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  • Global Catalog Reads

    Does Exchange need to have a global catalog server from each child domain located within its site for best performance?

    Background:

    Single Forest: corp.mytest.com

    Four Child Domains:
    Na.corp.mytest.com
    Emea.corp.mytest.com
    Ap.corp.mytest.com
    La.corp.mytest.com

    All Exchange Servers are members of the same AD domain (na.corp.mytest.com) and in the same AD site (Exchange-Site) with two dedicated DC’s that are both global catalog servers, located in the same (Exchange-Site) site and members of the na.corp.mytest.com domain.

    We have users with AD accounts in all of the domains that have their mailboxes located on the na.corp.mytest.com Exchange Servers.

    As expected, when watching various counters (i.e. LDAP Read call/Sec) on the CAS servers I see that Exchange is making calls to the two GDC’s located within its site as expected; I also can see that it is making calls to GDC's which are members of the emea, ap and la domains and located in different sites. Note: All sites links have the same cost (hub and spoke)

    From my understanding, Exchange should be able to retrieve all of its information from the two GDC located within the Exchange-Site site and not be making calls to other GDC’s in other sites. My only thought is that since the two GDC’s are both members of the NA domain, Exchange is looking for GDC’s that are members of the other domains for some reason.

    Has anyone else seen this behavior and is there something that I need to adjust so that Exchange doesn’t look at the GDC’s in the other domains? I have already hard-coded the local DC’s and GDC’s into the CAS servers which didn’t stop them from making calls to various emea, la and ap GDC’s.

    Any input is appreciated. Thanks.
    Last edited by BigDW; 22nd July 2009, 16:29.

  • #2
    Re: Global Catalog Reads

    Based on what Iím seeing, it seems as though we might benefit from having a GDC for each of our child domains in our Exchange site.

    This is for Exchange 2003 but many 2007 articles reference it.

    http://support.microsoft.com/kb/875427

    For scalability and for fault tolerance, we recommend that you configure at least two global catalog servers in each Active Directory site. If a site spans multiple domains, we recommend that you configure a global catalog for each domain where Exchange 2000 Server computers or Exchange Server 2003 computers and clients reside.

    Anyone have any thoughts on this?

    Comment


    • #3
      Re: Global Catalog Reads

      You need to have a global catalog as close to Exchange as you can get. It queries the GC a lot. If the nearest GC is over a WAN then you will soon have complaints from the users about poor email performance - probably within the first hour of the deployment.

      Is Exchange installed on any domain controller? That will cause problems.
      Exchange tends to talk to the first GC in its site that responds. If it is going across the site, then that would tend to indicate the sites are not setup correctly.
      Some traffic I believe will go across sites when Exchange reads the information for routing the email. Hub should be doing that. Also don't forget that there will be some availability information going across as well.

      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


      • #4
        Re: Global Catalog Reads

        Thanks for the response. All of our AD sites are based on physical location and all of our Exchange servers and two dedicated GDC’s are located in the same site (AD & physical) What I’m trying to determine is if there needs to be a GDC that is a member of each of the child domains in this same site?

        Thanks.
        Last edited by BigDW; 23rd July 2009, 16:47.

        Comment


        • #5
          Re: Global Catalog Reads

          Technically there does not need to be domain controller of the child domain in the site.
          Practically though, it can come down being needed.

          Exchange will still need to query the domain controllers in the child domains, but it should cache that information once it has done the query. However if the number of users is considerable, then you may look at putting a domain controller in the same site so that Exchange has a closer point to refer to.

          You then have issues of what happens when the WAN link goes down to the child domains, which may start to cause a problem.

          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


          • #6
            Re: Global Catalog Reads

            Why does Exchange need to query the domain controllers in the child domains if it has two local global catalog servers in its site?

            Does anyone know of any documentation that states whether a gdc that is a member of each child domain should be in the same Exchange site or not? I provided a link earlier that doesnít specifically fit our case because our site doesnít span multiple domains?

            The issue Iím seeing is that Exchange is querying the local site GDCís as expected but I also see it making calls to child domain GDCs not in its AD site as well.

            Comment


            • #7
              Re: Global Catalog Reads

              As far as I am aware, I have already answered the question.
              It is querying the child domain controllers because the local domain controllers do not have the information that it needs.

              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


              • #8
                Re: Global Catalog Reads

                Thank you for your response.

                Do you know of any documentation stating whether a child domain contoller and child domain global catalog is needed in the site?

                "Technically is does not need one but practically it does" isn't exactly what I'm looking for here.

                1) Why is it making call to out-of-site GDC's when there are GDC's in the site that should have the information needed?
                2) What information is it looking for that the on-site GDC's don't have?
                3) Why is it going to random off-site GDC's for the information and not using the GDC's closest to the Exchange site if the on-site GDC's don't have the information needed?

                Does anyone else have experience with this?
                Last edited by BigDW; 27th July 2009, 20:44.

                Comment


                • #9
                  Re: Global Catalog Reads

                  I am not really sure what else I can tell you.
                  Each domain has its own set of information. That is what the global catalogs hold. If the local cache does not have that information, then Exchange has to go and get it from the original source.
                  Furthermore it refreshes that information periodically to ensure that it has the latest information regarding mailbox permissions and settings.
                  Some information is refreshed live, others on a cache of approximately two hours.

                  It is not something that you are missing, it is not something that you can add. It is simply a fact that is how the domain works. Exchange needs to poll those domain controllers, you cannot stop it.

                  The reason for my comment about it technically not being needed is because Exchange will poll that information. However if your WAN links are over utilised then that can cause a delay which is only removed by having a domain controller with the Exchange server.

                  In effect this is the same way that most library systems work. You will have a copy of the most commonly accessed information in your local library, but if you want something not as frequently accessed then it comes from a central point. That copy is then stored locally for a period of time, if it is not used again then it is sent back (or for Exchange, discarded from the local cache).

                  If your goal is to try and stop Exchange from polling the remote domain controllers, then that is not going to happen. You can give up wasting your time. Exchange needs to be able to poll a global catalog in every domain where it has users.

                  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


                  • #10
                    Re: Global Catalog Reads

                    "If your goal is to try and stop Exchange from polling the remote domain controllers, then that is not going to happen. You can give up wasting your time. Exchange needs to be able to poll a global catalog in every domain where it has users"

                    Really.... Even though I have yet to find any documentation stating that is in fact the case. Let's say Exchange does need to poll child domain global catalog servers to get information that the NA global catalog servers in its own site can't provide; why is Exchange polling from global catalog server's that are in sites physically located 1000's of miles away and not using the child domain global catalog servers in the site that is physically located 10 feet away from it?

                    I find it hard to believe that Exchange would need to poll data from every global catalog server in every site for every domain.

                    Again, any documentation would be helpful.

                    Comment


                    • #11
                      Re: Global Catalog Reads

                      I think what he's saying is that Exchange needs to poll a GC for the domain, regardless of where those DCs are. If there's one in the same site as the Exchange server, then great. Otherwise, Exchange will be forced to poll one in another site. As for documentation... I Googled "exchange needs global catalog" and saw plenty of hits for you to take a look at.

                      http://social.technet.microsoft.com/...0-f661ca9b7c0c

                      http://www.petri.com/establishing-pr...t-strategy.htm
                      Last edited by gforceindustries; 27th July 2009, 22:24.
                      Gareth Howells

                      BSc (Hons), MBCS, MCP, MCDST, ICCE

                      Any advice is given in good faith and without warranty.

                      Please give reputation points if somebody has helped you.

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                      • #12
                        Re: Global Catalog Reads

                        Exchange doesn't know that the server is 1000 miles away. If it is polling a server a long way away then you need to look at your AD design, particularly with the costs for the site links.
                        The IT world has no concept of distance, other than what you would see with something like ping - yet I have seen servers on the other side of the world respond faster than servers in my own country.

                        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


                        • #13
                          Re: Global Catalog Reads

                          Thank you for the response. I really appreciate it and have read over those articles preeviously which lead me to post here for some clarification.
                          Last edited by BigDW; 28th July 2009, 21:28.

                          Comment


                          • #14
                            Re: Global Catalog Reads

                            While not a definitive document here's an event that I was able to track down...

                            Event Type: Information
                            Event Source: MSExchange ADAccess
                            Event Category: Topology
                            Event ID: 2129
                            Description:
                            Process w3wp.exe (OWA) (PID=5844). Exchange Active Directory Provider needs a Domain Controller in domain DC=emea,DC=corp,DC=mytestnetwork,DC=com. Found server EDCGDC153.emea.corp.mytestnetwork.com.

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