Disabling the Cisco Discovery Protocol


You don't want to allow adjacent devices to gain information about this router for security reasons.


You can disable CDP on a single interface by using the command no cdp enable interface configuration command:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#cdp run
Router1(config)#interface FastEthernet0/0
Router1(config-if)#no cdp enable

And you can disable all CDP on the router with the global configuration command, no cdp run:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#no cdp run



CDP can be an extremely useful feature because it tells you so much information about all of your neighboring devices. However, this can also represent a serious security problem. CDP packets are not encrypted in any way, so if somebody can just capture the CDP packets from a network segment as they pass between the routers, they can easily deduce a lot about your network architecture. And if they can get access to the router either via Telnet or SNMP, they can use the CDP tables to discover the entire topology of your network at Layer 2 and 3, including all IOS levels, router and switch model types, and IP addressing. If somebody was armed with this information and a Cisco bug list, they could launch a very effective attack against your network.

For this reason, many network engineers choose to disable CDP throughout their networks. In general, if you need to disable CDP for security reasons, you should probably disable it globally on the whole router, rather than on individual interfaces. If you disable CDP on a single interface, you will only prevent people from intercepting the CDP advertisement packets. But the CDP table information is easily accessible through Telnet and SNMP, so valuable topology information is still vulnerable to probing.

We would like to clarify that the security risk is that somebody will launch a deliberate and focused attack against your network either from the inside or from a directly connected network. We strongly recommend disabling CDP on any routers that connect to external networks, particularly the public Internet. However, for purely internal networks, it is important to remember that you would be protecting yourself against people who are already physically connected to the network in some way. At this point, you must balance the obvious usefulness of CDP against the risk of attack from people who probably have legitimate access to the network. Whether you disable CDP or not in this situation depends on how much you can trust your legitimate users not to launch a deliberate internal attack.

See Also

Recipe 2.5

Router Configuration and File Management

Router Management

User Access and Privilege Levels


IP Routing





Frame Relay

Handling Queuing and Congestion

Tunnels and VPNs

Dial Backup

NTP and Time


Router Interfaces and Media

Simple Network Management Protocol





First Hop Redundancy Protocols

IP Multicast

IP Mobility




Appendix 1. External Software Packages

Appendix 2. IP Precedence, TOS, and DSCP Classifications


Cisco IOS Cookbook
Cisco IOS Cookbook (Cookbooks (OReilly))
ISBN: 0596527225
EAN: 2147483647
Year: 2004
Pages: 505

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