Unidirectional Link Detection: Difference between revisions

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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.

'''U'''ni'''D'''irectionalUnidirectional Link Detection'''L'''ink ('''DUDLD'''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]]slinks. UDLD complements the [[Spanning Tree Protocol]] (STP) which is used to eliminate [[layerswitching 2loop]] loopss.

If two devices, A and B, are connected via a pair of [[optical fiber|fibres]], one used for sending from A to B, other from sending from B to A, it is a bidirectional link (two-way link).

:* Unidirectional Link Detection (UDLD) is one of two major features (UDLD and loop guard) in Cisco Switches to prevent Layer 2 loops.

If one of these fibres are broken, we're left with a unidirectional link (one-way link).

:* Spanning-Tree Protocol (STP) resolves redundant physical topology into a loop-free, tree-like forwarding topology via blocking one or more ports.

: However, Unidirectional Link failure can cause "[[Black hole (networking)|traffic blackholing]]" and loops in the Switch topology.

:* In order to detect the unidirectional links before the forwarding loop is created, UDLD works by exchanging protocol packets between the neighboring devices.

:* In order for UDLD to work, both switch devices on the link must support UDLD and have it enabled on respective ports.

==Description==

The goal of the UDLD protocol is to detect whenever a bidirectional link is either broken, such that [[packet (information technology)|packet]]s are sent only from one device to another, or connected wrongly, such that the fibres are connected to different [[computer port (hardware)|ports]].

If two devices, A and B, are connected via a pair of [[optical fiber]]s, 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 fibers 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 [[packet (information technology)|packets]] do not arrive at the receiver, or the fibers 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. Switchesbut fromHP, [[3ComExtreme Networks]], and its[[Avaya|AVAYA]] Enterprise division H3Call have a similar feature calledcalling it by a different name. HP calls theirs Device Link Detection Protocol (DLDP). Extreme SimilarNetworks functionalitycall init aExtreme standardizedLink formStatus isMonitoring provided(ELSM) asand partAVAYA ofcalls thetheirs, Link-state Tracking. [[EthernetBrocade_Communications_Systems|Brocade]] /[[OA&M|OAMRuckus Networks]] protocolICX thatSwitches isoffer definedthis feature as partUni-Directional of the [[Ethernet in the First Mile]] changes to [[802.3]]Link Detection(previously [[802.3ah]]UDLD).

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]]). [[D-Link]] has their DULD feature built on top of [[Ethernet]] [[OA&M|OAM]] function. [[Brocade Communications Systems|Brocade]] devices running Ironware support a proprietary form of UDLD.

== External links ==

The use of UDLD over 10GbE is augmented, as per 802.3ae/D3.2 standard, 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

== External links ==

* [http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a008009477b.shtml Understanding and Configuring the Unidirectional Link Detection Protocol Feature] at [[Cisco Systems]]

* {{cite web |url=https://www.iol.unh.edu/sites/default/files/knowledgebase/10gec/10GbE_fault_signaling.pdf |title=10Gig Link Fault Signaling Updated to IEEE Draft P802.3ae/D3.2}}

*http://docs.ruckuswireless.com/fastiron/08.0.30/fastiron-08030-l2guide/GUID-CE29A0CF-07A6-40C3-A999-9AF5FB8AC020.html

[[Category:Link protocols]]

[[Category:Cisco protocols]]

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