Understanding the impact of a DHCP server outage

Discussion in 'Windows Networking' started by Hugh, May 19, 2007.

  1. Hugh

    Hugh Guest

    We want to make sure we fully understand the impact to DHCP clients whenever
    we have a DHCP server outage. After doing to testing a tracing, it seems the
    breaking point for a DHCP client (at least in our environment) is controlled
    by the value of the Rebinding Time Value parameter in the DHCP Offer packet.

    If my testing was accurate, DHCP clients (Win XP SP2 clients, specifically)
    will continue to use their previously obtained IP address until they reach
    the value of the Rebinding Time Value (which is set automatically to 87.5% of
    the lease value). So at 50% of the lease time (set to 14 days in our
    environment), the client passes the renewal threshold. However, in my
    testing, the client still used its IP address. However, once I reached the
    Rebinding Time Value, the client would send DHCP Discover packets and, when
    no response was received from a DHCP server, the client would then revert to
    an APIPA address.

    The conclusion from our testing is that DHCP clients must meet two criteria
    before they will fail to obtain (or use an existing) IP address:

    First, prior to or during the failure, the DHCP client exceeds its Rebinding
    Time Value.
    Second, during the failure (and after exceeding the Rebinding Time Value),
    the DHCP client is restarted.

    So my question is simple: Are our conclusions correct? Thanks.
    Hugh, May 19, 2007
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  2. Hugh

    Jeremy Guest

    You are correct.

    At 50% of the lease time the DHCP client attempts to renew its lease from
    the DHCP server that it leased the address from via unicast. If this is not
    successful the DHCP client waits until 87.5% of the lease time has expired
    and then re-attempts to renew it address, again via unicast to the DHCP
    server. If this is unsuccessful the client attempts to obtain a new lease
    from any DHCP server via the standard broadcast method (DHCP DISCOVER). If
    no DHCP server responds the client falls back (assuming XP) to the alternate
    configuration, which is APIPA by default.
    Jeremy, May 20, 2007
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  3. Hugh

    Hugh Guest

    Great. Thanks very much for the response.
    Hugh, May 20, 2007
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