OSPF areas and why the design matters
Areas are not organisational tidiness. They control how much work every router has to do.
Writing
Design
Broadcast domains and why they still matter
Switching is cheap, so people build large flat networks. Here is what that costs you.
Spanning tree: what it protects you from
A layer 2 loop does not degrade a network. It stops it, completely, in seconds.
Designing for the link you cannot drive to
Satellite enforces habits that terrestrial networks let you skip: observability before convenience, validation before shipping, and a backup path that has actually carried traffic.