Networking
LLDP
Also known as: Link Layer Discovery Protocol
Protocol where devices advertise their identity to neighbors — how switches track ports and VoIP phones learn their VLAN.
LLDP (Link Layer Discovery Protocol) is a vendor-neutral protocol that lets network devices advertise their identity and capabilities to directly connected neighbors. It's how a switch knows what's plugged into each port — and how a VoIP phone learns which VLAN to use.
LLDP operates at Layer 2, sending periodic advertisements out each port containing information about the device: its hostname, port identifier, management IP address, capabilities (switch, router, phone), and for VoIP, the voice VLAN number.
The most practical use: LLDP-MED (Media Endpoint Discovery) is an extension of LLDP specifically for VoIP deployment. A phone boots up, hears an LLDP-MED advertisement from the switch, and learns which VLAN to use for voice traffic — no manual configuration on the phone required. This is the standard mechanism for VoIP VLAN assignment in modern deployments.
LLDP is also used by network management tools to build topology maps — automatically discovering which device is connected to which port on which switch, creating a picture of the physical network.
Cisco gear historically used CDP (Cisco Discovery Protocol) for this purpose; LLDP is the standards-based equivalent. Most Cisco devices now support both. For environments with mixed vendors, enable LLDP globally across all switches.