By default, DSAccess chooses the primary domain controller (PDC) emulator operations master role computer to handle requests in Microsoft Exchange. This action may result in poor performance if other non-Exchange programs are making heavy use of the PDC emulator.

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To resolve this problem, add the MinUserDC registry value to exclude the PDC emulator from the server list that Exchange can use.

Note For Exchange Server 2007, we recommend that you use a cmdlet to configure the DSAccess component to exclude a particular domain controller or a list of domain controllers from use. For more information about how to do this, see the "More Information" section.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
To add the MinUserDC registry value:

1.	Start Registry Editor (Regedt32.exe).
2.	Locate and click the following key in the registry: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeDSAccess\Profiles\Default Note In Exchange Server 2007, locate the MSExchangeADAccess subkey instead of the MSExchangeDSAccess subkey.
3.	On the Edit menu, click Add Value, and then add the following registry value: Value name: MinUserDC Data type: REG_DWORD Value data: As required Note The MinUserDC value determines how many total user domain controllers must be available for PDC emulator exclusion to turn on. The value data that is configured for the MinUserDC registry entry is the maximum number of domain controllers to contact before contacting the PDC emulator. For example, setting MinUserDC to 4 configures DSAccess to exclude the PDC emulator only when a total of four domain controllers are available. When this condition is met, the PDC emulator is excluded from use, and DSAccess communicates only with the remaining three domain controllers.
4.	Exit Registry Editor.

Notes

•	You have to apply the registry change that this article describes regardless of the service pack that is installed.
•	In Exchange Server 2007, Exchange Server 2003, and Exchange 2000 Server Enterprise Edition, the Profiles key and the Default key are unavailable. You must create these keys in Exchange Server 2003 and in Exchange 2000 Server Enterprise Edition.
•	When you determine a value for MinUserDC, consider the equation n - 1, where n is equal to the total number of domain controllers in the site. This number includes the PDC emulator. Subtract 1 from this number, and the sum should be the value that you enter for MinUserDC.

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Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

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In Exchange Server 2007, you can use a cmdlet to configure the DSAccess component to exclude a particular domain controller or a list of domain controllers from use. To do this, use the Set-ExchangeServer command together with the StaticExcludedDomainControllers option.

For more information about the Set-ExchangeServer command, visit the following Microsoft Web site:

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Example

The following example illustrates how to use the Set-ExchangeServer command to exclude one or more domain controllers from use. Additionally, this example describes how to verify the status of the Exchange environment after you run the Set-ExchangeServer command.

The following table describes the servers that this example uses.

Host name	Domain	Role
E2K7-1	contoso.com	Exchange Server 2007
DC-1	contoso.com	domain controller together with PDC operations master
DC-2	contoso.com	domain controller
DC-3	contoso.com	domain controller

To use the Set-ExchangeServer command to exclude the three domain controllers that are listed in this table from use for the DSAccess component, follow these steps:

1.	Start the Exchange Management Shell. To do this, click Start, point to All Programs, point to Microsoft Exchange Server 2007, and then click Exchange Management Shell.
2.	At the command prompt, type the following command, and then press ENTER: Set-ExchangeServer -identity E2K7-1.contoso.com -StaticExcludedDomainControllers:dc-1.contoso.com,dc-2.contoso.com,dc-3.contoso.com This command excludes DC-1, DC-2, and DC-2 from use by the server that is named E2K7-1. Note In this command, specify the fully qualified domain names of the individual domain controllers by using a comma-separated list that does not contain spaces between each entry.
3.	To verify the list of excluded domain controllers, type the following command, and then press ENTER: Get-ExchangeServer -identity E2K7-1.contoso.com -status | fl Name, StaticExcludedDomainControllers

Note If you want to remove the changes that you have made, and revert to the default behavior of Exchange, type the following command at the Exchange Management Shell prompt, and then press ENTER:

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For more information, click the following article number to view the article in the Microsoft Knowledge Base:

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