Explanation/Reference:
Section: Design Methodologies Explanation
Explanation:
Cisco Discovery Protocol (CDP) operates at the Data Link layer, or Layer 2, of the Open Systems Interconnection (OSI) model. CDP is a proprietary protocol used by Cisco devices to detect neighboring Cisco devices. For example, Cisco switches use CDP to determine whether a directly connected Voice over IP (VoIP) phone is manufactured by Cisco or by a third party. CDP packets are broadcast from a CDP-enabled device to a multicast address. Each directly connected CDP-enabled device receives the broadcast and uses that information to build a CDP table. The CDP table contains a significant amount of information, including the following:
The device ID of the neighboring device

The capabilities of the neighboring device

The product number of the neighboring device

The holdtime

The local interface

The remote interface

Although CDP does not operate at the Physical layer, or Layer 1, it relies on a fully operational Physical layer. CDP packets are encapsulated by the CDP process on a Cisco device and then passed to the Physical layer for transmission onto the Physical medium, typically as electrical or optimal pulses which represent the bits of data. If CDP information is not being exchanged between directly connected devices, you should first check for Physical layer connectivity issues before moving on to troubleshoot potential Data Link layer connectivity issues.
CDP does not operate at any OSI layer above the Data Link layer, such as the Network layer (Layer 3), Transport layer (Layer 4), or Application layer (Layer 7). One of the strengths of CDP is that its operation is network protocol agnostic? meaning that CDP is not dependent on any particular Network layer protocol addressing scheme, such as IP addressing. For example, two directly connected devices with misconfigured IP addressing can still communicate and share information.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 15, CDP, p. 629
Cisco: Configuring Cisco Discovery Protocol