Register an existing organizational unit with AWS Control Tower - AWS Control Tower

Register an existing organizational unit with AWS Control Tower

An efficient way to bring multiple, existing AWS accounts into AWS Control Tower is to extend governance by AWS Control Tower to an entire organizational unit (OU).

To enable AWS Control Tower governance over an existing OU that was created with AWS Organizations, and its accounts, register the OU with your AWS Control Tower landing zone. You can register OUs that contain up to 300 accounts. If an OU contains more than 300 accounts, you cannot register it in AWS Control Tower.

When you register an OU, its member accounts are enrolled into the AWS Control Tower landing zone. They are governed by the guardrails that apply to their OU.


If you don't already have an AWS Control Tower landing zone, start by setting up a landing zone, either in a new organization created by AWS Control Tower, or in an existing AWS Organizations organization. For more details about how to set up a landing zone, see Getting started with AWS Control Tower.

What happens to my accounts when I register my OU?

AWS Control Tower requires permission to establish trusted access between AWS CloudFormation and AWS Organizations on your behalf, so that AWS CloudFormation can deploy your stack to the accounts in your organization automatically.

  • The AWSControlTowerExecution role is added to all accounts with status Not enrolled.

  • Mandatory guardrails are enabled by default to your OU and all its accounts when you register your OU.

Partial enrollment of accounts after an OU is registered

It's possible to register an OU successfully, yet certain accounts may remain unenrolled. If so, these accounts do not meet some of the prerequisites for enrollment. If an account enrollment as part of the Register OU process does not succeed, the account status on the accounts page shows Enrollment failed. You may also see account information on your OU page such as 4 of 5, in the accounts field.

For example, if you see 4 of 5, it means that your OU has 5 accounts in total, and 4 of them enrolled successfully, but one account failed to enroll during the Register OU process. You can choose Re-Register OU to bring accounts into enrollment, after you make sure the accounts meet the enrollment prerequisites.

IAM user prerequisites for registering an OU

Your AWS Identity and Access Management (IAM) identity (user or role) must be included on the appropriate Account Factory portfolio when you perform the Register OU operation, even if you already have Admin permissions. Otherwise, the creation of the provisioned products will fail during registration. Failure occurs because AWS Control Tower relies upon the credentials of the IAM identity when registering an OU.

The relevant portfolio is one created by AWS Control Tower, called AWS Control Tower Account Factory Portfolio. Navigate to it by choosing Service Catalog > Account Factory > AWS Control Tower Account Factory Portfolio. Then select the tab called Groups, roles, and users to view your IAM identity. For more information on how to grant access, see the documentation for AWS Service Catalog.

Common causes of failure during registration or re-registration

If registration (or re-registration) of an OU or any of its member accounts fails, you can download a file containing a detailed report that shows which pre-checks did not pass. You can complete the download by choosing the Download button, which appears at the upper right of the registration area.

This section lists the types of errors you may receive if pre-checks fail, and how to correct the errors.

In general, when you register or re-register an OU, all accounts within that OU are enrolled in AWS Control Tower. However, it is possible that some accounts may fail to enroll, even if the OU as a whole is registered successfully. In these cases, you must resolve the pre-check failure related to the account and then try re-enrolling that account or OU.

Landing Zone error

  • Landing zone not ready

    Repair your current landing zone, or update it to the latest version.

OU errors

  • Exceeds maximum number of SCPs

    You may be over the limit for service control policies (SCPs) per OU, or you may have reached another quota. A limit of 5 SCPs per OU applies to all OUs in your AWS Control Tower landing zone. If you have more SCPs than the quota allows, you must delete or combine the SCPs.

  • Conflicting SCPs

    Existing SCPs may be applied to the OU or account, which prevent AWS Control Tower from enrolling the account. Check the applied SCPs for any policy that may prevent AWS Control Tower from working. Be sure to check the SCPs that are inherited from OUs higher in the hierarchy.

  • Exceeds stack set quota

    The stack set quota may have been exceeded. If you have more instances than the quota allows, you must delete some stack instances. For more information, see AWS CloudFormation quotas in the AWS CloudFormation User Guide.

  • Exceeds account limit

    AWS Control Tower limits each OU to 298 accounts during registration.

Account errors

  • Pre-checks prevented on accounts

    An existing SCP on the OU prevents AWS Control Tower from conducting pre-checks on your OU member accounts. To resolve this pre-check failure, update or remove the SCP from the OU.

  • Email address error

    The email address you specified for the account does not conform to the naming standards. Here is the regular expression (regex) that specifies which characters are allowed: [A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+[.]+[A-Za-z]+

  • Config recorder or delivery channel enabled

    The account may have an existing AWS Config configuration recorder or delivery channel. These must be deleted or modified through the AWS CLI in all AWS Regions where the AWS Control Tower management account has governed resources, before you can enroll an account.

  • STS disabled

    AWS Security Token Service (AWS STS) may be disabled in the account. AWS STS endpoints must be activated in the accounts for all Regions supported by AWS Control Tower.

  • IAM Identity Center conflict

    The AWS Control Tower home Region is not the same as the AWS IAM Identity Center (successor to AWS Single Sign-On) (IAM Identity Center) Region. If IAM Identity Center is already set up, the AWS Control Tower home region must be the same as the IAM Identity Center Region.

  • Conflicting SNS topic

    The account has an Amazon Simple Notification Service (Amazon SNS) topic name that AWS Control Tower needs to use. AWS Control Tower creates resources (such as SNS topics) with specific names. If these names are already taken, AWS Control Tower setup fails. This situation could occur if you are reusing an account previously enrolled in AWS Control Tower.

  • Suspended account detected

    This account has been suspended. It cannot be enrolled into AWS Control Tower. Remove the account from this OU, and try again.

  • IAM user not in portfolio

    Add the AWS Identity and Access Management (IAM) user to the AWS Service Catalog portfolio before registering your OU. This error pertains to the management account only.

  • Account does not meet prerequisites

    The account doesn’t meet prerequisites for account enrollment. For example, the account may be missing roles and permissions required to enroll it in AWS Control Tower. Instructions for adding a role are available in Manually add the required IAM role to an existing AWS account and enroll it.

As a reminder, AWS CloudTrail is auto-enabled on all of your AWS accounts when you enroll them in AWS Control Tower. If CloudTrail is enabled on an account previous to enrollment, you could experience double-billing unless you deactivate CloudTrail before you begin the enrollment process.