Confusion over “2003 AD” and “2008 AD” with Exchange Installs.

One of my colleagues ran into this one last week, so I thought I’d share the issue here, and detail the fix.  When you go to install various versions of Exchange Server (2003 and up) and/or try to activate certain Exchange functionality, you may find that your Active Directory (AD) domains are not at the correct functional level, even though they appear to be.  Here’s a case in point:

I checked, and my domain is at a 2003 domain level, but Exchange Server 2010 says that it isn’t…

At first glance, this is an infuriating problem, because my colleague definitely confirmed that the Domain Functional Level was “2003” and the installer just wouldn’t work. The issue is a matter of one word versus another – specifically “Mixed” versus “Native.”

Mixed mode domains are designed to allow earlier versions of Windows AD Servers to operate properly in an environment that is being upgraded to a new AD platform.  Today, it is not uncommon to see 2003, 2008 and 2008 R2 Domain Controllers (DC’s) in a single environment, and the Mixed mode of each of those AD versions can allow for interoperability.  Exchange Server 2010, however, cannot operate if there are components of AD/Domain Services from Server 2000 or NT.  As such, it requires that only Server 2003 or higher AD controllers exist.  This means that you must be in 2003 Native mode (or higher) to install Exchange Server 2010.

Native mode ensures that no DC’s of any version prior to the functional level can communicate with AD in this domain.  This allows software packages like Exchange Server to not have to worry about passing commands that might be incompatible with earlier versions of AD.  Additionally, some features of Exchange Server 2010 go one step further and require either 2008 Native mode or 2008 R2 Native mode to function properly, so you may be on 2003 Native and still get errors if you are attempting to enable those services.

If you get the error during the pre-flight check on the installer (or later when adding features), make sure that your AD systems are in the appropriate Native mode, instead of Mixed, and you should be just fine. Note: you will often see these modes referred to as “Legacy” and “2008 Only” or similar language.  Legacy, as the name implies, allows for earlier AD controllers to communicate.  2008 Only or 2008 R2 Only limit the DC’s to those levels and higher.

Sometimes, when it comes to Exchange Server, one word can really make all the difference.