Open shortest path first (OSPF) - Set 2 - GeeksforGeeks
OSPF Adjacency & Neighbor Forming Process. OSPF Hello Messages, OSPF Database Updates via Link State Requests (LSR & LSU)
The Ultimate Guide to Understanding and Configuring OSPF - Expert Network Consultant
routing - Router on one side of a GRE tunnel not adding OSPF route through the tunnel whilst the router on the other side is - Network Engineering Stack Exchange