Unidirectional Link Detection: Difference between revisions

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{{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 looploops 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.

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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 but [[HP Networking Products|HP]], [[Extreme Networks]], and [[Avaya|AVAYA]] all have a similar feature calling it by a different name. HP calls theirs Device Link Detection Protocol (DLDP). Extreme Networks call it Extreme Link Status Monitoring (ELSM) and AVAYA calls theirs, Link-state Tracking. [[Brocade_Communications_Systems|Brocade]]/[[Ruckus Networks]] ICX Switches offer this feature as Uni-Directional Link Detection(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 Firstfirst Milemile]] 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 use of UDLD over 10GbE is augmented, as per 802.3ae/D3.2 standard, when a fault is detected in the physical link:

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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/sites/default/files/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]]