8021ab

IEEE 802.1AB defines the Link Layer Discovery Protocol (LLDP), a vendor-neutral, data-link layer protocol used by Ethernet devices to advertise identity and capabilities to directly connected neighbors. LLDP enables network devices such as switches, routers, and IP phones to discover each other’s presence and basic characteristics, supporting network management, topology mapping, and troubleshooting.

LLDP operates through Type-Length-Value units called TLVs. An LLDPDU begins with mandatory TLVs for Chassis ID, Port ID, and Time to Live, followed by an End of LLDPDU TLV. Commonly used optional TLVs include Port Description, System Name, System Description, System Capabilities, and Management Address. Organizationally Specific TLVs allow vendor-specific or standardized extensions, including LLDP-MED for media endpoint devices such as VoIP phones and location/policy signaling.

LLDP frames are carried at the data-link layer with EtherType 0x88CC and are sent to the multicast address 01:80:C2:00:00:0E. A device typically maintains a local topology table correlating the received TLVs with attached hardware, allowing continuous or periodic updates as devices come and go on the network. LLDP is designed for ongoing use, with information refreshed according to the configured TTL values.

Security and deployment considerations: LLDP provides no built-in authentication or encryption, so enabling it on untrusted segments can introduce risks. Best practices include limiting LLDP to trusted network areas, using LLDP-MED where appropriate, and applying network controls such as VLAN segmentation and port security to mitigate potential misuse. LLDP remains widely supported and interoperable across major network equipment vendors.