Is Multi-forest Active Directory Environment Supported?
Resolution
Palo Alto Networks does not support a multi-forest Active Directory environment, even if there is a trust between the two forests.
For example, there are two different forests (forest A and forest B), and each forest has its own domain (domain 1 and domain 2). There is a trust between the two forests. When a security policy is created with the source user “domain1\auth_users” (there is a member domain2\user1 in that security group), this type of rule will not work.
Workaround
The best work around for this is to have a separate user ID agent installed for each domain in each forest. You must also have individual security polices for each set of groups within each domain.
See Also
What is a Global Catalog : http://technet.microsoft.com/en-us/library/cc728188(v=ws.10).aspx
The role of the Global Catalog: http://technet.microsoft.com/en-us/library/cc736934(v=ws.10).aspx
owner: rvanderveken