When trying to debug OSPF issues recently I found a set of excellent video tutorials from Darrell Root: https://www.youtube.com/c/DarrellRoot – I believe this is his website: https://networkmom.net/CiscoHandsOnTraining/
He has a playlist of videos called Cisco Hands-On Training: https://www.youtube.com/playlist?list=PLueKg14298FzreJSTGtMFIGdu10GLnT6z
I found his videos super helpful, and a very clear explanation of how OSPF works, and how to troubleshoot various issues, as well as different network designs you can use.
To get full value, you can follow along with your own cisco routers, or use GNS3 to simulate them. Be aware that, if you simulate in GNS3 with the standard iOSv routers, you will have a couple differences from Darrell’s setup:
- Links will be ethernet, not serial, so they will have a different cost (1 by default)
- Ethernet links are broadcast, not point-to-point, so extra network LSA’s show up in the OSPF database than what he has on his video.