LLDP

networking

Link Layer Discovery Protocol: neighbors introduce themselves on the wire - device, port, capabilities - turning 'what is on the other end?' into a show command.

The vendor-neutral successor to Cisco's CDP; IP phones extend it as LLDP-MED for voice VLANs and PoE hints.

LLDP lets a device advertise what it is and what port it is on, so a neighbour learns its identity without configuration. It is the vendor-neutral equivalent of CDP, and its practical value is that a network map can be discovered rather than drawn.

The extension worth knowing is LLDP-MED, which negotiates power for PoE devices and can push a voice VLAN to a phone, so an IP phone plugged into a data port configures itself. That is also why LLDP is worth leaving enabled internally and disabling on external-facing ports: the same advertisement that helps an administrator identify a switch tells an intruder the model, firmware and port assignment.

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