How CDP Works

As illustrated in Figure 3-35, CDP information is sent only between devices. In this figure, the person connected to Switch A can see the directly attached to Switch A; other devices are not visible via CDP.

would have to log in to Switch B to see Router C with CDP.

Figure 3-35 CDP Provides Information About Neighboring Cisco Devices

Figure 3-35 CDP Provides Information About Neighboring Cisco Devices

directly connected Cisco router and the two switches For example, the person

KEY POINT

Cisco devices never forward a CDP frame.

CDP is a hello-based protocol, and all Cisco devices that run CDP periodically advertise their attributes to their neighbors using a multicast address. These frames advertise a time-to-live value (the holdtime, in seconds) that indicates how long the information must be retained before it can be discarded. CDP frames are sent with a time-to-live value that is nonzero after an interface is enabled. A time-to-live value of 0 is sent immediately before an interface is shut down, allowing other devices to quickly discover lost neighbors.

Cisco devices receive CDP frames and cache the received information; it is then available to be sent to the NMS via SNMP. If any information changes from the last received frame, the new information is cached and the previous information is discarded, even if its time-to-live value has not yet expired.

CDP is on by default and operates on any operational interface. However, CDP can be disabled on an interface or globally on a device. Consequently, some caveats are indicated:

■ Do not run CDP on links that you do not want discovered, such as Internet connections.

■ Do not run CDP on links that do not go to Cisco devices.

For security reasons, block SNMP access to CDP data (or any other data) from outside your network and from subnets other than the management station subnet.

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