CDP nei question between 2 cisco devices

[mahesh18]

                   Hi all,

We have cisco router and switch which have direct connection between them.

From Router int say gi0/1 goes to non cisco device.

Switch interface say gi0/2 also goes to same non cisco device.

When i do sh cdp nei on switch  int  gi0/2   it shows Router as nei.

Need to know why it is like this?

Thanks

Mahesh

[John Blakley · Accepted solution]

Mahesh,

CDP is a l2 protocol, so if you have a Cisco router connected to a non-Cisco switch, and another Cisco switch connected to that one, the router will see the Cisco switch on the other side of the non-Cisco switch. The Cisco switch will see the router as a neighbor because the non-Cisco switch is still allowing the CDP traffic to pass.

HTH,

John

[mahesh18]

Hi John,

So does it mean that both cisco devices will see each other as nei?

Also what if we have layer 3 interface does will it make difference?

Thanks

Mahesh

[John Blakley · Accepted solution]

The Cisco devices will see each other as neighbors as long as CDP is running on those interfaces. CDP packets will pass through the non-Cisco switch unhindered. The downside is you don't know what device is between the 2 Cisco devices because they look like they're directly connected when in fact they're not.

You should still be able to see a device if it's connected to a routed port on a switch. Usually, you want to disable running cdp on an interface that leads to a service provider for security reasons.

HTH,

John

[mahesh18]

Hi John,

Many thanks again

MAhesh