Figure 4-1 OSPF Hierarchy
A I B
E D
G H F
C
Area 1 Area 2 Area 3
External
Routing
Domain
Autonomous System
Backbone Area
Introducing OSPF 141
OSPF uses a two-layer network hierarchy that has two primary elements:
?– Autonomous system: An autonomous system consists of a collection of networks under a
common administration that share a common routing strategy. An autonomous system,
sometimes called a domain, can be logically subdivided into multiple areas.
?– Area: An area is a grouping of contiguous networks. Areas are logical subdivisions of the
autonomous system.
Within each autonomous system, a contiguous backbone area must be de?¬?ned. All other
nonbackbone areas are connected off the backbone area. The backbone area is the transition area
because all other areas communicate through it. For OSPF, the nonbackbone areas can be
additionally con?¬?gured as stub areas, totally stubby areas, or not-so-stubby areas (NSSA) to help
reduce the link-state database and routing table size.
OSPF special areas such as NSSAs, totally stubby, and stub areas are beyond the scope of this text.
Routers that operate within the two-layer network hierarchy have different routing entities and
different functions in OSPF.
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