title: Passing parameters of type List<AWS::EC2::Subnet::Id> to nested CloudFormation template
body: <p>I'm trying to nest a CloudFormation template into another by using the <code>AWS::CloudFormation::Stack</code> resource type. The nested template has a parameter of type <code>List&lt;AWS::EC2::Subnet::Id&gt;</code>.</p>
  
  <p>Individually, the nested template runs just fine. But when embedding AWS Console says <em>Encountered unsupported property ELBSubnetList</em>.</p>
  
  <p>Changing the parameter's type to String/CommaSeparated list seems to be a workaround, as dicussed <a href="https://stackoverflow.com/questions/39770088/cloudformation-passing-a-listawsec2subnetid-parameter-as-a-comma-separa">here</a> but I'd loose the fancy UI when creating the template interactivly via AWS Console.</p>
  
  <p>Do you have any idea how to pass the list of subnet ids as a paramter?</p>
  
  <p>This is the embedded template:</p>
  
  <pre><code>{
      "AWSTemplateFormatVersion" : "2010-09-09",
      "Parameters" : {
          "ELBSubnetList" : {
              "Type" : "List&lt;AWS::EC2::Subnet::Id&gt;",
              "Description" : "Subnet List for Elastic Loadbalancer"
          },
          "ELBSecurityGroupList": {
              "Type": "List&lt;AWS::EC2::SecurityGroup::Id&gt;",
              "Description": "Security Group List for Elastic Loadbalancer"
          }
      },
      "Resources" : {
          "ELB" : {
              "Type" : "AWS::ElasticLoadBalancing::LoadBalancer",
              "Properties" : {
                  "Subnets": { "Ref": "ELBSubnetList" },
                  "CrossZone" : "true",
                  "SecurityGroups": { "Ref": "ELBSecurityGroupList" },
                  "LBCookieStickinessPolicy" : [ {
                      "PolicyName" : "CookieBasedPolicy",
                      "CookieExpirationPeriod" : "30"
                  }],
                  "Listeners" : [ {
                      "LoadBalancerPort" : "80",
                      "InstancePort" : "80",
                      "Protocol" : "HTTP",
                      "PolicyNames" : [ "CookieBasedPolicy" ]
                  } ],
                  "HealthCheck" : {
                      "Target" : "HTTP:80/wordpress/wp-admin/install.php",
                      "HealthyThreshold" : "2",
                      "UnhealthyThreshold" : "5",
                      "Interval" : "10",
                      "Timeout" : "5"
                  }
              }
          }
      }
  }
  </code></pre>
  
  <p>And the template that embedds:</p>
  
  <pre><code>{
      "AWSTemplateFormatVersion" : "2010-09-09",
      "Parameters": {
          "ChildTemplate": {
              "Type": "String",
              "Default": "https://s3.eu-central-1.amazonaws.com/cf-templates-xxxxxxxxxxx-eu-central-1/sample_child.template"
          },
          "ELBSubnetList" : {
              "Type" : "List&lt;AWS::EC2::Subnet::Id&gt;",
              "Description" : "Subnet List for Elastic Loadbalancer"
          },
          "ELBSecurityGroupList": {
              "Type": "List&lt;AWS::EC2::SecurityGroup::Id&gt;",
              "Description": "Security Group List for Elastic Loadbalancer"
          }
      },
      "Resources": {
          "Child": {
              "Type": "AWS::CloudFormation::Stack",
              "Properties": {
                  "TemplateURL": { "Ref": "ChildTemplate" },
                  "Parameters": {
                      "ELBSubnetList": { "Ref": "ELBSubnetList" },
                      "ELBSecurityGroupList": { "Ref": "ELBSecurityGroupList" }
                  }
              }
          }
      }
  }
  </code></pre>