For OSPF routers to form an adjacency on a broadcast or point-to-point network, their interfaces must be configured with IP addresses within the same subnet. This is a fundamental prerequisite for them to discover each other via Hello packets. Additionally, OSPF does not use TCP or UDP; it communicates directly over IP using protocol number 89. If a firewall policy or access control list between the routers is blocking IP protocol 89, the OSPF Hello packets will be dropped, preventing adjacency formation. Therefore, checking for subnet alignment and ensuring IP protocol 89 is permitted are critical troubleshooting steps.

A. TCP port 179 is used by the Border Gateway Protocol (BGP), not OSPF.

B. A static route is not required for adjacency on a directly connected link and its presence may indicate a network design that prevents adjacency.

1. Fortinet FortiOS 7.6.0 Troubleshooting Guide, "OSPF troubleshooting" section. This guide mentions checking for blocked OSPF traffic (IP protocol 89) and verifying basic IP connectivity and configuration as initial steps.

2. Fortinet FortiOS 7.6.0 Administration Guide, "OSPF" chapter. It details the requirements for OSPF neighbors to become adjacent, which includes having interfaces on the same network segment.

3. RFC 2328 - OSPF Version 2, Section 10, "The Neighbor Data Structure". This standard specifies that the interface's IP address and subnet mask are used to determine if a neighbor is on the same network.