If you take another look at the figure, you’ll notice that Router A connects to both Area 1 and Area 0. This makes Router A what is known as an Area Boundary Router (ABR). It connects to (at least) two areas, and maintains a link state database for both. The job of an ABR is important – it summarizes the routes within an area, and passes this information to other ABRs, who will then pass the information within their connected areas. The route summarization done by the ABRs helps to make routing tables much smaller than they would otherwise be – instead of passing information about 8 individual networks, the information might be summarized into a single routing table entry. For example, if an ABR had networks 192.168.8.0/24 through 192.168.15.0/24 configured within a connected area, it could summarize these routes via a single advertisement for network 192.168.8.0/21. For a review of how route summarization works, take a look back at the CIDR section of Chapter 5.
The final type of router in an OSPF implementation is an Autonomous System Boundary Router. The job of an ASBR is to act as a gateway to networks running other routing protocols, or different OSPF autonomous systems. This is often referred to as redistribution. For example, the ASBR in Figure 8-20 is connected to both AS 99 and AS 100. It would be the job of the ASBR to summarize the information learned about AS 100 and redistribute it into AS 99 and vice versa. This ultimately allows routers that are part of AS 99 to know about AS 100, without actually requiring any of the other routers within one AS to talk to routers in another AS directly. Ultimately, OSPF allows you to design your network hierarchically, and obtain a higher degree of routing efficiency.