CISCO Discovery Protocol

Cisco Discovery Protocol (CDP) is a proprietary layer 2 management protocol for networks.CDP helps administrators in collecting information about cisco devices.
 CDP provides network device inventory, connectivity information, and IP next hop info. It works on LANs and WANs. CDP also works no matter the layer 3 network in use. CDP is excellent for troubleshooting tool to see what Cisco devices are directly connected to the Cisco devices with are working on.

 

This protocol runs globally and on per-interface level by default on Cisco Routers and switches and is Cisco-proprietary. 

Cisco devices send periodic CDP announcements to the multicast destination address 01-00-0ccc- cc-cc, out each connected network interface. These multicast packets may be received by Cisco switches and other networking devices that support CDP into their connected network interface.

CDP contains important information about Cisco devices including:

• IOS version number
• Hardware platform
• Native VLAN ID
•  Duplex setting
•  Port ID
• Hardware capabilities
• Layer 3 address (IP address) of device
• Interface that generate CDP message
• Number of seconds for CDP advertisement is valid
• Name of device configured with hostname
•  Device type

CDP messages are generated in every 60 seconds. Hold down time for missing neighbor is 180 s

1. cdp run and cdp no run – to enable and disable CDP globally
2. cdp enable and no cdp enable – to enable and disable cdp per interface
3. show cdp neighbors – to view see what routers and switches are connected to you on the LAN or WAN.
4. show cdp interfaces – to see which of your interfaces are participating in the CDP exchange
5. show cdp entry – to look at a particular switch or router that is a neighbor
6. show traffic – to see statistics about how many CDP packets have been exchanged

Also, you can also set cdp timers and clear cdp statistics.

Let's run show CDP  to see if CDP is enable on the  first place .If you get global info, it's on, but if you don't , it's not ..

 We could see that in this situation CDP is enable by default, so we don't nmeed to run "cdp run" command.We could see that CDP is sending it's announcements every 60 seconds, and the holdtime is 180 seconds.To change those values  please follow the directions:

Now to check important information about connected devices to our switch:

These is my equipment and you can compare the right results...

So, to explain certain concepts:

• Holdtime:The number of seconds the local device will retains the contentents of the last CDP advertisement received from the remote host

• Capability: The type of device the remote  device is 

• Platform :The remote device platform, in our case an IP phone and a 3550 Cisco switch

• Port ID: The remote device interface involved

 Now let's check in case you want to keep enable CDP globally but disable  n int fa0/10

Now the results several times before expiring those 134 secs. :

Now if you want to check on more  details of the remaining device:

 You see here advertisement version 2, but CDP version 1 still available but doesn't have a lot of features like mismatched native VLAN's

A lot of network administrators disable CDP because it can provide a lot of information that can be accessible to everyone who can see the information  with the command "show cdp neighbor detail", and all that information is sent with clear text without any encryption;but the problem with disabling CDP is that many network management tools use data collected by CDP.

To reduce that risk make sure you know what you can do without it, and where it needs to be running and use the interface -level command to make sure that situation happens.

In case that you work in your network with non-Cisco device , the Link Layer Discovery Protocol (LLDP) can be very useful, which is the equivalent of CDP  and is defined by IEEE 802 parameters.

There is an usuful extension, LLDP for Media Endpoint devices (LLDP_MED) that can be handy when VOIP is in use, but CDP carry features that that LLDP-MED doesn't like MTU size VLAN trunking protocol Information, IP network prefix support, etc, etc.

To enable CDP globally:

 To enable CDP on the interface

I hope that this article was useful for you..If you understood it and  like if, please share it..