Terraform Error: creating Route in Route Table with destination (192.168.0.0/24): InvalidParameterValue

Question

I'm currently carrying out a task where I need to connect two VPCs (lets call them VPC_1 and VPC_2) via a Transit Gateway. VPC_1's task is to forward any 0.0.0.0/0 traffic onto the TGW, to VPC_2, where it can then go through the appropriate NATs and IGWs to access the internet. VPC_2's task is to send any local (192.168.0.0/24) traffic hitting the subnet back to VPC_1.

However, I'm getting an error when running terraform apply, Error: creating Route in Route Table (vpc_2_subnet_route_table) with destination (192.168.0.0/24): InvalidParameterValue. Not sure if it's something to do with my configuration of both VPCs share the same cidr_block, but any help in the right direction would be apppreciated.

Error:

Error: creating Route in Route Table (rtb-0a9c85b8ae7bc2208) with destination (192.168.0.0/24): InvalidParameterValue: The destination CIDR block 192.168.0.0/24 is equal to or more specific than one of this VPC's CIDR blocks. This route can target only an interface or an instance.

Here's my routing tables currently configured:

VPC_1: (cidr_block 192.168.0.0/16) Private subnet (192.168.00.0/24)

resource "aws_route_table" "vpc_1_subnet_route_table" {
  vpc_id = aws_vpc.user_vpc.id

  # 0.0.0.0/0 -> TGWA
  route {
    cidr_block         = "0.0.0.0/0"
    transit_gateway_id = aws_ec2_transit_gateway.tgw.id
  }

  tags = {
    Name = "user-route-table"
  }
}

TGW routes attached to TGW-route-table: (Forwards 0.0.0.0/0 traffic coming from VPC_1 to VPC_2, and 192.168.0.0/24 traffic coming from VPC_2 to VPC_1)

resource "aws_ec2_transit_gateway_route" "tgw_egress_route" {

  # 0.0.0.0/0 -> Internet VPC TGWA
  destination_cidr_block         = "0.0.0.0/0"
  transit_gateway_attachment_id  = aws_ec2_transit_gateway_vpc_attachment.vpc_2_tgwa.id
  transit_gateway_route_table_id = aws_ec2_transit_gateway_route_table.tgw_route_table.id
}

resource "aws_ec2_transit_gateway_route" "tgw_ingress_route" {

  # 192.168.0.0/24 --> User VPC TGWA
  destination_cidr_block         = "192.168.0.0/24"
  transit_gateway_attachment_id  = aws_ec2_transit_gateway_vpc_attachment.vpc_1_tgwa.id
  transit_gateway_route_table_id = aws_ec2_transit_gateway_route_table.tgw_route_table.id
}

VPC_2: (cidr_block 192.168.0.0/16) Private subnet (192.168.100/24)

resource "aws_route_table" "vpc_2_subnet_route_table" {
  vpc_id = aws_vpc.internet_vpc.id

  # 0.0.0.0/0 -> NAT
  route {
    cidr_block     = "0.0.0.0/0"
    nat_gateway_id = aws_nat_gateway.nat_gateway.id
  }

  # 192.168.0.0/24 -> TGWA
  route {
   cidr_block         = "192.168.0.0/24"
   transit_gateway_id = aws_ec2_transit_gateway.tgw.id
  }

  tags = {
    Name = "internet-private-route-table"
  }
}

Here's a diagram of what I'm expected to build, all of the infrastructure is done besides this error with the routes.

Answer 1

Ah, there was a definitely issue with how I was declaring my VPC + Subnet blocks. Has been fixed by correctly assigning the cidr_blocks.