6.1.5.5 Infrastructure FSMO Role
When an object in one domain is referenced by another object in another domain, it represents the reference as a dsname. There is one Infrastructure FSMO role per domain and application NC in a directory.
If all the domain controllers in a domain also host the GC, then all the domain controllers have the current data, and it is not important which domain controller owns the Infrastructure Master (IM) role. See section 3.1.1.5 for more information about the Infrastructure Master.
When the Recycle Bin optional feature is not enabled, the Infrastructure FSMO role owner is the DC responsible for updating a cross-domain object reference in the event that the referenced object is moved, renamed, or deleted. In this case, the Infrastructure Master role MUST be held by a domain controller that is not a GC server. If the Infrastructure Master runs on a GC server, it will not update object information, because it does not contain any references to objects that it does not hold. This is because a GC server holds a partial replica of every object in the forest.
When the Recycle Bin optional feature is enabled, every DC is responsible for updating its cross-domain object references in the event that the referenced object is moved, renamed, or deleted. In this case, there are no tasks associated with the Infrastructure FSMO role, and it is not important which domain controller owns the Infrastructure Master role.