Knowledge Base

Two PDCs in One Domain

Article ID: 112549

Article Last Modified on 11/1/2006

APPLIES TO

This article was previously published under Q112549

SUMMARY

When two different Windows NT Advanced Servers or Windows NT Servers in a domain think they are the Primary Domain Controller (PDC), the following error message may appear in the Event Log:
A domain controller is already running in this domain.

MORE INFORMATION

It may be necessary to promote a Backup Domain Controller (BDC) to PDC when the original PDC fails. In the event that the original PDC is unavailable during this process, it is necessary to promote the BDC without having the PDC on-line. This should only be done when the PDC is expected to be down for a long period of time because demotion of the original PDC to BDC will not occur. The following message occurs in Server Manager during promotion of a BDC to PDC when the original PDC is unavailable: 
Cannot find the Primary DC for Domain xxxxxx.
You can administer this domain, but certain domain-wide operations are disabled. Promoting the BDC to PDC should work properly at this point. When the original PDC is restarted, it attempts to also become the PDC, and two Windows NT Advanced Server computers will both think they are PDCs for the same domain. As a result, the NetLogon service fails on the original PDC because a PDC already exists in the domain. Server Manager shows two PDCs for the domain. The original PDC has a transparent icon representing a domain controller while the current PDC has the correct colored domain controller icon.

To demote one of the PDCs:
  1. Stop the NetLogon service on the current PDC by typing Net Stop NetLogon.
  2. Highlight the machine name of the original PDC and then on the Computer menu, select Demote To Server. This changes the role of the original PDC to BDC.
  3. Restart the NetLogon server by typing Net Start NetLogon.
The original PDC can be promoted to PDC in step 2, if you want. The current PDC needs to be demoted to a server immediately after this step.

For additional information on Windows NT 3.51 and 4.0, please see the following article in the Microsoft Knowledge Base:

167248 How to Demote to BDC When Two PDCs Present


Additional query words: demote duplicate

Keywords: kbnetwork KB112549


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