LLDP Link Layer Discovery Protocol
An open-standard (IEEE 802.1AB) network discovery protocol. It performs the exact same function as the Cisco Discovery Protocol (CDP), but because it is vendor-neutral, it allows a Cisco switch to discover and map connected devices from other manufacturers like Juniper, Arista, HP, or standard Linux servers.
The TLV Architecture
LLDP transmits information in highly structured blocks called TLVs (Type-Length-Value). A device packages multiple TLVs into a single LLDP ethernet frame and multicasts it out of its active interfaces.
- Type: Identifies what kind of information is being sent (e.g., Port ID, System Name, Management IP).
- Length: Specifies exactly how many bytes the value data will consume, so the receiving parser knows where the string ends.
- Value: The actual data payload (e.g., "GigabitEthernet0/1" or "192.168.1.5").
CCNA Exam Gotchas
State by Default
The CCNA tests heavily on the default states of discovery protocols. CDP is completely enabled by default on all Cisco devices. LLDP is disabled by default globally. You must type lldp run in global config mode before the router or switch will process LLDP frames.
Unidirectional Capabilities
Unlike CDP, which is essentially an all-or-nothing toggle on an interface, LLDP allows you to specify directionality. You can configure a port to lldp receive (listen to a connected server's information) while using no lldp transmit to prevent the switch from leaking its own network topology data back to the server for security purposes.