Guest VLAN on DHCP is aquireing Nonexistent IP's

[glamprecht1]

Not sure if this should be in the Cisco Switching room or here but here goes. ....

I Have a VLAN just for Wireless guest users. This VLAN is VLAN 10.
The VLAN has an ip assigned to it of 10.10.1.1 and uses a DHCP scope that allocates addresses from 10.10.1.21 - 10.10.1.254
This has been in use for about 3 months and has been working fine until this weekend.

I get a complaint that a user cannot access the internet.
In my investigation I see that I am picking up a 10.10.2.x address. The funny thing is that this subnet does not exist on my network and I can’t seem to see where these addresses are coming from.

My primary VLAN is unaffected, Thank God. The issue is only with the Guest Vlan that supports wired and wireless devices. Anything that connects Via VLAN 10 either by wireless or statically defined VLAN 10 ports will pick up this oddball network. If it were a 192.168.x.x address I would suspect that some idiot has plugged in a home router to a LAN switch port under their desk. But the nature of the IP address closely mimics our real addresses.

I can’t see any mention of this network any ware on the DHCP server nor the Core switch that Controls the VLAN info.

Any Ideas?? On how to figure out where these IP’s are coming from? How to stop them?

 

[Roadki11]

If you do an ipconfig /all on the affected host does it tell you what ip the dhcp address came from? it should if its a dhcp lease.

example:
DHCP Server . . . . . . . . . . . : 10.10.2.252

RoadKi11

 

[glamprecht1]

Thanks for the Reply RoadKi11

Yes an ipconfig /all will point to the core switch. Why? I don’t know.
I believe that the Core switchport is trunked on both private and public VLANS. That way the guest VLAN can use the internal DHCP server.

The address that shows up as the DHCP server is the IP that is assigned to the Guest VLAN on the Core switch. That would be 10.10.1.1

To my knowledge, I don’t think that I have ever seen a switch that will hand out IP addresses????????

 

[Roadki11]

Ive never used dhcp on switch but i thought some of the cisco switches could do it. I would bust open the config and check it out cuz it sure looks like your culprit.

RoadKi11

 

[Wilma]

yes many layer 3 switches do dhcp. i run a allied telesyn 9816gb that does.

If its a layer 3 switch - Do you have routing between the vlans?