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 has its destination MAC address typically 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 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:
| 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 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 | 0-510 octets |
Custom TLVs, 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 show 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.
Read more about this topic: Link Layer Discovery Protocol
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