Cisco Discovery Protocol (CDP) is a proprietary protocol developed by Cisco that discovers information about directly connected Cisco devices, such as routers, switches, and firewalls.
CDP is a Layer 2 protocol operating at the OSI model's Data Link Layer.
It runs on Cisco devices by default but can be turned off if desired. CDP is used to obtain information such as the neighbor’s IP address, device type, and the operating system version it is running.
This information can be useful for network management and troubleshooting, especially when management software tools can use the exchanged data to build a network topology map.
CDP also automates VoIP deployments. It allows the voice VLAN to automatically be assigned to Cisco IP phones, resulting in a less manual configuration required on the switches.
CDP sends hello (announcement) messages every 60 seconds through each interface by default. This information is then stored on the neighboring devices for 3 minutes, and this time interval is called hold time. If a device does not receive a hello message during the hold time period, it discards the device from the list of neighbors.
