The file replication service maintains identical sets of files and directories on different servers and workstations running Windows NT. When files are updated on one server, the file replication service replaces the corresponding files on other servers and workstations with the updated files. The replication process simplifies the task of updating and coordinating files, and maintains the integrity of the replicated data.

FRS relies on containers, objects, and attributes that are stored in Active Directory and that are replicated among domain controllers in a given domain to function. Critical objects include FRS member and subscriber objects. Required and optional attributes include the schedule, the file filters, the folder filters, and the database location. Schema definitions define the containers or the location where FRS objects are located. 

FRS supports two replica sets, DFS and SYSVOL. When you use Dcpromo.exe to promote domain controllers, containers, objects, and attributes for SYSVOL, replica sets are created (they are created indirectly). You can use the DFS snap-in (Dfsgui.msc) to create objects when you configure replication between two or more targets in a DFS root or link, or when you add new members to an existing FRS set.

NTFRSUTL.EXE is the diagnostic utility for the File Replication Service. It is included with the Support Tools.

Before you attempt to add a replica domain controller to your domain, ensure that your source DC is healthy and can replicate its SYSVOL share to the partner.

Install the Support Tools and run the NTFRSUTL command with two switches:

ntfrsutl ds > c:\ntfrsutlds.txt

ntfrsutl sets > c:\ntfrsutlsets.txt

The ntfrsutil ds output will show your subscription information for replica sets in your domain.  If you have not set up any DFS then you should only see the domain system volume.

The main thing to look for in the ntfrsutl ds output is the "Server Ref" value.  Ensure that the value is not set to "null".  If it is then you have a problem. It can be fixed.

The ntfrsutl sets shows active replica sets and deleted replica sets along with pertinent information about those replica sets.

A successful initialization of the domain system volume by the file replication service will return an Event ID 13516 in the File Replication Service log.  If you have not received this event in a while, do not add any additional domain controllers until your source domain controller is healthy.

Articles that you can reference for more information include the following:

296183 Overview of Active Directory Objects That Are Used by FRS
http://support.microsoft.com/default.aspx?scid=kb;EN-US;296183

312862 Recovering missing FRS objects and FRS attributes in Active Directory
http://support.microsoft.com/default.aspx?scid=kb;EN-US;312862