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?

Acupuncture

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.

Eades

basically right - but the "Type" can't be simply string for a VPC, it must be "AWS::EC2::VPC::Id", like here documented: docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/… – Ummersen

String should work fine (or at least did at the time when I wrote this code), but using a more specific type is better. Thanks. – Eades

at least in the latest awscli i'm using right now it will fail with a validation error on a template not using this specific type ;) – Ummersen

So if you do this and then delete this stack will it delete that VPC? That would be bad since it wasn't created by this stack... – Correction

If you delete the stack creating the VPC, the VPC will be deleted. If you delete a stack just referencing the VPC it will not. – Eades

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."
    },

Prissie

Answer 3

Reference it by name ie. "VpcId" : { "Ref" : "myVPC" }, In something like:

    {
   "Type" : "AWS::EC2::Subnet",
   "Properties" : {
      "AvailabilityZone" : String,
      "CidrBlock" : String,
      "Tags" : [ Resource Tag, ... ],
      "VpcId" : { "Ref" : String }
      }
    }  

Documentation here: http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet.html

Proximity

It looks like I need to have a Parameter or a Mapping and then hard-code the VPC Id and then reference it in the subnet script unless the VPC and Subnet all are created in the same script for me to be able to reference the VPC Id using "VpcId" : { "Ref" : "myVPC" }. – Acupuncture

If you already have a VPC it will have an Id simply put that in the ref. ie "VpcId" : {"Ref": "vpc-123456"} – Proximity

Doesn't work: Template validation error: Template format error: Unresolved resource dependencies ... – Raffinate

@benniej 's suggestion also does not work for me. Will have to try the param, and mapping option – Ledoux