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Is DAG what I need?

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  • Is DAG what I need?

    Hi all

    I have an exisiting Exchange 2010 environment in the office, all roles on one server.

    After a prolonged power issue in the office today and being without email, I got wondering if I could set a spare ML115 up at home (decent internet, no caps, probably more reliable for power than the office) as a 2nd server using DAG.

    I have read a few guides on how to set this up, as in what to press etc, but there are a few points I need clarifying.

    Am I right in thinking it will do all of the below if configured correctly:

    1. Store a copy of the mailboxes on both servers, continually replicating in real time
    2. If Server A (office) goes down, will remote Outlook clients automatically look for the 2nd server? This needs to be able to send/receive mail.
    3. When the Office comes back up, will it sync any changes (ie mail that came in during downtime) to the Office Server, and have that resume as "master"?

    Are there any other considerations I should be thinking of?



  • #2
    Re: Is DAG what I need?

    If you are thinking about setting up a server at home about how much mailboxes we are talking?

    In theory you are right with your points if setup and configured correctly.
    But in practice do you realy need high availability and if yes do you realy want to set this up depedning on a home installation?


    • #3
      Re: Is DAG what I need?

      About 6 mailboxes only at the moment.

      I thought I could live without HA but given the disruptions power issues have caused the last few months, if anything lasted longer than a day I'd really be in trouble...

      Thanks - off to research.


      • #4
        Re: Is DAG what I need?

        The first question has to be bandwidth.
        For a DAG to operate you need quite a bit. If you only have a regular ADSL connection, with the poor upload speed, then it may not be quick enough.

        Furthermore, a DAG isn't the only part of the equation. In the scenario that you have outlined, clients will not automatically connect to the second server, as clients don't connect to the mailbox role, they connect to the CAS role.
        You would need to have a CAS Array configured, with a DNS host involved. If the server goes down, the DNS entry would be changed to point to the second server.

        You would also need to have a domain controller in the second location. That is a separate domain controller, not Exchange installed on a domain controller. The reason for that is when Exchange is installed on a DC, all Exchange servers will only use that DC for their domain controller functionality, which would mean all DC traffic from Exchange (and there is a lot) would be going across whatever WAN connection you have, rather than using local.

        The most common scenario I have been deploying on small sites is to use VMWARE and a server in a data centre. The VMWARE server has a domain controller and Exchange server in separate virtual machines. This doesn't cost more to licence because you need to have Windows Enterprise edition to use the DAG, and Enterprise addition allows multiple virtual machines on the same piece of hardware.
        In the event of a loss, then failover occurs, and DNS entries are manually changed.

        Simon Butler
        Exchange MVP

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        • #5
          Re: Is DAG what I need?

          Hi Simon

          Thank you for your thoughts.

          I think bandwidth will be ok - cable connection at home, 50 down, about 1.6 up (mbps). 10mbps (soon to be 20) lease line at the office end. Is that likely to be sufficient in your opinion?

          My original thought (without considering the implications of not having a DC on the 2nd site) was to utilise this spare ML as a quick and cheap way of creating redundancy.

          However, this is not possible with the ML - it's too low spec to reliably run VM + 2 machines! Probably would not be worth the cost of upgrading either...need to check really.

          Your suggestion of Co-Lo is obviously the best idea.

          I'm going to go and look at costs for all of this and work out which is the most cost efficient way forward.

          Thank you.