Ignore DHCP requests from Cisco IP phones

I was wondering if anyone knew of a way to configure the DHCP server service
on a Windows 2003 Server to ignore DHCP requests from certain type of
clients, specifically Cisco IP phones?

In our branch office locations we share office space with our parent company
and utilize their legacy phone system and LAN infrastructure via a separate
VLAN which contains our PC's, printers and a W2K3 server which is a DC and
runs DHCP to hand out leases to our PC's and printers. The problem we are
running into is that our parent company is replacing the legacy phone systems
with a Cisco VOIP system and when they plug the phones into the network jacks
in our cubes they are getting DHCP leases from our local server which hoses
things up because the IP address is in a totally different subnet and it
doesn’t contain the necessary options the VOIP phones require to work. What
we would like to happen is have our server ignore the IP phones DHCP request
which will then cause the request to get relayed back to the parent companies
home office DHCP server(s) via a helper app on the router.

If the phones are not setup manualy w/ both an IP/subnet AND a TFTP
server (DHCP option 150) then they will always send a DHCP cast.

Unless you want to setup DHCP reservations for _ALL_ your hosts, then
your answer is no.

However, what you need to do is:

Cisco's phones kida expect to be pluged into Cisco switches. Not
nessicary if the switches being used can provide the sames
features...Interfaces on Cisco switchs have a speical command called
"Switchport Voice VLAN" in addition to "Switchport access vlan" (and
related command if interfaces are dot1q trunks)

The phones should be placed in thier own vlan, with intervlan routing
enalbed (depends on switch gear) so the phones can still talk to vaious
gatways, Callmanager/astrix, etc, etc, etc.

You need to setup an "ip helpper-address <DHCP server's IP Address
goes here>" for each virtual interface for each vlan on the switch.
Basicaly a DHCP relay Agent.

Then, setup a NEW scope in your DHCP server matching the subnet for the
voice vlan. You can then exlucde the enitre range if you wish.

The important points here is that IP phones SHOULD have their own VLAN.
(the exception being very small installs.)

Tell them jokers to setup the VLAN for thier phones properly?

the requests should always be caught by a DHCP relay agent (ip helper
address on vlan interface?) so that the correct DHCP server can repsond
using the correct DHCP scope, w/ the correct options.

msch...@pingless.com wrote:
> Tell them jokers to setup the VLAN for thier phones properly?
>
> the requests should always be caught by a DHCP relay agent (ip helper
> address on vlan interface?) so that the correct DHCP server can repsond
> using the correct DHCP scope, w/ the correct options.

Oh ya. I get it now. The new phones are also plugged into YOUR cubes?

You just need to set the right option, on your DHCP server. Your DHCP
server will still respond to the request even if you set up a relay
agent.

The phones will want "option 150" (TFTP server) specifiy the address
for (CallManager, if using this platform)

So long as the phones get the "option 150" they will get the rest when
they download thier config from the TFTP server.