DHCP and VLANs

[FatBoy]

Hi all,

 

We have two DHCP servers setup here at the school one on each VLAN. The VLANs are static (assigned to ports on switches) there is a default vlan (10.140.*) in school classrooms etc and our student boarder VLAN (10.143.*). Everything has worked fine for quite a while now but today the DHCP server from our student VLAN .143 started giving ip address to clients or our default vlan :confused: We have an external company working on the routers but they say they have done nothing and I can't work out why this would start happening. Has anyone got any idea's?

 

Any help, tips or things to check to try and isolate the problem would be great.

 

Thanks

FB

[Geoff]

Are any of your DHCP servers dual homed onto both VLANs? If so, check the DHCP Server bindings and make sure they are set to only the VLAN you want.

 

Otherwise, I'd blame your contracters. Messing up the router config would easily cause the problem your seeing. As would patching the wrong uplinks together. Or mucking up some VLAN tagging on ports.

[wrights]

If the DHCP servers aren't dual homed and the config hasn't been changed on your switches then my bet would be on someone connecting the two VLANs together with a misplaced network cable.

[wrights]

It might be worth connecting Wireshark to one of your network ports and looking to see where the incorrect DHCP replies are coming from.

If you can see the other DHCP server's MAC address then you can connect to the edge switch and start tracing back the route packets are taking from your end device to the server, you'll pretty quickly discover how it is connected.

[FatBoy]

Are any of your DHCP servers dual homed onto both VLANs? If so, check the DHCP Server bindings and make sure they are set to only the VLAN you want.

 

Otherwise, I'd blame your contracters. Messing up the router config would easily cause the problem your seeing. As would patching the wrong uplinks together. Or mucking up some VLAN tagging on ports.

 

Yes one of the DHCP servers is in a way.....The Server providing the DHCP to the student VLAN is a virtual server, the physical server it is residing on is on the normal default Vlan and this is on Network card 1. Then the virtual server has network card 2 assigned to the virtual server and this port plugs into the student VLAN. So even though the physical server is on both Vlan's the bindings have to be correct don't they as the virtual server can't see NW card 1, would I be correct in saying that?

 

If I am correct in what I have just explained I'll go over the main switch again just to be sure that all the network cables are correctly in the right Vlan ports

 

Thanks for your replys they have pointed me in the right direction I'll also take a look at Wireshark

[FatBoy]

Problem solved guys (I think) I found the problem, I found a cable on the student VLAN plugged into a port on the default vlan , I have unplugged it and at the moment it seems fine, fingers crossed!

 

Thanks for the help