Unidirectional Link Detection: Difference between revisions

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{{unreferencedno footnotes|date=JanuaryMay 20082019}}

{{More citations needed|date=January 2008}}

'''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 loop]]s.

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

:* 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==

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 fibresfibers 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 fibresfibers 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 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. Switchesbut fromHP, [[3ComExtreme Networks]], and its Enterprise division[[Avaya|AVAYA]] H3Call have a similar feature calledcalling it by a different name. HP calls theirs Device Link Detection Protocol (DLDP). Switches from [[Extreme Networks]] havecall a similar feature calledit Extreme Link Status Monitoring (ELSM). Similarand functionalityAVAYA incalls atheirs, standardizedLink-state form is provided as part of theTracking. [[Ethernet]] [[OA&MBrocade_Communications_Systems|OAMBrocade]] protocol that is defined as part of the /[[EthernetRuckus in the First MileNetworks]] changesICX toSwitches [[802.3]]offer (previously [[802.3ah]]). D-Link has their DULDthis feature builtas onUni-Directional topLink of [[Ethernet]] [[OA&M|OAM]] functionDetection(UDLD). Brocade devices also have support for 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.

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 needuse forof UDLD hasover been10GbE eliminatedis when operating over 10GbEaugmented, 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

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==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/servicessites/testingdefault/10gecfiles/knowledgebase/10gec/10GbE_fault_signaling.pdf |format=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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