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  • Same Subnet DHCP redundancy, reservations?

    Could someone help me with DHCP redundancy? I have a two DC's which also both provide DNS.

    I have the PDC setup as a DHCP server, scope xxx.xxx.xxx.110 to xxx.xxx.xxx.240. There are also a number of IP reservations setup on the server.

    How would I go about setting up the other DC as a DHCP server, splitting the scope (50/50?) whilst also being able to serve the reserved IPs should the initial DC/DHCP server go down?

  • #2
    Re: Same Subnet DHCP redundancy, reservations?

    Presumably the reservations are for member servers, printers etc. Giving them static addresses rather than DHCP reservations would be the recommended configuration.
    Gareth Howells

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    • #3
      Re: Same Subnet DHCP redundancy, reservations?

      Originally posted by gforceindustries View Post
      Presumably the reservations are for member servers, printers etc. Giving them static addresses rather than DHCP reservations would be the recommended configuration.
      I agree.

      Also, I've never tried it but I think you should try setting up the same reservations on both DHCP servers. At the end of the day, the client is only going to get it's reserved ip address from one of them, I don't think it matters which one. I don't think you'd have the same problems as you would have with non-reserved DHCP clients. With non-reserved DHCP clients if you have two DHCP servers with scopes that overlap you could get two DHCP clients allocated the same ip address and you'd have an ip address conflict on the network. In your case you're talking about a reservation (based on the MAC address) for the same MAC address, so there's no possibility of two DHCP clients getting the same ip address. The reserved DHCP client is going to get the address from one server or the other but there won't be any ip address conflict on the network.

      I use a 50\50 setup in my network. The only thing you should make sure is that:

      1. Your scopes don't overlap.
      2. You have enough ip addresses in each scope to service all of your clients.

      I had a problem with one of my DHCP servers last week and the second DHCP server didn't have a scope big enough for all of the client machines so I had some people who got ip addresses and some who didn't.

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      • #4
        Re: Same Subnet DHCP redundancy, reservations?

        For DHCP you normally use an 80/20 rule. I.e. have 80% of your reservation on one server and 20% on the other. If your leases are valid for 8 days then this means that you will typically have 4 days to rebuild a dhcp server in the event of a server goind down.

        You set up the same scope on both DHCP servers i.e. 10.1.1.10 - 250 and then reserve 20% from one and the other 80% from the other.

        http://technet.microsoft.com/en-us/l.../cc958936.aspx

        Michael
        Michael Armstrong
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        • #5
          Re: Same Subnet DHCP redundancy, reservations?

          The 80/20 rule never made sense when I first learnt it and that continues till today. You only need to know it for the exams.

          I would follow a 50/50 setup for the real world. The example of an 8 day lease woth 4 days to rebuild may also not work depending on how it has been setup. I have inherited networks that revert to the 169.254.x.x IP when DHCP becomes unavailable.
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          • #6
            Re: Same Subnet DHCP redundancy, reservations?

            Originally posted by biggles77 View Post
            The 80/20 rule never made sense when I first learnt it and that continues till today. You only need to know it for the exams.

            I would follow a 50/50 setup for the real world. The example of an 8 day lease woth 4 days to rebuild may also not work depending on how it has been setup. I have inherited networks that revert to the 169.254.x.x IP when DHCP becomes unavailable.
            I've seen the same thing regarding getting APIPA addresses as soon as the DHCP server goes down, even with a fresh DHCP install with all the default settings. That's what prompted me to move to a 50\50 split.

            On a side note, if you're running W2K8R2, there's a "split scope" function to assist in setting up a split DHCP scope amongst multiple DHCP servers.

            Microsoft seems to be getting better and better with this stuff, IMHO.

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            • #7
              Re: Same Subnet DHCP redundancy, reservations?

              My experience has been that the 50/50 design has worked best. I have used this design on networks servicing up to 600 subnets between 2 DHCP servers.

              Oh, and in regards to any confusion regarding DHCP reservations, yes, they must be defined on both servers. You cannot guarantee which DHCP server will service the client that is configured to use a reservation.

              Also, when creating your scopes, make sure you define the whole subnet and use exlcusions. With Windows 2008, you CANNOT create reservations outside of the pool.

              For example, if you are servicing the 192.168.1.x/24 subnet, both DHCP servers should be configured with the same pool 192.168.1.1-254. Then on the first server exclude the lower range and on the second server, exlcude the higher range. You will then be able to create the same reservations on both servers without a problem.

              If you are big on DHCP administration and have some coding skills, you can even build a web interface to have the DHCP reservation process fully automated from using a web form.
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              Last edited by [JM]; 24th January 2011, 18:13.
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