Reference an existing AWS VPC Id in CloudFormation script when creating subnets

Question

How do you reference the VPC Id of an existing VPC (which has been created before in a separate CloudFormation script) in CloudFormation script in order to create subnets in the VPC?

Answer 1

In the template defining the VPC, include the VPC ID in the outputs section:

"Outputs" : {
    "VPC" : {
        "Value" : {"Ref":"VPC"},
        "Description" : "VPC ID"
    },
    ...
}

In the template for the stack using the VPC, define a parameter for the VPC ID:

"Parameters" : {
    "VPC" : {
        "Type" : "String",
    },
    ...
}

When creating this stack, call describe-stack on the VPC-defining stack to get the ID from outputs, and pass it as the VPC parameter to create-stack.

Answer 2

Or get vpc id from input, such as

 "VpcId" : {
      "Type" : "AWS::EC2::VPC::Id",
      "Description" : "VpcId of your existing Virtual Private Cloud (VPC)",
      "ConstraintDescription" : "must be the VPC Id of an existing Virtual Private Cloud."
    },