Understanding When to Create a Forest Trust
Applies To: Windows Server 2008, Windows Server 2008 R2, Windows Server 2012
When to create a forest trust
You can create a forest trust between forest root domains if the forest functional level is Windows Server 2003 or higher. Creating a forest trust between two root domains with a forest functional level of Windows Server 2003 or higher provides a one-way or two-way, transitive trust relationship between every domain in each forest. Forest trusts are useful for application service providers, organizations undergoing mergers or acquisitions, collaborative business extranets, and organizations seeking a solution for administrative autonomy.
For more information about creating forest trusts, see Create a Forest Trust.
Using one-way, forest trusts
A one-way, forest trust between two forests allows members of the trusted forest to use resources that are located in the trusting forest. However, the trust operates in only one direction. For example, when a one-way, forest trust is created between forest A (the trusted forest) and forest B (the trusting forest), members of forest A can access resources that are located in forest B, but members of forest B cannot access resources that are located in forest A, using the same trust.
Using two-way, forest trusts
A two-way, forest trust between two forests allows members from either forest to use resources that are located in the other forest, and domains in each respective forest trust domains in the other forest implicitly. For example, when a two-way, forest trust is established between forest A and forest B, members of forest A can access resources that are located in forest B, and members of forest B can access resources in forest A, using the same trust.