Connectivity Guide
Protocol data units
LLDP devices exchange system information represented as type, length, and value (TLV) segments:
Type Information included in the TLV.
Length Value in bytes of the TLV after the Length eld.
Value System information the agent advertises.
tlv segment
LAN devices transmit LLDPDUs, which encapsulate TLVs, to neighboring LAN devices. LLDP is a one-way protocol and LAN devices
(LLDP agents) transmit and/or receive advertisements but they cannot solicit and do not respond to advertisements.
There are three mandatory TLVs followed by zero or more optional TLVs and the end of the LLDPDU TLV. The three mandatory TLVs must
be located at the beginning of the LLDPDU in the following order:
• Chassis ID TLV
• Port ID TLV
• Time-to-live TLV
0 — End of
LLDPDU
Marks the end of an LLDPDU.
1 — Chassis ID Identies the LAN agent.
2 — Port ID Identies a port through which the LAN device transmits LLDPDUs.
3 — Time-to-live Number of seconds that the recipient LLDP agent considers the information associated with this MAP identier to
be valid.
— Optional Includes sub-types of TLVs that advertise specic conguration information. These sub-types are management
TLVs, IEEE 802.1, IEEE 802.3, and TIA-1057 organization-specic TLVs.
288 Layer 2