OSPF Hierarchical Topology, Areas and Router Roles
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When the number of routers in an autonomous system (AS) is relatively small, using the basic topology described in the previous topic works well. Each router maintains a common picture of the network topology in the form of an identical link-state database (LSDB). The routers communicate as peers using link-state advertisements (LSAs). While changes in the AS may cause a router to temporarily have different information than its peers, routine exchanges of data will keep all the LSDBs synchronized and up-to-date, and not that much information needs to be sent around because the AS is small.
This simpler topology does scale reasonably well, and can support many smaller and even moderate-sized autonomous systems. However, as the number of routers increases, the amount of communication required to update LSDBs increases as well. In a very large internetwork with dozens or even hundreds of routers, having all the routers be OSPF peers using basic topology can result in performance degradation. This problem occurs due both to the amount of routing information that needs to be passed around, and also the need for each router to maintain a large LSDB containing every router and network in the entire AS.
To provide better support for these larger internetworks, OSPF supports the use of a more advanced, hierarchical topology. In this technique, the autonomous system is no longer considered a single, flat structure of interconnected routers all of which are peers. Instead, a two-level hierarchical topology is constructed. The autonomous system is divided into constructs called areas, each of which contains a number of contiguous routers and networks. These areas are numbered, and managed independently by the routers within them, so each area is almost as if it were an autonomous system unto itself. The areas are interconnected so that routing information can be shared between areas, across the entire AS.
The easiest way to understand this hierarchical topology is to consider each area like a sub-autonomous system within the autonomous system as a whole. The routers within any area maintain a link-state database containing information about the routers and networks within that area. Routers within more than one area maintain LSDBs about each area they are a part of, and also link the areas together to share routing information between them.
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