Q: How do I control which Amazon VPCs can communicate with each other?

A: You can segment your network by creating multiple route tables in an AWS Transit Gateway and associate Amazon VPCs. This will allow you to create isolated networks inside an AWS Transit Gateway similar to virtual routing and forwarding (VRFs) in traditional networks. The AWS Transit Gateway will have a default route table. The use of multiple route tables is optional.

Q: How does routing work in AWS Transit Gateway?

A: AWS Transit Gateway supports dynamic and static routing between attached Amazon VPCs. By default, Amazon VPCs are associated to the default route table. You can create additional route tables and associate Amazon VPCs with it.
The routes decide the next hop depending on the destination IP address of the packet. Routes can point to an Amazon VPC.

Q: How do routes get propagated into the AWS Transit Gateway?

A: Routes get propagated in the AWS Transit Gateway to/from Amazon VPCs: When you attach an Amazon VPC to an AWS Transit Gateway or resize an attached Amazon VPC, the Amazon VPC Classless Inter-Domain Routing (CIDR) will propagate into the AWS Transit Gateway route table using internal APIs (not BGP). CIDR is a method for allocating IP addresses and IP routing to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses. Routes in the AWS Transit Gateway route table will not be propagated to the Amazon VPC’s route table. Amazon VPC owner need to create static route to send Traffic to the AWS Transit Gateway.

Q: Can I connect Amazon VPCs with overlapping CIDRs?

A: AWS Transit Gateway doesn’t support routing between Amazon VPCs with overlapping CIDRs. If you attach a new Amazon VPC that has a CIDR which overlaps with an already attached Amazon VPC, AWS Transit Gateway will not propagate the new Amazon VPC route into the AWS Transit Gateway route table.

Performance and limits

Q: What are the service limits that I need to keep in mind while using AWS Transit Gateways?

A: The table below list the different service limits:

Limit Default
Number of AWS Transit Gateway attachments
Maximum bandwidth (burst) per VPC connection 50 Gbps
Number of AWS Transit Gateways per account
Number of AWS Transit Gateway attachments per VPC
Number of routes 10,000

Feature interoperability

Q: Does AWS Transit Gateway support IPv6?

A: Yes, AWS Transit Gateway supports attaching Amazon VPCs with IPv6 CIDRs.

Q: Which Amazon VPC features are not supported in the first release?

A: Security Group Referencing on Amazon VPC is not supported at launch. Spoke Amazon VPCs cannot reference security groups in other spokes connected to the same AWS Transit Gateway.

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Simple pricing based on the amount of data.

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