CDP does not "find fellow devices", it is more like a beacon, periodically
anouncing itself to anyone who would listen. If you would like to listen,
type in:
show cdp neighbors
If CDP is enabled on other Cisco network devices, you will see something
like:
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
069031368(5505a) Vlan1 131 T B S WS-C5500
7/1
069031369(5505b) Vlan1 131 T B S WS-C5500
7/2
069031275(5505c) Vlan1 131 T B S WS-C5500
7/3
069031219(5505d) Vlan1 131 T B S WS-C5500
7/4
In the above example, it found a switch. If it finds a router, and the
interface has an IP address assigned to it and the interface is up, CDP will
indicate the IP address of that interface. CDP only shows you Cisco devices
that are neighbors, you can't see the entire network....or can you?
You can telnet to the neighboring router, then from that router, enter "sh
cdp nei" and see who is adjacent to *that* router, and telnet to them and do
it over and over until you have mapped the entire network! But wait, the
downside is it doesn't always work. ;(
To see CDPs variables enter: sh cdp
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds
Even though Cisco calls it a "discovery" protocol, if there is going to be
any discovery, you are going to have to do the work.
HTH.....vlan2
> I am trying to figure out exactly what CDP does. IF I understand it
> correctly it simply finds fellow cisco devices on the network (switches,
> routers). What I am confused about is that I read somewhere that it can
> pick up the layer 3 (or was it layer 2?) address on interface cards on the
> devices it discovers. If this is true and 2 routers, who are in the same
> network, discover each other, could they route packets to each other
without
> a routing process installed? Thanks.
>
>
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