For every updated version of Windows Server, there are some new features incorporated in Active Directory that can only be utilised when all domain controllers in a domain (or forest) are upgraded to the same Windows Server version. To ensure the activation of Active Directory’s additional features, Windows Server 2000 first introduced the functional levels command as an extension of the mixed mode and the native mode concepts.
It is essential to check whether any of your domain controllers operate under an earlier (or lower) version of Windows Server before raising the functional level of the entire domain to that specified level. For example, if one domain controller in your domain has Windows Server 2003 whereas all the other domain controllers have Windows Server 2008, then that one domain controller would not work if the domain functional level is raised to Windows Server 2008.
It is possible to ‘lower’ the functional level of a domain in Windows Server 2008 R2 (not available in previous versions), but to Windows Server 2008 only. Therefore, it is strongly recommended that you are certain you want to raise your domain functional level.
Procedure: Raise Domain Functional Level
Prerequisite : Only users that are Domain Admins or Enterprise Admins, or equivalent, are able to raise the domain functional level.
Select Start – Administrative Tools – Active Directory Domains and Trusts.
Right click the domain you want to raise the functional level of (as in example below) and then select Raise Domain Functional Level.
In the Raise domain functional level select an available domain functional level from the drop down list. For example, if your current domain functional level is Windows 2000 native (as in example below), then you can either raise the domain functional level to Windows Server 2003 or Windows Server 2008.
Once you have made your choice, click the Raise button to continue.
Click OK to proceed to raise the functional level of the domain.
A message box appears informing you that the domain functional level was raised successfully.
Procedure: Raise Forest Functional Level
It is recommended that the functional level remain consistent between domains and forests. For example, if all your domain controllers are running Windows Server 2008, you should set the forest to the same functional level.
This process is similar to above, however, you access the raise forest functional level through the ‘root’ node in Active Directory Domains and Trusts (see example below).