In a hub-and-spoke topology, particularly over a Non-Broadcast Multi-Access (NBMA) network such as a typical IPsec VPN overlay, OSPF requires a specific network type to establish adjacencies correctly. Setting the network type to point-to-multipoint on the hub's interface is the standard and recommended configuration. This mode eliminates the need for a Designated Router (DR) and Backup Designated Router (BDR) election. Instead, the hub establishes a separate point-to-point adjacency with each spoke router, which is the intended behavior for this topology. This ensures stable neighbor relationships and proper route propagation without requiring manual neighbor configuration.

A. Set bfd enable in the router configuration

BFD (Bidirectional Forwarding Detection) is used for rapid link failure detection but is not a prerequisite for forming an OSPF neighbor adjacency.

C. Set rfc1583-compatible enable in the router configuration

This setting controls the calculation method for summary routes (Type 3 LSAs) for compatibility with older standards and does not affect neighbor adjacency formation.

D. Set virtual-link enable in the hub interface

Virtual links are used to connect a disconnected OSPF area to the backbone area (Area 0). The exhibit shows all devices are within Area 0, making a virtual link unnecessary.

1. Fortinet FortiOS 7.6.0 Administration Guide

Networking > OSPF > OSPF network types

p. 1031: "Point-to-multipoint: This mode is a set of point-to-point links. There is no DR or BDR. This mode is useful for partial-mesh NBMA networks." This directly applies to the hub-and-spoke scenario.

2. Fortinet FortiOS 7.6.0 Administration Guide

Networking > OSPF > OSPF configuration examples > OSPF point-to-multipoint configuration example

p. 1040: This section provides a step-by-step configuration for a hub-and-spoke topology

explicitly demonstrating the use of set network-type point-to-multipoint on the hub's interface. The example topology is identical to the one shown in the question's exhibit.