How DHCP decide which scope to give thourgh dhcp relay agent?

Discussion in 'Windows Networking' started by cs, Dec 30, 2006.

  1. cs

    cs Guest

    Hi ,

    I did some research and know that dhcp realay agent will modify the DHCP
    request info and add some info on the giaddr field to indetify the source so
    that DHCP will know which scope to give .

    Anyone have more details docs related on this design ? Like what is the info
    store in giaddr field ? how the dhcp server calculate and decide which scope
    to give ?

    If in multiple VLAN enviroment , my client pc is getting other VLAN IP .
    What is the possible causes ?
     
    cs, Dec 30, 2006
    #1
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  2. cs

    Bill Grant Guest

    DHCP is a very simple setup. The client broadcasts on the network with a
    request. If there is a DHCP server which can receive the request, it replies
    with an offer of an IP in the same subnet as the NIC in the server which
    received the broadcast request.

    If there is only a relay agent to receive the request, it will forward
    the request to a DHCP server. The DHCP server will make an offer in the same
    IP subnet as the NIC of the relay agent which recieved the original
    broadcast.
     
    Bill Grant, Dec 31, 2006
    #2
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  3. cs

    cs Guest

    Thanks for reply .. ok now .. What you mean by same Ip subnet ? If my
    enviroment is have multiple VLAN configure in the same switch and dhcp server
    at other vlan network . So what will be the ip subnet on client if it on vlan
    1 ?

    vlan 1 10.10.1.x 255.255.255.0
    vlan 2 10.10.2.x 255.255.255.0
    vlan 3 10.10.3.x 255.255.255.0 (dhcp server ip range)

    Because my understanding is client will first register as 0.0.0.0
    255.255.255.255 . So how do dhcp know it should issue which scope ?
     
    cs, Jan 4, 2007
    #3
  4. cs

    Bill Grant Guest

    It should not make any difference if you have VLANs. VLANs are just a
    construct built into the switch. As far as the networking protocols are
    concerned the network looks exactly the same, whether you have VLANs or
    simple hub/switches and routers. If a machine is in the VLAN which is using
    10.10.2.x and it broadcasts a DHCP request, that request will be received by
    a DHCP relay agent on an interface with a 10.10.2.x IP address. The
    broadcasts should not be crossing the "routers" between the subnets.
     
    Bill Grant, Jan 4, 2007
    #4
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