Design Considerations for Compatibility

In Windows Server 2003, each forest and each domain within a forest can be assigned a functional level. The functional level for a forest is referred to as the forest functional level. The functional level for a domain within a forest is referred to as the domain functional level.

Inside Out: Functional levels affect compatibility

Functional levels affect compatibility

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 backward compatibility with previous versions of Windows, Active Directory is configured by default to be compatible with Windows NT domains and clients. Windows NT domains have a primary domain controller and one or more backup domain controllers rather than multiple domain controllers that are all equally accountable. When a domain is operating in the default mode, a domain controller running Windows 2000 or later is designated as a primary domain controller (PDC) emulator. The PDC emulator role is a special operations master role that allows the domain controller to act as the primary domain controller for Windows NT clients in the domain.

Understanding Domain Functional Level

When a functional level is set for a domain, the level of functionality applies only to that domain. This means that other domains in the forest can have a different functional level.

As shown in Table 33-1, there are several domain functional levels. The default domain functional level is Windows 2000 mixed. Changing a functional level changes the operating systems that are supported for domain controllers. For example, in Windows 2000 mixed functional level, the domain can have domain controllers running Windows Server 2003 or Windows 2000, with backup domain controllers running Windows NT 4.

Table 33-1. Domain Functional Levels

Domain Functional Level

Supported Domain Controllers

Windows 2000 mixed (default)

Windows Server 2003Windows 2000Windows NT 4 backup domain controller (BDC)

Windows 2000 native

Windows Server 2003Windows 2000

Windows Server 2003 interim

Windows Server 2003Windows NT 4 BDC

Windows Server 2003

Windows Server 2003

Note

Although you can raise the domain functional level, you can never lower it. This means that if you raise the domain functional level to Windows Server 2003, only Windows Server 2003 domain controllers can be configured in the domain.

Understanding Forest Functional Level

Forest functional level is a bit simpler, as shown in Table 33-2. The default forest functional level is Windows 2000. Before you can raise the forest functional level to Windows Server 2003, all domains in the forest must be set to the Windows 2000 native or Windows Server 2003 functional level. Then, when you raise the forest functional level to Windows Server 2003, all domains using the Windows 2000 native domain functional level will automatically be raised to the Windows Server 2003 domain functional level. As with the domain functional level, once you raise the forest functional level, you cannot lower it.

Table 33-2. Forest Functional Levels

Forest Functional Level

Supported Domain Controllers

Windows 2000 (default)

Windows Server 2003Windows 2000Windows NT 4 BDC

Windows Server 2003 interim

Windows Server 2003Windows NT 4 BDC

Windows Server 2003

Windows Server 2003

Raising the Domain or Forest Functional Level

You can raise the domain or forest functional level using Active Directory Domains And Trusts. To raise the domain functional level, follow these steps:

  1. Click Start, choose Programs or All Programs as appropriate, choose Administrative Tools, and then select Active Directory Domains And Trusts.

  2. In the console tree, right-click the domain you want to work with, and then select Raise Domain Functional Level. The current domain name and functional level appear in the Raise Domain Functional Level dialog box.

  3. To change the domain functionality, select the new domain functional level using the selection list provided, and then click Raise.

    Warning

    You can't reverse this action. Once you raise the functional level, there's no going back, so you should consider the implications carefully before you do this.

  4. When you click OK, the new domain functional level will be replicated to each domain controller in the domain. This operation can take some several minutes or longer in a large organization.

You can raise the forest level functionality by completing the following steps:

  1. Click Start, choose Programs or All Programs as appropriate, choose Administrative Tools, and then select Active Directory Domains And Trusts.

  2. Right-click the Active Directory Domains And Trusts node in the console tree, and then select Raise Forest Functional Level. The current forest name and functional level appear in the Raise Forest Functional Level dialog box.

  3. To change the forest functionality, select the new forest functional level using the selection list provided, and then click Raise.

    Warning

    You can't reverse this action. Once you raise the functional level, there's no going back, so you should consider the implications carefully before you do this.

  4. When you click OK, the new forest functional level will be replicated to each domain controller in each domain in the forest. This operation can take several minutes or longer in a large organization.

As a planning option, you can determine the steps you need to take to raise the forest functional level by clicking Save As in the Raise Forest Functional Level dialog box. When you click Save As, a Save As dialog box appears, allowing you to select a save location for a log file. As shown in the following screen, the log file details show the following information:

  • The forest root domain and the current forest functional level.

  • The domains and the domain controllers in those domains that are running versions of Windows earlier than Windows Server 2003. These are the servers that need to be upgraded.

  • The domain functional level of each domain whose functional level must be raised. As long as the domain functional level of all domains is set to at least Windows 2000 native, you can raise the forest functional level—doing so raises the domain functional level in all the domains to Windows Server 2003 and sets the forest functional level to Windows Server 2003 as well.

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