The correct answer is Create a peering connection between the VPCs. Add a route table entry for the peering connection in both VPCs. Configure an inbound rule for the ElastiCache clusters security group to allow inbound connection from the applications security group.

VPC peering is the most cost-effective solution for enabling communication between two VPCs in the same AWS Region. Once the peering connection is established, route table entries must be added to both VPCs to direct traffic across the peering connection. Additionally, security groups should be configured to allow the appropriate traffic. ElastiCache supports VPC peering, making this a fully compatible and cost-efficient design.

Create a Transit VPC. Update the VPC route tables in the Cache VPC and the App VPC to route traffic through the Transit VPC. Configure an inbound rule for the ElastiCache cluster's security group to allow inbound connection from the applications security group is incorrect because a Transit VPC adds significant complexity and cost. It is more suitable for hub-and-spoke network designs or multi-account environments with many VPCs, not just two VPCs in the same Region.

Create a peering connection between the VPCs. Add a route table entry for the peering connection in both VPCs. Configure an inbound rule for the peering connections security group to allow inbound connection from the applications security group is incorrect because VPC peering itself does not have a security group. Security group rules must reference other security groups or CIDR blocks within VPCs, not the peering connection.

Create a Transit VPC. Update the VPC route tables in the Cache VPC and the App VPC to route traffic through the Transit VPC. Configure an inbound rule for the Transit VPCs security group to allow inbound connection from the applications security group is also incorrect for the same reasons as the previous Transit VPC option: it's costlier and unnecessary for this simple two-VPC setup.

Why this question matters: It tests your knowledge of inter-VPC connectivity options and cost-efficient network architecture. Choosing between VPC peering and Transit Gateway/Transit VPC solutions depends on scale, Region, and the number of VPCs involved.

Tip: For two VPCs in the same Region, VPC peering is typically the best choice in terms of simplicity and costjust remember to update the route tables and configure appropriate security group rules.

Full AWS Practitioner Certification Question