Setting Up CDP

Using CDP, the Cisco Discovery Protocol, is a good method of Cisco network management if you have VTY (Telnet) sessions open on the devices when troubleshooting (or "Telnetting around the network"). From CDP, you can get an idea of the network topology and feature characteristics of other devices that are directly connected around that local device, such as what kind of device is its neighbor or what kind of features are turned on in the neighboring devices. CDP is a useful tool when troubleshooting because most of a network operational engineer's time is spent at the command prompt of a Cisco device. The CLI for viewing CDP information on Cisco devices is show cdp neighbor detail. There are tools out there, such as Ciscoworks for Switched Internetworks or CWSI, which use CDP for autodiscovery.

CDP does produce a small amount of overhead on the network by using multicast data-link packets to exchange information between devices. It uses the destination multicast MAC address of 01-00-0c-cc-cc-cc. This minimal traffic is easily justified when uptime and network availability are so crucial. CDP is a quick method for troubleshooters to get a quick understanding of what's in the network around them, from where they are troubleshooting.

CDP is enabled by default on all Cisco devices and on all interfaces (cdp enable) except for ATM interfaces and other Non-Broadcast Media (NBMA), where CDP is not supported at all due the nature of the multicast traffic over ATM.

Here are the default characteristics of CDP:

• CDP holds on to learned CDP information for three minutes (Time-to-Live).

• CDP sends out updates every 60 seconds.

Using the default settings for CDP is recommended. The only exception is if you want to disable CDP on interfaces where you are certain that Cisco devices do not exist and you know you have full control of what goes in and out of your network.