By Edward Tetz

Cisco Discovery Protocol (CDP) is data link layer protocol that allows you to find information about the connections between devices. Cisco Discovery Protocol (CDP) is a nifty tool from Cisco that gives you a boost if your network is primarily made of Cisco devices because Cisco devices use this communication protocol to identify other Cisco devices. Most Cisco Enterprise devices support CDP.

CDP essentially allows you to identify Cisco devices on your network and see how they are connected.

CDP becomes very useful if you have inherited or been brought in to work with an unfamiliar network. Within a few minutes, you have a picture of the network with a level of clarity that the previous administrator who did not use CDP would have had because many network administrators make connections to devices at the spur of a moment and then forget that those connected devices exist.

CDP has been enabled by default on Cisco routers and switches since IOS 10.0 (circa 1996), so unless you have turned it off, it should be running on your network devices. At periodic intervals, the CDP device sends out a packet on all of its interfaces. By default, the CDP packets are sent to a multicast address every 60 seconds.

The receiving device’s holdtime (the amount of time which it retains the data) is 180 seconds by default. The size of these packets is less than 500 bytes, which means that although they are on your network often, they do not amass a large amount of data — no more bandwidth is consumed with CDP than with Spanning Tree Protocol (STP) management packets.

The following table shows the type of information found in the Type Length Value (TLV) definitions, which is the main data found in the CDP frame. TLV is a standard data storage structure designed to encode data inside of protocol frames.

CDP TLV Definitions
TLV Definition
Device-ID The name of the device that is stored as a string.
Address A list of network addresses of both transmitting and receiving
devices.
Port-ID Displays the port identifier used to send the CDP frame.
Capabilities Represents the type of devices, such as a switch or
router.
Version Displays the software release or version operating on the
device.
Platform Identifies the hardware platform for the transmitting device,
such as Cisco ASA 2200 or Catalyst 2950.
IP Network Prefix Contains a list of network prefixes that the transmitting
device can send IP packets. Data can be sent to this physical
interface as an interface type and port number, such as Eth
0/2.
VTP Management Domain VLAN Trunking Protocol (VTP) makes use of CDP to distribute
information, so this contains the name of the VTP management
domain, of which a switch can be a member of only one domain. This
allows the device to verify the VTP configuration of neighboring
devices.
Native VLAN Identifies the VLAN on the identified interface that will be
used when untagged data is received on that port as specified in
the IEEE 802.1Q specification.
Full/Half Duplex Shows the status of the Duplex configuration for the interface
used to send the CDP data. This information can be used for
troubleshooting.

The data being passed around the network allows you to build a topological map of the connected devices, and if you move from node to node of that map (by connecting to the CLI on each switch or router you discover), you can build an entire map of the network.

Therefore, if you are in the middle of troubleshooting and need to know how a few of the devices on your network are connected (or if you are documenting your entire network design), CDP is for you.