Resultant Set of Policy (RSoP) planning mode is not supported in a cross-forest scenario
This article describes limitations to the resultant set of policy (RSoP) planning mode when you try to use it across multiple forests.
Applies to: Windows 10 - all editions, Windows Server 2012 R2, Windows Server 2016, Windows Server 2019
Original KB number: 910206
You cannot use the RSoP planning mode to plan for scenarios that span forests in Windows Server 2003 and later versions. For example, you cannot plan for a scenario in which a user logs on to a workstation in Forest 1 from Forest 2. When you try to run RSoP planning mode in a cross-forest environment, you receive the following Group Policy error message:
Cross forest planning mode scenarios are not currently supported.
This issue occurs because RSoP planning mode does not support cross-forest scenarios. In many scenarios, RSoP cannot validate the information that is returned from a domain controller that is located in another forest. The Authenticated Users group must have read permissions for relevant policies to successfully read a particular policy in a cross-forest environment.