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There are no external routes in an OSPF stub area, so routes cannot be
redistributed from another protocol into a stub area. On the other hand, an NSSA
allows external routes from another protocol to be redistributed into its own area,
and then leaked to adjacent areas.
Routes that can be advertised with NSSA external LSAs include network
destinations outside the AS learned via OSPF, the default route, static routes, routes
derived from other routing protocols such as RIP, or directly connected networks that
are not running OSPF.
Also, note that unlike stub areas, all Type-3 summary LSAs are always imported into
NSSAs to ensure that internal routes are always chosen over Type-7 NSSA external
routes.
An NSSA can be used to simplify administration when connecting a central site
using OSPF to a remote site that is using a different routing protocol. OSPF can be
easily extended to cover the remote connection by defining the area between the
central router and the remote router as an NSSA.
Default Cost – This specifies a cost for the default summary route sent into a stub or
not-so-stubby area (NSSA) from an Area Border Router (ABR).
20-20
NSSA – A not-so-stubby area (NSSA) can be configured to control the use of default
routes for Area Border Routers (ABRs) and Autonomous System Boundary Routers
(ASBRs), or external routes learned from other routing domains and imported
through an ABR.
A
n NSSA is similar to a stub. It blocks most external routing information, and can be
configured to advertise a single default route for traffic passing between the NSSA
and other areas within the autonomous system (AS) when the router is an ABR.
A
n NSSA can also import external routes from one or more small routing domains
that are not part of the AS, such as a RIP domain or locally configured static routes.
This external AS routing information is generated by the NSSA’s ASBR and
advertised only within the NSSA. By default, these routes are not flooded onto the
backbone or into any other area by ABRs. However, the NSSA’s ABRs will convert
NSSA external LSAs (Type 7) into external LSAs (Type-5) which are propagated into
other areas within the AS.
default external
route for another
routing domain
A
SBR
7
NSSA
5
backbone
A
BR
Route
r
external network
default external
route for local AS
Unicast Routing
20