PDC Emulator (Domain Role)
Any Domain Controller performing an Operations Master (FSMO) function will not be the end of your environment; but each does have a potential for impact given a sufficient window of absence. In usual circumstances, however, the most crippling role to lose is the PDC Emulator.
In a mixed-mode environment, when the PDC Emulator goes down, you lose the bridgehead server for Windows NT 4.0 networks involved in trust relationships with that Active Directory domain. You also lose any down-level updating of the SAM for your Windows NT 4.0 Backup Domain Controllers, therefore Active Directory account changes such as password changes, login name changes etc. are never communicated.
Regardless of the mode or functional-level of the domain, you stand the risk of losing time-synch within the domain. The more complex and distributed the domain/forest, the greater the potential of Kerberos failures as the clocks fall apart from one-another on the Domain Controllers or clients which were directly dependent upon the defunct PDC Emulator.
Any Domain sitting above Mixed Mode would still also be susceptible to password changes not being communicated across the domain in a timely fashion.
RID Master (Domain Role)
If the Domain Controller performing as the RID Master goes down or becomes inaccessible, Windows 2000 and above domain controllers will have no place to acquire new RID pool assignments. As this function is only called upon sporadically , unless you are adding security principals in bulk, this outage may not become apparent for some time.
A more noticeable occurrence may be the failure of the movetree.exe command to function properly as it relies upon the RID Master present in the domain that the object is coming from to actually perform the move.
Infrastructure Master (Domain Role)
In the event that the Infrastructure Master Role holder is lost, the ramifications will vary based upon whether the forest is in itself a single domain, or if it contains multiple domains. If everything within your Active Directory forest is contained within a single domain, the Infrastructure Master really doesn’t have anything to do as there are no cross-domain references to be maintained.
In a forest with multiple domains, the Infrastructure Master Role holder plays a more vital role by maintaining cross-domain references (i.e. users from Domain A are members of a group in Domain B). Now the kicker here, any server in Domain B that is a Global Catalog Server will be automatically maintained due to the intercommunications of the GC processes forest-wide. This would make for an intermittent issue as some servers would have stale phantom references and others would be up-to-date.
Domain Naming Master (Forest Role)
If the Domain Naming Master role holder is lost, domains won’t be able to be added or removed from the Active Directory forest. DCPROMO is also affected, meaning that servers can neither be promoted nor demoted.
Though the loss of this role holder impacts some more common operations performed within an Active Directory forest and its contained domains, it is still doesn’t create highly visible issues within your environment.
Schema Master (Forest Role)
All Domain Controllers contain a copy of the Active Directory Schema. This Schema is essentially a template or listing of the various Active Directory object types and available attributes present within a given forest. This template is used to refresh the Active Directory database where the actual objects are stored.
The loss of the Schema Master Role holder in an Active Directory puts the forest into a state of stasis so no extensions (addition of object types and/or attributes) to the Schema can be made. This would impair activities such upgrading an Active Directory domain from Windows Server 2000 to Windows Server 2003, installing Microsoft Exchange, and/or adding new attributes to an object. All things considered, as this sort of activity does not happen on a daily basis, a forest could survive the loss of this role holder and continue with minimal inconvenience in most cases.
