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There is much room in an Exchange 5.5 to Exchange 2003 migration for something to go wrong -- the installation could bomb halfway through, the hardware might be inadequate to run Exchange 2003 at a sufficient speed, or the user mailboxes could get messed up.
There are other cons to a direct upgrade as well. For starters, it involves a considerable amount of downtime for users. The other disadvantage is that you are making major changes to a perfectly good mail server. If something goes wrong during the upgrade, your only hope of getting Exchange to a usable state is often to restore a backup.
Rather than directly upgrading legacy Exchange servers, I prefer to perform a multiphase migration. This type of migration offers much less downtime and doesn't place your existing Exchange servers in jeopardy. The disadvantage is it requires the purchase of at least one new server. But, in my opinion, the cost of a new server is well worth it when you consider the headaches you will be able to avoid.
Phase 1: Deploy Active Directory
Exchange 2003 is completely dependent on an Active Directory infrastructure and a DNS server that is aware of it. So the first step in the migration process is to ensure that at least one domain controller in your Exchange domain is running Active Directory.
You don't have to install an AD-based server operating system on every domain controller, but if you do decide to upgrade all of them, you will see benefits that you just can't get when your domain is running in mixed mode.
Phase 2: Install Exchange Server 2003
Once the Active Directory infrastructure is in place, it is time to begin the migration. You will start the process by installing Windows Server 2003 and Exchange Server 2003 onto your new server.
Phase 3: Install the Active Directory connector
In order to make a multiphase migration work, you will need functional Exchange 5.5 Exchange 2003 environments. The catch is that Exchange groups all servers under a common organization. This model requires that mail servers are able to communicate with each other and share user information. The problem is that all of Exchange 2003's user information is stored in Active Directory, while Exchange 5.5's user information is stored in the Exchange Directory file (DIR.EDB).
This is where the Active Directory connector comes in. The AD connector is an Exchange 2003 tool that allows you to establish a link between the Exchange 2003 and Exchange 5.5 portions of the organization.
The connector populates Active Directory with the information stored in the Exchange Directory. This is important because if a user on the Exchange 2003 server wants to send a message to the Exchange 5.5 user, they need to be able to find information about that user in Active Directory.
The AD connector not only populates Active Directory with this information, it keeps it synchronized with the Exchange Directory.
Phase 4: Create a test mailbox
Once the Active Directory connector is in place, the next step is to create a few test mailboxes on the Exchange 2003 server. You will want to verify that these test users can send and receive mail to and from Exchange 5.5 users and the outside world (SMTP mail). Once you have verified that the test mailboxes are completely functional and the server is stable, you will want to create all new user mailboxes on this server.
Phase 5: Migrate users
The next phase in the process is to migrate users to the new server. You can migrate the mailboxes by using the Move Mailbox command in the Exchange System Manager. You can move as many or as few mailboxes at a time as you want.
I recommend initially creating a few test mailboxes on the Exchange 5.5 server, moving them, and then verifying they still work prior to moving any real mailboxes. I also recommend initially only moving a couple of real mailboxes at first, just so that you can make sure that they are still functional after the move.
Once you feel comfortable that the Move Mailbox function is working as expected, you can migrate as many mailboxes as you want. You can even consolidate mailboxes from multiple Exchange 5.5 servers onto a single Exchange 2003 server if you like. Just be aware that unless you are using Exchange 2003 Enterprise Edition, the information store has a 16 GB size limit.
Phase 6: Decommissioning the old server
After all of the mailboxes have been moved off your old Exchange server, you should remove it from your Exchange organization.
If you have additional servers to migrate, you might be able to repurpose your old server by installing Exchange 2003 on it and then migrating mailboxes from a different server to it. If you choose to go this route, you will need to verify that the server has sufficient hardware resources to run Exchange 2003.
It is also best if you perform a clean install rather than trying to upgrade the existing copy of Exchange.
Step 7: Go native
Once all of the servers in your organization are running Exchange Server 2003, the last step is to switch Exchange to native mode. This enables you to take advantage of features in Exchange 2003 that you just can't get in mixed mode.
Just remember that switching to native mode is a one-way process. Once you go native, you can't ever add another Exchange 5.5 Server to your Exchange organization.
About the author: Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server and IIS. Brien has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. As a freelance technical writer he has written for Microsoft, CNET, ZDNet, TechTarget, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at http://www.brienposey.com.
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