cdp neighbor IP address & Vlan interfaces

linnea.wren · ‎07-28-2006

Hi,

No problem here, but I got curious after noticing an unexpected IP address in cdp neighborhood output.

We use Vlan999 for management vlan. Usually I see a Vlan999 ip address in cdp neighbors. Today, instead, I see a Vlan2 ip address for one of our 4506's. (cdp neighbor command executed on a 3550. connection between 3550 and 4506 is trunk.)

I look around some more, discover that most switches only have Vlan999 configured. The unusual switch has many vlans configured, starting with Vlan2 (Vlan1 is shutdown).

(And that makes sense. We have 5 sites. This 4506, with native IOS, is site core at a small site - all other site cores are 6509 with CatOS/IOS.)

So, it appears to me that CDP sees the IP address of the lowest interface it finds at the other end of a trunk.

Is that correct?

royalblues · ‎07-29-2006

Hi,

CDP generally dispalys the management VLAN.

Could you please check what is the management VLAN configured for the switch ?

Narayan

Roberto Salazar · ‎07-29-2006

If it's straight L2 device, then this is true since there is no other interface but the "management interface". but on platforms like 3550 or any other platfrom that is L3 capable and where it has multiple interface vlans, it will send the interface vlan with the lowest interface vlan number in the CDP information. After, all the interfaces are in effect the "management interface", you cannot expect it to send all those information on all of those interfaces, it has to chose one. If the lowest interface vlan is shut down it will send the next lowest interface vlan. so, this is normal for a trunk port. Now, if it's an access port the ip address showing for the neighbor will be the ip address of the interface vlan for that vlan.

royalblues · ‎07-30-2006

Thanks for the information robert.

I thought the CDP information always travels on the management VLAN regardless of L2 and L3 properties.

Narayan

Roberto Salazar · ‎07-30-2006

Yes, it does but when the switch is a L3 switch and it has multiple interface vlans, which one is management interface? Aren't they all management interface since you can telnet to the switch using anyone of the interface vlans ip address?