What Is an Active Directory Replication Failure?

Updated on September 29, 2025

An Active Directory replication failure occurs when changes made on one domain controller (DC) fail to synchronize correctly with other domain controllers in the forest. Replication is the process that ensures all DCs have a consistent copy of the Active Directory database.

When this process breaks down, it can lead to a state where different DCs hold conflicting information. This causes severe issues such as user authentication problems, group policy inconsistencies, and an inability to apply security changes. For IT professionals, diagnosing and resolving replication failures is a critical skill for maintaining the health and security of a distributed Active Directory environment.

The impact of replication failures extends beyond individual user experiences. These failures can compromise the integrity of your entire directory service, creating security vulnerabilities and operational disruptions that affect business continuity.

Definition and Core Concepts

A replication failure is any event that prevents Active Directory from successfully copying object and attribute changes from a source domain controller to a destination domain controller. The failure can be caused by a variety of issues, ranging from network connectivity problems to misconfigured settings or underlying system faults.

Understanding these foundational concepts is essential for effective troubleshooting:

• Replication Topology: The logical map of replication connections between domain controllers, automatically managed by the Knowledge Consistency Checker (KCC). This topology determines how changes propagate through your network.
• Update Sequence Number (USN): A monotonically increasing number that a DC assigns to every change. Replication failures often occur when there is a mismatch or a “rollback” in USNs. Each DC maintains its own USN counter, and these numbers are critical for tracking change sequences.
• Lingering Objects: Objects that have been deleted on all other DCs but persist on a DC that was offline for longer than the Tombstone Lifetime (TSL). These objects can cause replication to fail by creating conflicts during the synchronization process.
• Inbound vs. Outbound Replication: A DC can fail to either push changes to its partners (outbound) or pull changes from them (inbound). A failure is typically diagnosed from the destination DC’s perspective, so it’s always classified as an inbound failure.

How It Works

Active Directory replication uses a “pull” model, where a destination DC requests changes from a source DC. A failure can happen at any point in this process, and understanding these failure points is crucial for effective diagnosis.

• RPC Call Failure: The destination DC attempts to make a Remote Procedure Call (RPC) to the source DC. If there are network issues such as DNS resolution problems or firewall blocking, the RPC call will fail, and replication will not even start. This is often the first point of failure in the replication chain.
• Authentication/Authorization Failure: The destination DC might not have the correct permissions to request changes from the source DC, leading to an “Access Denied” error. This is often caused by a trust relationship issue or a password mismatch between the machine accounts of the domain controllers.
• Data Consistency Failure: Even if the connection is successful, the replication might fail due to data issues. For example, if a DC receives a change for an object that no longer exists on its own database due to a lingering object, the replication will fail to prevent data corruption. The system prioritizes data integrity over replication completion.
• System Health: A DC that is low on disk space, has corrupted database files, or has a services issue will fail to replicate. The destination controller cannot write the new changes to its local database, causing the entire replication attempt to abort.

Key Features and Components

Active Directory includes several built-in mechanisms and tools to manage and diagnose replication failures:

• Self-Healing: Active Directory has some self-healing capabilities. For example, the KCC can reroute replication traffic around a failed DC. However, persistent failures often require manual intervention. The system can temporarily work around connectivity issues but cannot resolve fundamental configuration problems automatically.
• Event Logging: The primary source for diagnosing replication failures is the Directory Service event log on the domain controllers. Specific event IDs provide detailed information about the failure, including the source and error codes. Event ID 1925 typically indicates replication link failures, while Event ID 1311 points to knowledge inconsistencies.
• Command-Line Tools: Tools like repadmin are essential for troubleshooting Active Directory replication failure scenarios. The repadmin /showrepl command shows the replication status and errors for all naming contexts, while dcdiag /test:replications runs a comprehensive test across the entire forest topology.

Troubleshooting and Considerations

Effective troubleshooting requires a systematic approach that addresses the most common failure points:

• DNS is Everything: The most common cause of replication failure is a DNS problem. All DCs must be able to correctly resolve the IP addresses of their replication partners. DNS issues manifest as RPC server unavailable errors and can be diagnosed using nslookup and dcdiag /test:dns commands.
• Firewall Configuration: Firewalls between sites must allow the necessary RPC ports for Active Directory replication. Port 135 is required for the RPC endpoint mapper, and dynamic RPC ports above 1024 must be accessible. Modern configurations often use port 3268 for Global Catalog replication.
• Lingering Objects: These represent a severe form of replication failure that can propagate incorrect data across the forest. They require manual intervention and cleanup using tools like repadmin /removelingeringobjects. The presence of lingering objects often indicates that a DC was offline beyond the tombstone lifetime period.
• Topology Mismatch: While rare due to the KCC’s automatic management, manual intervention can cause topology issues that result in replication failures. It is best to let the KCC manage the topology unless specific business requirements necessitate manual configuration.

Key Terms Appendix

• Domain Controller (DC): A server that stores a copy of the Active Directory database and provides authentication services to the network.
• Replication: The process of synchronizing changes between DCs to maintain consistency across the Active Directory forest.
• Repadmin: A command-line tool for diagnosing and troubleshooting Active Directory replication issues and viewing replication metadata.
• Lingering Object: A deleted object that persists on a DC that was offline for longer than the tombstone lifetime, potentially causing replication conflicts.
• Update Sequence Number (USN): A unique, incrementing number assigned by each DC to track the sequence of changes made to the directory database.