OSPF virtual link and ABRs
Refer to the following topology:
In order for Area 2 to operate correctly, a virtual link must be created between R2 and R1. R2 becomes the ABR between Area 2 and the backbone area. R2 does not become an ABR between Area 1 and Area 2 directly, since by definition, an ABR exchanges routes between the backbone and a non-backbone area.