TGW: How to solve overlapping VPC CIDR problem

Question

I am trying to create a private network with a vendor using AWS TGW.

My Org

1. VPC-A: 
      CIDR: 10.50.0.0./16
          Subnets: 
            Subnet-1: 10.50.1.0./24
            Subnet-2: 10.50.2.0./24
2. VPC-B: 
      CIDR: 10.50.0.0/16
          Subnets: 
            Subnet-3: 10.50.3.0./24
            Subnet-4: 10.50.4.0./24

Vendor

1. VPC-C 
     CIDR 10.60.0.0/16

My Org: TGW with ARM TGW 1. TGW-Attachment for VPC-A, VPC-B, VPC-C

As you might have already observed, VPC A and VPC B have an overlapping CIDR and TGW doesn't like that.

What I want is:

1. Traffic to route between VPC-A and VPC-C
2. Traffic to route between VPC-B and VPC-C
3. Traffic is not required to route between VPC-A and VPC-B

Changing the CIDR is not an option.

What are my options?

Can I achieve the above by creating a new TGW? So That TGW-1-> VPC-A and VPC-C and TGW-2 -> VPC-B and VPC-C

Answer 1

for my understanding and if don't want to deal with NAT gateways, you can achieve your requirements by creating two separate Transit Gateways (TGWs), one for VPC-A and VPC-C, and another for VPC-B and VPC-C.

By creating two separate TGWs, you can avoid the overlapping CIDR range issue (as per Amazon Transit Gateway FAQs), and route traffic between VPC-A and VPC-C via TGW-1, and traffic between VPC-B and VPC-C via TGW-2.

You would need to create two separate TGWs, and attach VPC-A and VPC-C to TGW-1, and VPC-B and VPC-C to TGW-2. You can then create TGW peering between TGW-1 and TGW-2 to allow communication between VPC-C and both VPC-A and VPC-B.

Here are the high-level steps to achieve this:

• Create TGW-1: Create a new TGW and attach VPC-A and VPC-C to it.
• Create TGW-2: Create another new TGW and attach VPC-B and VPC-C to it.
• Create TGW Peering: Create TGW peering between TGW-1 and TGW-2 to enable communication between VPC-C and both VPC-A and VPC-B.
• Update Routing: Update the routing tables in VPC-A, VPC-B, and VPC-C to route traffic to the correct TGW.