user manual

Chapter 3 Configuring Open Shortest Path First (OSPF)
OSPF Overview
3-4
Cisco Content Services Switch Routing and Bridging Configuration Guide
OL-4580-01
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 3-1 shows 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.