In OSPF, which area is designated as the backbone that connects all other areas?

• A. Area 0 (backbone)
• B. Area 1
• C. Area 2
• D. Area 3

Answer: (A)

Area zero acts as the backbone in OSPF, the central conduit that all other areas connect through. This backbone creates a hierarchical routing structure: inter-area routes are learned and distributed across areas via ABRs (Area Border Routers) that connect each area to the backbone. Because all inter-area traffic must pass through the backbone, routing stays consistent and scalable, and routes can be summarized as they move between areas. If an area isn’t connected to the backbone (directly or through a virtual link), it can’t participate in standard inter-area routing. The other areas mentioned are non-backbone areas that rely on the backbone to exchange routes with one another. So, the backbone area is Area 0.