DHCP and secondary networks

Discussion in 'Cisco' started by branigan, Mar 13, 2007.

  1. branigan

    branigan Guest

    I have a layer 3 switch that I need to support some secondary
    networks. I also have a DHCP server on one of the secondary networks.
    The problem is, I cannot get DHCP to work on any of these networks. If
    the network are remote or on another vlan everything works fine. Is
    there something that can be done on the layer3 switch that I need to
    set to support this type of configuration?

    thx.
     
    branigan, Mar 13, 2007
    #1
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  2. branigan

    Thrill5 Guest

    "ip helper-address x.x.x.x" on the routing interfaces for your subnets that
    don't have a DHCP server where x.x.x.x is the ip address of your DHCP
    server.

    Scott

    "branigan" <> wrote in message
    news:...
    >I have a layer 3 switch that I need to support some secondary
    > networks. I also have a DHCP server on one of the secondary networks.
    > The problem is, I cannot get DHCP to work on any of these networks. If
    > the network are remote or on another vlan everything works fine. Is
    > there something that can be done on the layer3 switch that I need to
    > set to support this type of configuration?
    >
    > thx.
     
    Thrill5, Mar 14, 2007
    #2
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  3. branigan

    branigan Guest

    Thank you for replying. My problem is or so it seems is as follows:

    I have a layer3 switch with vlan100. On this vlan I have 3
    secondaries. One of those seondaries has the dhcp server on it. I have
    the helper under vlan100, pointing to that dhcp server. For some
    reason dhcp wont work when using that dhcp server. So i moved it to
    another one, not at the same vlan/secondary setup. That dhcp server
    worked fine. I was just wondering if there was a command or parameter
    that would be necessary for dhcp in this configuration.

    thx again.
     
    branigan, Mar 14, 2007
    #3
  4. On Wed, 14 Mar 2007 19:23:24 -0400, branigan wrote:

    > Thank you for replying. My problem is or so it seems is as follows:
    >
    > I have a layer3 switch with vlan100. On this vlan I have 3
    > secondaries. One of those seondaries has the dhcp server on it. I have
    > the helper under vlan100, pointing to that dhcp server. For some
    > reason dhcp wont work when using that dhcp server. So i moved it to
    > another one, not at the same vlan/secondary setup. That dhcp server
    > worked fine. I was just wondering if there was a command or parameter
    > that would be necessary for dhcp in this configuration.
    >


    If your DHCP server is in the same vlan as the clients, then it is going
    to see the client's broadcast before it sees the relayed request from the
    router. The server will want to give the client an address in the same
    network that the server is on since they are in the same broadcast domain.

    In any case if you want to have secondary networks and expect dhcp relay
    to work, you need to configure "ip dhcp smart-relay" on the relay agent.
    without that configured the relay will only use the primary interface
    address as the giaddr and you will never see any addresses offered in the
    secondary networks.

    With smart-relay the relay agent works through the primary and secondary
    networks in order, filling each one before moving on to the next. Full means
    the DHCP servr stops responding to requests for a given network.

    These debugs, debug ip udp, and debug ip dhcp server packet, will show
    you what the relay agent is doing with the requests, and give you some
    idea of what sort of response, if any, it gets from the dhcp server. Any
    half decent dhcp server should have some sort of logging you can look at
    to see what it thinks is going on.

    I've been round this same issue once before, and I think we concluded it
    probably wouldn't and shouldn't work.

    --
    Rgds,
    Martin
     
    Martin Gallagher, Mar 15, 2007
    #4
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