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Exchange native mode is a commonly misunderstood concept. This article explains what it is, how it works and what its benefits are.
What is native mode?
It all started when Microsoft released Windows 2000 Server. Since most IT shops weren't going to convert their networks to Windows 2000 in one fell swoop, Microsoft created the mixed mode/native mode switch.
When Windows 2000 is running in mixed mode, it is backwards compatible with NT domains. Once an organization has upgraded all its domain controllers to Windows 2000, it can then change over to native mode. This activates all of Windows 2000's new features, but prohibits any NT domain controllers from being on the network.
Shortly after Windows 2000 was released, Microsoft released Exchange 2000 Server. Exchange 2000 faced similar issues, because it was the first version of Exchange to depend on Active Directory instead of its own built-in directory service.
Exchange 2000 was designed so that it could co-exist with Exchange 5.5 as long as a special Active Directory connector was in place. But many Exchange 2000 features were not backwards compatible with Exchange 5.5. So Microsoft therefore introduced the mixed mode/native mode concept, first unveiled for Windows 2000, to Exchange 2000 as well. This meant that organizations could adopt Exchange 2000, but still maintain Exchange 5.5 compatibility as long as necessary.
Once all Exchange 5.5 servers were removed from the network, or upgraded to Exchange 2000, Exchange could then be set to native mode. This would activate the new Exchange 2000 features, but disallow any Exchange 5.5 servers from existing within the Exchange organization.
Fast forward a few years. Microsoft released Windows Server 2003. Although Windows Server 2003 has a similar architecture to Windows 2000, it does offer Active Directory functionality that Win2k doesn't support. Since Microsoft had already used the concept of mixed mode and native mode in Windows 2000, it needed a new and different mechanism to make a distinction between Windows 2000 domains and Windows 2003 domains. So Microsoft introduced the new concepts of domain functional levels and forest functional levels.
Setting the domain functional level
Setting the domain functional level is similar to choosing between mixed mode and native mode, except that you have four choices. These choices include:
These settings are domain specific and do not affect the overall forest. It is possible to have one domain running Windows 2003 mode, while another domain is running Windows 2000 mixed mode. The various settings allow you to gradually raise the domain's functionality as you phase out various legacy domain controller operating systems.
Setting the forest functional level
Setting the forest functional level works similarly to setting the domain functional level, except that you only have three choices:
These settings allow you to raise the functionality of the entire forest. Some Active Directory features apply to the domain level, while others apply to the forest level. Having separate settings for domains and forests allows those domains that are running only Windows 2003 domain controllers to take advantage of new features, while still allowing the forest to contain domains running older operating systems.
Shortly after releasing Windows Server 2003, Microsoft released Exchange Server 2003. You will be happy to know that the whole concept of functional levels does not apply to Exchange Server 2003. Exchange Server 2003 does have a mixed mode and native mode switch. However, this switch only determines whether or not Exchange 5.5 will be supported by the Exchange organization. You can run Exchange in native mode, even if you are running a mixture of Exchange 2000 and Exchange 2003 servers.
Benefits of native mode
Now that I have explained how mixed mode and native mode work, you are probably wondering what the benefits are of running Exchange in native mode and how to make the switch. In native mode, Exchange Server 2003 allows you to:
Making the switch
First, to be clear, the mode in which Exchange is running is completely independent from the mode in which Windows is running. Windows can be running in native mode while Exchange is running in mixed mode, and vice versa.
To switch Exchange Server 2003 to native mode, make sure no Exchange 5.5 servers currently exist in your Exchange organization and understand that you cannot add any Exchange 5.5 servers once you make the switch. Now, open the Exchange System Manager, right click on the Organization name, and select Properties. Then just click on the Change Operation Mode button.
A word of caution: This is a one way operation. Once you go native, there is no going back. Also, this operation will require all Exchange servers to be rebooted or will require all Information Stores to be remounted.
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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This was first published in January 2005