7.3. OSPF Areas
OSPF is almost always the preferred IGP for multi-area topologies. Multi-area OSPF networks are easier to design, configure, and administer than multi-area IS-IS networks. Although you can implement almost all the same intra- and inter-area behaviors in IS-IS as you can in OSPF, doing so often involves the use of routing policies that are not as easy to understand as the more straightforward OSPF mechanisms.
7.3.1. Backbone and Non-Backbone Areas
Normal OSPF design uses a loop-free inter-area topology, as discussed in the introduction to this chapter, by ensuring that all inter-area traffic originates in, terminates in, or passes through the backbone area. OSPF identifies the backbone area with an all-zero AID (0.0.0.0), commonly ...