interval which ensures that UDLD is detected before STP forward delay expires.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1538-1545). . Kindle Edition.