Updating active directory schema
AD admins who didn't pay close attention to the documentation or who didn’t read this article will be in for a big surprise, as the Forestprep and domain preparation happens automatically as a result of initiating a Server 2012 DC installation—not beforehand, and with no warning at all.
(See image #14 in the screenshot gallery.) This means that if one of your administrators gets the idea to put a Server 2012 DC into a domain, before you know it Adprep will have run its course. The other way to view it, of course, is that smaller shops won't have to worry about it.
Adprep is the utility—included in the OS installation media—that performs several crucial functions to upgrade AD to support that OS.
The utility has three major options: /forestprep, /domainprep, and /rodcprep.
Extending the AD schema to support either a new OS release or Active Directory (AD)-integrated applications that require it (e.g., Exchange Server) is a regular—if infrequent—administrative task.
And these kinds of schema upgrades have always made AD administrators nervous.
You can now also specify separate credentials for Adprep /forestprep (Enterprise Admins membership) or /domainprep or /rodcprep (Domain Admins membership) using the /user and /userdomain parameters.
With Server 2012, Adprep has been integrated into the Active Directory Domain Services (AD DS) role installation process.
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Schema upgrades aren’t bothersome only to AD admins; they also irritate Microsoft's Customer Support Services (CSS), because confusion about the process is a high call generator.
Further, adoption of many new AD-related and other features in a new OS was demonstrably slower if the feature depended on a schema upgrade because of the care (and resulting delay) associated with the process.