CDP and LLDP

CDP and LLDP are neighbor discovery protocols that operate at Layer 2 of the OSI model. Devices running them periodically send advertisements out their interfaces describing themselves — device identifier (hostname), the local port sending the message, hardware platform, capabilities (router, switch, phone), and often management IP address and software version. A device receiving these messages builds a neighbor table, letting an engineer map how devices are physically interconnected without tracing cables.

CDP (Cisco Discovery Protocol) is Cisco proprietary and enabled by default on Cisco devices. It sends advertisements every 60 seconds with a 180-second holdtime — if no update arrives before the holdtime expires, the neighbor entry is removed. LLDP (Link Layer Discovery Protocol) is the vendor-neutral IEEE 802.1AB standard, making it the choice in multivendor networks. LLDP typically sends advertisements every 30 seconds with a 120-second holdtime, and on Cisco devices it usually must be enabled manually. Commands like "show cdp neighbors" and "show lldp neighbors" display the learned neighbor tables.

These protocols are invaluable for documenting and troubleshooting a network — verifying which switch port connects to which device, discovering a neighbor's management address, or confirming a link is cabled as designed. They carry a security tradeoff, however: because advertisements reveal device details in plaintext, best practice is to disable them on interfaces facing untrusted networks.

The CCNA exam tests CDP and LLDP within network management topics, including their default timers, CDP's Cisco-proprietary versus LLDP's open-standard status, and interpreting the output of neighbor verification commands.