Router as DHCP server and using ip helper at the same time?

Discussion in 'Cisco' started by Rick, Mar 3, 2006.

  1. Rick

    Rick Guest

    Hello all,

    Has anybody here been successful with or know how to set up the following?

    Using the Cisco router to hand out DHCP addresses to all but one device on a
    That one device needs its boot request to be forwarded to a server on a
    different subnet.

    I can get either the router to hand out addresses OR forward with ip helper
    but not at the same time.

    Thanks in advance.
    Rick, Mar 3, 2006
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  2. Rick

    ciscodagama Guest

    My educated guess on this matter is that this will not be possible. My
    first thought was some kind of policy routing to pick out the one
    device on the subnet that needs special treatment and maybe do
    something sneaky like send it into a loopback interface which has
    perhaps an "ip helper-address" configured and hope that the helper
    address command overrides the DHCP server running on the same system.

    However, note that the one device that needs this special treatment
    does not even have an IP address yet! So, policy routing is out. The
    only way to identify that device would be its layer 2 address and I
    can't see how with that constraint we could do this mix of DHCP server
    and DHCP relay functionality on the same box.

    Cisco da Gama
    ciscodagama, Mar 3, 2006
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  3. Rick

    Merv Guest

    What are the requesting devices attached to - a switch ?
    If so are the swiutch and the router capable of trunking ?

    Please explain the requirement to have the boot request for one device
    to go to a different server
    Merv, Mar 4, 2006
  4. Rick

    Rick Guest

    Yes, they are attached to a switch that is capable of trunking to the
    One device needs to get its address from a DHCP server that will then load
    an OS to it. All other devices get there address from the router no problem.
    Rick, Mar 4, 2006
  5. Rick

    Merv Guest

    so you could put the switch port for that device on a separate vlan and
    enable trunking between switch ant the router.

    Then on one vlan the router can be the local DHCP server, while the
    speical device is helpered to the desired server
    Merv, Mar 4, 2006
  6. Rick

    ciscodagama Guest

    The problem originally stated that a solitary device on a subnet needed
    this special treatment with a different DHCP server while other devices
    on that same subnet should use the IOS DHCP server. Moving the device
    to a separate vlan would mean moving it out of that subnet and putting
    it in a subnet of its own. It does seem reasonable to do something
    like that for this special case and so if you can afford to have two
    subnets instead of the one initially specified, the above suggestion
    should work and is very nice.

    I was earlier assuming that for some reason you need to keep all those
    devices on the same subnet.

    Cisco da Gama
    ciscodagama, Mar 4, 2006
  7. Rick

    Merv Guest

    Another thing I would investigate is if the server can be configured to
    unicast instead of broadcast its bootp request. AN IP helper would
    not be required in the case.
    Merv, Mar 4, 2006
  8. Rick

    Rick Guest

    So put this one device in a new vlan by itself, same IP address but trunk
    the old and new vlan on the same router port?
    Rick, Mar 4, 2006
  9. Rick

    Merv Guest

    Using a new vlan will mean a new IP subnet for the device using bootp.

    BTW what is the device in question (make,model, OS version)
    Merv, Mar 4, 2006
  10. Rick


    Jul 20, 2009
    Likes Received:
    Yes, this can be done.

    Yes. I am doing this with a Cisco 2621 running IOS (tm) C2600 Software (C2600-IK9O3S3-M), Version 12.3(12), RELEASE SOFTWARE (fc3)

    I am running vlans with subinterfaces on my FastEthernet 0/0. I also have FastEthernet 0/1, but don't use it with vlans or DHCP, as it is a standart ethernet (access) port that connects to my ISP.

    My applicable config is:

    Global config:
    ip dhcp excluded-address
    ip dhcp pool RepairVirusLAN
    domain-name RepairVirusLAN.internal

    ip dhcp-server

    Interface config on the interface that I want the ip helper to send dhcp broadcasts to my windows dhcp server:
    ip helper-address

    No extra config needed on the interface that I have the Cisco router doing DHCP for. As long as that interface has an ip address assigned that is in the range defined my my dhcp pool (shown above), then it just works. My router ip on that interface is, which is why I have excluded it from the dhcp pool.

    I'm not sure whether all of these commands are necessary. I have seen some posts that indicate that the ip dhcp-server command may not be, but I'm not sure. I haven't tried to disable it. But in any case, what I currently have works exactly as you have requested.

    Good luck!!

    dkmort, Jul 20, 2009
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