DHCP Multiple VLAN problem

Simon Hobson dhcp1 at thehobsons.co.uk
Thu Jul 19 23:05:33 UTC 2007

Scott F wrote:

>Basically we have 2 subnets set up in a shared network. One is for the
>VLAN1 data traffic and the other is for the VLAN 10 voice traffic. The
>DHCP Server is in VLAN1 and there is an IP Helpr set up to forward the
>DHCP traffic from VLAN1 to 10 (on the 3com 5500).

OK, this is NOT a shared network then !

>  On the VLAN 1 subnet
>all the computers are statically assigned addresses by MAC. I have set
>up two pools of addresses, one of VLAN1 range and the other on VLAN 10
>range. When the phones start up they are on VLAN1 and broadcast for an
>address, they get one along with other options that tell it that it
>should be on VLAN10, the phone releases its address and comes up on
>VLAN10 where it broadcasts and gets an address in the VLAN10 range,
>because the request comes from the helper, all seems to work ok.
>However, work want to have the pools set so people can't just plug in
>any device and get an address, but whenever I put any sort of
>conditional statement in DHCPd sees this as the most specific option
>and goes to the subnet containing it ignorning whether it has come by
>the helper or not.

As I read it, you have incorrectly configured a shared network. VLANs 
are not shared networks.

I suspect that if you take out your shared network declaration then 
things will work better. You will still need to identify the phones 
when they first come up in VLAN1 (and give them the right config 
options), but once they switch to VLAN10 then the server will have no 
problem because it will know from the GIAddr field in teh relayed 
packet that they are now on a different network.

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