How to show cfnoutput from nested stack in parent stack?

[mavwolverine]

Is there any way to show the CfnOutput from nested stack in parent stack output?

For eg. If I have LB as nested stack with CfnOutput the dns name, is there any way for the parent stack to print it automatically without having to explicitly again do a CfnOutput?

[MentalOfCrow]

CloudFormation does not automatically promote every child output into the root stack's Outputs section. An output belongs to the template/stack in which it is declared, so it will be visible on the nested stack, not duplicated on the parent.

There are two separate use cases:

1. The parent only needs to consume the value

Expose the resource attribute as a property of your NestedStack class and use that property in the parent. CDK will create the internal nested-stack output/reference automatically; you do not need to declare a child CfnOutput for this plumbing.

interface LoadBalancerNestedStackProps extends cdk.NestedStackProps {
  vpc: ec2.IVpc;
}

class LoadBalancerNestedStack extends cdk.NestedStack {
  public readonly dnsName: string;

  constructor(scope: Construct, id: string, props: LoadBalancerNestedStackProps) {
    super(scope, id, props);

    const lb = new elbv2.ApplicationLoadBalancer(this, 'LB', {
      vpc: props.vpc,
      internetFacing: true,
    });

    this.dnsName = lb.loadBalancerDnsName;
  }
}

class ParentStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props: cdk.StackProps) {
    super(scope, id, props);

    const vpc = new ec2.Vpc(this, 'Vpc');
    const networking = new LoadBalancerNestedStack(this, 'Networking', { vpc });

    // Use networking.dnsName in another construct in the parent.
  }
}

The NestedStack API explicitly states that cross-references between parent and nested stacks are translated automatically into parameters and outputs.

2. You want the value displayed as a root-stack output

Then the root template must contain an output, so one explicit parent CfnOutput is required:

new cdk.CfnOutput(this, 'LoadBalancerDnsName', {
  value: networking.dnsName,
  description: 'Public DNS name of the application load balancer',
});

This is not creating or copying the load balancer value at synthesis time; it creates the parent output whose value resolves through the nested-stack reference. Underlying CloudFormation represents that as approximately:

Outputs:
  LoadBalancerDnsName:
    Value: !GetAtt NetworkingNestedStack.Outputs.<generated-output-name>

CloudFormation documents that nested outputs are addressed as Fn::GetAtt [NestedStackResource, Outputs.OutputName] in the AWS::CloudFormation::Stack reference. CDK generates that low-level expression for you when you pass the child's token to the parent output.

If you do not want any extra root output, the alternative is operational rather than declarative: list the child stacks and read their individual outputs with DescribeStacks. There is no CloudFormation setting that recursively flattens all child outputs into the parent automatically.

So the clean CDK pattern is: expose resource attributes from the nested-stack class, let CDK synthesize private cross-stack plumbing, and declare only the small set of parent CfnOutputs that humans or automation actually need at the root.

If this clarifies why the output is not promoted automatically, please mark it as the accepted answer so future CDK users can find the pattern quickly.