25 CDP and LLDP
This chapter covers
- The purpose of layer 2 discovery protocols
- How neighboring Cisco devices use CDP to share information
- CDP’s industry-standard equivalent: LLDP
In a perfect world, all networks would be perfectly documented, with up-to-date network diagrams and other documents detailing the routers, switches, firewalls, and other devices that make up the network infrastructure, how they are connected, their IP addresses, etc. In reality, that’s often not the case: people cut corners, changes go undocumented, or perhaps the network was never properly documented in the first place.
In either case, the result is often that the network documentation doesn’t accurately reflect the current state of the network; this is not an ideal state of affairs. Fortunately, layer 2 discovery protocols—the topic of this chapter—can help you remedy this situation. Layer 2 discovery protocols, such as Cisco Discovery Protocol (CDP) and Link Layer Discovery Protocol (LLDP), enable devices to share information about themselves with their directly connected neighbors. Using this information, you can map out the network by identifying the devices in the network, their hardware models, how they are connected to each other, their IP addresses, and other information.