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Restrictions on CDP
The restrictions on CDP are contained in this section.
The switch supports CDP Version 2.
Information About CDP
CDP is a device discovery protocol that runs over Layer 2 (the data link layer) on all Cisco-manufactured devices (routers, bridges, access servers, controllers, and switches) and allows network management applications to discover Cisco devices that are neighbors of already known devices. With CDP, network management applications can learn the device type and the Simple Network Management Protocol (SNMP) agent address of neighboring devices running lower-layer, transparent protocols. This feature enables applications to send SNMP queries to neighboring devices.
CDP runs on all media that support Subnetwork Access Protocol (SNAP). Because CDP runs over the data-link layer only, two systems that support different network-layer protocols can learn about each other.
Each CDP-configured device sends periodic messages to a multicast address, advertising at least one address at which it can receive SNMP messages. The advertisements also contain time-to-live, or holdtime information, which is the length of time a receiving device holds CDP information before discarding it. Each device also listens to the messages sent by other devices to learn about neighboring devices.
On the switch, CDP enables Network Assistant to display a graphical view of the network. The switch uses CDP to find cluster candidates and maintain information about cluster members and other devices up to three cluster-enabled devices away from the command switch by default.
CDP and Device Stacks
A switch stack appears as a single switch in the network. Therefore, CDP discovers the switch stack, not the individual stack members. The switch stack sends CDP messages to neighboring network devices when there are changes to the switch stack membership, such as stack members being added or removed.
Default CDP Configuration
The table below shows the default CDP configuration.
CDP global state
CDP interface state
CDP timer (packet update frequency)
CDP holdtime (before discarding)
CDP Version-2 advertisements
How to Configure CDP
Configuring CDP Characteristics
The CDP characteristics that are configured with these instructions are the following:
The frequency of CDP updates
The amount of time to hold the information before discarding it
Whether or not to send Version-2 advertisements
Steps 2 through 4 are all optional and can be performed in any order.
Beginning in privileged EXEC mode, follow these steps to configure these characteristics:
Command or Action
Switch# configure terminal
Enters the global configuration mode.
Switch(config)# cdp timer 20
(Optional) Sets the transmission frequency of CDP updates in seconds.
The range is 5 to 254; the default is 60 seconds.
Switch(config)# cdp holdtime 60
(Optional) Specifies the amount of time a receiving device should hold the information sent by your device before discarding it.
The range is 10 to 255 seconds; the default is 180 seconds.
Switch(config)# cdp advertise-v2
(Optional) Configures CDP to send Version-2 advertisements.
This is the default state.
Returns to privileged EXEC mode.
The following example shows how to configure CDP characteristics: