LLDP packets

LLDP agents transmit information about a sending device/port in packets called LLDP Data Units (LLDPDUs). All the LLDP information to be communicated by a device is contained within a single 1500 byte packet. A device receiving LLDP packets is not permitted to combine information from multiple packets.

As shown in the following figure, each LLDPDU has three mandatory TLVs, an End of LLDPDU TLV, plus optional TLVs as selected by network management.

LLDPDU packet format

Each LLDPDU consists of an untagged Ethernet header and a sequence of short, variable length information elements known as type, length, value (TLV).

TLVs have Type, Length, and Value fields, where:

• Type identifies the kind of information being sent
• Length indicates the length (in octets) of the information string
• Value is the actual information being sent (for example, a binary bit map or an alpha-numeric string containing one or more fields).