10.8. Routing across Multiple Areas
The OSPF extensions discussed earlier apply to a single OSPF area. For reasons of scalability, an OSPF routing domain may consist of multiple areas. In this section, we discuss how optical routing extensions to OSPF can be extended to multiple OSPF areas.
10.8.1. Packet Forwarding in IP Networks
Let us first consider how basic packet routing works in a two-level OSPF hierarchy. This hierarchy was described in the last chapter, where the notion of OSPF areas was discussed. Specifically, the OSPF protocol requires that each of the areas (including the backbone area) be internally connected. Following this rule, there are two general OSPF topology configurations possible. The first is the strictly hierarchical ...