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Network discovery with CDP

Cisco Discovery Protocol (CDP) is a powerful network monitoring and troubleshooting tool. CDP is an information-gathering tool used by network administrators to get information about directly connected Cisco devices. CDP is a proprietary tool that enables you to access a summary of protocol and address information about Cisco devices that are directly connected. By default, each Cisco device sends periodic messages, which are known as CDP advertisements, to directly connected Cisco devices. These advertisements contain information such as the types of devices that are connected, the router interfaces they are connected to, the interfaces used to make the connections, and the model numbers of the devices.

Most network devices, by definition, do not work in isolation. A Cisco device frequently has other Cisco devices as neighbors on the network. Information gathered from other devices can assist you in making network design decisions, troubleshooting, and making changes to equipment. CDP can be used as a network discovery tool, helping you to build a logical topology of a network when such documentation is missing or lacking in detail.

Familiarity with the general concept of neighbors is important for understanding CDP as well as for future discussions about dynamic routing protocols.

Layer 3 Neighbors

At this point in our topology configuration, we only have directly connected neighbors. At Layer 3, routing protocols consider neighbors to be devices that share the same network address space.

Layer 2 Neighbors

CDP operates at Layer 2 only. Therefore, CDP neighbors are Cisco devices that are directly connected physically and share the same data link. In the CDP Protocol figure, the network administrator is logged in to S3. S3 will receive CDP advertisements from S1, S2, and R2 only.