CDP Neigh command question

DerekAustin26

if you enter a "sho cdp nei" command in IOS on a router and you have 3 links. 2 serial & 1 fastethernet. But CDP only gives you the output of 2 of them.

Why would this command not give you the output of the 3rd link other than the fact that the remote device is not a Cisco device. What other reason would cause this???

Anyone know?

Selfmade

What is the fastethernet port connected to? if it's a computer, then your answer is obvious.

If there's a Cisco device on all 3 links that's connected to that router, then it's possible CDP is disabled on one of them.

if you have a router connected to a switch on the fastethernet port, and then you have only ONE serial link up, and a router is on that end, and your other serial link is dead, and that router on the other end has a switch connected to it

you would only see the switch connected to your router, and you'd see the other router using the command, but NOT the switch connected to the other router.

That's the definition of neighbors, it only tells you what YOU are directly connected to.

CDP is enabled by default, so, if you are DIRECTLY connected to some Cisco devices but you do a show cdp neighbors <detail> and you do not see the device on the list, then it indicates a layer 1 or 2 problem. CDP operates at the data link layer, so any problem with seeing a device indicates there's a problem at the first 2 layers.

Remember, incorrect clock signals indicate a problem at what layer?

I'll let you answer that question