H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 241)

25-7
On the left of the figure, RIP routes are translated into Type-5 LSAs by the ASBR of Area 2 and
distributed into the OSPF AS. However, Area 1 is an NSSA area, so these Type-5 LSAs cannot travel to
Area 1.
Like stub areas, virtual links cannot transit NSSA areas.
Figure 25-5 NSSA area
Route summarization
Route summarization: An ABR or ASBR summarizes routes with the same prefix with a single route and
distribute it to other areas.
Via route summarization, routing information across areas and the size of routing tables on routers will
be reduced, improving calculation speed of routers.
For example, as shown in the following figure, in Area 1 are three internal routes 19.1.1.0/24,
19.1.2.0/24, and 19.1.3.0/24. By configuring route summarization on Router A, the three routes are
summarized with the route 19.1.0.0/16 that is advertised into Area 0.
Figure 25-6 Route summarization
OSPF has two types of route summarization:
1) ABR route summarization
To distribute routing information to other areas, an ABR generates Type-3 LSAs on a per network
segment basis for an attached non-backbone area. If contiguous network segments are available in the
area, you can summarize them with a single network segment. The ABR in the area distributes only the
summary LSA to reduce the scale of LSDBs on routers in other areas.
2) ASBR route summarization
If summarization for redistributed routes is configured on an ASBR, it will summarize redistributed
Type-5 LSAs that fall into the specified address range. If in an NSSA area, it also summarizes Type-7
LSAs that fall into the specified address range.
If this feature is configured on an ABR, the ABR will summarize Type-5 LSAs translated from Type-7
LSAs.
Route types
OSPF prioritize routes into four levels:
z
Intra-area route
z
Inter-area route