Areas; Backbone Area; Stub Area; Not-So-Stubby Area - Cisco ASR 9000 Series Routing Configuration Manual

Implementing OSPF

Areas

Areas allow the subdivision of an autonomous system into smaller, more manageable networks or sets of
adjacent networks. As shown in
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
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 route advertisements 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
OSPF Routing Components, on page
cannot be a stub area.

Not-so-Stubby Area

A Not-so-Stubby Area (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.
Figure 17: OSPF Routing Components, on page
Figure 17: OSPF Routing Components, on page
410, Area 2 is a stub area that is reached only through ABR 2. Area 0
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
OSPF Routing Components
410, autonomous system
410, Area 0 is an OSPF backbone
Figure 17:
411