CloudFormation unknown tag !<!Cidr>

Question

I'm trying to use Amazon CloudFormation to build a VPC and subnets, but I keep running into issues trying to use the !Cidr function to specify the CidrBlock of the subnets.

Minimal example, I'm using a CF template sourced directly from Amazon documentation for the !Cidr function. I save it to a file, create a new stack, choose to load in that file, then View in Designer. I get this error:

Cannot render the template because of an error.: YAMLException: unknown tag !<!Cidr> at line 18, column 75: ... Att ExampleVpc.CidrBlock, 1, 8 ]] ^

Here is the template I'm using:

AWSTemplateFormatVersion: "2010-09-09"
Description: "Try to create VPC and Subnet using Cidr function"
Resources:
  ExampleVpc:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: "10.0.0.0/16"
  IPv6CidrBlock:
    Type: AWS::EC2::VPCCidrBlock
    Properties:
      AmazonProvidedIpv6CidrBlock: true
      VpcId: !Ref ExampleVpc
  ExampleSubnet:
    Type: AWS::EC2::Subnet
    DependsOn: IPv6CidrBlock
    Properties:
      AssignIpv6AddressOnCreation: true
      CidrBlock: !Select [ 0, !Cidr [ !GetAtt ExampleVpc.CidrBlock, 1, 8 ]]
      Ipv6CidrBlock: !Select [ 0, !Cidr [ !Select [ 0, !GetAtt ExampleVpc.Ipv6CidrBlocks], 1, 64 ]]
      VpcId: !Ref ExampleVpc

Is there something wrong with my template, or is the CloudFormation Template Designer buggy?

Answer 1

You have to re-organize the template a bit. Designer syntax parser must have some bugs. This should work:

AWSTemplateFormatVersion: "2010-09-09"
Description: "Try to create VPC and Subnet using Cidr function"
Resources:
  ExampleVpc:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: "10.0.0.0/16"
  IPv6CidrBlock:
    Type: AWS::EC2::VPCCidrBlock
    Properties:
      AmazonProvidedIpv6CidrBlock: true
      VpcId: !Ref ExampleVpc
  ExampleSubnet:
    Type: AWS::EC2::Subnet
    DependsOn: IPv6CidrBlock
    Properties:
      AssignIpv6AddressOnCreation: true
      CidrBlock:
        !Select
            - 0
            - Fn::Cidr: [ !GetAtt ExampleVpc.CidrBlock, 1, 8 ]
      Ipv6CidrBlock:
        !Select
          - 0
          - Fn::Cidr: [ !Select [ 0, !GetAtt ExampleVpc.Ipv6CidrBlocks], 1, 64 ]
      VpcId: !Ref ExampleVpc