Cisco Discovery Protocol (CDP) is a Layer 2 protocol that connects lower physical media and upper network layer protocols, as shown in Figure . CDP is used to obtain information about neighboring devices, such as 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 Version 2 (CDPv2) is the most recent release of the protocol. Cisco IOS (Release 12.0(3)T or later) supports CDPv2. CDP Version 1 (CDPv1) is enabled by default with Cisco IOS (Release 10.3 to 12.0(3)T).

When a Cisco device boots up, CDP starts up automatically and allows the device to detect neighboring devices that are also running CDP. It 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-to-live” or holdtime information, indicating the length of time that receiving devices should hold CDP information before discarding it. Additionally, each device listens to periodic CDP messages sent by others in order to learn about neighboring devices.

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