Documenting a Network Topology Using CDP

As the title of this section implies, I'm now going to show you how to document a sample network by using CDP. You'll learn to determine the appropriate router types, interface types, and IP addresses of various interfaces using only CDP commands and the show running-config command. And you can only console into the Lab_A router to document the network. You'll have to assign any remote routers the next IP address in each range. Given figure is what you'll use to complete the document.

In this output, you can see that you have a router with four interfaces: Two FastEthernet and two Serial. First, determine the IP addresses of each interface by using the show running-config command.

Lab_A#sh running-config

Building configuration...

Current configuration : 960 bytes

!

version 12.2

service timestamps debug uptime

service timestamps log uptime

no service password-encryption

!

hostname Lab_A

!

ip subnet-zero

!

!

interface FastEthernet0/0

ip address 192.168.21.1 255.255.255.0

duplex auto

!

interface FastEthernet0/1

ip address 192.168.18.1 255.255.255.0

duplex auto

!

interface Serial0/0

ip address 192.168.23.1 255.255.255.0

!

interface Serial0/1

ip address 192.168.28.1 255.255.255.0

!

ip classless

!

line con 0

line aux 0

line vty 0 4

!

end

With this step completed, you can now write down the IP address of the Lab_A router's four interfaces. Next, You need to determine the type of devices on the other end of each of these interfaces. It's easy to do this--Just use the show cdp neighbors command:

You've got a good deal of information now! By using both the show running-config and show cdp neighbors commands, you know about all the IP addresses of the Lab_A router plus the types of routers connected to each of the Lab_A router's links and all the interfaces of the remote routers.

And by using all the information gathered from show running-config and show cdp neighbors, we can now create the topology figure.

If we needed to, we could've also used the show cdp neighbors detail command to view the neighbor's IPaddresses. But since we know the IP addresses of each link on the Lab_A router we already know that the next available IP address is going to be.