CDP: Cisco Discovery Protocol

Cisco Discovery Protocol (CDP) is primarily used to obtain protocol addresses of neighboring devices and discover the platform of those devices. CDP can also be used to show information about the interfaces your router uses. CDP is media- and protocol-independent, and runs on all Cisco-manufactured equipment including routers, bridges, access servers, and switches.
CDP runs on all media that support Subnetwork Access Protocol (SNAP), including local-area network (LAN), Frame Relay, and Asynchronous Transfer Mode (ATM) physical media. CDP runs over the data link layer only. Therefore, two systems that support different network-layer protocols can learn about each other.
Each device configured for CDP sends periodic messages, known as advertisements, to a multicast address. Each device advertises at least one address at which it can receive SNMP messages. The advertisements also contain time-to-live, or holdtime, information, which indicates the length of time a receiving device should hold CDP information before discarding it. Each device also listens to the periodic CDP messages sent by others in order to learn about neighboring devices and determine when their interfaces to the media go up or down. 
CDP is a Cisco proprietary Layer 2 protocol that is media- and protocol-independent, and runs on all Cisco-manufactured equipment that includes:
  • routers
  • bridges
  • access servers
  • switches
A Cisco device enabled with CDP sends out periodic interface updates to a multicast address in order to make itself known to neighbors. Since it is a layer two protocol, these packets (frames) are not routed. Use of SNMP with the CDP MIB allows network management applications to learn the device type and the SNMP agent address of neighboring devices, and to send SNMP queries to those devices.
Let us apply CDP on packet tracer.
In the above topology, we have different devices attached with each other. So, if we want to look for the information of neighboring devices, we will apply the following command on Router enabled mode, “show cdp neighbors”.

And on Router 2 and 3 as well, as shown in figures below.


It can be seen clearly that it gives us the information of the neighboring devices. Note one thing here, that it gives us the information of the routers and switches that are directly attached to its ports. However, it does not gives us information about the hosts that are attached to it directly.




  1. I think devices should have to use the same network for the purpose of sharing data, For the same network you don't need to change the IP addresses of the devices for sending and receiving data and data communication purposes.

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