View source for Link Layer Discovery Protocol

{{short description|Protocol used by network devices for advertising their identity}}
{{Redirect|LLDP|the left lateral decubitus position|Lying (position)}}

{{Internet protocol suite|link=LLDP}}

The '''Link Layer Discovery Protocol''' ('''LLDP''') is a vendor-neutral [[link layer]] protocol used by [[network device]]s for advertising their identity, capabilities, and neighbors on a [[local area network]] based on [[IEEE 802]] technology, principally [[wired Ethernet]].<ref>{{cite web
|title=802.1AB-REV - Station and Media Access Control Connectivity Discovery
|url=http://www.ieee802.org/1/pages/802.1AB-rev.html
|publisher=IEEE
|access-date=2009-10-17
}}</ref> The protocol is formally referred to by the IEEE as ''Station and Media Access Control Connectivity Discovery'' specified in '''IEEE 802.1AB''' with additional support in IEEE 802.3 section 6 clause 79.<ref>{{cite web|url=https://standards.ieee.org/ieee/802.1AB/6047/|title=IEEE 802.1AB-2016 - IEEE Standard for Local and metropolitan area networks - Station and Media Access Control Connectivity Discovery}}</ref>

LLDP performs functions similar to several [[proprietary protocol]]s, such as [[Cisco Discovery Protocol]], [[Foundry Discovery Protocol]], [[Nortel Discovery Protocol]] and [[Link Layer Topology Discovery]].

==Information gathered==
Information gathered with LLDP can be stored in the device [[management information base]] (MIB) and queried with the [[Simple Network Management Protocol]] (SNMP) as specified in {{IETF RFC|2922}}. The topology of an LLDP-enabled network can be discovered by ''crawling'' the hosts and querying this database. Information that may be retrieved include:
* System name and description
* [[Port (computer networking)|Port]] name and description
* [[VLAN]] name
* IP management address
* System capabilities ([[Network switch|switching]], [[routing]], etc.)
* [[Medium access control|MAC]]/[[PHY]] information
* [[Power over Ethernet|MDI power]]
* [[Link aggregation]]

==Applications==
The Link Layer Discovery Protocol may be used as a component in [[network management]] and [[network monitoring]] applications.

One such example is its use in [[data center bridging]] requirements. The '''{{visible anchor|Data Center Bridging Capabilities Exchange Protocol}}''' (DCBX) is a discovery and capability exchange protocol that is used for conveying capabilities and configuration of the above features between neighbors to ensure consistent configuration across the network.<ref>{{cite web
|title=DCB Capabilities Exchange Protocol Base Specification, Rev 1.01
|url=http://www.ieee802.org/1/files/public/docs2008/az-wadekar-dcbx-capability-exchange-discovery-protocol-1108-v1.01.pdf
|author=Qlogic
|display-authors=etal
|publisher=IEEE 802
}}</ref>

LLDP is used to advertise [[power over Ethernet]] capabilities and requirements and negotiate power delivery.

===Media endpoint discovery extension {{anchor|LLPD-MED}} ===
''Media Endpoint Discovery'' is an enhancement of LLDP, known as '''LLDP-MED''', that provides the following facilities:
* Auto-discovery of LAN policies (such as VLAN, [[802.1q|Layer 2 Priority]] and [[Differentiated services]] (Diffserv) settings) enabling [[plug and play]] networking.
* Device location discovery to allow creation of location databases and, in the case of [[Voice over Internet Protocol]] (VoIP), [[Enhanced 911]] services.
* Extended and automated power management of [[Power over Ethernet]] (PoE) endpoints.
* Inventory management, allowing network administrators to track their network devices, and determine their characteristics (manufacturer, software and hardware versions, serial or asset number).

The LLDP-MED protocol extension was formally approved and published as the standard ANSI/TIA-1057 by the [[Telecommunications Industry Association]] (TIA) in April 2006.<ref name="tia1057">{{cite web
 | url=http://www.tiaonline.org/standards/technology/voip/documents/ANSI-TIA-1057_final_for_publication.pdf
 | title=ANSI/TIA-1057 standard
 | archive-url = https://web.archive.org/web/20101008234510/https://tiaonline.org/standards/technology/voip/documents/ANSI-TIA-1057_final_for_publication.pdf
 | archive-date = 2010-10-08
 }}</ref>

== System Capability Codes ==
{| class="wikitable"
|+
!Code
!Capability
|-
|B
|Bridge (Switch)
|-
|C
|DOCSIS Cable Device
|-
|O
|Other
|-
|P
|Repeater
|-
|R
|Router
|-
|S
|Station
|-
|T
|Telephone
|-
|W
|WLAN Access Point
|}

==Frame structure==
LLDP information is sent by devices from each of their interfaces at a fixed interval, in the form of an [[Ethernet frame]].  Each frame contains one LLDP Data Unit (LLDPDU). Each LLDPDU is a sequence of [[type–length–value]] (TLV) structures.

The Ethernet frame used in LLDP typically has its destination [[MAC address]] set to a special [[multicast address]] that [[802.1D]]-compliant bridges do not forward. Other multicast and unicast destination addresses are permitted. The [[EtherType]] field is set to 0x88cc.

Each LLDP frame starts with the following mandatory TLVs: ''Chassis ID'', ''Port ID'', and ''Time-to-Live''. The mandatory TLVs are followed by any number of optional TLVs. The frame optionally ends with a special TLV, named ''end of LLDPDU'' in which both the ''type'' and ''length'' fields are 0.<ref>IEEE 802.1AB-2016 chapter 8.4</ref>

Accordingly, an Ethernet frame containing an LLDPDU has the following structure:

{| class="wikitable" style="text-align: center;"
|+ LLDP Ethernet frame structure
|-
! [[Preamble (communication)|Preamble]]
! Destination MAC
! Source MAC 
! [[Ethertype]]
! Chassis ID TLV
! Port ID TLV
! Time to live TLV
! Optional TLVs
! Optional End of LLDPDU TLV
! [[Frame check sequence]]
|-
| 
| align="left" | {{MACaddr|01:80:c2:00:00:0e}},&nbsp;or<br /> {{MACaddr|01:80:c2:00:00:03}},&nbsp;or<br /> {{MACaddr|01:80:c2:00:00:00}}
| Station's address
| 0x88CC 
| Type=1
| Type=2
| Type=3
| Zero or more complete TLVs
| Type=0, Length=0 
|
|}

Each of the TLV components has the following basic structure:

{| class="wikitable" style="text-align: center;"
|+ TLV structure
! Type !! Length !! Value
|-
| 7 bits || 9 bits || 0-511 octets
|}

{| class="wikitable"
|+ TLV type values<ref>IEEE 802.1AB ''8.4 Basic TLV Format''</ref>
! TLV type !! TLV name !! Usage in LLDPDU
|-
| 0 || End of LLDPDU || Optional
|-
| 1 || Chassis ID || Mandatory
|-
| 2 || Port ID || Mandatory
|-
| 3 || Time To Live || Mandatory
|-
| 4 || Port description || Optional
|-
| 5 || System name || Optional
|-
| 6 || System description || Optional
|-
| 7 || System capabilities || Optional
|-
| 8 || Management address || Optional
|-
| 9–126 || Reserved || -
|-
| 127 || Custom TLVs || Optional
|}

Custom TLVs<ref group=note>Termed ''Organizationally Specific TLVs'' by IEEE 802.1AB</ref> are supported via a TLV type 127.  The value of a custom TLV starts with a 24-bit organizationally unique identifier and a 1 byte organizationally specific subtype followed by data.  The basic format for an organizationally specific TLV is shown below:

{| class="wikitable" style="text-align: center;"
|+ Organizationally specific TLV
! Type 
! Length 
! Organizationally unique identifier (OUI) 
! Organizationally defined subtype 
! Organizationally defined information string
|-
| 7 bits&mdash;127 
| 9 bits 
| 24 bits 
| 8 bits 
| 0-507 octets
|}

According to IEEE Std 802.1AB, §9.6.1.3, "The Organizationally Unique Identifier shall contain the organization's OUI as defined in IEEE Std 802-2001."  Each organization is responsible for managing its subtypes.

==Notes==
<references group=note/>

==References==
{{reflist}}

==External links==
*[https://www.eetimes.com/document.asp?doc_id=1272069 Tutorial on the Link Layer Discovery Protocol] on [[EE Times]]
* [http://www.ieee802.org/1/pages/802.1ab.html 802.1AB - Station and Media Access Control Connectivity Discovery] on [[IEEE 802.1]]
*[https://gitlab.com/wireshark/wireshark/-/wikis/LinkLayerDiscoveryProtocol Link Layer Discovery Protocol] on The [[Wireshark]] Wiki

[[Category:Device discovery protocols]]
[[Category:Ethernet standards]]
[[Category:IEEE standards]]
[[Category:Link protocols]]
[[Category:Logical link control]]