The recommended best practice for a multi-region SD-WAN deployment is a two-tier BGP design. Each region is configured with a unique Autonomous System (AS) number and uses IBGP for routing within the region (intra-region). Peering over loopback interfaces is a standard practice to ensure stability, as loopbacks are not tied to the state of a physical interface. To connect the different regions, EBGP is used between the regional hubs. This architecture provides excellent scalability, clear administrative boundaries, and robust policy control for inter-region traffic, which is ideal for the described scenario.

A: OSPF is not the recommended overlay protocol for large, multi-region SD-WAN due to scalability and policy limitations compared to BGP.

C: Using IBGP between regions is not best practice; it complicates the design by requiring a full mesh or route reflectors across regions.

D: A single AS with IBGP for the entire deployment is less scalable and manageable than a multi-AS design using EBGP between regions.

1. Fortinet Document Library, "SD-WAN Architecture for Enterprise 7.4", Page 26, Section: Multi-Region / Multi-Datacenter, Subsection: Routing design.

This document explicitly states: "A two-tier BGP design is recommended for multi-region deployments. Tier 1 is the inter-region routing where eBGP is used to advertise routes between regions... Tier 2 is the intra-region routing where iBGP is used to advertise routes between the hubs and from the spokes to the hubs." It also recommends using a unique private AS for each region and establishing BGP peering between loopback interfaces.

2. FortiOS 7.6.0 Administration Guide, "SD-WAN", Section: BGP for SD-WAN.

This guide details the use of BGP as the preferred dynamic routing protocol for SD-WAN overlays. It highlights BGP's advantages in scalability and attribute-based policy control, which are critical for managing routing in complex, multi-region enterprise networks. The principles support using an EBGP/IBGP hierarchical design for administrative separation and control.