Two projects: project-a with vpc-a (10.0.0.0/16) and project-b with vpc-b (10.8.0.0/16). Frontend is in vpc-a, backend APIs in vpc-b. Enable efficient, cost-effective communication between projects following Google-recommended practices. What should you do?

VPC Network Peering is the recommended and most efficient way to connect two VPC networks, even across different projects, within Google Cloud. It allows internal IP address communication between resources in the peered networks, behaving as if they are on the same network. This provides low-latency, high-bandwidth connectivity and is cost-effective as traffic stays within Google's network. OpenVPN is an external solution, adding complexity and latency. Cloud Routers are used for VPNs or Cloud Interconnect, not direct VPC-to-VPC peering. Cloud Interconnect is for connecting on-premises networks to Google Cloud, not for inter-VPC communication within Google Cloud.