Unidirectional Link Detection (UDLD) is a

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

protocol from

Cisco Systems Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational corporation, multinational digital communications technology conglomerate (company), conglomerate corporation headquartered in San Jose, California. Cisco develo ...

to monitor the physical configuration of the cables and detect unidirectional links. UDLD complements the

Spanning Tree Protocol The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also al ...

which is used to eliminate

switching loop A switching loop or bridge loop occurs in computer networks when there is more than one layer 2 path between two endpoints (e.g. multiple connections between two network switches or two ports on the same switch connected to each other). The 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 " 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 An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...

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 packets do not arrive at the receiver, or the fibers are connected to different

ports A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...

). 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,

Extreme Networks Extreme Networks is an American networking company based in San Jose, California. Extreme Networks designs, develops, and manufactures wired and wireless network infrastructure equipment and develops the software for network management, policy, a ...

, and

AVAYA Avaya Holdings Corp., often shortened to Avaya (), is an American multinational technology company headquartered in Durham, North Carolina, that provides cloud communications and workstream collaboration services. The company's platform includ ...

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 Brocade is a class of richly decorative shuttle-woven fabrics, often made in colored silks and sometimes with gold and silver threads. The name, related to the same root as the word "broccoli", comes from Italian ''broccato'' meaning "embos ...

Ruckus Networks Ruckus Networks (formerly known as Ruckus Wireless) is a brand of wired and wireless networking equipment and software owned by CommScope. Ruckus offers Network switch, Switches, Wi-Fi Wireless access point, access points, Citizens Broadband Radi ...

ICX Switches offer this feature as Uni-Directional Link Detection(UDLD). Similar functionality in a standardized form is provided as part of the

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

OAM protocol that is defined as part of the

Ethernet in the First Mile Ethernet in the first mile (EFM) refers to using one of the Ethernet family of computer network technologies between a telecommunications company and a customer's premises. From the customer's point of view, it is their first mile, although from th ...

changes to 802.3 (previously 802.3ah).

D-Link D-Link Corporation is a Taiwanese multinational networking equipment manufacturing corporation headquartered in Taipei, Taiwan. It was founded in March 1986 in Taipei as ''Datex Systems Inc.'' History D-Link Corporation changed its name fr ...

has their DULD feature built on top of

OAM function.

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: * 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

Understanding and Configuring the Unidirectional Link Detection Protocol Feature
at

* {{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 Link protocols Cisco protocols