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. (using the trademark Cisco) is an American multinational corporation, multinational digital communications technology conglomerate (company), conglomerate corporation headquartered in San Jose, California. Cisco develops, m ...

to monitor the physical configuration of 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 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 loo ...

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, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...

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 Ports collections (or ports trees, or just ports) are the sets of makefiles and Patch (Unix), patches provided by the BSD-based operating systems, FreeBSD, NetBSD, and OpenBSD, as a simple method of installing software or creating binary packages. T ...

). 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, Inc. is an American networking company based in Morrisville, North Carolina. Extreme Networks designs, develops, and manufactures wired and wireless network infrastructure equipment and develops the software for network managem ...

, and

AVAYA Avaya LLC(), formerly Avaya Inc., is an American multinational technology company headquartered in Morristown, New Jersey, that provides cloud communications and workstream collaboration services. The company's platform includes unified commun ...

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 (weaving), shuttle-woven fabrics, often made in coloured silks and sometimes with gold and silver threads. The name, related to the same root as the word "broccoli", comes from Italian langua ...

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 switches, Wi-Fi access points, CBRS access points, controllers, management systems, cloud m ...

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 t ...

changes to

802.3 IEEE 802.3 is a working group and a collection of standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of the Institute of Electrical and Electro ...

(previously 802.3ah).

D-Link D-Link Systems, Inc. (formerly Datex Systems, Inc.) is a Taiwanese multinational manufacturer of networking hardware and telecoms equipments. It was founded in 1986 and headquartered in Taipei, Taiwan. History Datex Systems was founded i ...

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