SMC Networks SMC6724L3 manual 193

IP ROUTING

•By default, a stub can only pass traffic to other areas in the autonomous system via the default external route. However, you also can configure an area border router to send Type 3 summary link advertisements into the stub.

NSSA – A not-so-stubby area (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). However, an 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 area border routers. 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.

5

default external route for another routing domain

7

backbone ABR NSSA ASBR

default external route for local AS

AS

Router

external network

•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.

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).

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