Re: DHCP Leasing

Discussion in 'MCSE' started by Pedro Simoes, Nov 24, 2005.

  1. Pedro Simoes

    Pedro Simoes Guest

    In article <>,
    says...
    > Client01 one leases an IP and so does Client02.
    > One hour after their leasing, the DHCP server goes down.
    > Client01 and Client02 have to reboot for any reason.
    >
    > After rebooting, Client01 maintains its IP address, as per the leasing


    > expiration. Client02 releases the IP (remember they both have the same


    > leasing period) for some reason.
    >
    > Both clients are Windows XP, and their behavior is expected to be the

    same,
    > since they have the same TCP/IP configuration. They are in the same

    subnet
    > and both can find the default gateway. No alternate IP is assigned.
    >
    > Any clue about why Client02 would release its IP before the leasing

    time
    > expires?
    > Remember Client01 has the same config and does not release the IP.


    This is not quite how DHCP leases work. DHCP leases are controlled by
    the DHCP server, not the client.

    When a DHCP client reboots, after the IP stack is started, it issues a
    DCHP Discovery packet, if the DHCP servers that receive the broadcast
    respond with a Acknowledge packet. If the DHCP server that acknowledges
    the client was the one that issued the address the client has cached,
    the client will request it in the Request packet, if the lease is still
    active, the DHCP server will respond with a Offer package for the
    address and the lease time is reset.

    If the DHCP server is not online when the client reboots, the client
    will use an APIPA address and continue to rebroadcast a Discovery packet

    every 5 minutes (2000 and later).

    As for why the two XP clients are behaving differntly, my guess is that
    the network IP properties on each client are configured differently.
    Most likely, APIPA is turned off on Client01. Client02 is working as
    expected.

    If you would like to learn more about this I suggest reading the RFC on
    DHCP (while boring as heck, if there is no better source for learning
    DHCP, which is a pretty simple protocol) and/or installing a network
    monitor and watching DHCP traffic.
    Pedro Simoes, Nov 24, 2005
    #1
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