In "Tracking AD Replication," November 2001, I explained how Active Directory (AD) uses update sequence numbers (USNs) to track changes to each domain controller's (DC's) copy of AD's contents. Each server remembers the most recent USN that the server obtained from its replication partners (i.e., the high watermark) and uses that USN to control replication. For example, DC1 might say to DC2, "Tell me what updates you've made since your USN 10,000."

Whe...