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Computer Network 2026

6.4 Introduction to CDP

Cisco Discovery Protocol (CDP) is a Layer 2 protocol that connects lower physical media
and upper network layer protocols. CDP is used to obtain information about neighboring devices.
This information shows the types of devices connected, the router interfaces they are connected
to, the interfaces used to make the connections, and the model numbers of the devices. CDP is
media and protocol independent, and runs on all Cisco equipment, over the Subnetwork Access
Protocol (SNAP). CDP is a proprietary protocol native to Cisco networking devices and will run
only on Cisco networking devices.

When a Cisco device boots up, CDP starts up automatically and allows the device to
detect neighboring devices that are also running CDP. CDP runs over the data link layer and
allows two systems to learn about each other, even if they are using different network layer
protocols. Each device configured for CDP sends periodic messages, known as advertisements,
to multiple routers. Each device advertises at least one address at which it can receive Simple
Network Management Protocol (SNMP) messages. The advertisements also contain Time-ToLive
(TTL) or hold time information. This hold time information indicates the length of time that
receiving devices hold CDP information before discarding it. Additionally, each device listens to
periodic CDP messages sent by others to learn about neighboring devices.

6.4.1 Obtaining Information with CDP

The primary use of CDP is to discover all Cisco devices that are directly connected to a
local device. A CDP frame can be small, yet can retrieve many useful information about
neighboring routers and switches. Use the command show cdp entry.

Notice that the output from this command includes all the Layer 3 addresses present in
the neighbor router, Router B. An administrator can view the IP addresses of the targeted CDP
neighbor (Router B) with the single command entry on Router A. The hold time value indicates
the amount of elapsed time since the CDP frame arrived with this information. The show cdp
entry [device name] command includes abbreviated version information about Router B.
Knowing the version of neighboring devices provides you with an idea of the exact physical
topology of the network to properly configure the devices.

6.4.2 Showing CDP Neighbors

Figure 6-7 shown a CDP Collecting Information Process displays an example of how CDP
delivers its collection of information to a network administrator. Each router that is running CDP
exchanges protocol information with its neighbors. The network administrator displays the
results of this CDP information exchange on a console that is connected to a local router.

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