Glossary

LLDP

Link Layer Discovery Protocol

What is LLDP

LLDP (Link Layer Discovery Protocol), standardized in IEEE 802.1ab, is a Layer 2 discovery protocol — a way for two directly connected devices to exchange information about themselves. Through LLDP, a switch can advertise its own configuration and identity to whatever's plugged into the other end of a link, and learn the same information back, without any manual documentation or third-party discovery tool.

That makes LLDP a foundational piece of network visibility: it's how monitoring tools, topology maps, and troubleshooting workflows figure out what's actually connected to what, automatically and continuously, rather than relying on cabling diagrams that go stale the moment something gets moved or re-patched.

How LLDP Works

An LLDP frame carries a destination MAC address (one of three defined multicast addresses, each scoping how far the frame can propagate — nearest bridge, nearest non-TPMR bridge, or nearest customer bridge), a source MAC, the LLDP ethertype (0x88CC, which tells a receiving switch to hand the frame to its LLDP module), and an LLDPDU (LLDP Data Unit) — the actual payload carrying the information being exchanged.

The LLDPDU is built from a series of TLVs (Type-Length-Value structures) — a type identifying what kind of information it is, a length, and the value itself. Every LLDPDU starts with the same three mandatory TLVs — Chassis ID (the sending device's MAC address), Port ID (identifies the specific port), and Time to Live (how long a neighbor should keep this information before it's considered stale) — and always ends with an End of LLDPDU TLV. Beyond those four mandatory fields, a range of optional TLVs can be included: System Name, System Description, System Capabilities, Management Address, and Port Description among the basic set, plus organization-specific TLVs defined by IEEE 802.1 (VLAN and port information), IEEE 802.3 (port speed/duplex, PoE capability, link aggregation, maximum frame size), and LLDP-MED (voice-device-specific information like Voice VLAN ID, network policy, and location data, aimed at interoperability between VoIP equipment from different vendors).

Once populated, these TLVs let a device advertise a genuinely useful profile of itself — not just "something is connected here," but its name, its capabilities, its management IP, and enough port-level detail (like negotiated speed and duplex, or maximum supported frame size) to catch a surprising range of misconfigurations before they cause a problem.

Why LLDP is Beneficial

  • Automatic, always-current topology discovery: Because LLDP information is exchanged continuously between directly connected devices, network diagrams and monitoring tools stay accurate without manual updates every time a cable gets moved.

  • Vendor-neutral by design: As an IEEE standard rather than a proprietary protocol, LLDP works consistently across switches, routers, and other devices from different manufacturers, unlike vendor-specific discovery protocols.

  • Surfaces mismatches before they cause problems: TLVs carrying speed/duplex settings, VLAN configuration, and maximum frame size let administrators catch configuration mismatches between neighboring devices during troubleshooting, rather than after they've caused a connectivity or performance issue.

  • Extends naturally to voice and specialized endpoints: LLDP-MED TLVs let voice devices exchange VLAN, priority, and location information directly with the switch, simplifying deployment of VoIP equipment from any vendor that supports the standard.

  • Lightweight and low-risk: LLDP's periodic advertisement and aging-based cleanup keep it simple to run continuously in the background, with defined destination-MAC scoping that keeps discovery information appropriately local.

At Asteraix

What We Can Do at Asteraix

AsterNOS ships LLDP enabled by default, with the full TLV set and CLI-level control over exactly what gets advertised.

  • Broad TLV coverage out of the box: AsterNOS supports the full set of basic TLVs (Chassis ID, Port ID, Time to Live, System Name/Description/Capabilities, Management Address, Port Description) alongside IEEE 802.1, IEEE 802.3, and LLDP-MED organization-specific TLVs — covering VLAN information, port speed/duplex/PoE/frame-size details, and voice-device network policy in a single implementation.

  • Sensible defaults, fully overridable: LLDP runs in rx-and-tx mode by default, sending advertisements every 30 seconds with a 120-second aging time, and both management-address and capabilities advertisement enabled — matching what most networks need without any configuration at all, while remaining adjustable when they don't.

  • Direct control over advertised identity: lldp port-id-subtype {local|ifname|macaddress} lets operators choose exactly how a port identifies itself to neighbors, and specific advertisements — like management-address or capabilities TLVs — can be disabled individually where an operator wants tighter control over what's exposed on a given link.

  • Tunable timing for different network needs: lldp message-transmission interval and lldp message-transmission hold let operators adjust how frequently LLDP advertises and how long neighbor information is considered valid (with TTL calculated as the minimum of 65535 and interval × hold), matching faster-changing or more static network segments as appropriate.

  • Detailed neighbor visibility from the CLI: show lldp neighbor summary gives a quick view of every discovered neighbor, while show lldp neighbor interface <name> drills into full detail for one link — chassis ID, system name and description, management IP, capabilities, and port-level information — everything needed to verify what's actually connected without leaving the command line.