VPC Peering vs Transit Gateway
Two ways to connect AWS VPCs: VPC peering is a direct, non-transitive 1-to-1 link — simple and cheap for a few VPCs. A Transit Gateway is a routed hub every VPC attaches to — the scalable answer once you have many. The decision is mesh maths.
Side by side
| VPC Peering | Transit Gateway | |
|---|---|---|
| Topology | 1-to-1 between two VPCs | Hub-and-spoke, many attachments |
| Transitive? | No — A-B and B-C does not give A-C | Yes — everything via the hub |
| Scale | Full mesh grows as n(n-1)/2 | Thousands of VPCs/VPNs on one hub |
| Cost | Data transfer only | Per-attachment + data processing |
| Best for | A few VPCs | Many VPCs, hybrid, growth |
The non-transitive trap
The exam-and-real-life gotcha: peering is not transitive. If A peers with B and B peers with C, A still cannot reach C — you must peer A–C directly. Ten VPCs in a full mesh need 45 peerings, each with route-table entries. That administrative explosion is exactly what the Transit Gateway removes.
How the Transit Gateway works
A TGW is effectively a regional cloud router: every VPC (and site-to-site VPN or Direct Connect) attaches once, and TGW route tables decide who reaches whom — hub-and-spoke, just like the hierarchical designs you know on-prem. You pay per attachment-hour plus data processing, which is the price of escaping mesh management.
Choosing in practice
2–4 VPCs, stable: peering — simpler and cheaper. Growing environment, hybrid connectivity, shared services: Transit Gateway from the start; retrofitting a mesh later hurts. Foundations first: AWS networking basics.
Frequently asked questions
What is the difference between VPC peering and Transit Gateway?
Peering is a direct, non-transitive connection between exactly two VPCs. A Transit Gateway is a central routed hub that many VPCs and VPNs attach to, with transitive routing between them.
Why is VPC peering non-transitive?
AWS designed peering as a strict 1:1 relationship — traffic cannot flow through an intermediate VPC. Any two VPCs that need to talk must be directly peered.
When should I use a Transit Gateway?
When you have (or expect) many VPCs, need hybrid connectivity via VPN or Direct Connect through one hub, or want centralised route control instead of managing a peering mesh.
Which is cheaper, peering or Transit Gateway?
Peering — it has no hourly charge, only data transfer. TGW adds per-attachment-hour and data-processing fees, which buy you scale and manageability.
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