Recently I was working through one of the CBT nuggets OSPF NBMA labs by memory to try and reinforce the configuration and while I was practising I found that I was having issues with the multipoint OSPF area.
R3 is the hub router and R5 and R6 are the spoke routers.
R5 and R6 interfaces are left as the default OSPF network type which is point to point.
For this scenario the hello-interval is changed on R3’s interface to match the point to point hello interval, which is 10 seconds by default and this allows the neighbourship to form.
What I noticed is that the ip ospf network point-to-multipoint command had not been enabled on R3’s interface. The neighbourship formed but R3 was not receiving any routes from R5 or R6, as you can see from the above picture the 18.104.22.168/24 and 22.214.171.124/24 networks are not in the routing table.
Once the ip ospf network point-to-multipoint command is added, R3 is then able to correctly learn routes from R5 and R6 (the 126.96.36.199/24 and 188.8.131.52/24 networks).
I spent some time troubleshooting this problem, including running debugs but none of the debug commands gave me any feedback which lead me towards finding the issue. It was not until I went back and watched the video that I found the interface type had not been correctly configured and this was the reason R3 was not receiving these routes.
I wrote this post as a reminder to myself when configring OSPF over NBMA, even if the neighbours have formed and everything looks fine with the configuration if you still see issues with routes being learned then check the network type as this can cause problems, like the one mentioned here.