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Exchange & Domain Controllers - Help with Topology

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  • Exchange & Domain Controllers - Help with Topology

    Hello,

    Could anyone help with this ...
    I am trying to find a document or a table which describes the various
    topologies for Exchange, relating several typical factors and with
    comments on the various solutions proposed :

    Number of Domain Controllers, Number of GC Servers, Number of Users,
    Type of traffic etc. ===> Exchange Server(s) installed on which type of server(s) ?

    eg: It's wel known that Exchange on a single DC is not advised but some
    firms cannot afford the extra (licence, HW, maintenance) and
    so install Exchange on their one Server. There is no choice.

    Then a firm might have 2 Server licences, 1 DC + 1 Member Server
    and not want yet another third server and decide to install on this second
    Server, but the Member server might not permit AD replication as a
    redundancy solution fro the Domain.
    This redundancy could be considered just as fundamental as the
    functionality of a Mail Server (Exchange) on one single member server.
    What is the best choice, if there is one ?

    A firm might have a third or forth licence, old 2K Servers, which could
    be used for certain options (supposing that the Exchange is not 2007).

    Looking at TechNet I have not yet found a paper mentioing such comparisons
    and describing the advantages and disadvantages of each method.
    I immagine it often depends on the customer.

    In the particular case I am considering now, the situation is reallly very
    modest, about 50 ca. users with reasonable traffic and the use of max 3
    licences (basically 2)

    Could anyone indicate how to make the various choices ?

  • #2
    Re: Exchange & Domain Controllers - Help with Topology

    For your scenario -- 50 users -- look seriously at SBS (the one time you CAN put Exchange on a DC and get away with it!)
    Tom Jones
    MCT, MCSE (2000:Security & 2003), MCSA:Security & Messaging, MCDBA, MCDST, MCITP(EA, EMA, SA, EDA, ES, CS), MCTS, MCP, Sec+
    PhD, MSc, FIAP, MIITT
    IT Trainer / Consultant
    Ossian Ltd
    Scotland

    ** Remember to give credit where credit is due and leave reputation points where appropriate **

    Comment


    • #3
      Re: Exchange & Domain Controllers - Help with Topology

      Hi Ossian,

      Thanks for the response but :

      1. I was aware of SBS and have already installed it elsewhere.
      2. Some years ago this firm bought 2 2003 Servers and and an
      Exchange Server Licence.
      The set was therefore 'fait accompli' when I arrived thus SBS is
      not really a solution in this case.

      I am still interested in your comments about the general case
      since there seem to be quite a number of possible solutions and
      opinions each with their Pros and Cons.

      Comment


      • #4
        Re: Exchange & Domain Controllers - Help with Topology

        Its quite simple.

        If Exchange is installed on a domain controller, it will ONLY use itself for DC functionality. It will never use any other domain controllers.
        If you have Exchange on a member server, then ideally you want two domain controllers, because if you have just one and it goes bang, then Exchange is dead in the water.

        With Exchange, you either use one server or three. Two is a waste of time. Gains you nothing other than a copy of the domain, but still leaving you with a complex DR of a combined domain controller and Exchange server.

        50 users is past the point where I recommend multiple DCs and a dedicated Exchange server. I usually start talking about it at 30. Obviously depends on the load, but if everyone is local, or there is heavy use of RPC over HTTPS, EAS and Blackberry, it is easily justified. Hardware and the licence is cheap - just means the CEO will have to delay replacing his desk for a month.

        Or put it another way - cost of downtime. How much will cost the company to be down hard for four hours while a recovery is carried out?

        Simon.
        --
        Simon Butler
        Exchange MVP

        Blog: http://blog.sembee.co.uk/
        More Exchange Content: http://exchange.sembee.info/
        Exchange Resources List: http://exbpa.com/
        In the UK? Hire me: http://www.sembee.co.uk/

        Sembee is a registered trademark, used here with permission.

        Comment


        • #5
          Re: Exchange & Domain Controllers - Help with Topology

          Thank you for participating and responding so accurately.

          Originally posted by Sembee View Post
          Its quite simple.

          If Exchange is installed on a domain controller, it will ONLY use itself for DC functionality. It will never use any other domain controllers.
          If you have Exchange on a member server, then ideally you want two domain controllers, because if you have just one and it goes bang, then Exchange is dead in the water.

          With Exchange, you either use one server or three. Two is a waste of time. Gains you nothing other than a copy of the domain, but still leaving you with a complex DR of a combined domain controller and Exchange server.
          If I am not mistaken, you are suggesting that if the firm is small then one accepts the small deal and does not try to go with half measures.
          After which one should move immedaitely to several servers (3+ depending on the nature of the firm) each with its own well defined role.

          Sometimes however directors, temporary economical problems or internal politics dictate a course of action which does not really follow such a sound
          reasoning.
          I have found that with smallish firms the presence of only two servers is quite common.

          Part of the reason for the question was in relation to being able to access the Domain if the first DC fails.
          Are you suggesting that replication between two 'mixed' (DC-DC Exch) servers can often be prone to error ?

          I have seen a case where there were only 2 servers for a small firm and Exchange was on one of them. One day, when the PDU of the first Server
          failed, this fall back solution seemed to work sufficiently well.
          The users were able to continue their work until the substitution without too much upset.
          Vice versa if the second server with exchange should ever fail then the users would simply be without their mail service for a while.
          When firms, with a moderately small number of users, do not have a person who is dedicated to full time maintenance, often they resort to semi solutions/compromises.

          Do you have by any chance a link to some page on TechNet, MSDN etc. where one can read a crtical review on Topologies, comparing the various combinations of servers : users : traffic etc. and explaining the Pros and Cons ?

          I have not yet found one.
          Thanks again.
          Last edited by Sembee; 10th February 2010, 18:50. Reason: Post formatting corrected for readability.

          Comment


          • #6
            Re: Exchange & Domain Controllers - Help with Topology

            There will be nothing from Microsoft in this regard, because Microsoft have their best practise and that is it.

            If you decide to operate with Exchange on a domain controller then you have to take the risks that involves, particularly around disaster recovery. Unfortunately most companies do not consider DR at all in their IT planning, preferring to put their head in the sand and hoping it never happens.

            The problems have nothing to do with replication. It is simply down to the complication of doing a DR on a domain controller with Exchange. For example, most DR for a domain controller says to delete the machine account. Do that when Exchange is involved you make recovery of Exchange a lot more complicated.

            If the domain is alive and Exchange is on a member server, then recovery of Exchange is very straightforward because the information is stored in the domain. However the Exchange databases are very sensitive to the machine name and therefore deleting the machine account is a bad move.

            If you are looking for content from Microsoft to backup arguments then you will not find it. Microsoft simply does not produce those kinds of documents. All you will find from Microsoft is one thing - they do not recommend Exchange is installed on a domain controller. That is it.

            Simon.
            --
            Simon Butler
            Exchange MVP

            Blog: http://blog.sembee.co.uk/
            More Exchange Content: http://exchange.sembee.info/
            Exchange Resources List: http://exbpa.com/
            In the UK? Hire me: http://www.sembee.co.uk/

            Sembee is a registered trademark, used here with permission.

            Comment


            • #7
              Re: Exchange & Domain Controllers - Help with Topology

              Hello,
              Thanks for the reply.
              I was interested in the reasoning in general behind the planning of the
              various topologies, the question of Exchange on a DC has in fact been
              discussed and criticised all over the place.
              A colleague of yours, Paul Robichaux, with others has written an
              interesting book (Exchange Server Cookbook) which mentions many
              of your arguments.
              The example I referred to is perhaps a border line case since it is a
              moderately small firm. It could in no way compare to the examples
              given by most of the readers in the forum (and MS), who may manage
              hundreds or thousands of users and a large number of servers.

              May I enquire on your opinions regarding regular Server Imaging
              as being a safe, efficient and a swift means for disaster recovery ?

              In my modest experience I have found that this this can be really
              quite a valuable technique and in some cases it can perform
              remarkably well.

              Thank you

              Comment


              • #8
                Re: Exchange & Domain Controllers - Help with Topology

                I don't use imaging at all for servers.
                The image is out of date the second it is completed. Particularly with email, the most valuable email is the content received in the last 24 hours. If your image finishes at 6am and then fails at midnight the next day, are you just going to reimage and lose that data?

                I manage a lot of servers across a lot of clients, and yet in the years I have been using servers I have not had a single instance where imaging would have helped. I have done the odd DR, but that was all planned in advance.

                What I tend to find is that imaging is used by administrators who don't know what they are doing. It is an easy fix, used to cover up other problems. I would have to ask why they got to the position where a reimage is required. Poorly specified server, bad choice of software, fiddling around and not knowing what they are doing with the server.
                That may well be a rather broad assumption, but I have seen it many many times.

                A server correctly configured, built and managed will give very little problems. I have servers I have not had to do anything significant with since they went in to production four or five years ago.

                Simon.
                --
                Simon Butler
                Exchange MVP

                Blog: http://blog.sembee.co.uk/
                More Exchange Content: http://exchange.sembee.info/
                Exchange Resources List: http://exbpa.com/
                In the UK? Hire me: http://www.sembee.co.uk/

                Sembee is a registered trademark, used here with permission.

                Comment

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