HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Routing Confi - Page 62

NSSA area Similar to a stub area, an NSSA area does not import AS external LSAs (Type-5 LSAs), but can import Type-7 LSAs generated by the NSSA ASBR. The NSSA ABR translates Type-7 LSAs into Type-5 LSAs and advertises the Type-5 LSAs to other areas. In the following figure, the OSPF AS contains Area 1, Area 2, and Area 0. The other two ASs run RIP. Area 1 is an NSSA area. The ASBR redistributes RIP routes in Type-7 LSAs into Area 1. Upon receiving these Type-7 LSAs, the NSSA ABR translates them to Type-5 LSAs, and then advertises the Type-5 LSAs to Area 0. The ASBR of Area 2 redistributes RIP routes in Type-5 LSAs into the OSPF routing domain. However, Area 1 does not receive these Type-5 LSAs because it is an NSSA area. Virtual links cannot transit NSSA areas. Figure 16 NSSA area Comparison between the areas Figure 17 Comparison between the areas A Type 3 default route can be distributed in the area, while Type 3 and Type 5 LSAs cannot be distributed in the area Totally Stub area Permits Type 3 LSAs Stub area Permits Type 7 LSAs within the area NSSA area Does not permit Type 3 LSAs Totally NSSA area Figure 17 shows the comparison of the areas: • In a totally stub area, the ABR distributes a Type 3 default route, rather than external routes and inter-area routes. • A stub area can import inter-area routes, but a stub area cannot. • An NSSA area can import external routes in Type 7 LSAs through the ASBR, but a stub area cannot. • A totally NSSA area cannot import inter-area routes but an NSSA area can. 52