Ignore DHCP requests from Cisco IP phones

Discussion in 'Windows Networking' started by Surety, Jul 19, 2006.

  1. Surety

    Surety Guest

    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.
    Surety, Jul 19, 2006
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  2. Surety

    mschunk Guest

    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.)
    mschunk, Jul 20, 2006
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  3. Surety

    mschunk Guest

    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.
    mschunk, Jul 20, 2006
  4. Surety

    mschunk Guest

    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

    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.
    mschunk, Jul 20, 2006
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