What Is a Transit Gateway?
AWS Transit Gateway (TGW) is a network transit hub that connects Virtual Private Clouds (VPCs), VPN connections, and AWS Direct Connect attachments through a single gateway. It replaces the need to create a mesh of VPC peering connections by acting as a central router. Each attachment connects to the Transit Gateway, and route tables control how traffic flows between them.
This planner helps network engineers design Transit Gateway topologies before deploying them in production. You can define attachments by name, VPC CIDR range, type (VPC, VPN, Direct Connect, or Peering), account ID, and region. The tool detects overlapping CIDR ranges between attachments and generates route table configurations automatically.
A common use case is multi-account networking. When you have VPCs spread across multiple AWS accounts and regions, each with its own CIDR range, the risk of overlap is significant. This planner catches those collisions before you connect everything, preventing routing asymmetries and connectivity failures that can be difficult to troubleshoot in a live environment.
Transit Gateway supports up to 5000 attachments per gateway, 20 route tables per gateway, and 1000 routes per route table. Each attachment can be associated with a single route table and can propagate to multiple route tables. Understanding these limits and designing your topology accordingly is critical for large-scale AWS networking architectures.
The planner also assigns an ASN to each Transit Gateway (defaulting to 64512, within the private BGP ASN range). This ASN is used when the Transit Gateway establishes BGP sessions with VPN connections or Direct Connect virtual interfaces. You can customize the ASN per attachment to match your existing BGP peering strategy.
For each route table, the tool lists all routes with their destination CIDRs and associated attachments. This gives you a complete picture of the traffic flow before you write a single line of Terraform or click a button in the AWS Console. It is particularly useful for enterprises managing large-scale hybrid cloud deployments with multiple regions and connectivity back to on-premises data centers.
Use this tool alongside the Cloud IP Planner and VPC Peering Check for a complete AWS network design workflow. Together they cover VPC subnet allocation, cross-region peering validation, and Transit Gateway route table design.