SCCM Multi Forest Design

Configuration Manager in Multiple Active Directory Forests
https://technet.microsoft.com/en-us/library/bb694003.aspx
 
Best Practices for Hierarchy Security
https://technet.microsoft.com/en-us/library/bb680386.aspx
 
Configuration Manager Site to Site Communications
https://technet.microsoft.com/en-us/library/bb694289.aspx