title: How to specify subnets when creating an ecs.LoadBalancedFargateService
body: <p>I am trying to create a Fargate ECS service on an existing VPC but get the following error after running <code>cdk deploy</code></p>
  
  <p><code>CREATE_FAILED        | AWS::ElasticLoadBalancingV2::LoadBalancer | exms-service/LB (exmsserviceLB259DA1C7) At least two subnets in two different Availability Zones must be specified (Service: AmazonElasticLoadBalancingV2; Status Code: 400; Error Code: ValidationError;</code></p>
  
  <p>The vpc is imported correctly and the ecs cluster creates successfully according to the cloudformation logs. the failure occurs when the load balanceer is being created</p>
  
  <pre><code>const vpc = ec2.VpcNetwork.import(this, "TB-DEV", {
        vpcId: 'vpc-xxxxxx',
        availabilityZones: ['eu-west-1G', 'eu-west-1b', 'eu-west-1c'],
        privateSubnetIds: ['subnet-xxxxxxx', 'subnet-xxxxx', 'subnet-xxxx', 'subnet-xxxxx', 'subnet-xxxxx', 'subnet-xxxxx']
  //this is a list of 1 private and 1 public subnet on each of the specified availability zones
      })
  
  const cluster = new ecs.Cluster(this, "TB-ECS-DEV", {
        clusterName: "TB-DEV",
        vpc: vpc,
  
      })
  
  const repo = ecr.Repository.import(this, 'EXMS-REPO', {
        repositoryName: "expense-type-mapper-dev"
      })
  
  new ecs.LoadBalancedFargateService(this, "EXMS", {
        cluster: cluster,
        image:ecs.ContainerImage.fromEcrRepository(repo),
      })
  
  </code></pre>
  
  <p>I expected the load balancer to make use of the subnets specified in the VPC definition but that does not seem to be happening.
  Do I need to define which subnets to use somewhere in the LoadBalancedFargateService definition?</p>