Cisco Systems OL-4580-01 manual Autonomous System, Areas, Backbone Area

Chapter 3 Configuring Open Shortest Path First (OSPF)

OSPF Overview

Autonomous System

The autonomous system (AS) is a collection of networks, under the same administrative control, that share the same routing information with each other. An AS is also referred to as a routing domain. Figure 3-1shows two ASs: AS A and AS B. An AS can consist of one or more OSPF areas.

Areas

Areas allow the subdivision of an AS into smaller, more manageable networks or sets of adjacent networks. As shown in Figure 3-1, AS A consists of three areas: area 0.0.0.0, area 1.1.1.1, and area 1.1.1.2.

OSPF hides the topology of an area from the rest of the AS. An area’s network topology is visible only to routers inside that area; the network topology is not visible to routers outside the area. When OSPF routing is within an area, this is called intra-area routing. This routing limits the amount of link-state information flooding onto the network, thereby reducing routing traffic. OSPF routing also reduces the size of the topology information in each router, which conserves processing and memory requirements in each router.

Conversely, the routers within an area cannot see detailed network structures outside the area. Because of this restriction of topological information, you can control traffic flow between areas and reduce routing traffic when the entire autonomous system is a single routing domain.

Backbone Area

A backbone area is responsible for distributing routing information between the areas of an autonomous system. When OSPF routing occurs outside of an area, this is called inter-area routing.

The backbone itself has all the properties of an area. It consists of ABRs, and routers and networks only on the backbone. As shown in Figure 3-1, area 0.0.0.0 is an OSPF backbone area. Note that a designated OSPF backbone area has a reserved ID of 0.0.0.0.

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