If your company plans to install Exchange 2000 or 2003, then you have probably read about all of the cool new features that have been introduced since Exchange version 5.5. What you might not realize, however, is that in the interest of preserving backward compatibility with older versions of Exchange many of the new features are disabled by default.
When Microsoft introduced Windows 2000, it included the Active Directory Service, which was not compatible with Windows NT's SAM (Security Accounts Manager). Because so many people had Windows NT networks in place, Microsoft knew that it had to make Windows 2000 compatible with Windows NT. Microsoft did this by creating something called "mixed mode."
Mixed mode was the default installation mode for Windows 2000. It disabled a few of the new features, but allowed Windows 2000 to interoperate with Windows NT domains. This is the same thing that happened when Microsoft released Exchange 2000. Exchange 2000 was designed to integrate itself into the Windows Active Directory and wasn't natively compatible with older versions of Exchange. To make Exchange 2000 backward compatible with previous versions, Microsoft created a mixed mode configuration that allows backward compatibility at a cost of sacrificing a few product features. Exchange 2003 can also be run in either mixed mode or in native mode.
So how do you know which mode Exchange is running in? There are several ways that you can tell. Missing features might be a good hint that Exchange is running in mixed mode, but there are easier ways to tell. Simply open the Exchange System Manager, right click on your organization and select the Properties command from the resulting shortcut menu to reveal the organization's properties sheet. When you do, the properties sheet's General tab will tell you whether the server is running in mixed mode or in native mode.
The General tab will also let you convert the Exchange organization to native mode with the click of a button. Switching to native mode is a non-reversible operation and should not be performed until all Exchange 5.5 servers have been removed from the organization and there is no chance of you ever installing another legacy Exchange Server.
How was the mode chosen?
You might also wonder why Exchange is running in the particular mode that it is. At the time that you install Exchange, the Setup program looks to see if there are any other Exchange Servers in the organization. If there are other Exchange Servers present, then Setup checks their version. If the servers are Exchange version 5.5 or below, then Setup will automatically install itself in mixed mode so that it can co-exist with those servers.
If Exchange does get installed in mixed mode, it will remain in mixed mode until you manually convert it to native mode, even if all legacy Exchange Servers have been removed from the organization. The reason for this is that Microsoft knows that there is a chance that you may wish to install an Exchange 5.5server into the organization at a later time.
The basic differences
Microsoft imposes three primary limitations on Exchange Server 2003 when it is running in mixed mode.
When Exchange Server 2003 is running in mixed mode, you will find that the basic Exchange 2003 structure is adapted to work with Exchange 5.5. For example, Exchange 2000 and 2003 treat any Exchange 5.5 sites that might exist as if they were administrative groups. Likewise, your Exchange 5.5 Servers will treat any Exchange 2000 or 2003 administrative groups as sites. Finally, an Exchange 2000 or 2003 routing group can only contain servers that exist within an administrative group.
In addition to ironing out these little quirks, native mode offers you a few things that you just can't get in mixed mode. For example, in native mode, routing groups can contain servers from multiple administrative groups. Likewise, if you are not happy with a server's routing group membership, you can easily move it to a different routing group.
There are also some performance improvements that you will get when running in native mode. For example, in native mode routing group bridgehead servers use 8-bit MIME data transfers. This offers a substantial improvement in the amount of data that you can send across a routing group connector. The default routing protocol has also been changed to SMTP.
Another cool feature that's only available in Exchange 2003 native mode is that you can store Lightweight Directory Access Protocol (LDAP) queries, which allows you to create a query based distribution group. The idea behind this is that distribution groups no longer have to have static memberships. The memberships can change depending on the query results.
When possible, I strongly recommend you use native mode. One thing to keep in mind, however, is that Exchange's mixed mode and native mode are in no way related to the Windows level mixed mode or native mode. It is possible to run Windows in mixed mode, but run Exchange in native mode, or visa versa.
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server and IIS. Brien has served as the 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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