Passing parameters from parent stack to child stack in nested stack - Cloudformation

Question

I am trying to pass the paramters to internet facing elb using root. I have created a nested stack with a root.yml acting as root and vpc and elb are the child stack. I am trying to pass the parameters from root to elb.yml but it gives an error Parameter values specified for a template which does not require them while creating elb. The nested stack runs fine till vpc but when it gets to elb it gives an error

Root.yml:

AWSTemplateFormatVersion: 2010-09-09
Parameters:
  bucketname:
    Type: String
    Description: Path to the bucket
    Default: webserver
  bucketpath:
    Type: String
    Description: Path to the bucket
    Default: /nested-stack
Resources:
  Vpcstack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: !Sub "https://${bucketname}.s3.us-east-2.amazonaws.com${bucketpath}/vpc1.yml"

  elb:
    DependsOn: Vpcstack
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: !Sub "https://${bucketname}.s3.us-east-2.amazonaws.com${bucketpath}/internetfacing-elb.yml"
      Parameters:
        SubnetA: !GetAtt Vpcstack.Outputs.SubnetA
        SubnetB: !GetAtt Vpcstack.Outputs.SubnetB
        VpcID: !GetAtt Vpcstack.Outputs.VpcID

Vpc stack:

---
AWSTemplateFormatVersion: 2010-09-09
Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: 11.0.0.0/16
      EnableDnsSupport: true
      EnableDnsHostnames: true
      InstanceTenancy: default
  InternetGateway:
    Type: AWS::EC2::InternetGateway
  VPCGatewayAttachment:
    Type: AWS::EC2::VPCGatewayAttachment
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway
  SubnetA:
    Type: AWS::EC2::Subnet
    Properties:
      AvailabilityZone: us-east-2a
      VpcId: !Ref VPC
      CidrBlock: 11.0.0.0/24
      MapPublicIpOnLaunch: true
  SubnetB:
    Type: AWS::EC2::Subnet
    Properties:
      AvailabilityZone: us-east-2b
      VpcId: !Ref VPC
      CidrBlock: 11.0.1.0/24
      MapPublicIpOnLaunch: true
  SubnetC:
    Type: AWS::EC2::Subnet
    Properties:
      AvailabilityZone: us-east-2a
      VpcId: !Ref VPC
      CidrBlock: 11.0.2.0/24
      MapPublicIpOnLaunch: false
  SubnetD:
    Type: AWS::EC2::Subnet
    Properties:
      AvailabilityZone: us-east-2b
      VpcId: !Ref VPC
      CidrBlock: 11.0.3.0/24
      MapPublicIpOnLaunch: false
  RouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
  RouteTable2:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
  InternetRoute:
    Type: AWS::EC2::Route
    DependsOn: VPCGatewayAttachment
    Properties:
      DestinationCidrBlock: 0.0.0.0/0
      GatewayId: !Ref InternetGateway
      RouteTableId: !Ref RouteTable
  SubnetARouteTableAssociation:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      RouteTableId: !Ref RouteTable
      SubnetId: !Ref SubnetA
  SubnetBRouteTableAssociation:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      RouteTableId: !Ref RouteTable
      SubnetId: !Ref SubnetB
  SubnetCRouteTableAssociation:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      RouteTableId: !Ref RouteTable2
      SubnetId: !Ref SubnetC

  SubnetDRouteTableAssociation:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      RouteTableId: !Ref RouteTable2
      SubnetId: !Ref SubnetD
  SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupName: "Internet Group"
      GroupDescription: "SSH traffic in, all traffic out."
      VpcId: !Ref VPC
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: "22"
          ToPort: "22"
          CidrIp: 0.0.0.0/0
      SecurityGroupEgress:
        - IpProtocol: -1
          CidrIp: 0.0.0.0/0
  NAT:
    Type: AWS::EC2::NatGateway
    Properties:
      AllocationId:
        Fn::GetAtt:
          - EIP
          - AllocationId
      SubnetId:
        Ref: SubnetA
      Tags:
        - Key: Name
          Value: ng-nat
  EIP:
    DependsOn: VPCGatewayAttachment
    Type: AWS::EC2::EIP
    Properties:
      Domain: VPC
  Route:
    Type: AWS::EC2::Route
    Properties:
      RouteTableId:
        Ref: RouteTable2
      DestinationCidrBlock: 0.0.0.0/0
      NatGatewayId:
        Ref: NAT
Outputs:
  VpcID:
    Description: VPC id
    Value: !Ref VPC
    Export:
      Name: "VpcID"
  SubnetA:
    Description: public subnet
    Value: !Ref SubnetA
    Export:
      Name: "SubnetA"
  SubnetB:
    Description: public subnet 2
    Value: !Ref SubnetB
    Export:
      Name: "SubnetB"
  SubnetC:
    Description: priavte subnet
    Value: !Ref SubnetC
    Export:
      Name: "SubnetC"
  SubnetD:
    Description: private subnet 2
    Value: !Ref SubnetD
    Export:
      Name: "SubnetD"

internet facing elb:

---
AWSTemplateFormatVersion: 2010-09-09
Resources:
  elb:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupName: sg-elb
      VpcId:
        Fn::ImportValue: "VpcID"
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0
          Description: For traffic from Internet
        - IpProtocol: tcp
          FromPort: 80
          ToPort: 80
          CidrIp: 0.0.0.0/0
          Description: For traffic from Internet
      GroupDescription: Security Group for demo server

  MyLoadBalancer:
    Type: AWS::ElasticLoadBalancing::LoadBalancer
    Properties:
      Listeners:
        - LoadBalancerPort: "80"
          InstancePort: "80"
          Protocol: HTTP
      SecurityGroups:
        - !Ref elb
      LoadBalancerName: elb
      Subnets:
        - Fn::ImportValue: "SubnetA"
        - Fn::ImportValue: "SubnetB"
      HealthCheck:
        Target: HTTP:80/SamplePage.php
        HealthyThreshold: "3"
        UnhealthyThreshold: "5"
        Interval: "30"
        Timeout: "5"
Outputs:
  ec2:
    Description: ec2
    Value: !Ref MyLoadBalancer
    Export:
      Name: "MyLoadBalancer"
  lgsg:
    Description: lg-sg
    Value: !GetAtt elb.GroupId
    Export:
      Name: "lgsg"

Answer 1

The issue comes from the fact that you are passing 3 parameters to elb stack:

        SubnetA: !GetAtt Vpcstack.Outputs.SubnetA
        SubnetB: !GetAtt Vpcstack.Outputs.SubnetB
        VpcID: !GetAtt Vpcstack.Outputs.VpcID

However, the elb template does not accept any parameters.

To rectify the issue, you should add Parameters section to the elb template. For example:

Parameters:

  SubnetA:
    Type: String

  SubnetB:
    Type: String

  VpcID:
    Type: String

Also, in the elb template you should be using !Ref instead of !ImportValue to reference the new parameters.