ERROR_DS_FLAT_NAME_EXISTS_IN_FOREST - 8635 (0x21BB)
The flat name of the trusted domain already exists in the forest.
Updated: Feb 21, 2026
Technical Meaning
This error code indicates that an attempt was made to add a trusted domain with a flat name that already exists within the forest. The flat name is used in Active Directory for identifying domains, and it must be unique across all domains in the forest.
Error Details
The error ERROR_DS_FLAT_NAME_EXISTS_IN_FOREST (0x21BB) is returned when an operation to add a trusted domain with a specified flat name fails because that name already exists within the current forest. This can occur during forest trust configuration or domain management operations.
Usage Context
This error typically arises in scenarios where administrators attempt to establish or modify forest trusts, particularly when specifying the flat names of domains involved in these configurations. It is important for administrators to ensure that all specified flat names are unique within the forest to avoid this error.
Developer Interpretation
Developers should interpret this error as a validation failure during domain trust operations. The operation will fail if the flat name provided already exists, indicating a potential configuration issue or an attempt to duplicate existing trust relationships.
Related Errors
ERROR_DS_DOMAIN_NAME_EXISTS_IN_FOREST(0x21B9): Indicates that a domain with the specified name already exists in the forest.ERROR_DS_FLAT_NAME_NOT_FOUND(0x21C7): Indicates that the flat name provided does not exist in the forest.
FAQ
Q: What causes this error?
A: This error occurs when attempting to add a trusted domain with a flat name that already exists within the forest.
Q: How can I resolve this issue?
A: Ensure that the specified flat name is unique across all domains in the forest before performing the operation.
Summary
The ERROR_DS_FLAT_NAME_EXISTS_IN_FOREST error (0x21BB) signifies a validation failure during domain trust operations. It indicates that the flat name of the trusted domain already exists within the current forest, preventing the successful establishment or modification of the trust relationship.