The Cisco Discovery Protocol (CDP) is a proprietary network protocol at layer 2 developed by Cisco Systems which runs on most Cisco 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. In networking, a communications protocol or network protocol is the specification of a set of rules for a particular type of communication. ... The Open Systems Interconnection Reference Model (OSI Model or OSI Reference Model for short) is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. ... A Cisco ASM/2-32EM router deployed at CERN in 1987. ... An operating system (OS) is a computer program that manages the hardware and software resources of a computer. ... An IP address (Internet Protocol address) is a unique address that devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... ODR is a simple routing protocol for hub and spoke networks. ... This article describes routing in computer networks, a method of finding paths from origins to destinations, along which information can be passed. ...

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 a device's information is discarded after three missed announcements from that device (after 180 seconds using the default 60 second announcement interval). Routing Schemes anycast broadcast multicast unicast Multicast is sometimes also used to refer to a multiplexed broadcast, although that is a very different thing and should not be confused. ... In computer networking a multicast address is an identifier for a group of hosts that have joined a multicast group. ... VTP stands for VLAN Trunking Protocol, a protocol used for configuring and administering VLANs on Cisco network devices. ... Cisco IOS (originally Internetwork Operating System) is the software used on the vast majority of Cisco Systems routers and all current Cisco network switches. ... The SubNetwork Access Protocol (SNAP) is a standard for the transmission of IP datagrams over IEEE 802 networks. ... Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks (LANs). ... In the context of computer networking, frame relay (also found written as frame-relay) consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... Asynchronous Transfer Mode (ATM) is a cell relay network protocol which encodes data traffic into small fixed-sized (53 byte; 48 bytes of data and 5 bytes of header information) cells instead of variable sized packets (sometimes known as frames) as in packet-switched networks (such as the Internet Protocol...

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. An operating system (OS) is a computer program that manages the hardware and software resources of a computer. ... A hostname (occasionally also, a sitename) is the unique name by which a network attached device ( which could consist of a computer, file server, network storage device, fax machine, copier, cable modem, etc. ... An IP address (Internet Protocol address) is a unique address that devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions. ... VTP stands for VLAN Trunking Protocol, a protocol used for configuring and administering VLANs on Cisco network devices. ... A virtual LAN, commonly known as a VLAN, is a logically segmented network mapped over physical hardware. ... A PoE injector (PSE) puts power into the cable and a PoE power splitter (PD) draws power from the cable. ... Within data communication protocols optional information may be encoded as a type-length-value or TLV element inside of the protocol. ... In telecommunications, a frame is a packet which has been encoded for transmission over a particular link. ...

External links

• CDP packet definition from Cisco
• Additional CDP info from Cisco
• Breakdown and explanation of a CDP packet by Wireshark packet sniffer.