H3C Technologies H3C S12500 Series Switches - Manual (Page 75)

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• Virtual links cannot transit (totally) stub areas.
NSSA area
Similar to a stub area, an NSSA area does not import AS external LSA (Type-5 LSA) 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
Figure 20 , the OSPF AS contains three areas: Area 1, Area 2, and Area 0. The other two ASs run RIP.
Area 1 is an NSSA area where 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 20 NSSA area
Comparison between the areas
Figure 21 Comparison between the areas
Figure 21 shows the following comparison of the areas:
• In a totally stub area, the ABR can distribute a Type-3 default route, while it does not distribute
external routes and inter-area routes.
• Compared with a totally stub area, a stub area can import inter-area routes.
• Compared with a stub area, an NSSA area can import external routes through Type-7 LSAs
advertised by the ASBR.
• Compared with an NSSA area, a totally NSSA area does not import inter-area routes.