Unidirectional Link Detection: Difference between revisions

Revision as of 17:13, 22 May 2009

Revision as of 20:16, 2 March 2011

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'''U'''ni'''D'''irectional '''L'''ink '''D'''etection (UDLD) is a [[data link layer]] [[Communications protocol|protocol]] from [[Cisco Systems]] to monitor the physical configuration of the cables and detect [[unidirectional link]]s. 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 [[optical fiber|fibres]]s, one used for sending from A to B, and other fromfor sending from B to A, itthe link is a bidirectional link (two-way link).

If one of these fibres areis broken, we'rethe leftlink withhas abecome unidirectional link (one-way link)or unidirectional.

The goal of the UDLD protocol is to detect whenever a broken bidirectional link is either broken, such(e.g. thattransmitted [[packet (information technology)|packetpackets]]s aredo sentnot onlyarrive fromat onethe device to anotherreceiver), or connected wrongly, such that the fibres are connected to different [[computer port (hardware)|ports]].

For each device and for each port, a UDLD packet is sent to the port it links to. The packet contains informationsender onidentity who sendsinformation (device and port), and whomexpected it'sreceiver supposedidentity to be sent toinformation (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). Similar functionality in a standardized form is provided as part of the [[Ethernet]] [[OA&M|OAM]] protocol that is defined as part of the [[Ethernet in the First Mile]] changes to [[802.3]] (previously [[802.3ah]]).