Amazon provided infrastructure architects a transformative VPC component in its November 2018 introduction of the AWS Transit Gateway (TGW). The TGW can be used to simplify previously complex and management-intensive architectures such as those involving Transit VPCs. In this post, I’ll use terraform to create a full mesh of connectivity between four VPCs in a single region.

Wouldn’t using the TGW to implement an inter-region VPC full mesh be more useful? Definitely. At the time of writing, the TGW cannot be used to connect VPCs between regions. In Nick Matthews’s AWS Transit Gateway & Transit VPCs, Ref Arch for Many VPCs (NET402) talk at re:Invent 2018, he communicated that inter-region peering with the TGW is “coming soon.”

Let’s consider the following simply use case that I’ve constructed primarily to demonstrate the TGW automation. You’ve requested that AWS increase its default five VPCs per region limit to a large number. You want a full mesh of connectivity between VPCs. Network engineers familiar with internal BGP with recognize that this requires N(N-1)/2 VPC peerings.

The following diagram depicts a simplified four VPC full mesh using VPC peering.