aws_ec2: limitation with allocateSubnetsCidr method of AwsIpam class

[fradetjulien]

Describe the bug

I want to deploy the following ec2.Vpc with a CIDR allocation done with IPAM.

ec2.Vpc(
    self,
    "VPC",
    vpc_name=f"vpc-{uuid4()}",
    ip_addresses=ec2.IpAddresses.aws_ipam_allocation(
        ipv4_ipam_pool_id=ipam_pool_id,
        ipv4_netmask_length=16,
        default_subnet_ipv4_netmask_length=21,
    ),
    nat_gateway_provider=ec2.NatProvider.gateway(),
    nat_gateways=1,
    max_azs=3,
    subnet_configuration=[
        ec2.SubnetConfiguration(
            name="public1",
            subnet_type=ec2.SubnetType.PUBLIC,
            cidr_mask=24,
        ),
        ec2.SubnetConfiguration(
            name="private1",
            subnet_type=ec2.SubnetType.PRIVATE_WITH_EGRESS,
            cidr_mask=24,
        ),
        ec2.SubnetConfiguration(
            name="transit1",
            subnet_type=ec2.SubnetType.PRIVATE_ISOLATED,
            cidr_mask=28,
        ),
    ],
)

However, it is impossible for me to have a successful deployment if the ipv4_netmask_length is < 20.

I can successfully deploy the vpc using an ec2.IpAddresses.aws_ipam_allocation resource with an ipv4_netmask_length >= 20

It seems that the count variable computed here is greater than the maximum allowed value 256 defined here. Causing an error when calling the Fn.cidr method here.

This looks normal to me because :

const count = Math.pow(2, netmask - rootNetmask);
math.pow(2, 28 - 16) = 4096
math.pow(2, 24 - 16) = 256
math.pow(2, 28 - 20) = 256

In my case, I think I need to generate the following CIDRs :

- !Cidr 10.0.0.0/16, 256, 24
- !Cidr 10.0.0.0/16, 4096, 28

But the range limitation from the Fn.cidr method forbids it. I don't understand why..

When I use the ec2.IpAddresses.cidr resource, I have no problem because the allocateSubnetsCidr it is not using the Fn.cidr method. See here.

Expected Behavior

I expect the ec2.IpAddresses.aws_ipam_allocation resource to behave like the ec2.IpAddresses.cidr resource. It should not limit the count value to a maximum value of 256.

Current Behavior

I get the following error :

Error: Fn::Cidr's count attribute must be between 1 and 256, 4096 was provided.
      at new FnCidr (/private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/jsii-kernel-1CU08O/node_modules/aws-cdk-lib/core/lib/cfn-fn.js:618:19)
      at Fn.cidr (/private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/jsii-kernel-1CU08O/node_modules/aws-cdk-lib/core/lib/cfn-fn.js:166:37)
      at /private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/jsii-kernel-1CU08O/node_modules/aws-cdk-lib/aws-ec2/lib/ip-addresses.js:95:64
      at Array.map (<anonymous>)
      at AwsIpam.allocateSubnetsCidr (/private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/jsii-kernel-1CU08O/node_modules/aws-cdk-lib/aws-ec2/lib/ip-addresses.js:93:44)
      at Vpc.createSubnets (/private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/jsii-kernel-1CU08O/node_modules/aws-cdk-lib/aws-ec2/lib/vpc.js:674:55)
      at new Vpc (/private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/jsii-kernel-1CU08O/node_modules/aws-cdk-lib/aws-ec2/lib/vpc.js:570:14)
      at Kernel._create (/private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/tmpx0t0nvxv/lib/program.js:9964:29)
      at Kernel.create (/private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/tmpx0t0nvxv/lib/program.js:9693:29)
      at KernelHost.processRequest (/private/var/folders/s8/t9pzpf1s2b19qg675763jwgc0000gn/T/tmpx0t0nvxv/lib/program.js:11544:36)

Reproduction Steps

ec2.Vpc(
    self,
    "VPC",
    vpc_name=f"vpc-{uuid4()}",
    ip_addresses=ec2.IpAddresses.aws_ipam_allocation(
        ipv4_ipam_pool_id=ipam_pool_id,
        ipv4_netmask_length=16,
        default_subnet_ipv4_netmask_length=21,
    ),
    nat_gateway_provider=ec2.NatProvider.gateway(),
    nat_gateways=1,
    max_azs=3,
    subnet_configuration=[
        ec2.SubnetConfiguration(
            name="public1",
            subnet_type=ec2.SubnetType.PUBLIC,
            cidr_mask=24,
        ),
        ec2.SubnetConfiguration(
            name="private1",
            subnet_type=ec2.SubnetType.PRIVATE_WITH_EGRESS,
            cidr_mask=24,
        ),
        ec2.SubnetConfiguration(
            name="transit1",
            subnet_type=ec2.SubnetType.PRIVATE_ISOLATED,
            cidr_mask=28,
        ),
    ],
)

Possible Solution

Avoid using the Fn.cidr method inside the allocateSubnetsCidr method from the AwsIpam class.

Additional Information/Context

No response

CDK CLI Version

2.77.0 (build 06a0b19)

Framework Version

2.77.0

Node.js Version

v18.16.0

OS

Amazon Linux 2

Language

Python

Language Version

3.11.0

Other information

No response

[pahud]

Thanks for your report. We will look into this issue.

[rix0rrr]

Unfortunately this is a limitation of CloudFormation's Fn::Cidr function: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-cidr.html

There is not much we can do about this.

Given that from an IPAM allocation, we can't know the CIDR range at synth time, we can't precalculate the ranges at synth time either using the logic that just parses any CIDR. Instead, we have to defer the CIDR calculation to deployment time (where CloudFormation does it), which is causing this error.

You can report the issue to CloudFormation: https://github.com/aws-cloudformation/cloudformation-coverage-roadmap

Someone could also go and implement a patch to Fn.cidr() where we do a nested CIDR calculation if the we need to split more than 256 times:

// I think this is correct but it needs double checking, this math requires a lot of thinking
const split1 = Fn.cidr('10.0.0.0/16', 256, 24);
const split2 = Fn.cidr(Fn.select(0, split1), 16, 28);

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[jonahweissman]

@fradetjulien IPAM service team member here. The intended way to get an IPAM allocated cidr block for your VPC is to provide an Ipv4IpamPoolId in the VPC properties. The IPAM allocation resource should only be used for custom allocations.