Very cool lesson, though I’m wondering if there is a diagram error at a couple points.
It appears R5 accidentally gets drawn as connected to the DR in the “middle” for the 123 subnet. This happens at the end of the part going thru R1 (https://cdn.networklessons.com/wp-content/uploads/2014/08/R1-OSPF-full-picture-links.png ) , disappears for awhile, and then comes back in the R5 section (e.g., https://cdn.networklessons.com/wp-content/uploads/2014/08/OSPF-LSDB-view-of-R5.png and most that follow).
It seems the stub network of 15.0/24 (corresponding to point-to-point link up to R1) is inadvertently connected all the way to the DR instead of just being a stub.
