AD RMS across forests with external AD trust

Question

The RMS servers is deployed in the resource AD forest abc.com . My client also wants to let the users in other AD forests to use the RMS services. However, they only have “external trust” type with
the resource AD. I can find Microsoft document to support the “forest trust” scenario.
http://technet.microsoft.com/en-us/library/ee918789%28v=ws.10%29.aspx
But I cannot find any document that Microsoft will support the “external trust” scenario. Can anyone confirm whether this scenario work and any potential issue?
Do note that we already deployed FIM for directory synchronization. There are contact objects in the resource forest to present users/groups in the account forests.
William Yang

Answer 1

Hi William,
"Only one Active Directory Rights Management Services (AD RMS) root cluster is permitted in each forest. If your
organization wants to use rights-protected content in more than one forest, you must have a separate AD RMS root cluster for each forest. " 
Ref. http://technet.microsoft.com/en-us/library/dd772648(v=ws.10).aspx
So if you want to use two forests with ADRMS, deploy it to each forest and create TUD or TPD. Also you
can think of having federation services with ADFS together with ADRMS.
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