On the Source Domain Controller page of the Active Directory Domain Services Installation Wizard, you can select which domain controller will be used as a source for data that must be replicated during installation, or you can have the wizard select which domain controller will be used as the source for this data. This wizard page appears only if you select the Use advanced mode installation check box on the Welcome to the Active Directory Domain Services Installation Wizard page and you are adding an additional domain to a forest or adding an additional domain controller to an existing domain.
Even if you choose to install from media (IFM), some network communication must occur between the new domain controller and an existing domain controller, which is sometimes known as an installation partner or a "helper domain controller."
The Active Directory Domain Services Installation Wizard lists all the domain controllers that are eligible to be used as an installation partner. If you are creating an additional domain, any domain controller can be the installation partner. However, the following restrictions apply to the domain controllers that can be used as an installation partner in other situations:
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A read-only domain controller (RODC) can never be an installation partner.
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If you are installing an RODC, only a writable domain controller that runs Windows Server 2008 or Windows Server 2008 R2 can be an installation partner.
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If you are installing an additional domain controller for an existing domain, only a domain controller for that domain can be an installation partner.
If you specify a domain controller to be used as an installation partner, specify a domain controller that has a low number of inbound and outbound connections and ensure that it is not a significant originator or forwarder of changes with File Replication Service (FRS) replication partners.
If you choose not to use IFM, a new NTDS Settings object and a new computer account is created or modified on the installation partner. The installation partner also has the first opportunity to replicate the contents of the SYSVOL shared folder to the new domain controller.