When you promote a member server to Domain Controller using Dcpromo.exe, the Dcpromo.exe contacts a domain controller available in the domain to replicate the AD Database. If there is only one DC then it always contacts that DC for AD database but what if there are more than one DC?
The Dcpromo process always contacts the DC available in its own site first. If there is no DC in its own site then a DC from the nearest site is contacted. You can check the Dcpromo process and see which DC it contacted during the promotion by opening the DCPROMO.LOG file found at %SystemRoot%\Debug folder. The log file will contain the following lines for the source DC:
07/30 11:58:24 [INFO] Located domain controller DCNAME.domain_name.com for domain domain_name.com
07/30 11:58:24 [INFO] Using site Default-First-Site-Name for server \\DCNAME.domain_name.com
