Cisco NCS 5500 Series

Autonomous Systems
The autonomous system is a collection of networks, under the same administrative control, that share routing
information with each other. An autonomous system is also referred to as a routing domain. Figure 4: OSPF
Routing Components, on page 103 shows two autonomous systems: 109 and 65200. An autonomous system
can consist of one or more OSPF areas.
Areas
Areas allow the subdivision of an autonomous system into smaller, more manageable networks or sets of
adjacent networks. As shown in Figure 4: OSPF Routing Components, on page 103, autonomous system 109
consists of three areas: Area 0, Area 1, and Area 2.
OSPF hides the topology of an area from the rest of the autonomous system. The network topology for an
area is visible only to routers inside that area. When OSPF routing is within an area, it is called intra-area
routing. This routing limits the amount of link-state information flood into the network, reducing routing
traffic. It also reduces the size of the topology information in each router, conserving processing and memory
requirements in each router.
Also, the routers within an area cannot see the detailed network topology outside the area. Because of this
restricted view 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 multiple areas of an autonomous
system. OSPF routing occurring outside of an area is called interarea routing.
The backbone itself has all properties of an area. It consists of ABRs, routers, and networks only on the
backbone. As shown in Figure 4: OSPF Routing Components, on page 103, Area 0 is an OSPF backbone area.
Any OSPF backbone area has a reserved area ID of 0.0.0.0.
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
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Implementing OSPF
Autonomous Systems