The Cisco Discovery Protocol (CDP) is a proprietary layer 2 network protocol developed by Cisco Systems which runs on most Cisco and HP equipment and is used to share information about other directly connected Cisco equipment such as the operating system version and IP address. CDP can also be used for On-Demand Routing (ODR) which is a method of including routing information in CDP announcements so that dynamic routing protocols do not need to be used in simple networks. Cisco devices send CDP announcements to the multicast destination address 01-00-0c-cc-cc-cc (which is also used for other Cisco proprietary protocols such as VTP). CDP announcements (if supported and configured in IOS) are sent by default every 60 seconds on interfaces which support Subnetwork Access Protocol (SNAP) headers, including Ethernet, Frame Relay and ATM. Each Cisco device that supports CDP stores the information received from other devices in a table which can be viewed using the show cdp neighbors command. The CDP table's information is refreshed each time an announcement is received, and the holdtime for that entry is reset. The holdtime specifies how long an entry in the table will be kept - if no announcements are received from a device and the holdtime timer expires for that entry, the device's information is discarded (default 180 seconds). The information contained in CDP announcements varies by the type of device and the version of the operating system running on it. Information contained includes the operating system version, hostname, every address for every protocol configured on the port where CDP frame is sent eg. IP address, the port identifier from which the announcement was sent, device type and model, duplex setting, VTP domain, native VLAN, power draw (for Power over Ethernet devices), and other device specific information. The details contained in these announcements is easily extended due to the use of the type-length-value (TLV) frame format. See external links for a technical definition.
