In “Troubleshooting Active Directory Replication,” Sean Deuby presented several strategies for solving Active Directory (AD) replication problems. To troubleshoot AD replication at a deeper level, it helps to have an in-depth understanding of how replication works when changes occur in the directory. AD was one of the first LDAP directories to introduce multi-master replication, whereby changes can originate in any instance of the directory (i.e., on any domain controller—DC). Previously, such as with Windows NT 4.0, changes could originate on only one DC, the Primary Domain Controller (PDC). Multi-master replication has numerous inherent benefits, but it presents a complex problem: how to coalesce and replicate changes to the directory. Throughout this article, Brian Desmond will refer back to the simple three-DC domain.