If the primary domain controller (PDC) is not available, and you configure a new PDC rather than promoting an existing backup domain controller, a new security identifier (SID) will be created for the domain. If the old PDC is then brought back on-line, it will not be able to process logon requests because it does not know the domain SID. To use both servers on the domain, you will need to reinstall Windows NT on one of them, while the other is available as a PDC. For more information see "How Domains Are Identified Internally" in Chapter 3, "How Network Security Works" of the Windows NT Server Concepts and Planning Guide.