Multiple DHCP servers

Discussion in 'Server Networking' started by jtdrummerboy, Jun 5, 2007.

  1. jtdrummerboy

    jtdrummerboy Guest

    I have 2 DHCP servers setup in my domain using the 80/20 rule that Microsoft
    recommends. I have problems with the 2nd DHCP server however, because it
    continues to run out of available address leases as it would appear clients
    default to pulling their addresses from the DHCP server with fewer addresses.
    Is there a way to default clients to pull their address from a specific DHCP
    server first and when unavailable, from the second DHCP server?
    jtdrummerboy, Jun 5, 2007
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  2. In
    Sounds like the faster server. Switch them around.


    This posting is provided "AS-IS" with no warranties or guarantees and
    confers no rights.

    Ace Fekay, MCSE 2003 & 2000, MCSA 2003 & 2000, MCSE+I, MCT, MVP
    Microsoft MVP - Directory Services
    Microsoft Certified Trainer

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    Ace Fekay [MVP], Jun 5, 2007
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  3. jtdrummerboy

    jtdrummerboy Guest

    Ace, If I wanted to set priority to one over the other, is there a way of
    doing that? The faster of the 2 servers is indeed the one that keeps running
    out of addresses, however, I want the other server to be the primary to
    distribute addresses. Does this make sense?
    jtdrummerboy, Jun 5, 2007
  4. jtdrummerboy

    Scott Lowe Guest

    It does indeed make sense, but AFAIK there is no way to do this (not
    if both servers are on the same subnet as the clients). In a routed
    environment where you are using DHCP relays, this is probably
    possible; however, we don't know if this would apply to your network.

    Scott Lowe, Jun 5, 2007
  5. jtdrummerboy

    jtdrummerboy Guest

    Hi Scott, thanks for your quick response. Both of these servers reside on
    the same subnet so I agree with your theory that it would be difficult to
    give priority to one over the other. I do also agree with Ace's response to
    give the larger pool to the one that acknowledges more requests...I was just
    trying to avoid this as this server is more heavily used.
    jtdrummerboy, Jun 5, 2007
  6. Hi..! I have this message in my DHCP server.. event id 1020

    Scope,, is 100 percent full with only 0 IP addresses remaining.

    I have running Windows 2000 Server .. the case is that when I try to
    put a new pc and it try to take dhcp automatally it present the error and I
    have to put ip address manually..!
    I have 10 pcs on my domain

    How can I solve this warning?
    please, send me a email.:
    DHCP Error Event ID 1020, Jun 5, 2007
  7. jtdrummerboy

    net_admin Guest

    You should adapt you scope with a bigger range of IP addresses.
    We do not email people, that's why this is a forum, to share knowledge.
    net_admin, Jun 6, 2007
  8. jtdrummerboy

    net_admin Guest

    Personally I don't like this rule because of issues like the one you are
    Another option is to setup 2 different ranges in the same subnet with enough
    net_admin, Jun 6, 2007
  9. jtdrummerboy

    net_admin Guest

    I mean, 2 # ranges, on in each server, of course.
    net_admin, Jun 6, 2007
  10. jtdrummerboy

    Scott Lowe Guest

    I wouldn't be too terribly worried about server utilization; the
    overhead for DHCP is fairly minimal, and adding addresses to an
    existing scope would be, I think, less of a worry than adding DHCP
    from scratch to an existing workload.

    Scott Lowe, Jun 6, 2007
  11. In
    Sorry, unfortunately it doesn't work that way. Put the larger pool on the
    faster box, as has been suggested.

    Ace Fekay [MVP], Jun 7, 2007
  12. jtdrummerboy

    Ryan Hanisco Guest


    When a DHCP client decides to look for a new address or refresh the one it
    has (at 50% of the lease duration) it does a broadcast on the network looking
    for available servers. In all cases, it will accept the first one that
    replies back -- this is why ACE knew it was the faster server. Since
    broadcasts don't cross routed boundaries by default only servers on the
    subnet are in the running. You can use IP Helper (IP Helper-address in cisco)
    to allow for DHCP requests to cross the boundary.

    An 80/20 scenario is implemented to provide higher availability for DHCP in
    the case that one server goes down. It should never be used to extend the
    scope. You should use larger scopes or superscopes to grow capacity. When
    the primary server goes down, you have the time that it takes the 20% to get
    used to fix the problem before you start impacting the users.

    It should also be noted that you shoudl authorize both servers to host the
    ENTIRE range and use exclusions to mask the 80/20 scenario. Otherwise you
    get NAK poisoning when a client goes to refresh and hits the "other" server
    for some reason. You want it to respond with a differnt valid response
    rather than issue a NAK telling the client that the range isn't valid. In
    some cases, the client doesn't try again and the IP stack goes down.

    Hope this helps.
    Ryan Hanisco
    MCSE, MCTS: SQL 2005, Project+
    Chicago, IL

    Remember: Marking helpful answers helps everyone find the info they need
    Ryan Hanisco, Jun 22, 2007
  13. In
    NAK poisoning, I like that. :)

    Your suggestions of putting DHCP elsewhere such as on another subnet and
    using IP helpers (similar to DHCP relay agent in Windows) is a great idea.

    Ace Fekay [MVP], Jun 23, 2007
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