AWS Transit Gateway is a service that acts as a central router for connecting multiple Amazon Virtual Private Clouds (VPCs), on-premises networks, and remote sites through a single gateway. It enables centralized network management, consistent routing policies, and simplified connectivity across complex hybrid environments.
Transit Gateway replaces the need for manual VPC peering setups between individual VPCs. Instead of creating dozens of peerings, you attach VPCs and on-premises networks to a single Transit Gateway resource. This reduces configuration complexity and eliminates the limitations of VPC peering, such as the 50 VPC limit per account and the inability to route traffic between non-directly connected VPCs.
For example, a company with five VPCs-development, staging, production, analytics, and security-can connect all of them to one Transit Gateway. Traffic between development and analytics flows through the gateway, not via direct peering. This setup simplifies troubleshooting and enforces uniform security and routing rules across all connections.
Transit Gateway operates as a regional service. You create one Transit Gateway in a specific AWS Region, then attach VPCs and VPN connections to it. Each attachment creates a dedicated path for traffic, which the gateway routes based on route tables you define.
When a VPC is attached, AWS automatically assigns a unique route table ID. You configure static or changing routes within that table. For example, you can set a route to send all traffic destined for 10.0.0.0/16 to a specific VPC attachment. You can also use BGP to dynamically exchange routes with on-premises networks using AWS Site-to-Site VPN or Direct Connect.
Transit Gateway supports cross-account and cross-region connectivity. With Transit Gateway Peering, you can link gateways in different AWS Regions. This allows a VPC in Oregon to communicate with a VPC in Ireland through a shared Transit Gateway in each region, connected via peering. This capability is essential for global applications requiring low-latency access across regions.
Transit Gateway supports several attachment types, each serving a distinct purpose:
| Type | Use Case | Connection Method | Limitations |
|---|---|---|---|
| VPC Attachment | Connects VPCs to the gateway | Direct attachment via AWS console or API | Max 50 attachments per gateway |
| VPN Attachment | Links on-premises networks via Site-to-Site VPN | IPsec tunnel with customer gateway | Requires public IP on customer gateway |
| Direct Connect Attachment | Integrates with AWS Direct Connect | Dedicated physical connection | Higher cost, lower latency |
| Peering Attachment | Connects Transit Gateways across regions | Inter-region peering | Not available in all regions |
Each attachment type uses a different routing mechanism. VPC attachments rely on route propagation and static routes. VPN attachments use BGP to advertise routes dynamically. Direct Connect attachments support both static and changing routing. Peering attachments require explicit route propagation between gateways.
For instance, a financial services firm with offices in New York, London, and Tokyo uses Direct Connect for high-throughput, low-latency access to their AWS environment. They attach each office's on-premises network to a Transit Gateway via Direct Connect. The gateway routes traffic between the offices and their respective VPCs-such as a compliance VPC in Frankfurt and a data warehouse in Virginia-using BGP-advertised routes.
Many users confuse Transit Gateway with VPC peering. While both enable inter-VPC communication, VPC peering requires a direct, one-to-one connection between two VPCs. Creating peering relationships for ten VPCs requires 45 individual peering connections. Transit Gateway avoids this by providing a hub-and-spoke model with a single gateway.
Another misconception is that Transit Gateway replaces all forms of VPC peering. That's incorrect. VPC peering still has valid use cases-like low-latency, secure communication between two VPCs in the same region where you don't want to route through a central gateway. However, for large-scale or multi-account architectures, Transit Gateway is far more efficient.
A less obvious distinction: Transit Gateway does not provide encryption by default. Data traversing between VPCs or from on-premises to AWS travels unencrypted unless you use encrypted tunnels (e.g., IPsec for Site-to-Site VPN or TLS for application-layer encryption). Always verify encryption settings when handling sensitive data.
I deployed a Transit Gateway for a healthcare provider managing patient records across three AWS accounts: one for clinical systems, one for research, and one for compliance. Each account had multiple VPCs. Without Transit Gateway, they'd need hundreds of VPC peering connections and complex route tables.
Instead, I created a single Transit Gateway in the central account. I attached VPCs from all three accounts using cross-account permissions. I configured route tables to allow only approved traffic-research VPCs could access anonymized datasets but not live patient data. On-premises clinics connected via Site-to-Site VPN, with routes advertised through BGP.
The result was a single control plane for routing, centralized logging via CloudWatch, and audit-ready change tracking. Traffic flow became predictable. When a new VPC was added, I simply attached it to the gateway and updated the route table-no need to reconfigure peering relationships across accounts.
According to AWS documentation, Transit Gateway supports up to 50 VPC attachments and 50 VPN attachments per gateway. These limits are per gateway, not per account. If you exceed them, you must create additional gateways or use Transit Gateway peering to scale across regions.
Transit Gateway is not a replacement for all networking patterns. It excels in centralized, multi-account, hybrid cloud scenarios. For small teams with two or three VPCs, VPC peering may still be simpler. But for enterprises scaling beyond a few VPCs, Transit Gateway delivers a scalable, manageable, and secure foundation.