route-tables.tf

# aws_vpc creates a default rtb. We can map the Internet Gateway so the vpc has internet.
resource "aws_default_route_table" "terraform-vpc-default-route-table" {
  default_route_table_id = aws_vpc.terraform-vpc.default_route_table_id

  route {
    # associated subnet can reach everywhere
    cidr_block = "0.0.0.0/0"
    # RTB uses this IGW to reach internet
    gateway_id = aws_internet_gateway.terraform-vpc-igw.id
  }

  route {
    # ipv6
    ipv6_cidr_block = "::/0"
    # RTB uses this IGW to reach internet
    gateway_id = aws_internet_gateway.terraform-vpc-igw.id
  }

  tags = {
    Name = "terraform-vpc-default-rtb"
  }
}

# Associate above RTB to VPC subnet. This use case uses one RTB for all subnets.
resource "aws_route_table_association" "vpc-rtb-public-ap-southeast-1-assoc" {
  count          = length(aws_subnet.terraform-vpc-subnet-public-ap-southeast-1.*.id)
  subnet_id      = element(aws_subnet.terraform-vpc-subnet-public-ap-southeast-1.*.id, count.index)
  route_table_id = aws_default_route_table.terraform-vpc-default-route-table.id
  depends_on     = [aws_subnet.terraform-vpc-subnet-public-ap-southeast-1]
}


# Route table for private subnet
resource "aws_route_table" "terraform-vpc-rtb-private-ap-southeast-1a" {
  vpc_id = aws_vpc.terraform-vpc.id

  # NAT instead of IGW for private subnets
  route {
    # associated subnet can reach everywhere
    cidr_block     = "0.0.0.0/0"
    nat_gateway_id = aws_nat_gateway.terraform-nat-public-ap-southeast-1a.id
  }

  # ipv6
  route {
    ipv6_cidr_block = "::/0"
    # RTB uses this EGW to reach internet
    egress_only_gateway_id = aws_egress_only_internet_gateway.terraform-vpc-egw.id
  }

  tags = {
    Name = "terraform-vpc-rtb-private-ap-southeast-1a"
  }
}

# Associate above RTB to VPC subnet. This use case uses one RTB for all subnets.
resource "aws_route_table_association" "vpc-rtb-private-ap-southeast-1-assoc" {
  count          = length(aws_subnet.terraform-vpc-subnet-private-ap-southeast-1.*.id)
  subnet_id      = element(aws_subnet.terraform-vpc-subnet-private-ap-southeast-1.*.id, count.index)
  route_table_id = aws_default_route_table.terraform-vpc-default-route-table.id
  depends_on     = [aws_subnet.terraform-vpc-subnet-private-ap-southeast-1]
}