Routing Within a Hierarchical Structure
There are three types of routes that may be used by OSPF: intra-area, inter-area, and external routes. The sections that follow provide general descriptions of these route types.
Intra-area routing is the name used to describe routing within a logical area. These types of routes are described by Router LSAs (Type 1). In order for packets to be routed within a single area, intra-area routing is used. When displayed in the OSPF routing table, these types of links are designated with an O.
Inter-area routing is the name used to describe routing between two or more logical areas that still fall within the source Autonomous System. These types of routers are described by Summary LSAs (Types 3 and 4). When routing packets between two non-backbone areas, the backbone will be used. This means that inter-area routing has pieces of intra-area routing along its path, for example:
Put these three routes together and you will have an inter-area route. Of course, the SPF algorithm will calculate the lowest cost between these two points. When these types of routes are displayed in the OSPF routing table, these types of routes are indicated with an IA.
AS (Autonomous System) External Routes
External routing information can be gained by OSPF through a number of means as discussed. This information must then be made available throughout the Autonomous System in order for it to be of use. The ASBR routers will summarize the information and flood this information throughout the AS. Every router will receive this information with the exception of stub areas.
There are two specific types of external routes, which are as follows:
This chapter discussed in detail the background and evolution of the OSPF protocol by tracing the RFCs relating to OSPF. The OSPF Overview section of this chapter discussed the earliest appearance and creation of the shortest path first algorithm. From the established beginnings of OSPF, the evolution and modifications applied to it will be traced through the RFCs. Each RFC relating directly and indirectly to the protocol is summarized and discussed as needed.
The OSPF Functional Environment section of this chapter helped you understand the internetworking environment of the protocol so as to design a network in which it will operate properly. The functional environment is a key element in understanding OSPF. Within this section are several basics to the protocol that are discussed and explored: network types, router identification, adjacencies, Designated Routers, protocols within OSPF, and LSAs.
The OSPF Routing Hierarchy section demonstrated how the protocol allows for a variety of different configurations and implementations. This capability requires many different levels or areas. OSPFs capability to perform as a hierarchical routing protocol allows it to be considered in networks of varying sizes. As a result of this, OSPF supports a variety of techniques and designations that make operation and design much smoother. Within this section, you learned about the types of OSPF routers and hierarchical design techniques. The latter of which included how OSPF separates the hierarchy through the use of areas and Autonomous Systems.
Case Study: Adding a New OSPF Router to a Network
This case study attempts to provide a scenario that will cover most of the information presented in this chapter. Suppose that a new OSPF router is added to a network. With this scenario in hand, follow along with the case study to understand the ramifications of how adding a new OSPF router would affect an operating network. Refer to Figures 4.19a-4.19d, which detail each step of the process as it occurs in the following sequence:.