Elements of OSPF Routing 7-3
Elements of OSPF
Routing
OSPF routing uses the following network elements:
■ Autonomous Systems
■ Areas
■ Neighbors
■ Protocol Packets
■ Router Types
■ Interface Characteristics
■ Stub Default Metrics
■ Virtual Links
Autonomous Systems An autonomous system consists of a set of OSPF routers that exchange
routing information. The network shown in Figure 7-1 contains two
autonomous systems.
Using identical topology databases, each router in an autonomous system
calculates shortest-path routes from itself to every known destination in
the autonomous system. The routers create their topology databases
using the data in link state advertisements (LSAs) from other routers in
the autonomous system.
Areas You can reduce the amount of routing information that travels through a
network, and the corresponding size of OSPF routers’ topology
databases, by subdividing OSPF autonomous systems into areas. The
routers in an area maintain and use identical LSA databases.
The network shown in Figure 7-1 contains four OSPF areas within
autonomous system A. There are three types of OSPF areas:
■ Transit — An area through which network traffic can pass to reach
other areas in the autonomous system. In Figure 7-1, the backbone
area and areas 1 and 3 are transit areas.
■ Stub — An area with only one entry or exit router. In Figure 7-1,
area 2 is a stub area that is reached only through area border router 1.
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