For SD-WAN deployments that span multiple regions—where most traffic is intra-region and there is no requirement for inter-region ADVPN—the best practice is to use IBGP with BGP on loopback interfaces for routing within each region and EBGP between the regions. This approach ensures robust and scalable routing, isolates regional routing domains, and enables policy control at region boundaries. BGP on loopback is preferred for its reliability and flexibility, as it enables peering that is not tied to specific physical interfaces. EBGP between regions allows each region to maintain independent routing policies and summarization, optimizing performance and manageability. By separating IBGP (intra-region) and EBGP (inter-region), you create a modular architecture that scales easily and simplifies fault isolation and troubleshooting.