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CDP nei question between 2 cisco devices

mahesh18
Frequent Contributor
Frequent Contributor

                   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

2 Accepted Solutions

Accepted Solutions

John Blakley
VIP Alumni
VIP Alumni

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

HTH, John *** Please rate all useful posts ***

View solution in original post

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

HTH, John *** Please rate all useful posts ***

View solution in original post

4 Replies 4

John Blakley
VIP Alumni
VIP Alumni

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

HTH, John *** Please rate all useful posts ***

mahesh18
Frequent Contributor
Frequent Contributor

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

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

HTH, John *** Please rate all useful posts ***

mahesh18
Frequent Contributor
Frequent Contributor

Hi John,

Many thanks again

MAhesh

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