Visual overview
Transit Gateway centralizes attachments and routing when point-to-point VPC relationships become a mesh.
Technical reference
Topology determines whether direct links or a transit hub remain manageable.
Connects two VPCs privately; matching or overlapping CIDRs are not allowed.
A↔B and B↔C do not create A↔C reachability.
VPCs, VPNs and supported hybrid connections attach to a central transit router.
Transit gateway route tables control attachment reachability and can create isolated routing domains.
VPC peering is a direct relationship
A VPC peering connection privately connects two VPCs so resources can communicate using private IP addresses as if they were part of a larger routed network. Peering can connect VPCs in the same or different AWS accounts and can span Regions. The VPC CIDR blocks cannot overlap, and both sides need route-table entries that send the peer's address range to the peering connection together with appropriate security authorization.
Peering is not transitive. If VPC A peers with B and B peers with C, A does not automatically gain a network path to C through B. This property keeps the relationship conceptually direct but creates a mesh-management problem as the number of interconnected VPCs grows. Treat every required pair as an explicit connectivity and routing decision.
Transit Gateway centralizes many network attachments
AWS Transit Gateway acts as a Regional network transit hub for connecting VPCs and supported hybrid network attachments. Instead of creating a full mesh of peering links, VPCs attach to the transit gateway. Transit gateway route tables then determine which attachment receives traffic for a destination, and route propagation can learn supported routes from attached VPCs, VPN connections and Direct Connect gateways.
The hub model makes connectivity easier to govern at scale and can intentionally segment networks. Attachments associated with one transit gateway route table can be isolated from attachments in another while still reaching a shared-services attachment when routes are designed for it. This is not 'automatic connectivity to everything': attachment association, route propagation/static routes and the VPC-side route tables all need to line up.
Choose by topology, scale and control
A small number of VPCs that need simple direct connectivity may fit peering well. A larger environment with many VPCs, centralized egress, inspection, VPN or Direct Connect integration often benefits from a transit hub. Transit Gateway introduces its own attachment and data-processing cost model, so architecture should balance operational simplicity and centralized control against the cost and requirements of a smaller peering design.
CIDR planning remains foundational in either case. Overlap blocks VPC peering and complicates routed connectivity more broadly. Organizations that expect multi-account or hybrid growth should treat IP address management as shared infrastructure rather than allowing every workload team to choose overlapping private ranges independently.
Key takeaways
- 01
VPC peering directly connects two non-overlapping VPC networks and does not provide transitive routing.
- 02
Transit Gateway provides a hub-and-spoke model using attachments and transit gateway route tables.
- 03
Transit Gateway route propagation and static routes control reachability; attachments do not imply unrestricted connectivity.
- 04
Topology, governance, hybrid requirements and cost determine whether peering or a transit hub is the better fit.
Official AWS sources
Use these primary AWS resources for the source material behind this article and for deeper reference.