Whew! My brain is full from 2 weeks in Atlanta at the Microsoft Exchange Conference 99 (MEC99). Not only was MEC99 a lot to digest, but I also attended Compaq’s pilot Platinum Academy held the week before MEC, where consultants learned the ins and outs of Exchange 2000. It's clear that we face some challenges in selecting the optimal migration path to Exchange 2000. Two basic approaches come to mind: parallel deployment and upgrade migration.
My favorite method is the parallel deployment approach. With this approach, an organization deploys a parallel Windows 2000 (Win2K) architecture separate from the existing Exchange Server 5.5 and Windows NT 4.0 environment. The Active Directory Connector (ADC) uses the Exchange Server 5.5 directory to populate and synchronize the Active Directory (AD). Next, administrators deploy Exchange 2000 servers into existing Exchange Server 5.5 sites, but as member servers or domain controllers in the Win2K domain. This configuration is supported in mixed mode operation for Exchange 2000 where servers can actually join the Exchange Server 5.5 sites. You can even administer Exchange Server 5.5 via the Exchange System Manager Microsoft Management Console (MMC) snap-in from Exchange 2000. As various workgroups are ready to move to Exchange 2000, you can move mailboxes and public folders from Exchange Server 5.5 to the Exchange 2000 server. After the move is complete, you enable the user’s account in the AD, allowing authentication to the AD when accessing the Exchange 2000 server. Obviously, this approach can be more complex than I have quickly outlined. However, assuming the client is Messaging API (MAPI)-based (which is the case for most Exchange deployments), this method should be the least painful.
A less desirable approach is to first upgrade existing Exchange Server systems running NT 4.0 to Win2K. Exchange Server 5.5 Service Pack 3 (SP3) provided Win2K support for this reason. The next step is to upgrade Exchange Server 5.5 to Exchange 2000. With a new version, the upgrade process will take a while, and you must plan and test before undertaking the process. Also, be sure that you have good backups in case you need to go back.
Both methods will have variations, and some organizations will end up developing a hybrid approach. In addition, because Microsoft has not yet released Exchange 2000 and Win2K and users have not yet deployed these products, we haven’t learned all the secrets or scratched the surface on best practices yet. One thing is certain, however: Be sure you understand AD, ADC, and Exchange 2000 interoperability with Exchange Server 5.5. Armed with this knowledge, you can successfully plan your migration path to Exchange 2000.