Active directory conditional forwarding

Active directory has a feature where by one or more IP address can be specified to forward name resolutions to that are not handled by the local DNS server.  The conditional forwarder definitions are also replicated via Active Directory. Together with the forward and reverse lookup zones in the active directory these can be set via the DNS mmc management console. The DNS servers are usually primary or secondary in nature. The primary stores all the records of the zone and the secondary gets the contents of its zone from the primary.  Each update can flow from the primary to the secondary or the secondary may pull the updates periodically or on demand. All updates have to be made to the primary. Each type of server can resolve name queries that come from hosts for the zones.   The contents of the zone file can also be stored in the active directory in a hierarchical structure.  The DNS structure can be replicated among all DCs of the domain, each DC holds a writeable copy of the DNS data.  The DNS objects stored in the Active Directory could be updated on any DC via LDAP operations or through DDNS against DCs that act as DNS servers when the DNS is integrated with the Active Directory.
The DNS "island" issue sometimes occurs due to improper configuration. AD requires proper DNS resolution to replicate changes and when using integrated DNS, the DC replicates DNS changes throught AD replication.  This is the classic chicken and egg problem. If the DC configured as name server points to itself and its IP address changes, the DNS records will successfully be updated locally but other DCs cannot resolve this DC's IP address unless they point to it. This causes replication fail and effectively renders the DC with the changed IP address an island to itself. This can be avoided when the forest root domain  controllers that are the name servers are configured to point at root servers other than themselves.