Functional levels determine the features of Active Directory Domain Services (AD DS) that are enabled in a domain or forest. They also restrict which Windows Server operating systems can run on domain controllers in the domain or forest. However, functional levels do not affect which operating systems can run on workstations and member servers that are joined to the domain or forest.
When you create a new domain or a new forest, set the domain and forest functional levels to the highest values that you know your environment can support. This way, you can take advantage of as many AD DS features as possible. For example, if you are sure that no domain controllers that run Windows Server 2008 (or any earlier operating system) will ever be added to the domain or forest, select the Windows Server 2008 R2 functional level. On the other hand, if it is possible that you will retain or add domain controllers that run Windows Server 2008 or earlier, select the Windows Server 2008 functional level during installation. You can raise the functional level after the installation, when you are sure that no such domain controllers will be added or are still in use.
When you install a new forest, you are prompted to set the forest functional level and then the domain functional level. You cannot set the domain functional level to a value that is lower than the forest functional level. For example, if you set the forest functional level to Windows Server 2008 R2, you can set the domain functional level only to Windows Server 2008 R2. The Windows 2000, Windows Server 2003, and Windows Server 2008 domain functional level values will not be available on the Set domain functional level wizard page. In addition, all domains that you subsequently add to that forest will have the Windows Server 2008 R2 domain functional level by default.
After you set the domain functional level to a certain value, you cannot roll back or lower the domain functional level, with one exception: when you raise the domain functional level to Windows Server 2008 R2 and if the forest functional level is Windows Windows Server 2008 or lower, you have the option of rolling the domain functional level back to Windows Server 2008. You can lower the domain functional level only from Windows Server 2008 R2 to Windows Server 2008. If the domain functional level is set to Windows Server 2008 R2, it cannot be rolled back, for example, to Windows Server 2003.
After you set the forest functional level to a certain value, you cannot roll back or lower the forest functional level, with one exception: when you raise the forest functional level to Windows Server 2008 R2 and if Active Directory Recycle Bin is not enabled, you have the option of rolling the forest functional level back to Windows Server 2008. You can lower the forest functional level only from Windows Server 2008 R2 to Windows Server 2008. If the forest functional level is set to Windows Server 2008 R2, it cannot be rolled back, for example, to Windows Server 2003.
The following sections explain the sets of features that are enabled at the different domain and forest functional levels.
Features that are enabled at domain functional levels
The following table lists the enabled features and supported domain controller operating systems for each domain functional level.
Domain functional level | Enabled features | Supported domain controller operating systems |
---|---|---|
Windows 2000 native |
All default Active Directory features, plus the following features:
|
Windows 2000 Windows Server 2003 Windows Server 2008 Windows Server 2008 R2 |
Windows Server 2003 |
All default Active Directory features, all features from the Windows 2000 native domain functional level, plus the following features:
|
Windows Server 2003 Windows Server 2008 Windows Server 2008 R2 |
Windows Server 2008 |
All default Active Directory features, all features from the Windows 2000 native and the Windows Server 2003 domain functional levels, plus the following features:
|
Windows Server 2008 Windows Server 2008 R2 |
Windows Server 2008 R2 |
All default Active Directory features, all features from the Windows 2000 native, Windows Server 2003, and Windows Server 2008 functional levels, plus the following feature:
|
Windows Server 2008 R2 |
Features that are enabled at forest functional levels
The following table lists the enabled features and supported domain controller operating systems for each forest functional level.
Forest functional level |
Enabled features |
Supported domain controller operating systems |
Windows 2000 |
All default Active Directory features |
Windows 2000 Windows Server 2003 Windows Server 2008 Windows Server 2008 R2 |
Windows Server 2003 |
All default Active Directory features, plus the following features:
|
Windows Server 2003 Windows Server 2008 Windows Server 2008 R2 |
Windows Server 2008 |
All the features that are available at the Windows Server 2003 forest functional level, but no additional features. All domains that are subsequently added to the forest, however, will operate at the Windows Server 2008 domain functional level by default. If you plan to include only domain controllers that run Windows Server 2008 or Windows Server 2008 R2 in the entire forest, you might choose this forest functional level for administrative convenience. |
Windows Server 2008 Windows Server 2008 R2 |
Windows Server 2008 R2 |
All of the features that are available at the Windows Server 2003 forest functional level, plus the following feature:
All domains that are subsequently added to the forest will operate at the Windows Server 2008 R2 domain functional level by default. If you plan to include only domain controllers that run Windows Server 2008 R2 in the entire forest, you might choose this forest functional level for administrative convenience. If you do, you will never have to raise the domain functional level for each domain that you create in the forest. |
Windows Server 2008 R2 |