Example: Appliance in a shared services VPC - Amazon VPC

Example: Appliance in a shared services VPC

You can configure an appliance (such as a security appliance) in a shared services VPC. All traffic that's routed between transit gateway attachments is first inspected by the appliance in the shared services VPC. When appliance mode is enabled, a transit gateway selects a single network interface in the appliance VPC, using a flow hash algorithm, to send traffic to for the life of the flow. The transit gateway uses the same network interface for the return traffic. This ensures that bidirectional traffic is routed symmetrically—it's routed through the same Availability Zone in the VPC attachment for the life of the flow. If you have multiple transit gateways in your architecture, each transit gateway maintains its own session affinity, and each transit gateway can select a different network interface.

You must connect exactly one transit gateway to the appliance VPC to guarantee flow stickiness. Connecting multiple transit gateways to a single appliance VPC does not guarantee flow stickiness because the transit gateways do not share flow state information with each other.

  • Traffic in appliance mode is routed correctly as long as the source and destination traffic are coming to a centralized VPC (Inspection VPC) from the same transit gateway attachment. Traffic can drop if the source and destination are on two different transit gateway attachments. Traffic can drop if the centralized VPC receives the traffic from a different gateway — for example, an Internet gateway — and then sends that traffic to the transit gateway attachment after inspection.

  • Enabling appliance mode on an existing attachment might affect that attachment's current route as the attachment can flow through any Availability Zone. When appliance mode is not enabled, traffic is kept to the originating Availability Zone.


The following diagram shows the key components of the configuration for this scenario. The transit gateway has three VPC attachments. VPC C is a shared services VPC. Traffic between VPC A and VPC B is routed to the transit gateway, then routed to a security appliance in VPC C for inspection before it's routed to the final destination. The appliance is a stateful appliance, therefore both the request and response traffic is inspected. For high availability, there is an appliance in each Availability Zone in VPC C.

An appliance in a shared services VPC

You create the following resources for this scenario:

  • Three VPCs. For information about creating a VPC, see Creating a VPC in the Amazon Virtual Private Cloud User Guide.

  • A transit gateway. For more information, see Create a transit gateway.

  • Three VPC attachments - one for each of the VPCs. For more information, see Create a transit gateway attachment to a VPC.

    For each VPC attachment, specify a subnet in each Availability Zone. For the shared services VPC, these are the subnets where traffic is routed to the VPC from the transit gateway. In the preceding example, these are subnets A and C.

    For the VPC attachment for VPC C, enable appliance mode support so that response traffic is routed to the same Availability Zone in VPC C as the source traffic.

    The Amazon VPC console supports appliance mode. You can also use the Amazon VPC API, an AWS SDK, the AWS CLI to enable appliance mode, or AWS CloudFormation. For example, add --options ApplianceModeSupport=enable to the create-transit-gateway-vpc-attachment or modify-transit-gateway-vpc-attachment command.


Flow stickiness in appliance mode is guaranteed only for source and destination traffic that originate towards the Inspection VPC.

Stateful appliances and appliance mode

If your VPC attachments span multiple Availability Zones and you require traffic between source and destination hosts to be routed through the same appliance for stateful inspection, enable appliance mode support for the VPC attachment in which the appliance is located.

For more information, see Centralized inspection architecture in the AWS blog.

Behavior when appliance mode is not enabled

When appliance mode is not enabled, a transit gateway attempts to keep traffic routed between VPC attachments in the originating Availability Zone until it reaches its destination. Traffic crosses Availability Zones between attachments only if there is an Availability Zone failure or if there are no subnets associated with a VPC attachment in that Availability Zone.

The following diagram shows a traffic flow when appliance mode support is not enabled. The response traffic that originates from Availability Zone 2 in VPC B is routed by the transit gateway to the same Availability Zone in VPC C. The traffic is therefore dropped, because the appliance in Availability Zone 2 is not aware of the original request from the source in VPC A.

Dropped response traffic to an appliance


Each VPC has one or more route tables and the transit gateway has two route tables.

VPC route tables


VPCs A and B have route tables with 2 entries. The first entry is the default entry for local IPv4 routing in the VPC. This default entry enables the resources in this VPC to communicate with each other. The second entry routes all other IPv4 subnet traffic to the transit gateway. The following is the route table for VPC A.

Destination Target




The shared services VPC (VPC C) has different route tables for each subnet. Subnet A is used by the transit gateway (you specify this subnet when you create the VPC attachment). The route table for subnet A routes all traffic to the appliance in subnet B.

Destination Target



The route table for subnet B (which contains the appliance) routes the traffic back to the transit gateway.

Destination Target



Transit gateway route tables

This transit gateway uses one route table for VPC A and VPC B, and one route table for the shared services VPC (VPC C).

The VPC A and VPC B attachments are associated with the following route table. The route table routes all traffic to VPC C.

Destination Target Route type

Attachment ID for VPC C


The VPC C attachment is associated with the following route table. It routes traffic to VPC A and VPC B.

Destination Target Route type

Attachment ID for VPC A


Attachment ID for VPC B