如何在CloudFormation中获取用户选择的VPC的CIDR块？

Question

I am asking the user to choose an existing VPC:

Parameters:
  MyVPC:
    Type: AWS::EC2::VPC::Id

Then in my 'Resources' block I am trying to get the CIDR block of that VPC.
When I try this:

      - Fn::GetAtt:
        - MyVPC
        - CidrBlock

I get:

Template error: instance of Fn::GetAtt references undefined resource MyVPC

Which I understand I guess because technically MyVPC is a parameter not a resource.

So how can I get the CIDR block of the VPC selected by the user?

Answer 1

I don't think it is possible.

The Parameter is merely a VPC ID , not the VPC itself.

The GetAtt command is expecting a Resource in the template, which is not there.

You would either need to add the CIDR block as another parameter, or create a Lambda-backed Custom Resource that would fetch the information and make it available to the template.