The LLDP (Link Layer Discovery Protocol) is a manufacturer-independent Layer 2 protocol defined according to the IEEE 802.1AB standard and offers the possibility of exchanging information between neighbouring devices.
On each device that supports LLDP, a software component works, the so-called LLDP agent, which sends information about itself at periodic intervals and constantly receives information from neighboring devices. This is completely independent of each other, which is why LLDP is called a "one-way protocol" that does not establish communication with other devices.
The information received via the LLDP-DUs (Data Units) is stored locally on each device in a data structure, the Management Information Base (MIB). This information can then be accessed via SNMP. LLDP messages are sent in a Layer 2 frame (OSI) to the multicast address "01:80:C2:00:00:0E" with the Ethertype "88-CC".
The LLDPDU consists of TLVs (Type-Length-Value). Each TLV has a type and, depending on the data (value), the corresponding length.
Application in the field of Voice over IP
LLDP-MED (Link Layer Discovery Protocol - Media Endpoint Devices) is an extension of LLDP developed by TIA (ANSI/TIA-1057) to support interoperability of VoIP devices with other devices on the network (e.g. a PC).
LLDP-MED focuses mainly on the detection of devices such as IP phones running between network devices (switches, routers) and terminals.
Another good link to the topic: https://www.dfn.de/fileadmin/3Beratung/Betriebstagungen/bt57/LLDP_und_LLDP-MED_-_Management_von_VoIP-Infrastrukturen.pdf