通过CloudFormation为路由表关联多个子网遇JSON错误

错误原因 #

AWS::EC2::SubnetRouteTableAssociation 资源不支持在单个实例中配置多个SubnetId，每个该类型资源只能关联一个子网与一个路由表——你尝试的两种格式都违背了这个属性定义，SubnetId字段仅接受单个字符串值，而非数组或嵌套结构，这是触发JSON错误的核心原因。

正确配置方式 #

方式1：为每个子网创建独立的关联资源

直接为每个需要关联的子网单独定义SubnetRouteTableAssociation资源，示例代码如下：

RouteTableAssociationOpsA:
  Type: AWS::EC2::SubnetRouteTableAssociation
  Properties:
    SubnetId: !ImportValue !Sub '${AWS::StackName}-OpsSubA'
    RouteTableId: !Ref PublicRouteTable

RouteTableAssociationProdA:
  Type: AWS::EC2::SubnetRouteTableAssociation
  Properties:
    SubnetId: !ImportValue !Sub '${AWS::StackName}-ProdSubA'
    RouteTableId: !Ref PublicRouteTable

RouteTableAssociationDemoA:
  Type: AWS::EC2::SubnetRouteTableAssociation
  Properties:
    SubnetId: !ImportValue !Sub '${AWS::StackName}-DemoSubA'
    RouteTableId: !Ref PublicRouteTable

RouteTableAssociationDevA:
  Type: AWS::EC2::SubnetRouteTableAssociation
  Properties:
    SubnetId: !ImportValue !Sub '${AWS::StackName}-DevSubA'
    RouteTableId: !Ref PublicRouteTable

方式2：使用批量创建语法（需模板版本≥2021-09-09）

如果子网数量较多，可以用CloudFormation的Fn::ForEach语法批量生成关联资源，简化代码：

AWSTemplateFormatVersion: "2021-09-09"
Resources:
  Fn::ForEach::SubnetAssociations:
    - SubnetName
    - [OpsSubA, ProdSubA, DemoSubA, DevSubA]
    - RouteTableAssociation${SubnetName}:
        Type: AWS::EC2::SubnetRouteTableAssociation
        Properties:
          SubnetId: !ImportValue !Sub '${AWS::StackName}-${SubnetName}'
          RouteTableId: !Ref PublicRouteTable

内容的提问来源于stack exchange，提问作者Michaey Tenorio