AWS Transit Gateway

AWS Transit Gateway is a highly available and scalable service that provides interconnectivity between VPCs and your on-premises network. Within a Region, AWS Transit Gateway provides a method for consolidating and centrally managing routing between VPCs with a hub-and-spoke network architecture.

Between Regions, AWS Transit Gateway supports inter-regional peering with other transit gateways. It does this to facilitate routing network traffic between VPCs of different Regions over the AWS global backbone. This removes the need to route traffic over the internet. AWS Transit Gateway also integrates with hybrid network configurations when a Direct Connect or AWS Site-to-Site VPN connection is connected to the transit gateway.

AWS Transit Gateway concepts

Attachments

AWS Transit Gateway supports the following connections: 

• One or more VPCs
• A compatible Software-Defined Wide Area Network (SD-WAN) appliance
• A Direct Connect gateway
• A peering connection with another transit gateway
• A VPN connection to a transit gateway

AWS Transit Gateway MTU

AWS Transit Gateway supports an MTU of 8,500 bytes for:

• VPC connections
• Direct Connect connections
• Connections to other transit gateways
• Peering connections

AWS Transit Gateway supports an MTU of 1,500 bytes for VPN connections.

AWS Transit Gateway route table

A transit gateway has a default route table and can optionally have additional route tables. A route table includes dynamic and static routes that decide the next hop based on the destination IP address of the packet. The target of these routes can be any transit gateway attachment. 

Associations

Each attachment is associated with exactly one route table. Each route table can be associated with zero to many attachments.

Route propagation

A VPC, VPN connection, or Direct Connect gateway can dynamically propagate routes to a transit gateway route table. With a Direct Connect attachment, the routes are propagated to a transit gateway route table by default.

With a VPC, you must create static routes to send traffic to the transit gateway.

With a VPN connection or a Direct Connect gateway, routes are propagated from the transit gateway to your on-premises router using BGP.

With a peering attachment, you must create a static route in the transit gateway route table to point to the peering attachment.

AWS Transit Gateway inter-regional peering

AWS offers two types of peering connections for routing traffic between VPCs in different Regions: VPC peering and transit gateway peering. Both peering types are one-to-one, but transit gateway peering connections have a simpler network design and more consolidated management. 

Suppose a customer has multiple VPCs in three different Regions. As the following diagram illustrates, to permit network traffic to route between each VPC requires creating 72 VPC peering connections. Each VPC needs 8 different routing configurations and security policies. 

With AWS Transit Gateway, the same environment only needs three peering connections. The transit gateway in each Region facilitates routing network traffic to all the VPCs in its Region. Because all routing can be managed by the transit gateway, the customer only needs to maintain three routing configurations, simplifying management.

Cheers

Osama