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:

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…