Q: How to enable NAAS area in OSPF for MA5200G?
A: Let's look at SUB area before discussing NSSA area:
Stub area is specific, in which ABR does not transmit fifth-category LSA (AS-External-LSA). Here, transmitting means the routes outside the autonomous area received cannot enter NSSA area, and the direct route and static route imported into the router in area cannot come out from NSSA area. Thus, the scale of routing table and the amount of routing information on the router will reduce greatly. Both direct and static routes imported into OSPF are required in principle, but ABR in Stub area cannot forward the routing information out. So, Stub area is rarely used in fact. To reduce the amount of routing information in an area, NSSA is recommended.
Not-So-Stubby Area (NSSA) is an extension of Stub area; as the name implies, it is a non-stubby SUB area. Stub area contains no route information outside AS, ensuring the destination at outside reachable through default route, in order to improve the flexibility of networking (both direct route and static route imported into router in area could be forwarded out) while keeping the strengths of Stub area. The difference between NSSA and Stub area lies in that NSSA could import the route outside AS in a limited manner and forward it out.
AS external routes in NSSA use a brand-new type of LSA--Type-7 LSA, which is very similar to Type-5 LSA. Once the router in area generates Type-7 LSA, it will be advertised to NSSA area for transmission; once reaching NSSA ABR, Type-7 LSA will be transformed to Type-5 LSA by ABR and be transmitted over the entire autonomous system. Consequently, the routes imported into the area could be forwarded out, and the fifth-category LSA is limited to enter the area, reducing the scale of routing table and the amount of routing information of the router in area greatly.
Look at the following example: suppose the area-number of NSSA is 2, so how to realize the functionalities of NSSA at MA5200G:
default-route-advertise Generate default route to NSSA with type 7 LSA
no-import-route ABR does not import external routes into the NSSA
no-summary Disable sending the summary LSA to an NSSA
Where, there are three parameters:
? default-route-advertise: The parameter generates Type-7 LSA to advertise a default route to NSSA area, and the route is also the default route outside the autonomous area. In other words, it tells the other else routers in area that it has a route to outside. The parameter could only be used at ABR of NSSA area.
? no-summary: Its application is the same to that in Stub area. After using it, NSSA ABR uses Summary-LSA of Type-3 to advertise a default route to the area, not other else Summary-LSAs to the area again, so the area is called NSSA totally stub. This parameter could only be configured at ABR of NSSA.
? no-import-route: It denies advertising of AS external route to NSSA area by means of Type-7 LSA.
MA5200G does not only serve as a gateway for access to network, but also as a router similar to NE40, and it is often located at the convergence layer or access layer of MAN. In most networks, MA5200G will be used as ABR in NSSA area mostly.