Two L2 protocols let neighboring devices announce who they are and what ports they're on. CDP is vendor-proprietary; LLDP is the IEEE standard (802.1AB). This chapter covers both, including configuration, verification, and which is on by default.
When you walk into a wiring closet with a half-labelled patch panel, CDP and LLDP tell you what's on the other end of every cable: device name, model, firmware version, port ID, IP address, VLAN, and PoE draw. They are L2 multicast frames that don't cross routers.
IP phones use CDP/LLDP-MED to learn their voice VLAN automatically from the upstream switch. Many WLCs use CDP to map APs back to their physical port.
CDP is on by default on the devices that support it, sends an advertisement every 60 s (default), and ages out neighbors after 180 s (holdtime). It uses MAC 01:00:0c:cc:cc:cc.
! Global toggle
R1(config)# cdp run ! default on
R1(config)# no cdp run ! kill CDP everywhere
! Per-interface
R1(config-if)# cdp enable / no cdp enable
! Verify
R1# show cdp neighbors
R1# show cdp neighbors detail ! also shows IP, firmware version
R1# show cdp interface
R1# show cdp entry R22 more sections in this chapter, plus a 2-question quiz and a hands-on lab.
You can see the title, summary, learning objectives and the opening sections. The rest of this chapter, like all of modules 2–8, comes with the Course & Lab Pass and All-Access Max. A free account opens Module 1: Network Foundations end-to-end and saves your labs — start there, no card required.
NetForge-AI is an independent educational platform and is not affiliated with, endorsed by, or sponsored by any networking vendor or certification body. All product names, protocols, and standards referenced are the property of their respective owners and are used for descriptive, educational purposes only.
Free account · no card required · compare plans any time