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Unable to use deploy-time parameters for privateSubnetIds for Vpc.fromVpcAttributes

See original GitHub issue

Unable to pass CommaDelimitedList CfnParameter to VPC.fromVpcAttributes privateSubnetIds

Reproduction Steps

When trying to use existing VPC for a lambda function

const vpcSubnetId = new CfnParameter(this, 'VPCSubnetId', {
            description: 'VPC Subnet Id',
            type: 'CommaDelimitedList',
const vpc = Vpc.fromVpcAttributes(this, 'GeoVPC', {
       vpcId: vpcId.valueAsString,
       availabilityZones: ['eu-west-2a', 'eu-west-2b', 'eu-west-2c'],
       privateSubnetIds: vpcSubnetId.valueAsList,

When runing npn run build npx synth for a stack with the above in it it will declare the following error:

Number of privateSubnetIds (1) must be a multiple of availability zones (3).


  • **CLI Version 😗*1.8.0
  • **Framework Version:**1.8.0
  • **OS 😗*MacOs 10.14.6
  • **Language 😗*typescript

Issue Analytics

  • State:open
  • Created 4 years ago
  • Reactions:1
  • Comments:13 (5 by maintainers)

github_iconTop GitHub Comments

wchawscommented, Jan 31, 2021

Here is another nice workaround

const vpcIdParam = new CfnParameter(this, 'YourVpcId', { type: 'AWS::EC2::VPC::Id' });
const pubSubnetsParam = new CfnParameter(this, 'PubSubnets', { type: 'List<AWS::EC2::Subnet::Id>' });
const privSubnetsParam = new CfnParameter(this, 'PrivSubnets', { type: 'List<AWS::EC2::Subnet::Id>' });

const azs = ['eu-west-2a', 'eu-west-2b', 'eu-west-2c'];

const vpc = ec2.Vpc.fromVpcAttributes(this, 'VpcAttr', {
  vpcId: vpcIdParam.valueAsString,
  vpcCidrBlock: Aws.NO_VALUE,
  availabilityZones: azs,
  publicSubnetIds:, index) =>, pubSubnetsParam.valueAsList)),
  privateSubnetIds:, index) =>, privSubnetsParam.valueAsList)),
eruvanoscommented, Jan 20, 2021


we faced this issue multiple times, as a workaround we wrap the subnetIds with select statements:

const vpcSubnetIdsRef = Token.asList(Fn.Split(",", Fn.importValue('VPCSubnetId')))
// or
// const vpcSubnetIdsRef = new CfnParameter(this, 'VPCSubnetId', {
//            description: 'VPC Subnet Id',
//            type: 'CommaDelimitedList',
// });

const azs = ['eu-west-2a', 'eu-west-2b', 'eu-west-2c']

const subnetIds = [];
azs.forEach((_, index) => {
        subnetIds.push(, vpcSubnetIdsRef))

const vpc = Vpc.fromVpcAttributes(this, 'GeoVPC', {
       vpcId: vpcId.valueAsString,
       availabilityZones: azs,
       privateSubnetIds: subnetIds,

This will create a list with three tokens, which will be resolved in cfn later.

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