OSPF Inter-Area Routing

How a route crosses from one area to another through the ABRs, with and without an area range

R1A's pathR1D's pathABR-2's path 1 router LSA 2 network LSA 3 summary LSA

Area 0 as the hub

A distance vector can loop: a router taking a neighbor's cost cannot tell whether that cost was worked out through itself. OSPF rules this out by the shape the summaries travel in.

Summaries go from an area into area 0, and from area 0 out to the other areas. Nowhere else.
An ABR uses only the summaries it hears in area 0. ABR-1A hears ABR-1B's summary of 10.2.100.0/24 inside area 1, and ignores it.
So the areas form a star around area 0, and a route cannot come back to where it started. It is also why every area must touch area 0.

The full explanation ↗

Database

Routes