Routing Types Within an OSPF Network

OSPF can use three types of routes:

• Intra-area
• Inter-area
• External

The following sections provide general descriptions of these route types. As you move further into designing and implementing OSPF networks, you will be exposed to these different types of routes.

Intra-Area Routing

Intra-area routing describes routes to destinations within a logical OSPF area. Intra-area routes in OSPF are described by router (Type 1) and network (Type 2) LSAs. When displayed in the OSPF routing table, these types of intra-area routes are designated with an "O."

Inter-Area Routing

Inter-area routing describes destinations that require travel between two or more OSPF areas and still fall within the same AS. These types of routes are described by network (Type 3) summary LSAs. When routing packets between two nonbackbone areas, the backbone is used. This means that inter-area routing has pieces of intra-area routing along its path, for example:

1. An intra-area path is used from the source router to the area border router.
2. The backbone is then used from the source area to the destination area.
3. An intra-area path is used from the destination area's area border router to the destination.

When you put these three routes together, you have an inter-area route. Of course, the SPF algorithm calculates the lowest cost between these two points. When displayed in the OSPF routing table, these types of routes are indicated with an "O IA,".