Link Layer Discovery Protocol
Internet protocol suite |
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Application layer |
Transport layer |
Internet layer |
Link layer |
The Link Layer Discovery Protocol (LLDP) is a vendor-neutral link layer protocol used by network devices for advertising their identity, capabilities, and neighbors on a local area network based on IEEE 802 technology, 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 with additional support in IEEE 802.3 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 can be stored in the device management information base (MIB) and queried with the Simple Network Management Protocol (SNMP) as specified in Template:IEFT RFC. 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
Applications[edit]
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 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.[3]
LLDP is used to advertise power over Ethernet capabilities and requirements and negotiate power delivery.
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.[4]
System Capability Codes[edit]
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[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 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.[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 their subtypes.
Notes[edit]
- ^ Termed Organizationally Specific TLVs by IEEE 802.1AB
References[edit]
- ^ "802.1AB-REV - Station and Media Access Control Connectivity Discovery". IEEE. Retrieved 2009-10-17.
- ^ "IEEE 802.1AB-2016 - IEEE Standard for Local and metropolitan area networks - Station and Media Access Control Connectivity Discovery".
- ^ Qlogic; et al. "DCB Capabilities Exchange Protocol Base Specification, Rev 1.01" (PDF). IEEE 802.
- ^ "ANSI/TIA-1057 standard" (PDF).
- ^ IEEE 802.1AB-2016 chapter 8.4
- ^ IEEE 802.1AB 8.4 Basic TLV Format