Area border routers    A router that attaches to multiple areas.  Area border routers run multiple copies of the basic algorithm, one copy  for each attached area. Area border routers condense the topological information of their attached areas for  distribution to the backbone.  The backbone in turn  distributes the information to the other areas.

根据上一段描述，只要连接到多个区域就可以叫做ABR。这是大部分资料所讲述的，我曾经对此也深信不疑。

The OSPF backbone is the special OSPF Area 0 (often written as Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP addresses). The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing        information between non-backbone areas. The backbone must be        contiguous. However, it need not be physically contiguous;       backbone connectivity can be established/maintained through the        configuration of virtual links.

再看一下这句话，OSPF骨干包含所有的ABR！

那到底什么是ABR呢，ABR其实确实是多个区域之间，并且，有AREA0！并且AREA0里面有UP的接口！这句话你明白了吗？可以做实验加深理解。