Setting the Functional Level of Domains and Forests
As discussed in "Establishing Functional Levels" in Chapter 2, each forest and each domain within a forest can be assigned a functional level. Functional levels affect the inner workings of Active Directory and are used to enable features that are compatible with the installed server versions of the Windows operating system. For example, with the domain functional level set as Windows Server 2008, the domain can use only domain controllers running Windows Server 2008 or later.
You raise the domain or forest functional level by using Active Directory Domains and Trusts. Although you can raise the functional level of a domain or forest, you can never lower it.
Because raising the forest functional ...