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 which is used to eliminate switching loops.
If two devices, A and B, are connected via a pair of optical fibers, one used for sending from A to B and other for sending from B to A, the link is bidirectional (two-way). If one of these fibres is broken, the link has become one-way or unidirectional. The goal of the UDLD protocol is to detect a broken bidirectional link (e.g. transmitted packets do not arrive at the receiver, or 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 sender identity information (device and port), and expected receiver identity information (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. Switches from 3Com and its Enterprise division H3C have a similar feature called Device Link Detection Protocol (DLDP). Switches from Extreme Networks have a similar feature called Extreme Link Status Monitoring (ELSM). 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). D-Link has their DULD feature built on top of Ethernet OAM function. Brocade devices also have support for UDLD.
The need for UDLD has been eliminated when operating over 10GbE, as per 802.3ae/D3.2, when a fault is detected in the physical link:
- The local device signals local fault is signaled by PHY
- The local device ceases transmission of MAC frames and transmits remote fault
- The remote device receives remote fault and stops sending frames and continuously generates idle frames
- Understanding and Configuring the Unidirectional Link Detection Protocol Feature at Cisco Systems
- "10Gig Link Fault Signaling Updated to IEEE Draft P802.3ae/D3.2" (pdf).
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