As defined in OSPF, stub areas cannot import external routes. This mechanism prevents external routes from consuming the bandwidth and storage resources of routers in stub areas. Stub areas thus cannot meet the requirement of the scenario when external routes need to be imported and resource consumption caused by external routes also needs to be avoided. Therefore, the NSSA concept is proposed.
OSPF not-so-stubby area (NSSA) is a new type of OSPF area. There are many similarities between NSSAs and stub areas. However, different from stub areas, NSSAs can import AS external routes into the OSPF AS and advertise the imported routes in the OSPF AS without learning external routes from other areas on the OSPF network.
The same area type must be configured for all routers in an area. A router uses the N-bit carried in a Hello packet to identify the area type that it supports. If routers have different area types, they cannot establish OSPF neighbor relationships. Some vendors' devices do not comply with standard protocols, but the N-bit is also set in OSPF Database Description (DD) packets. You can manually set the N-bit on a router to interwork with the vendors' devices.
Type 7 LSAs, which describe imported external routes, are introduced to support NSSAs. Type 7 LSAs are generated by an ASBR in an NSSA and advertised only within the NSSA. After an ABR in an NSSA receives Type 7 LSAs, it selectively translates Type 7 LSAs into Type 5 LSAs to advertise external routes to other areas on an OSPF network.
To advertise external routes imported by an NSSA to other areas on the OSPF network, a translator must translate Type 7 LSAs into Type 5 LSAs. Figure 1 shows the translation process.
The propagate bit (P-bit) in Type 7 LSAs is used to notify a router whether Type 7 LSAs need to be translated.
By default, the translator is the ABR with the largest router ID in the NSSA.
Only Type 7 LSAs with the P-bit set and a non-zero forwarding address (FA) can be translated into Type 5 LSAs. An FA indicates that packets to a destination address will be forwarded to the address specified by the FA.
An FA indicates that packets to a destination address will be forwarded to the address specified by the FA.
The loopback interface address in an area is preferentially selected as the FA. If no loopback interface exists, the address of the interface that is up and has the smallest logical index in the area is selected as the FA.
The P-bit is not set for default routes in Type 7 LSAs generated by an ABR.
Multiple ABRs may be deployed in an NSSA. To prevent routing loops caused by default routes, ABRs do not calculate the default routes advertised by each other.