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Implementing OSPF on Cisco IOS XR Software
Information About Implementing OSPF on Cisco IOS XR Software
RC-133
Cisco IOS XR Routing Configuration Guide
Areas
Areas allow the subdivision of an autonomous system into smaller, more manageable networks or sets
of adjacent networks. As shown in Figure 6, autonomous system A 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 6, Area 0 is an OSPF backbone area. Any OSPF backbone area has a
reserved area ID of 0.0.0.0.
Stub Area
A stub area is an area that does not accept or detailed network information external to the area. A stub
area typically has only one router that interfaces the area to the rest of the autonomous system. The stub
ABR advertises a single default route to external destinations into the stub area. Routers within a stub
area use this route for destinations outside the area and the autonomous system. This relationship
conserves LSA database space that would otherwise be used to store external LSAs flooded into the area.
In Figure 6, Area 2 is a stub area that is reached only through ABR 2. Area 0 cannot be a stub area.
Not-so-Stubby Area (NSSA)
NSSA is similar to the stub area. NSSA does not flood Type 5 external LSAs from the core into the area,
but can import autonomous system external routes in a limited fashion within the area.
NSSA allows importing of Type 7 autonomous system external routes within an NSSA area by
redistribution. These Type 7 LSAs are translated into Type 5 LSAs by NSSA ABRs, which are flooded
throughout the whole routing domain. Summarization and filtering are supported during the translation.
Use NSSA to simplify administration if you are a network administrator that must connect a central site
using OSPF to a remote site that is using a different routing protocol.
Before NSSA, the connection between the corporate site border router and remote router could not be
run as an OSPF stub area because routes for the remote site could not be redistributed into a stub area,
and two routing protocols needed to be maintained. A simple protocol like RIP was usually run and
handled the redistribution. With NSSA, you can extend OSPF to cover the remote connection by
defining the area between the corporate router and remote router as an NSSA. Area 0 cannot be an
NSSA.