· Schema Master - While the first three FSMO roles described above are domain-specific, the Schema Master role and the one following are forest-specific and are found only in the forest root domain (the first domain you create when you create a new forest). This means there is one and only one Schema Master in a forest, and the purpose of this role is to replicate schema changes to all other domain controllers in the forest. Since the schema of Active Directory is rarely changed however, the Schema Master role will rarely do any work. Typical scenarios where this role is used would be when you deploy Exchange Server onto your network, or when you upgrade domain controllers from Windows 2000 to Windows Server 2003, as these situations both involve making changes to the Active Directory schema.
· Domain Naming Master - The other forest-specific FSMO role is the Domain Naming Master, and this role resides too in the forest root domain. The Domain Naming Master role processes all changes to the namespace, for example adding the child domain vancouver.mycompany.com to the forest root domain mycompany.com requires that this role be available, so you can\'t add a new child domain or new domain tree, check to make sure this role is running properly.
Domain-wide changes · RID Master - This is another domain-specific FSMO role, that is, every domain in your forest has exactly one domain controller holding the RID Master role. The purpose of this role is to replenish the pool of unused relative IDs (RIDs) for the domain and prevent this pool from becoming exhausted. RIDs are used up whenever you create a new security principle (user or computer account) because the SID for the new security principle is constructed by combining the domain SID with a unique RID taken from the pool. So if you run out of RIDS, you won\'t be able to create any new user or computer accounts, and to prevent this from happening the RID Master monitors the RID pool and generates new RIDs to replenish it when it falls beneath a certain level.
· PDC Master:- PDC Emulator - This role is the most heavily used of all FSMO roles and has the widest range of functions. The domain controller that holds the PDC Emulator role is crucial in a mixed environment where Windows NT 4.0 BDCs are still present. This is because the PDC Emulator role emulates the functions of a Windows NT 4.0 PDC. But even if you\'ve migrated all your Windows NT 4.0 domain controllers to Windows 2000 or Windows Server 2003, the domain controller that holds the PDC Emulator role still has a lot to do. For example, the PDC Emulator is the root time server for synchronizing the clocks of all Windows computers in your forest. It\'s critically important that computer clocks are synchronized across your forest because if they\'re out by too much then Kerberos authentication can fail and users won\'t be able to log on to the network. Another function of the PDC Emulator is that it is the domain controller to which all changes to Group Policy are initially made. For example, if you create a new Group Policy Object (GPO) then this is first created in the directory database and within the SYSVOL share on the PDC Emulator, and from there the GPO is replicated to all other domain controllers in the domain. Finally, all password changes and account lockout issues are handled by the PDC Emulator to ensure that password changes are replicated properly and account lockout policy is effective. So even though the PDC Emulator emulates an NT PDC (which is why this role is called PDC Emulator), it also does a whole lot of other stuff. In fact, the PDC Emulator role is the most heavily utilized FSMO role so you should make sure that the domain controller that holds this role has sufficiently beefy hardware to handle the load. Similarly, if the PDC Emulator role fails then it can potentially cause the most problems, so the hardware it runs on should be fault tolerant and reliable. Finally, every domain has its own PDC Emulator role, so if you have N domains in your forest then you will have N domain controllers with the PDC Emulator role as well.
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