- Index
- Preface
- Overview
- Using the Command-Line Interface
- Assigning the Switch IP Address and Default Gateway
- Managing Switch Stacks
- Clustering Switches
- Administering the Switch
- Configuring SDM Templates
- Configuring Switch-Based Authentication
- Configuring 802.1x Port-Based Authentication
- Configuring Interface Characteristics
- Configuring Smartports Macros
- Configuring VLANs
- Configuring VTP
- Configuring Private VLANs
- Configuring Voice VLAN
- Configuring 802.1Q and Layer 2 Protocol Tunneling
- Configuring STP
- Configuring MSTP
- Configuring Optional Spanning-Tree Features
- Configuring Flex Links
- Configuring DHCP Features and IP Source Guard
- Configuring Dynamic ARP Inspection
- Configuring IGMP Snooping and MVR
- Configuring Port-Based Traffic Control
- Configuring CDP
- Configuring UDLD
- Configuring SPAN and RSPAN
- Configuring RMON
- Configuring System Message Logging
- Configuring SNMP
- Configuring Network Security with ACLs
- Configuring QoS
- Configuring EtherChannels
- Configuring IP Unicast Routing
- Configuring HSRP
- Configuring IP Multicast Routing
- Configuring MSDP
- Configuring Fallback Bridging
- Troubleshooting
- Supported MIBs
- Working with the Cisco IOS File System, Configuration Files, and Software Images
- Unsupported Commands in Cisco IOS Release 12.2(25)SE
Configuring CDP
This chapter describes how to configure Cisco Discovery Protocol (CDP) on the Catalyst 3750 switch. Unless otherwise noted, the term switch refers to a standalone switch and to a switch stack.
Note For complete syntax and usage information for the commands used in this chapter, see the command reference for this release and the "System Management Commands" section in the Cisco IOS Configuration Fundamentals Command Reference, Release 12.2.
This chapter consists of these sections:
•Monitoring and Maintaining CDP
Understanding 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, 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.
The switch supports CDP Version 2.
CDP and Switch 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.
Configuring CDP
These sections include CDP configuration information and procedures:
•Configuring the CDP Characteristics
•Disabling and Enabling CDP on an Interface
Default CDP Configuration
Table 25-1 shows the default CDP configuration.
Configuring the CDP Characteristics
You can configure the frequency of CDP updates, the amount of time to hold the information before discarding it, and whether or not to send Version-2 advertisements.
Beginning in privileged EXEC mode, follow these steps to configure the CDP timer, holdtime, and advertisement type.
Note Steps 2 through 4 are all optional and can be performed in any order.
Use the no form of the CDP commands to return to the default settings.
This example shows how to configure CDP characteristics.
Switch# configure terminal
Switch(config)# cdp timer 50
Switch(config)# cdp holdtime 120
Switch(config)# cdp advertise-v2
Switch(config)# end
For additional CDP show commands, see the "Monitoring and Maintaining CDP" section.
Disabling and Enabling CDP
CDP is enabled by default.
Note Switch clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange CDP messages. Disabling CDP can interrupt cluster discovery and device connectivity. For more information, see "Clustering Switches" and see Getting Started with Cisco Network Assistant, available on Cisco.com.
Beginning in privileged EXEC mode, follow these steps to disable the CDP device discovery capability:
|
|
|
---|---|---|
Step 1 |
configure terminal |
Enter global configuration mode. |
Step 2 |
no cdp run |
Disable CDP. |
Step 3 |
end |
Return to privileged EXEC mode. |
Beginning in privileged EXEC mode, follow these steps to enable CDP when it has been disabled:
|
|
|
---|---|---|
Step 1 |
configure terminal |
Enter global configuration mode. |
Step 2 |
cdp run |
Enable CDP after disabling it. |
Step 3 |
end |
Return to privileged EXEC mode. |
This example shows how to enable CDP if it has been disabled.
Switch# configure terminal
Switch(config)# cdp run
Switch(config)# end
Disabling and Enabling CDP on an Interface
CDP is enabled by default on all supported interfaces to send and receive CDP information.
Beginning in privileged EXEC mode, follow these steps to disable CDP on a port:
Beginning in privileged EXEC mode, follow these steps to enable CDP on a port when it has been disabled:
This example shows how to enable CDP on a port when it has been disabled.
Switch# configure terminal
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# cdp enable
Switch(config-if)# end
Monitoring and Maintaining CDP
To monitor and maintain CDP on your device, perform one or more of these tasks, beginning in privileged EXEC mode.