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Second DHCP server

Discussion in 'Software' started by nugget, Aug 19, 2008.

  1. nugget
    Honorary Member

    nugget Junior toady

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    Just wondering how you guys in larger companies get on with DHCP servers. I'll be setting up a second DC this week and would also like to put DNS and DHCP roles on it too. As far as I can work it out the DNS server role won't be a problem but the DHCP likely as not will be.

    All information so far points to the well known saying "you shouldn't have 2 DHCP servers on the same network" that we all know.

    How the heck do you have a second DHCP server on the same network for redundancy in case the first one goes belly up? I thought about exporting the config from the first one and importing it into the second one but just not activating the scope. Would this work?

    Any advice appreciated.
     
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  2. Qs

    Qs Semi-Honorary Member Gold Member

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    This may help.

    Saves me re-writing it all. :p

    Hope this helps. :)

    Qs
     
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  3. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

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    I think that the standard rule for implementing a second DHCP server is the 20/80 rule. that is 20% of the addresses available go onto the second DHCP server for redundancy, whilst the othe 80% are on the main DHCP server. Of course you could choose to implement it 50/50, but you cannot control which DHCP server the clients will connect to, so 90% of clients may well be using one server which is only distributing 50% of the available addresses.
     
  4. BrizoH

    BrizoH Byte Poster

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    That's exactly how I set ours up, using the 80/20 rule
     
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  5. Qs

    Qs Semi-Honorary Member Gold Member

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    Yep, that's what I'd suggest too - though there's nothing wrong with other ratios.
     
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  6. nugget
    Honorary Member

    nugget Junior toady

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    Okay, got it now. Thanks guys.

    So, in short
    1 configure the same exclusions on both dhcp servers
    2 configure the same reservations on both dhcp servers
    3 configure 1st dhcp server with a scope with 80% of the dhcp addresses
    4 configure 2nd dhcp server with a scope with 20% of the dhcp addresses
     
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    WIP: MCSA, 70-622,680,685
  7. zcapr17

    zcapr17 Nibble Poster

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    Hi Nugget,

    First of all, if posssible you should avoid putting DHCP on DCs due to the security risk. See http://windowsecurity.com/articles/DHCP-Security-Part1.html

    The golden rule when running multiple DHCP servers is to ensure that the lease ranges do not overlap.

    In my current company, we have two active DHCP servers (which are in different datacentres) and issue IPs to around 100 other WAN sites. The WAN router on each site is configured for DHCP forwarding which forward requests to both of the two central DHCP servers. This has the effect of load-balancing requests accross the two servers.

    We use a 50/50 split for the IP ranges, but ensure that one server has enough IPs to satisfy any requests (i.e. one of the DHCP servers can be offline for a period without resulting in a shortage of IPs).

    A quick example:
    WAN site has the range: 10.1.0.0/22
    10.1.0.1 - 10.1.0.255 = Reserved for Static devices.
    10.1.1.0 - 10.1.1.255 = DHCP Range for DHCP01
    10.1.2.0 - 10.1.2.255 = DHCP Range for DHCP02
    10.1.3.0 - 10.1.3.254 = Reserved.

    You can apply the same strategy to a single LAN also.

    Regards,
     
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