Migrating to Exchange Server 2007 - Coexistence Migration
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The first thing we need to understand is what coexistence means. It is basically a state when there are two Exchange Servers within an environment, say Exchange 2003 and Server 2007. Coexistence is interoperability. The other key differences between the newer and older versions of Exchange Server concern its management tools.
Exchange Server 2007 comes with a set of management tools that do offer some sort of backward compatibility. But to easily remember, the rule of thumb should be like managing the appropriate mailboxes with their management tools. For example, you should not manage mailboxes belonging to Exchange 2007 with old-fashioned 2003 tools. This means the way you "work with" mailboxes changes slightly.
With Exchange 2003 you've been accustomed to managing your mailboxes with the MMC snap-in extension for the ADUC (Active Directory Users & Computers). Every time you right-clicked a user, there you could find the Exchange-related tabs. During the coexistence state, those will not disappear, but they cannot edit/manage properly Exchange 2007 mailboxes.
And once Exchange Server 2007 is set up, don't look for a renowned version of that MMC snap-in extension. The way you manage mailboxes has changed. Now you're given two options: either via management console (GUI-based) or via shell (text). You can also find plenty of tools from Microsoft, but always double-check which kind of Exchange mailboxes they are written for.
Earlier we've mentioned backwards compatibility regarding tools. Most of the tools written for Exchange 2007 mailboxes can manage older mailboxes (2003), except for creating them. But if you're migrating, you won't need to create any new mailboxes on the old platform. During the coexistence state, certain configurations are considered global and, therefore, are shared between both versions.
Our advice is keeping the coexistence state only as a backup, and once you've done the migration and the new platform works as expected, you don't need to maintain co-existence anymore. You can remove the old Exchange server to alleviate system resources and clean up the clutter.
In the case of an Exchange migration, coexistence is the only option, since a direct upgrade is not possible. The process requires you to set up another Exchange Server, of the new version (2007) within the organization. After that you can move the mailboxes and server roles. Moreover, if you check the change-log of Exchange 2007, you'll find out that the Hub Transport Server role has become the main role that handles communication.
On earlier versions you could install an Exchange server without that role. Things have changed, since the architecture of Exchange 2007 is different. Right now communication between Exchange servers happens on the RPC topology, unlike SMTP as it did in earlier versions. Since RPC is handled by the Hub Transport Server role, this makes it required.
Exchange 2007 makes the following server roles available: the Hub Transport Server role that we discussed above, the Mailbox Server role (this role hosts the mailboxes), the Client Access Server role (this one enhances the accessibility of the server by providing more protocol types, such as POP3/IMAP4, does the OWA, and so forth), the Unified Message Server role (VoIP and IP-PBX capabilities), and Edge Transport (enhances security).
Please read the official documentation regarding these server roles to decide which are necessary for your custom scenario. One of the typical setups involves installing 3 roles: Mailbox Role, Hub Transport, and Client Access. These can be installed on the same physical machine. The same goes for Unified Messaging if you really need those telephone-related capabilities. However, the Edge Transport role needs to be alone as a server role.
Now that we know what we're dealing with-let's get to it!
Next: Coexistence Migration, Continued >>
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