OSPF scales by splitting a network into areas. Smaller areas mean smaller link-state databases, less SPF computation, and contained flooding. But areas need a common meeting point — and that's area 0, the backbone.

The golden rule

Every non-backbone area (area 1, area 2, …) must connect to area 0. All traffic between areas transits the backbone. You can't route directly from area 1 to area 2 without passing through area 0. This deliberate hub-and-spoke topology is what keeps inter-area routing loop-free.

ABRs: the doorways

An Area Border Router (ABR) has interfaces in area 0 and at least one other area. It's the doorway between them: the ABR takes the routes it learns in one area, summarizes them, and injects them into the other as Type 3 (Summary) LSAs. Without an ABR sitting on area 0, an area is stranded.

What breaks without it

If an area gets physically separated from area 0 — say a new area is added off another area instead of the backbone — its routes won't reach the rest of the OSPF domain. The fix is a virtual link, which tunnels the disconnected area's traffic across an intermediate area back to area 0. Virtual links are a patch, not a design goal; the right answer is to connect to area 0 directly.

Single-area is fine too

Small networks often run single-area OSPF — and that single area is area 0. You only introduce additional areas when the topology grows large enough to benefit from segmentation.