Placing FSMO Server Roles
So where are these FSMO server roles found? Is there a one to one relationship between the server roles and the number of servers that house them?
The first domain controller that is installed in a Windows 2000 domain, by default, holds all five of the FSMO server roles. Then, as more domain controllers are added to the domain, the FSMO roles can be moved to other domain controllers. Moving a FSMO server role is a manual process, it does not happen automatically. But what if you only have one domain controller in your domain? That is fine. If you have only one domain controller in your organization then you have one forest, one domain, and of course the one domain controller. All 5 FSMO server roles will exist on that DC. There is no rule that says you have to have one server for each FSMO server role.
However, it is always a good idea to have more than one domain controller in a domain for a number of reasons. Assuming you do have multiple domain controllers in your domain, there are some best practices to follow for placing FSMO server roles.
The Schema Master and Domain Naming Master should reside on the same server, and that machine should be a Global Catalog server. Since all three are, by default, on the first domain controller installed in a forest, then you can leave them as they are.
Note: According to MS, the Domain Naming master needs to be on a Global Catalog Server. If you are going to separate the Domain Naming master and Schema master, just make sure they are both on Global Catalog servers.
The Infratructure Master should not be on the same server that acts as a Global Catalog server.
The reason for this is the Global Catalog contains information about every object in the forest. When the Infrastructure Master, which is responsible for updating Active Directory information about cross domain object changes, needs information about objects not in it's domain, it contacts the Global Catalog server for this information. If they both reside on the same server, then the Infratructure Master will never think there are changes to objects that reside in other domains because the Global Catalog will keep it contantly updated. This would result in the Infrastructure Master never replicating changes to other domain controllers in it's domain.
Note: In a single domain environment this is not an issue.
Microsoft also recommeds that the PDC Emulator and RID Master be on the same server. This is not mandatory like the Infrastructure Master and the Global Catalog server above, but is recommended. Also, since the PDC Emulator will receive more traffic than any other FSMO role holder, it should be on a server that can handle the load.
It is also recommended that all FSMO role holders be direct replication partners and they have high bandwidth connections to one another as well as a Global Catalog server.