Cloudformation: VPC Routing table with No Route for Internet Gateway

Question

I am creating a whole stack using CloudFormation. I have noticed that even though I do have a routing rule for 0.0.0.0/0 to access an internet gateway in my cloud formation template, it is not being created.

VPC:

"vpc": {
  "Type": "AWS::EC2::VPC",
  "Properties": {
    "CidrBlock": "172.31.0.0/16",
    "InstanceTenancy": "default",
    "EnableDnsSupport": "true",
    "EnableDnsHostnames": "true",
    "Tags": [
      {
        "Key": "Environment",
        "Value": {
          "Ref": "Env"
        }
      }
    ]
  }

Routing table:

"rtb": {
  "Type": "AWS::EC2::RouteTable",
  "Properties": {
    "VpcId": {
      "Ref": "vpc"
    }
  },
  "Metadata": {
    "AWS::CloudFormation::Designer": {
      "id": "65297cdc-8bcd-482d-af40-b0fef849b8c2"
    }
  }
}

VPCGatewayAttachment:

"gw1": {
  "Type": "AWS::EC2::VPCGatewayAttachment",
  "Properties": {
    "VpcId": {
      "Ref": "vpc"
    },
    "InternetGatewayId": {
      "Ref": "ig"
    }
  },
  "Metadata": {
    "AWS::CloudFormation::Designer": {
      "id": "aa69d6c0-3b11-43be-a8c1-7e79176f8c89"
    }
  }
}

Route:

"route1": {
  "Type": "AWS::EC2::Route",
  "Properties": {
    "DestinationCidrBlock": "0.0.0.0/0",
    "RouteTableId": {
      "Ref": "rtb"
    },
    "GatewayId": {
      "Ref": "ig"
    }
  },
  "DependsOn": "gw1",
  "Metadata": {
    "AWS::CloudFormation::Designer": {
      "id": "a68dd12e-3c14-4fa9-ba36-e0046374a0e9"
    }
  }
}

Internet Gateway:

"ig": {
  "Type": "AWS::EC2::InternetGateway",
  "Properties": {},
  "Metadata": {
    "AWS::CloudFormation::Designer": {
      "id": "9f9b4ce3-b994-43ff-9155-04aeb7ab2edf"
    }
  }
}

All of the items are being created, except the IG routing rule for the VPC. There are no errors in the cloudformation stack creation.

The routing table:

Destination: 172.31.0.0/16
Target: local

Expected routing table:

Destination: 172.31.0.0/16
Target: local
Destination: 0.0.0.0/0
Target: igw-********

Note that I can add the rule by myself directly after cloudformation stack creation.

Is there something I am missing?

Answer 1

After contacting AWS support, it turned out that each VPC creates a routing table automatically and it is set by default for all of its subnets. The solution to that would be to use a SubnetRouteTableAssociation to associate my new route table with each subnet.

    "subnet0RTA": {
      "Type" : "AWS::EC2::SubnetRouteTableAssociation",
      "Properties" : {
        "RouteTableId" : {"Ref" : "rtb"},
        "SubnetId" : {"Ref" : "subnet0"}
      }
    },
    "subnet1RTA": {
      "Type" : "AWS::EC2::SubnetRouteTableAssociation",
      "Properties" : {
        "RouteTableId" : {"Ref" : "rtb"},
        "SubnetId" : {"Ref" : "subnet1"}
      }
    },