With CDK is it possible to modify the default subnets after they have been created?

Question

I've created a VPC like this

    vpc = new Vpc(theStack, vpcName,
        VpcProps.builder()
            .cidr("10.0.0.0/16")
            .build());

In eu-west-1, by default I get 3 public and 3 private subnets. The private subnets will have a NAT Gateway.

Now, we're trying to remove the NAT Gateways (because of cost), so I tried this

    vpc = new Vpc(theStack, vpcName,
        VpcProps.builder()
            .maxAzs(3)
            .cidr("10.0.0.0/16")
            .subnetConfiguration(List.of(
                SubnetConfiguration.builder()
                    .subnetType(SubnetType.PUBLIC)
                    .name("Public")
                    .cidrMask(24)
                    .build(),
                SubnetConfiguration.builder()
                    .subnetType(SubnetType.ISOLATED)
                    .name("Private")
                    .cidrMask(24)
                    .build()))
            .build());

Creating this in a fresh stack works fine. I get a VPC with the same subnets as before and no NAT GW:s. But, running this to modify the VPC created above, results in name clashes.

Is there some way I can get cdk/cloudformation to understand that I want to modify the existing private subnets and not create new ones?

score 1 · Accepted Answer · answered Sep 29 '21 at 06:25

I double-checked the subnets that where created without specifying the subnets. The cdir-mask was /19 not /24 like i entered in the second version.

So, changing the cidrMask to 19 works fine. Now CloudFormation doesn't create new subnets and deletes the NAT Gateways.

With CDK is it possible to modify the default subnets after they have been created?

1 Answers1