Introduction to CDP (Cisco Discovery Protocol)

Let’s talk a bit about network management. Perhaps not the most exciting topic but I’m going to show you how you can use CDP (Cisco Discovery Protocol) to help you build network maps and what other information it can reveal.

Most networks have multiple switches and/or routers and to make our life easier it’s good to have a network map that shows us how everything is connected to each other, what kind of devices we have, to what VLAN they belong and the IP addresses that we are using. CDP is a Cisco protocol that runs on all Cisco devices that helps us discover Cisco devices on the network. CDP is Cisco proprietary, runs on the data-link layer and is enabled by default.

Let’s take a look at a network map:

cdp demo topology

Above we have 3 routers. Now if I had no idea what the network looked like we could use CDP to build the network map that you see above. Let me show you how:

R1#show cdp neighbors 
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
R2               Ser 0/0            167        R S I      3640      Ser 0/0

Use the show cdp neighbors command to see all directly connected neighbors. Above you see that R1 is connected to R2 and you can also see the platform (3640 router) and the interfaces on both sides. Let me show you the other routers as well:

R2#show cdp neighbors 
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
R1               Ser 0/0            144        R S I      3640      Ser 0/0
R3               Fas 1/0            164        R S I      3640      Fas 1/0
R3#show cdp neighbors 
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
R2               Fas 1/0            135        R S I      3640      Fas 1/0

Now we have all the information we need to build a network map with the router names and interfaces. CDP can tell us even more however…

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Forum Replies

  1. Rene,

    It’s always so good read your articles! I can understand them and make me feeling smarter :smiley: Thanks!!!

    Hug

  2. Hi Rene,

    I work as technical support in Internet Services & Communications company and we don’t use Cisco products only, we also use other network products from different vendors like Ubiquiti, mikrotik, Exalt, etc. and we connect all these different products with each other!!
    so when I use CDP to show which devices connected to cisco switch or router I noticed that information of different vendors products also appears to me!!
    My question is why I show this info ? I know that CDP is Cisco proprietary so how it’s able to identify the products from other vendors???

  3. Rene,

    Cisco ASA doesn’t seem to have CDP available. Is it maybe configured a different way?

    Chris

  4. Hello @ReneMolenaar ,

    I work with Cisco ASRs devices running cisco XR OS, and they don’t have CDP enabled by default. You have to enable it globally withcdp run command and that’s not enough to make it work. You still have to enable it per interface as well with cdp enable command.

    Regards.

  5. @chrismchavez,

    ASAs don’t have CDP feature and it make sense since they are firewall devices. :slight_smile: but there is a trick way to do this, I will lab it and show you the commands.

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