TCP/IP Tutorial and Technical Overview - IBM Redbooks

In this example, the ASBR is redistributing the 10.99.5.0/24 route into the OSPF
network. This subnet is located within the RIP network. The route is announced
into OSPF with an external cost of 50. This represents the cost for the portion of
the path traversing the RIP network:
► If the ASBR redistributed the route as an E1 route, R1 will contain an external
route to this subnet with a cost of 60 (50 + 10). R2 will have an external route
with a cost of 65 (50 + 15).
► If the ASBR redistributed the route as an E2 route, both R1 and R2 will
contain an external route to this subnet with a cost of 50. Any costs
associated with traversing segments within the OSPF network are not
included in the total cost to reach the destination.
5.6.5 OSPF stub areas
OSPF allows certain areas to be defined as a stub area. A stub area is created
when the ABR connecting to a stub area excludes AS external LSAs from being
flooded into the area. This is done to reduce the size of the link state database
maintained within the stub area routers. Because there are no specific routes to
external networks, routing to these destinations is based on a default route
generated by the ABR. The link state databases maintained within the stub area
contain only the default route and the routes from within the OSPF environment
(for example, intra-area and inter-area routes).
Because a stub area does not allow external LSAs, a stub area cannot contain
an ASBR. No external routes can be generated from within the stub area.
Stub areas can be deployed when there is a single exit point connecting the area
to the backbone. An area with multiple exit points can also be a stub area.
However, there is no guarantee that packets exiting the area will follow an
optimal path. This is due to the fact that each ABR generates a default route.
There is no ability to associate traffic with a specific default routes.
All routers within the area must be configured as stub routers. This configuration
is verified through the exchange of hello packets.
Not-so-stubby areas
An extension to the stub area concept is the not-so-stubby area (NSSA). This
alternative is documented in RFC 3101. An NSSA is similar to a stub area in that
the ABR servicing the NSSA does not flood any external routes into the NSSA.
The only routes flooded into the NSSA are the default route and any other routes
from within the OSPF environment (for example, intra-area and inter-area).
However, unlike a stub area, an ASBR can be located within an NSSA. This
ASBR can generate external routes. Therefore, the link state databases
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