05. Routing

Glossary

Routing: Routing is the action of applying routing rules to your network, in this case, to your VPC.
Routing rule: Resources follow the routing rule, which defines what resource has access to communicate with another resource. It blocks traffic from resources that do not follow the routing rule.

#### Route Tables

We create RouteTables for VPCs so that we can add Routes that we later associate with Subnets. The following is the syntax used to define a RouteTable:

Type: AWS::EC2::RouteTable
Properties: 
  Tags: 
    - Tag
  VpcId: String

The only required property for setting up a RouteTable is the VpcId. Here is an example table from the video lesson:

PublicRouteTable:
        Type: AWS::EC2::RouteTable
        Properties: 
            VpcId: !Ref VPC
            Tags: 
                - Key: Name 
                  Value: !Sub ${EnvironmentName} Public Routes

#### Routes

The following is the syntax used to set up our Route:

Type: AWS::EC2::Route
Properties: 
  DestinationCidrBlock: String
  DestinationIpv6CidrBlock: String
  EgressOnlyInternetGatewayId: String
  GatewayId: String
  InstanceId: String
  NatGatewayId: String
  NetworkInterfaceId: String
  RouteTableId: String
  VpcPeeringConnectionId: String

The DestinationCidrBlock property is used for destination matching and a wildcard address (0.0.0/0) to reference all traffic. So in the following example, when we use the wildcard address 0.0.0.0/0, we are saying for any address that comes through this route, send it to the referenced GatewayId:

DefaultPublicRoute: 
        Type: AWS::EC2::Route
        DependsOn: InternetGatewayAttachment
        Properties: 
            RouteTableId: !Ref PublicRouteTable
            DestinationCidrBlock: 0.0.0.0/0
            GatewayId: !Ref InternetGateway

#### SubnetRouteTableAssociation

In order to associate Subnets with our Route Table we will need to use a SubnetRouteTableAssociation. SubnetRouteTableAssociations are defined using the following syntax:

Type: AWS::EC2::SubnetRouteTableAssociation
Properties: 
  RouteTableId: String
  SubnetId: String

This only takes two properties, which are the id's used for our RouteTable and our Subnet. You can see references used in the example from our video lesson above:

PublicSubnet1RouteTableAssociation:
        Type: AWS::EC2::SubnetRouteTableAssociation
        Properties:
            RouteTableId: !Ref PublicRouteTable
            SubnetId: !Ref PublicSubnet1

Important Note: Routes should be defined starting with the most specific rule and transitioning to the least specific rule.

PublicRouteTable:
        Type: AWS::EC2::RouteTable
        Properties: 
            VpcId: !Ref VPC
            Tags: 
                - Key: Name 
                  Value: !Sub ${EnvironmentName} Public Routes

    DefaultPublicRoute: 
        Type: AWS::EC2::Route
        DependsOn: InternetGatewayAttachment
        Properties: 
            RouteTableId: !Ref PublicRouteTable
            DestinationCidrBlock: 0.0.0.0/0
            GatewayId: !Ref InternetGateway

    PublicSubnet1RouteTableAssociation:
        Type: AWS::EC2::SubnetRouteTableAssociation
        Properties:
            RouteTableId: !Ref PublicRouteTable
            SubnetId: !Ref PublicSubnet1

    PublicSubnet2RouteTableAssociation:
        Type: AWS::EC2::SubnetRouteTableAssociation
        Properties:
            RouteTableId: !Ref PublicRouteTable
            SubnetId: !Ref PublicSubnet2


    PrivateRouteTable1:
        Type: AWS::EC2::RouteTable
        Properties: 
            VpcId: !Ref VPC
            Tags: 
                - Key: Name 
                  Value: !Sub ${EnvironmentName} Private Routes (AZ1)

    DefaultPrivateRoute1:
        Type: AWS::EC2::Route
        Properties:
            RouteTableId: !Ref PrivateRouteTable1
            DestinationCidrBlock: 0.0.0.0/0
            NatGatewayId: !Ref NatGateway1

    PrivateSubnet1RouteTableAssociation:
        Type: AWS::EC2::SubnetRouteTableAssociation
        Properties:
            RouteTableId: !Ref PrivateRouteTable1
            SubnetId: !Ref PrivateSubnet1

    PrivateRouteTable2:
        Type: AWS::EC2::RouteTable
        Properties: 
            VpcId: !Ref VPC
            Tags: 
                - Key: Name 
                  Value: !Sub ${EnvironmentName} Private Routes (AZ2)

    DefaultPrivateRoute2:
        Type: AWS::EC2::Route
        Properties:
            RouteTableId: !Ref PrivateRouteTable2
            DestinationCidrBlock: 0.0.0.0/0
            NatGatewayId: !Ref NatGateway2

    PrivateSubnet2RouteTableAssociation:
        Type: AWS::EC2::SubnetRouteTableAssociation
        Properties:
            RouteTableId: !Ref PrivateRouteTable2
            SubnetId: !Ref PrivateSubnet2

#### Resources

• Route Tables Overview
• Route Table Documentation
• Route Documentation
• Subnet Route Table Association Documentation