Using CDP to create network topology diagram

[miguel.desantiago]

Hello All,

I'm a beginning Network Specialist and just thought I would throw this question out and see if I can get some answers.  Well I want to know how I can use CDP to discover our network topology so I can create layouts and diagrams in Visio or if you know of any free network discovery programs out there that can help?  Well any feedback will be appreciated so thanks in advance and I look forward to hearing something!

[Jeff Van Houten]

Cdp only sees devices directly connected and therefore cannot see something 2 "hops" away. There are plenty of network diagram tools built around snmp.

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[Alessio Andreoli]

CDP is THE protocol if you want to start documenting seriously a network. It's 13 years I am in networking but if I need to be sure about a topology CDP is the only protocol I would still use. One of the many benefits is that you can match the CDP entries against the arp table ones. It is true that it will not see two hops away, but a real diagram describing the physical topology does not need to know what is two hops far away!!!!

Hope this helps

Alessio

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[miguel.desantiago]

Thanks Alessio! Appreciate the input! I work nearly in a complete Cisco environment. So when I run sh cdp neighbors or other commands what am I looking for to identify the topology? How can I use what I find to get to directly connected routers or switches?

[Jon Marshall · Accepted solution]

When you run "sh cdp neighbors" it shows you all direclty connected devices. If you then do "sh cdp neighbor detail" for each entry it will show you the IP address of the connected device. You can then connect to this and do the same all the way throiugh your network.

I think CDP can be useful for this but your are relying on CDP being run on all devices. Sometimes it has been disabled for security reasons so you always need to be aware that there may be devices that are not showing.

SNMP as mentioned is another alternative but again this assumes SNMP has been setup on esch device.

So just be aware you may not always get the full picture but a combination of -

1) CDP

2) SNMP

3) running traceroutes will show router hops but not L2 switches.

you should get most of the information. Be aware also that internal firewalls tend to mess these sort of things as well.

Jon

[johnlloyd_13]

hi,

the most useful to me when creating visio network diagrams are the show cdp neighbors and show ip interface brief commands.

i used solarwinds LAN surveyor tool a couple of times, but i still prefer doing it manually. it makes me more familiar with the network (espcially for LAN).

[Jigar Dave]

Just to add 2 cents on John's explanation.

CDP will give you later 2 topology, this restricts only to Cisco devices connected to other Cisco devices (CDP stands for

Cisco Discovery Protocol), so it searches only Cisco gears.

I am not sure about your set up you are trying to put it on Visio diagrams. but if other devices are present on your environment, like lets take an example of Juniper device ( connected to Cisco device) then you need to enable LLDP (Link layer Discovery Protocol) on cisco device to get to know about adjacent Juniper gear. you can run "show lldp neighbor" command on Cisco once you enable lldp on Cisco. (For Juniper, lldp is already enabled).

for layer 3 topology, you need to see by various commands, like if you are running ospf in Switching environment then

show ip ospf neighbor

show ip ospf interface

if you are running BGP then show ip bgp summary or "show run | beg er bgp"

if you are running EIGRP, (I think you are not) but then - show ip eigrp neighbors.

hope this helps.

JD...

[Zaaf Aba]

Hi

Although this is an old thread but may be someone else find it useful.

I have used following commands to draw the network diagram on paper

- show ip route

- show ip interface brief

- sh cdp entry * protocol (very helpful command to only list the neighbouring switch name and its ip address)

Regards

Zaaf