Area 0 serves as what in OSPF?

• A. The backbone that connects other areas
• B. An area reserved for remote sites
• C. A testing area
• D. An area for external routes only

Answer: (A)

In OSPF, area 0 is the backbone that all other areas must connect to. It carries inter-area routing information, and ABRs sit between area 0 and the other areas to summarize and flood LSAs between them. Because inter-area traffic has to pass through the backbone, non-backbone areas cannot directly exchange routes with each other without going through area 0. External routes are not limited to this area—they’re injected by ASBRs into area 0 and then distributed into other areas via ABRs.