Mixed Mode Versus Native Mode

Windows 2000 domains can operate in either mixed mode or native mode. Mixed mode is used to provide backward compatibility with Windows NT domain controllers during a migration from NT to Windows 2000.

When you're upgrading a domain to Windows 2000, the first server to be converted should be the primary domain controller (PDC). When the upgrade is complete, this server is a Windows 2000 domain controller and also holds all appropriate Operations Master roles. (Operations Master roles are discussed in detail in Chapter 9, "Designing a Physical Active Directory Topology.") The critical Operations Master role in a mixed mode domain is the PDC Emulator. The Windows 2000 domain controller with this role acts as a Windows NT PDC to all the down-level (NT) backup domain controllers (BDCs) in the domain, thus providing notification of updates and sending updates when requested by the BDCs.

While in mixed mode, universal groups are not available, nor can group nesting be performed. Also, you need to monitor the size of Active Directory for domains operating in mixed mode; domain size should be limited to 40,000 objects as long as Windows NT BDCs are still active.

When no more Windows NT domain controllers exist in the domain, the conversion to native mode is easily accomplished from the Active Directory Domains and Trusts MMC snap-in. Simply right-click the domain to be upgraded, select Properties from the drop-down list, and click the button labeled Change Mode on the General tab of the Properties page.

Once in native mode, all the new security group features of Windows 2000 are available. The domain controller with the PDC Emulator role no longer advertises itself as an NT PDC and can no longer provide SAM updates to down-level NT BDCs.

The conversion from mixed mode to native mode is one-way and cannot be reversed .