In an OSPF deployment, why is the backbone area essential?

• A. It is used only for intra-area routing within a single area.
• B. It connects all other areas but does not carry inter-area routing.
• C. It carries inter-area routing and connects all other areas.
• D. It is optional and used only for external routes.

Answer: (C)

The main idea here is that OSPF uses a central backbone area to move routing information between different areas. This backbone, Area 0, acts as the shared highway for inter-area communication. Routers in non-backbone areas connect to the backbone through area border routers, and those ABRs summarize routes from their area into the backbone (and then distribute them into other areas as inter-area routes). Because all inter-area routes must pass through Area 0, networks in one area are reachable from networks in another area only via the backbone. If an area isn’t connected to the backbone (or connected via a virtual link when needed), inter-area reachability breaks. While there are ways to repair a discontiguous backbone, the core role remains that the backbone carries inter-area routing and ties all areas together. That’s why the best description is that it carries inter-area routing and connects all other areas.