Office Communications Server 2007 and Office Communications Server R2 have two multi forest topologies that’ve been tested by and are supported by Microsoft. One of these topologies is OCS Resource forest \ User forest topology and other is OCS Central forest and User forest topology. Focus of this post’ll be to discuss implementations of OCS Resource forest \ User forest topology that allows resources like Exchange 2007, Exchange 2003, Sharepoint services and OCS which’ll reside in Resource forest to be accessed by users that reside in User forest. Configuration of Resource forest \ User forest topology requires prerequisites be completed prior to configuration of domain user account information that’ll be hosted in local Active Directory of each forest that’s part of this topology.

Full Article: OCS blog