What is the role of the backbone in OSPF?

• A. It interfaces with external routing protocols.
• B. It prevents routing loops through perimeter segmentation.
• C. It holds per-interface route metrics.
• D. It serves as the core area to which all other OSPF areas must connect.

Answer: (D)

The backbone in OSPF is the central hub for inter-area routing. Area 0, the backbone, is where all inter-area information flows, and every other area must connect to it so routes can be exchanged between areas. Routers that touch multiple areas, called ABRs, attach to the backbone and propagate routing information across area boundaries through it. Any inter-area path between two non-backbone areas must transit through the backbone, which keeps the routing structure organized and loop-free; if a direct connection to the backbone isn’t possible, virtual links can extend backbone connectivity. So the backbone serves as the core area to which all other OSPF areas must connect, enabling scalable and consistent inter-area routing.