| Internet protocol suite |
|---|
| Application layer |
| Transport layer |
| Internet layer |
| Link layer |
TheLink Layer Discovery Protocol (LLDP) is a vendor-neutrallink layer protocol used bynetwork devices for advertising their identity, capabilities, and neighbors on alocal area network based onIEEE 802 technology, principallywired Ethernet.[1] The protocol is formally referred to by the IEEE asStation and Media Access Control Connectivity Discovery specified inIEEE 802.1AB with additional support in IEEE 802.3 section 6 clause 79.[2]
LLDP performs functions similar to severalproprietary protocols, such asCisco Discovery Protocol,Foundry Discovery Protocol,Nortel Discovery Protocol andLink Layer Topology Discovery.
Information gathered with LLDP can be stored in the devicemanagement information base (MIB) and queried with theSimple Network Management Protocol (SNMP) as specified inRFC 2922. The topology of an LLDP-enabled network can be discovered bycrawling the hosts and querying this database. Information that may be retrieved include:
The Link Layer Discovery Protocol may be used as a component innetwork management andnetwork monitoring applications.
One such example is its use indata center bridging requirements. TheData 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.[3]
LLDP is used to advertisepower over Ethernet capabilities and requirements and negotiate power delivery.
Media Endpoint Discovery is an enhancement of LLDP, known asLLDP-MED, that provides the following facilities:
The LLDP-MED protocol extension was formally approved and published as the standard ANSI/TIA-1057 by theTelecommunications Industry Association (TIA) in April 2006.[4]
| Code | Capability |
|---|---|
| B | Bridge (Switch) |
| C | DOCSIS Cable Device |
| O | Other |
| P | Repeater |
| R | Router |
| S | Station |
| T | Telephone |
| W | WLAN Access Point |
LLDP information is sent by devices from each of their interfaces at a fixed interval, in the form of anEthernet frame. Each frame contains one LLDP Data Unit (LLDPDU). Each LLDPDU is a sequence oftype–length–value (TLV) structures.
The Ethernet frame used in LLDP typically has its destinationMAC address set to a specialmulticast address that802.1D-compliant bridges do not forward. Other multicast and unicast destination addresses are permitted. TheEtherType field is set to 0x88cc.
Each LLDP frame starts with the following mandatory TLVs:Chassis ID,Port ID, andTime-to-Live. The mandatory TLVs are followed by any number of optional TLVs. The frame optionally ends with a special TLV, namedend of LLDPDU in which both thetype andlength fields are 0.[5]
Accordingly, an Ethernet frame containing an LLDPDU has the following structure:
| 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 |
|---|---|---|---|---|---|---|---|---|---|
| 01:80:C2:00:00:0E, or 01:80:C2:00:00:03, or 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:
| Type | Length | Value |
|---|---|---|
| 7 bits | 9 bits | 0-511 octets |
| 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[note 1] 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:
| 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, §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.