Link Layer Discovery Protocol

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The Link Layer Discovery Protocol (LLDP) is a vendor-neutral link layer protocol in the Internet Protocol Suite used by network devices for advertising their identity, capabilities, and neighbors on an IEEE 802 local area network, principally wired Ethernet.[1] The protocol is formally referred to by the IEEE as Station and Media Access Control Connectivity Discovery specified in IEEE 802.1AB and IEEE 802.3-2012 section 6 clause 79.[2]

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

Information gathered[edit]

Information gathered with LLDP is stored in the device as a management information database (MIB) and can be queried with the Simple Network Management Protocol (SNMP) as specified in 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 name and description
  • VLAN name
  • IP management address
  • System capabilities (switching, routing, etc.)
  • MAC/PHY information
  • MDI power
  • Link aggregation


The Link Layer Discovery Protocol may be used as a component in network management and monitoring applications. One such example is its use in data center bridging requirements.[3]

The 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.[4]

Frame structure[edit]

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 has its destination MAC address typically set to a special multicast address that 802.1D-compliant bridges do not forward.[note 1] 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 ends with a special TLV, named end of LLDPDU in which both the type and length fields are 0.

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

LLDP Ethernet frame structure
Preamble Destination MAC Source MAC Ethertype Chassis ID TLV Port ID TLV Time to live TLV Optional TLVs End of LLDPDU TLV Frame check sequence
01:80:c2:00:00:0e, or
01:80:c2:00:00:03, or
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:

TLV structure
Type Length Value
7 bits 9 bits 0-511 octets
TLV type values[5]
TLV type TLV name Usage in LLDPPDU
0 End of LLDPDU Mandatory
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[note 2] 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:

Organizationally specific TLV
Type Length Organizationally unique identifier (OUI) Organizationally defined subtype Organizationally defined information string
7 bits—127 9 bits 24 bits 8 bits 0-507 octets

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

Media endpoint discovery extension [edit]

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, 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) end points.
  • 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.[6]

See also[edit]


  1. ^ IEEE 802.1AB-2009 suggests three such addresses, 01:80:c2:00:00:0e, 01:80:c2:00:00:03 and 01:80:c2:00:00:00.
  2. ^ Termed Organizationally Specific TLVs by IEEE 802.1AB


  1. ^ "802.1AB-REV - Station and Media Access Control Connectivity Discovery". IEEE. Retrieved 2009-10-17. 
  2. ^ "IEEE standard 802.1AB-2009" (PDF). 
  3. ^ "Data Center Bridging Task Group". Retrieved 2012-03-10. 
  4. ^ Qlogic; et al. "DCB Capabilities Exchange Protocol Base Specification, Rev 1.01" (PDF). IEEE 802. 
  5. ^ IEEE 802.1AB 8.4 Basic TLV Format
  6. ^ "ANSI/TIA-1057 standard" (PDF). 

External links[edit]