Q. How do I enable clients to find a domain controller (DC) in the next closest site?

A. By default, clients will try to find a DC in their local site; if no DC is found, clients will search for any DCs in the enterprise. You can configure Windows Vista and Windows Server 2008 to search for a DC in the next closest site if no local DC is available via a Group Policy change, as follows:
1. Start the Group Policy Management Console (Start - Programs - Administrative Tools - Group Policy Management).
2. Expand the forest, expand the Domains, then select the domain. Right click "Default Domain Policy" and select Edit.
3. Navigate to Computer Configuration - Policies - Administrative Templates - System - Net Logon - DC Locator DNS Records. 
4. Double click "Try Next Closest Site" and set to Enabled. Click OK.

At the next Group Policy refresh, clients will use the new policy.  You can also set this directly in the registry by creating a DWORD value for the HKEY_LOCAL_MACHINESystemCurrentControlSetServicesNetlogonParametersTry Next Closest Site subkey and set the value to 1.