Unidirectional Link Detection
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UniDirectional Link Detection (UDLD) is a data link layer protocol from Cisco Systems to monitor the physical configuration of the cables and detect unidirectional links. UDLD complements the Spanning Tree Protocol (STP) which is used to eliminate layer 2 loops.
If two devices, A and B, are connected via a pair of fibres, one used for sending from A to B, other from sending from B to A, it is a bidirectional link (two-way link). If one of these fibres are broken, we're left with a unidirectional link (one-way link).
The goal of the UDLD protocol is to detect whenever a bidirectional link is either broken, such that packets are sent only from one device to another, or connected wrongly, such that the fibres are connected to different ports.
For each device and for each port, a UDLD packet is sent to the port it links to. The packet contains information on who sends (device and port), and whom it's supposed to be sent to (device and port). Each port checks that the UDLD packets it receives, contain the identifiers of his own device and port.
UDLD is a Cisco-proprietary protocol. Similar functionality in a standardized form is provided as part of the Ethernet OAM protocol that is defined as part of the Ethernet in the First Mile changes to 802.3 (previously 802.3ah).
