In OSPF, all areas must connect to which part of the network design?

• A. The backbone
• B. A hub-and-spoke central site
• C. An external ASBR region
• D. A directly connected router

Answer: (A)

In OSPF, routing is built around a central backbone area that interconnects all other areas. This backbone, Area 0, must be the transit path for all inter-area traffic. ABRs (Area Border Routers) connect each non-backbone area to the backbone, carrying routing information between the backbone and its attached area. This setup keeps the design scalable and ensures that routes between different areas are exchanged properly. If an area isn’t connected to the backbone (directly or via a virtual link to maintain backbone contiguity), inter-area routing can fail. So, the backbone is the required anchor for the network design. The other options describe different concepts that aren’t required by OSPF’s fundamental structure.