I need to install fallback DHCP

Posted on 2011-10-18
Last Modified: 2012-05-12
As DHCP is given by AD, if AD goes down, what happens to DHCP?

Question by:techgyan
    LVL 10

    Expert Comment

    Although DHCP is authorised by AD it does not really rely on it and is a separate function on the server.

    To build in redundancy you can either use Microsoft Cluster Services and have two or more servers managing DHCP, or the poor mans way is to just have 2 DHCP servers both giving slightly different ranges as each PC will only get an address from one server.

    LVL 8

    Expert Comment

    DHCP is a separate service, but while integrated it should still work if the AD goes down, however you could have a secondary Domain controller on your network with DHCP service installed. This will mean that if your server goes down you still have an active Domain controller and you can just set the secondary dhcp servers scope to active to give out IP addresses. Generally you only want one dhcp scope active at a time so you would have the primary server active and when it goes down enable the second one.

    However, devices will keep there  ips and still keep working if the server goes down, its just new clients that will have problems if they try to connect to the network while your server has issues.

    Author Comment

    DHCP and DC running on same server and I have no Secondary ADC on my network
    LVL 8

    Expert Comment

    well you could implement the poor mans dhcp with a cheap router as WayneATaylor suggested and just keep the dhcp scopes in seperate ranges eg. on the server and on the router. Then when you have an issue with the server just power the router on, and it will take over the dhcp. However if your having issues with the domain controller then I think you will be having bigger things to deal with than any user that joins the network after that point.

    You understand that any clients currently on the network wont lose there ips. They will continue to use the same ips they already have allocated and will only have issues if they reboot. So its possible to reboot the dc and have the users still being able to connect to the internet  ( based on you having a secondary DNS server and a gateway separate to the DC)
    LVL 39

    Expert Comment

    by:Krzysztof Pytko
    Hi, if you have only one DC,DNS,DHCP server the nothing will save you when AD would crash :) No AD authentication, no DNS names resolution and the Internet access and no IPs issued by DHCP server :)

    You need for that redundant server with AD,DNS and DHCP. On 2003 it's a liitle bit difficult :/ You need to split scope between 2 DHCP servers manually and make exclusion range on both of them. I woud suggest (if it's possible to use 2008 R2 for that DHCP servers). Then you can use split-scope wizard. If you wish you can check how it looks like on my blog at

    if you cannot, you need to split scope between 2 DHCP servers by yourself :|

    LVL 16

    Accepted Solution

    You need a secondary DC...
    As iSiek wrote, with a single DC, you have a "single point of failure" for all your AD functions.

    You can have as many DHCP servers as you want, even on the same subnet, and without scope exclusions etc, if you have AD, as long as the DHCP servers are authorized in AD. They will then share the same configuration (stored in AD)
    Check the following links:

    1/ Install a second DHCP server (member server or DC. I recommend DC)
    2/ authorize both servers in AD

    You now have "DHCP redundancy".


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