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Troubleshooting AD Replication error 8418: The replication operation failed because of a schema mismatch

Note

Home users: This article is intended for only technical support agents and IT professionals. If you're looking for help with a problem, ask the Microsoft Community.

Original KB number:   2734946

Active Directory replication error 8418 occurs when domain controllers (DCs) have inconsistent schema definitions. The schema partition defines the structure of all objects and attributes in the Active Directory forest. When the schema partition differs between DCs, replication fails because the destination DC can't process objects that reference schema definitions it doesn't recognize.

This article helps you:

  • Identify the symptoms of error 8418.
  • Understand the common causes of schema mismatch errors.
  • Determine whether the issue is transient or persistent.
  • Collect diagnostic data to isolate the cause.
  • Apply the appropriate resolution.

Symptoms

You observe one or more of the following symptoms:

  • During a regular replication cycle, one or more on-screen errors, logged events, or diagnostics indicates that a schema mismatch occurred. The error text might include any or all of the following information.

    Decimal Hex Symbolic Error String
    8418 0x20e2 ERROR_DS_DRA_SCHEMA_MISMATCH The replication operation failed because of a schema mismatch between the servers involved.

  • When you try to promote a member server to a domain controller (DC), you receive the following message:

    ---------------------------  
Active Directory Domain Services Installation Wizard  
---------------------------  
The operation failed because: Active Directory Domain Services could not replicate the directory partition <DN path of partition> from the remote
Active Directory Domain Controller <helper DC FQDN>.  
"The replication operation failed because of a schema mismatch between the servers involved."  
---------------------------  
OK  
---------------------------

    If the error occurs before the DC promotion process reaches the noncritical replication phase, the DCPromo.log file records entries that resemble the following example:

    MM/DD/YYYY HH:MM:SS [INFO] NtdsInstall for <hostname.dns domain name.<top level domain> returned 8418  
MM/DD/YYYY HH:MM:SS [INFO] DsRolepInstallDs returned 8418  
MM/DD/YYYY HH:MM:SS [ERROR] Failed to install to Directory Service (8418)  
MM/DD/YYYY HH:MM:SS [INFO] Starting service NETLOGON

    If the error occurs during the noncritical replication phase, the new DC restarts. After the restart, DCPromo.log records entries that resemble the following example:

    MM/DD HH:MM:SS [WARNING] Non critical replication returned 8418
MM/DD HH:MM:SS [INFO] Cleaning up old Netlogon information
MM/DD HH:MM:SS [INFO] Stopped the DS
MM/DD HH:MM:SS [INFO] The attempted domain controller operation has completed

  • The directory service event log records one or more events that resemble the following examples.

    Event source Event ID or
    Event ID/Error status    		 Event string
    Microsoft-Windows-ActiveDirectory_DomainService 1203 The directory service could not replicate the following object from the source directory service at the following network address because of an Active Directory Domain Services schema mismatch.
    Microsoft-Windows-ActiveDirectory_DomainService 1309/8418 The Knowledge Consistency Checker (KCC) has detected that successive attempts to replicate with the following directory service has consistently failed.
    Microsoft-Windows-ActiveDirectory_DomainService 1791/8418 Replication of application directory partition <DN path> from source <NTDS Settings object GUID> from source DC (<source DC FQDN>) has been aborted. Replication requires consistent schema but last attempt to synchronize the schema had failed. It is crucial that schema replication functions properly. See previous errors for more diagnostics. If this issue persists, contact Microsoft Product Support Services for assistance. Error 8418: The replication operation failed because of a schema mismatch between the servers involved.
    NTDS KCC (Knowledge Consistency Checker) 1925/8418 The attempt to establish a replication link for the following writable directory partition failed.
    NTDS Replication 1203 The local domain controller could not replicate the following object from the source domain controller at the following network address because of an Active Directory schema mismatch
    NTDS Replication 1791/8418 Replication of Naming Context <DN path> from source <NTDS Settings object guid> has been aborted. Replication requires consistent schema but last attempt to sync the schema had failed. It is crucial that schema replication functions properly. See previous errors for more diagnostics. If this issue persists, contact Microsoft Product Support Services for assistance. Error 8418: The replication operation failed because of a schema mismatch between the servers involved.

  • You manually trigger replication by using the Replicate now command in the Active Directory Sites and Services console. The operation fails, and generates an error message that resembles the following example:

    --------------------------- Replicate Now ---------------------------
The following error occurred during the attempt to synchronize naming context <Naming context> from Domain Controller <source DC> to Domain Controller <destination DC>: The replication operation failed because of a schema mismatch between the servers involved. This operation will not continue.
--------------------------- OK ---------------------------

  • You run one of the following Repadmin commands (repadmin.exe), and you see messages that resemble the following examples.

    Command Message
    Repadmin /ShowReps Last attempt @ YYYY-MM-DD HH:MM:SS failed, result 8418 (0x20e2):
    The replication operation failed because of a schema mismatch between the servers involved.
    Repadmin /SyncAll repadmin /syncall /AePdq Syncing all NC's held on <DC Name>.
    Syncing partition: DC=contoso,DC=com SyncAll reported the following errors:
    Error issuing replication: 8418 (0x20e2):
    The replication operation failed because of a schema mismatch between the servers involved.
    Repadmin /ReplSum repadmin /replsummary Replication Summary Start Time: YYYY-MM-DD HH:MM:SS
    Beginning data collection for replication summary, this may take a while:
    .....
    Source DSA largest delta fails/total %% error <Source DC> xxh:yym:zzs 4 / 5 80 (8418)
    The replication operation failed because of a schema mismatch between the servers involved.
    Destination DSA largest delta fails/total %% error <dest DC> 04h:02m:16s 4 / 5 80 (8418)
    The replication operation failed because of a schema mismatch between the servers involved.

Cause

Replication of any Active Directory data between DCs in a forest depends on all DCs having a consistent view of the definitions of objects and attributes. These definitions are stored in the Schema partition of the Active Directory database. The directory replication engine prioritizes the Schema partition so that any change to a schema definition replicates before any data that uses the schema definition can replicate.

Schema mismatches can occur under the following circumstances:

  • DCs replicate the schema partition, and the source DC has different schema information than the destination DC.
  • DCs replicate a nonschema partition, and the data on the source DC uses a different schema than the data on the destination DC.

Scenarios

In order to resolve a schema mismatch issue, you have to understand the scenario in which the issue occurred.

Scenario 1

The issue occurs after the Active Directory schema is updated. In many cases, this issue is transient and resolves itself.

Important

Test any schema update thoroughly before you introduce it into your production environment.

Scenario 2

The issue occurs when you try to promote a member server to a domain controller (DC). The promotion operation fails.

Scenario 3

The issue occurs during normal replication. Typically, this behavior means that an underlying issue is preventing Active Directory from resolving issues that would usually be transient. This scenario has multiple possible causes, including (but not limited to) the following issues:

  • A DC is quarantined, or might have lingering objects
  • Communication stopped because of a DNS or RPC issue
  • A DC stopped replicating for other reasons
  • Objects can't replicate because their nTSecurityDescriptor attributes are too large

Scenario 4

A less common scenario is one in which the schema partition on one or more DCs has improper attribute definitions. Possible schema definition issues that can trigger a schema mismatch error include the following issues:

  • OID clash
  • Invalid OM syntax values
  • Invalid MayContain values
  • Object attributes that contain data but whose schema definition is marked as defunct

For scenarios in which schema replication fails because of improper attribute schema definitions, contact Microsoft Support for assistance.

Transient issues versus persistent issues

Schema mismatch issues typically fall into one of two categories: Transient and persistent.

Transient schema mismatch issues meet the following criteria:

  • The error messages appear on different DCs throughout the forest, in a pattern that matches the Active Directory replication topology and schedule. This kind of pattern typically occurs when you install an update to the schema.
  • On any particular DC, the issue disappears after one replication cycle per replication partner. This process might take twice the amount of time that's needed for an object update to replicate from one DC to all other DCs in the forest. The more replication partners that a DC has, the longer this process takes.

Persistent schema mismatch issues don't meet these criteria, and don't resolve themselves. Such issues indicate one or more of the following issues:

  • An underlying issue is interfering with typical replication. The replication service can't resolve the schema mismatch until the underlying issue is corrected. In some cases, you can identify and resolve the underlying issues. In other cases, you might need assistance from Microsoft Support. For example, any of the following issues can block replication:
    • Database corruption
    • Memory constraints
    • Replication quarantine
    • Strict replication consistency enforced
    • Disabled replication
    • DNS (Domain Name Resolution) issues
    • RPC communication issues
    • Local or network firewalls

Collect diagnostic data to help isolate possible causes

Some schema mismatch issues have straightforward resolutions. However, in many cases you'll, need assistance from Microsoft Support. This section guides you in determining the type of issue that you have and whether a solution is available.

Level 1: Review the overall replication status

We recommend that you start your investigation by collecting replication data for all the DCs in the forest. Collecting this data helps identify any pockets of replication failure. To collect data and export it to a .csv file for analysis, run a command that resembles the following example at the command prompt:

Repadmin showrepl * /csv > allrepl.csv

Review this data, identifying all the DCs that experience any replication issues.

If you recently applied a schema update to the forest, see Resolution 1: Following a schema update, issues persist.

If you didn't recently update the schema, continue to the next section to collect diagnostic event logs.

Level 2: Collect and review diagnostic logs

Increase the diagnostic logging level

Important

This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For protection, back up the registry before you modify it so that you can restore it if a problem occurs. For more information about how to back up and restore the registry, see How to back up and restore the registry in Windows.

Increase the level of event logging on both source and destination DCs. To change the logging level, follow these steps:

  1. In Registry Editor, locate the following subkey:

    HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NTDS\Diagnostics

  2. Update the values of the registry entries as described in the following table.

    Name Type Value
    5 Replication DWORD 5
    9 Internal Processing DWORD 5
    24 Schema DWORD 5
    7 Internal Configuration DWORD 5

Review the errors and event logs for any replication issues

Wait for the issue to occur, or repeat the action that triggers the issue (for example, promote a member server to a domain controller). After the schema mismatch occurs, review the event logs on the affected DCs and their replication partners. Look for any information in the errors or events that identifies the source of the issue or which resource the issue affects.

  • Look for the Event ID (if applicable) and the error code.
  • Does the error or event refer to one or more computers? Replication errors typically list a source DC and a destination DC. For events, the destination DC typically logs the event. Look for the DN, NetBIOS name, or the directory service agent GUID (DSA GUID) for each DC.
  • Does the error or event identify a particular object or attribute that triggers the issue? Look for the object DN or GUID, or the attribute display name or GUID.
  • Look for any internal or extended error data.

The following table lists examples of error codes that indicate issues that prevent Active Directory from resolving a schema mismatch.

Note

  • In addition to appearing in logged events, these error codes can occur when you use command-line tools such as repadmin, dcdiag, or dcpromo.
  • If you receive the schema mismatch error when you try to promote a member server to a DC, collect the DCpromo and DCpromoUI log files. This kind of issue is almost always persistent, and doesn't resolve itself.
  • Some events are associated with more than one error code. When you review your event log for these events, make sure that you check the error code that's listed in the event so that you can interpret the event correctly.
Error code Event ID and source, if appropriate More information
1203 Microsoft-Windows-ActiveDirectory_DomainService or NTDS Replication
1340 1450 NTDS SDProp Inherited access ACL or ACE couldn't be built
1722 Active Directory replication error 1722
1753 Active Directory Replication Error 1753
8614 1925 NTDS KCC
2042 NDTS Replication		 Troubleshoot Active Directory replication error 8614
Active Directory replication Event ID 2042
8606 1988 NTDS Replication Active Directory Replication Error 8606
8456 or 8457 1308 NTDS KCC
1393 NTDS General
1586 NTDS Replication
1925 NTDS KCC
1926 NTDS KCC
2023 NTDS Replication
2095 Microsoft-Windows-ActiveDirectory_DomainService
2103 Microsoft-Windows-ActiveDirectory_DomainService or NTDS General		 Active Directory replication error 8456 or 8457
8524 1308 NTDS KCC
1655 NTDS General
1865 NTDS KCC
1925 NTDS KCC
1926 NTDS KCC
2023 Microsoft-Windows-ActiveDirectory_DomainService
2087 NTDS Replication
2088 NTDS Replication		 Active Directory Replication Error 8524

For additional common error codes, see Troubleshoot common Active Directory replication errors.

If you can isolate object or attribute identifiers from the error or event information, continue to the next section to collect object and attribute metadata.

If the replication events that cite error code 8418 contain any Extended or Internal errors, compare those values to known issues.

Level 3: Export and review object and attribute metadata

If the events or error codes identify a particular object (or set of objects) or a particular object attribute as the trigger of the replication issue, the object or attribute metadata might provide more information about the issue.

Objects and attributes can trigger replication issues in several manners. The causes of such issues might not appear to relate directly to the schema. One such potential issue is the size of an object's security descriptor (the nTSecurityDescriptor attribute). If the attribute gets too large, it can block replication. Fortunately, error code 1340 specifically indicates that this issue occurred.

Other object and attribute issues are more complex. In this case, you might have to contact Microsoft Support for assistance.

Export the object and attribute metadata

To retrieve this data, run repadmin /showobjmeta on both the source DC and the destination DC, as shown in the following examples:

If you have the DN of the object that caused the issue, open an administrative Command Prompt window, and then run the following command:

Repadmin /showobjmeta <Target_DC> "<DN_of_Trigger_Object>"

If you have the GUID of the object or attribute that caused the issue, run the following command:

`Repadmin /showobjmeta <Target_DC> "GUID=<ObjectGuid_of_Trigger_Object>"`

Note

  • In these commands, <Target_DC> represents the DC whose data you're retrieving (the source DC or the destination DC).
  • You can use <ObjectGuid_of_Trigger_Object> to represent the GUID of an object or of one of the object's attributes.
  • If the issue occurred when you promoted a member computer to a DC, the destination DC is the computer that you tried to promote. It might not yet have the object.

You can use this method to identify "candidate" attributes that might be causing the issue.

If you only have an identifier for an object, dump all the attribute values of the object by running a set of commands that resembles the following example:

Ldifde -f results.txt -d "<DN_of Trigger_Object>" -s <Target_DC> 
Ldifde -f results.txt -d "GUID=<ObjectGuid_of Trigger_Object>" -s <Target_DC>
Repadmin /showattr <Target_DC> "<DN_of Trigger_Object>"
Repadmin /showattr <Target_DC> "GUID=<ObjectGuid_of Trigger_Object>"

You can also use ldp commands to extract replication metadata.

Review the object and attribute data

To determine whether the data replicated to each partner, you can compare the metadata for the same object on the source DC and its replication partners.

Tip

The metadata of objects or attributes that are waiting to be replicated shows a higher version number on the source DC than on the destination DC.

LDIFDE, Repadmin, and LDP all provide slightly different views of object and attribute metadata. You can compare these views to identify discrepancies in the metadata. For example, consider the following repadmin output:

USN   DSA Org                              USN Org. Time/Date  Version Attribute  
24260 f4617e99-9688-42a6-8562-43fdd2d5cda4 18085395 <DateTime> 2  
24260 f4617e99-9688-42a6-8562-43fdd2d5cda4 18086114 <DateTime> 3

The output should include attribute identifiers, but those values are missing.

The following example shows the same metadata as viewed by using the ldp tool. This output shows attribute identifiers in the AttID column.

AttID  Ver Loc.USN Originating DSA                      Org.USN  Org.Time/Date  
250000 2   24260   f4617e99-9688-42a6-8562-43fdd2d5cda4 18085395 <DateTime>  
250004 3   24260   f4617e99-9688-42a6-8562-43fdd2d5cda4 18086114 <DateTime>

Such discrepancies in the metadata might help you determine what's causing the issue. In most cases, however, you should contact Microsoft Support for further assistance.

Important

Make sure that you have on hand the data that's listed in Data to provide to Microsoft Support, including the exported schema data.

Resolution

Resolution 1: Following a schema update, issues persist

After the Active Directory schema updates, schema mismatch issues typically appear and disappear on various DCs throughout the forest. This behavior typically occurs in a pattern that matches the replication topology and schedule. These transient schema mismatches are the expected behavior. Monitoring software typically reports the issues but doesn't require administrative intervention.

If schema mismatch issues persist for an extended period, such as more than twice the time that's needed for an update to replicate through the entire topology, then you should investigate the issue. A single DC might have to restart, the update might not be applied correctly, or an underlying issue might be blocking replication.

Step 1: Verify that the schema version values match in Active Directory and the registry

Each DC stores the version of the schema that it uses in two locations: The registry, and in its local Active Directory replica. During normal operation, the two values should be in sync and correctly reflect the schema version of the forest. This schema version is defined by the DC that owns the schema Flexible Single Master Operation (FSMO) role. If the values don't match for a particular DC, that DC might not have received the schema update yet. Check the status values on DCs that report persistent schema mismatches, and on their replication partners.

Note

To make sure that SchemaVersion updates correctly, install only the updates of the Active Directory Schema that Microsoft provides.

To review the version number in the registry, open Registry Editor, and locate the following subkey: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NTDS\Parameters\SystemSchemaVersion

To view the version number in Active Directory, use one of the following methods:

  • In ADSIEdit, connect to the Schema naming context for the affected forest, right-click the schema object (for example, cn=Schema,cn=configuration,dc=contoso,dc=com), and then select Properties. Look for the value of the ObjectVersion attribute.

  • In a Command Prompt window on a DC, run a command that resembles the following example:

    Ldifde -f schemaver.ldf -d Cn=Schema,cn=configuration,dc=contoso,dc=com -l ObjectVersion

  • At the DC command prompt, run a command that resembles the following example:

    dsquery * cn=schema,cn=configuration,dc=contoso,dc=com -scope base -attr objectVersion

For reference, the following table lists the schema versions for different versions of Windows Server.

Operating system Schema version
Windows 2000 13
Windows Server 2003 30
Windows Server 2003 R2 31
Windows Server 2008 43
Windows Server 2008R2 47
Windows Server 2012 56
Windows Server 2012R2 69
Windows Server 2016 87
Windows Server 2019 88
Windows Server 2022 88
Windows Server 2025 91

Step 2: Restart the source DC that's not replicating the update

You can use this step if the following conditions apply:

  • The Active Directory schema was recently updated.
  • You identify one or more DCs that report a persistent schema mismatch issue.
  • These DCs use the same source DC for inbound replication.
  • The source DC has the expected schema version information in the registry and in its Active Directory replica. This information matches the updated forest version (the source DC has the correct update installed).

Restart the source DC.

In some cases, a DC might not correctly reload the in-memory schema version after it receives the schema update. In this case, restart the DC to resolve the issue.

If the issue persists, see Data to provide to Microsoft Support, and then contact Microsoft Support for assistance.

Resolution 2: The inherited access control list (ACL) or access control entry (ACE) could not be built (Event 1450, error code 1340)

This error indicates that the identified object's security descriptor (the nTSecurityDescriptor attribute) is too large.

The security descriptor contains the object's ACL information. This information includes ACEs that are set directly on the object and ACEs that are inherited from the parent object. Windows limits the attribute to 65,535 bytes.

To resolve this issue, you have to reduce the size of the object's security descriptor by reducing the number of ACEs. This process involves manually checking the security descriptor to identify the directly applied ACEs and the inherited ACEs, and the sources of the inherited ACEs. If multiple objects trigger error code 1340, you can efficiently collect this information by using a script. For an example of such a script, see Scripts to calculate the sizes of object security descriptors.

Data to provide to Microsoft Support

If you need assistance from Microsoft support, we recommend that you collect information by following the steps mentioned in Gather information by using TSS for Active Directory replication issues.

Additionally, gather the following information to enable the Microsoft Support staff to help you diagnose the causes of the schema mismatch. (Much of this information was gathered in earlier sections of this article.)

  • DCpromo logs from destination DC (if appropriate for the scenario)

  • Repadmin /showrepl output from the source DC and the destination DC

  • Directory Services event logs from the source and destination domain controller

  • Replication metadata of any problem object that the errors or events identified

  • Exported LDIFDE data of any problem object that the errors or events identified

  • Exported schema partition LDIFDE data from both the source DC and the destination DC. To export this information to a file, run the following command at a command prompt:

    Ldifde -f schema <Target_DC>.ldf -d cn=schema,cn=configuration,dc=<Domain>,dc=com -s <Target_DC>

    Note

    In this command, <Target_DC> represents the name of the DC that holds the replica that you want to export, and <Domain> is the name of the root domain to which the schema belongs.

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