AWS Transitive routing with Transit Gateways in the same region

Gilles Chekroun
Lead VMware Cloud on AWS Specialist
At the AWS Re:invent 2019 conference, the long waited TGW peering functionality was announced and available in a few AWS regions. This is an INTER-REGION peering only meaning that the Transit Gateways need to be in different regions.
Hoping that AWS will soon release an INTRA-REGION capability I discussed with a few AWS Solutions Architects in Las Vegas, among them Tom Adamski, about using a VPC as a bridge between two TGWs in the same region. Tom assured me that it is possible opening for the first time the transitive routing capability in AWS networking.

Well . . . I needed to test that and see by myself.

Test bed

For a simple test, I will use 2 TGWs in the same regions with 2 VPCs attached each and another VPC as "bridge" connected to both TGWs. Yes you can connect a VPC up to 5 TGWs.
The whole idea is to use this "bridging VPC" and point the default route of the TGWs to it. To do that I create 2 subnets on the bridging VPC and adapt the route tables as described below.

Step 1 - Create 2 TGWs in the same region

If TGWs were in different regions, then TGW peering is available since early Dec 2019. But here we want to concentrate on TGWs in the same region.
Give separate ASN to both TGWs and keep default route propagation and association enabled.

Step 2 - Create VPCs and attach them respectively

For each VPC, I have 1 subnet in 1 AZ and an EC2 to do the networking tests. I also attach an IGW so I can SSH to my EC2 from outside.
CIDR will be 172.201, 202, 203, 204.0.0/16

Step 3 - Create "Bridging VPC"

The Bridging VPC will need 2 subnets and will be VPC attached to each TGW. No EC2. No NAT. just simple routing tables.

Step 4 - VPC Route Tables

For VPCs 1 and 2, all routes will point to TGW_A. For VPCs 3 and 4, all routes will point to TGW_B. Default  route points to IGW.

Step 5 - Bridging VPC Route Table

On the bridging VPC, the route table will point VPC1 and 2 to TGW_A and VPC3 and 4 to TGW_B

TGWs Route Table Associations and Propagations

This is really where we need to be careful. We should leave the TGW Route table association and propagation ON for VPC1,2,3,4 but DISABLE Route Table propagation for the Bridging VPC
And similarly for TGW_B

Add a Global Static Route to point to Bridging VPC

On both TGW_A and TGW_B we need to add a global static route that will point to the Bridging VPC.

Ping Tests

Ping from EC2 in VPC1 to EC2 in VPC3
Ping from EC2 in VPC4 to EC2 in VPC2

Re-deploy the test bed easily

I am a big fan of Terraform and did a complete setup of this test bed using AWS provider.
Complete Terraform code here. Please update the terraform.tfvars with your credentials.


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