In an OSPF deployment, which statement best describes the backbone area in relation to other areas?

• A. The backbone area is used for intra-area routing within a single area.
• B. A stub area serves as the backbone.
• C. The backbone area connects all other areas and carries inter-area routing.
• D. Any area can function as the backbone if it is designated as area 0.

Answer: (C)

The backbone area is the central hub that interconnects all other areas for inter-area routing. In OSPF, Area 0 carries the routing information that needs to travel between areas, using summary routes that ABRs translate between an area and the backbone. All non-backbone areas must connect to this backbone, either directly or via virtual links, so that networks in different areas can reach each other through a common path.

This is why the statement that the backbone connects all other areas and carries inter-area routing is the best description. Intra-area routing happens within a single area and isn’t the backbone’s job. A stub area is a special type of non-backbone area, not the backbone itself. And only Area 0 is the backbone; other areas can connect to it but cannot themselves serve as the universal backbone, though a virtual link can connect a non-backbone area to Area 0 if a direct connection isn’t present.