Can a server running Windows Server 2012 R2 (file server, app server, etc.) exist on a network with Windows Server 2008 R2 domain controllers, or do the DC's need to be upgraded first?
Windows Server 2012 R2 in a Windows Server 2008 R2 domain
active-directorydomain-controllerwindowswindows-server-2008-r2windows-server-2012-r2
Best Answer
Yes. For that matter, you can join practically any version of Windows (from 2000 onward) to that domain without any problems, or touching the domain controllers. The operating system on your domain controllers has essentially nothing to do with which operating system(s) the member servers can run.
The main impact of the domain controllers' operating systems is the domain and forest functional levels of your domain and forest, and those determine which Active Directory features are available.