Cisco Systems OL-8241-02 Manual De Usuario

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. 

 shows two ASs: AS A 

and AS B. An AS can consist of one or more OSPF areas.

Areas allow the subdivision of an AS into smaller, more manageable networks or 
sets of adjacent networks. As shown in 

, 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. 

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 

, 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.