Sunday, June 5, 2011

Migration from FRS to DFRS with RODC

Traditionally, File Replication Service (FRS) is used for replicating the contents of the SYSVOL share between Windows domain controllers (a.ka. AD replication). A new AD DS feature in Windows Server 2008 can use the DFS Replication (DFSR) service for replicating the contents of the SYSVOL share. DFSR offers several advantages over FRS, including
  1. Efficient, scalable and reliable file replication protocol which has been tested extensively to ensure data consistency in multi-master replication scenarios.
  2. Differential replication of changes to files using the Remote Differential Compression (RDC) algorithm, which enhances efficiency in branch office scenarios.
  3. Flexible scheduling and bandwidth throttling mechanisms.
  4. Self-heals from USN journal wraps and database corruptions – end user intervention and monitoring requirement is minimal.
  5. Provides built in health monitoring tools for ease of monitoring deployments.
  6. Improved support for Read Only Domain Controllers.
Click here to find detailed step-by-step migration guide, which comprises a 5-part series. A quicker step-by-step can be found here. Generally, migration is pretty easy and straightforward. However, if you have promoted a Read-Only Domain Controller (RODC) before the migration is complete, you will see the following error:

The RODC will never move to the next step beyond 'Start'. To resolve it, perform the following command on the PDC emulator:
  • dfsrmig /DeleteRoNtfrsMember: Since Read Only Domain Controllers cannot delete the FRS related global settings from their own Active Directory, this operation needs to be performed by the Primary Domain Controller in its Active Directory. When these changes eventually replicate in to the Read Only Domain Controller (after applicable AD Replication latencies), the DFS Replication service on the RODC is able to proceed with the process of migration.
When you run dfsrmig /getGlobalState again, the RODC entry will disappear. Issue repadmin /syncall on PDC emulator to sync with all domain controllers (including RODCs).

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