Discover the device at the other end
IEEE 802.1AB defines LLDP, a Layer 2 advertisement protocol. A device sends information about itself on a local link. Chassis ID, port ID and time to live identify the advertiser and how long to retain the entry. Optional fields can include a system name, capabilities and a management address.
Advertisements are independent in each direction. A device can learn a neighbor even if it is not advertising itself. A listed management address is information from the neighbor, not proof that the address is reachable.
LLDP-MED for media endpoints
LLDP Media Endpoint Discovery extends the base protocol for devices such as IP phones. A network policy can tell a compatible endpoint which VLAN and priority settings apply to voice traffic. Those fields are optional extensions. A device can participate in ordinary LLDP without supporting LLDP-MED.
Try it in a lab
- Connect two LLDP-capable devices and verify that each learns the other’s chassis and remote port on the expected local interface.
- On a link to a compatible phone, inspect the LLDP-MED voice network policy. Compare the advertised VLAN and priority with what the endpoint actually sends.