Structure : Physical structure and replication (2)

In a multi-domain forest the AD database becomes partitioned. That is, each domain maintains a list of only those objects that belong in that domain. So, for example, a user created in Domain A would be listed only in Domain A's domain controllers. Global catalog (GC) servers are used to provide a global listing of all objects in the Forest. The Global catalog is held on domain controllers configured as global catalog servers. Global Catalog servers replicate to themselves all objects from all domains and hence, provide a global listing of objects in the forest. However, in order to minimize replication traffic and to keep the GC's database small, only selected attributes of each object are replicated. This is called the partial attribute set (PAS). The PAS can be modified by modifying the schema and marking attributes for replication to the GC.

Replication of Active Directory uses RPCs (Remote Procedure Calls). Between Sites you can also choose to use SMTP for replication, but only for changes in the Schema or Configuration. SMTP cannot be used for replicating the Domain partition. In other words, if a domain exists on both sides of a WAN connection, you must use RPCs for replication.