An NSSA is a special type of OSPF area. Neither the NSSA nor the stub area transmits routes learned from other areas over the OSPF network. However, unlike the stub area, the NSSA does allow AS external routes to be imported and forwarded throughout the entire AS.

If you want to import AS external routes to an area and prevent these routes from consuming resources, configure the area as an NSSA.

Type 7 LSAs are used in the NSSA to carry information about the imported AS external routes. Type 7 LSAs are generated by ASBRs of NSSAs, and are only flooded in the NSSAs where ASBRs reside. The ABR in an NSSA selects Type 7 LSAs from those received, and translates them into Type 5 LSAs in order to advertise external routes to other areas over the OSPF network.

On the OSPF network shown in Figure 5-23, area 2 is a stub area, through which an external network requires access to the OSPF network. In this case, AS external routes need to be imported and advertised within the entire AS. One method is to enable DeviceA to import AS external routes into the OSPF AS. DeviceA then becomes an ASBR, indicating that area 2 is no longer a stub area. Another method is to configure an NSSA.

Figure 5-23 NSSA and totally NSSA

An NSSA differs from a stub area in that it allows AS external routes to be imported and advertised within the entire OSPF AS without learning routes from other areas on the OSPF network.

To ensure the reachability of AS external routes, the ABR in an NSSA generates a default route and advertises it to the other devices in this NSSA.

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.

The same area type must be configured for all devices in an area, and each device uses the N-bit carried in a Hello packet to identify the area type it supports. If devices have different area types, they cannot establish OSPF neighbor relationships. However, devices from some vendors do not comply with the OSPF implementation. These devices also have the N-bit set in DD packets. To enable a Huawei device to interwork with such devices, run the nssa command with the set-n-bit parameter specified on the Huawei device to set the N-bit in DD packets.

Similar to a totally stub area, OSPF defines the totally NSSA to further reduce the number of LSAs transmitted within an NSSA.