H3C S3610-28P Operation Manual page 376

Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches Chapter 3 OSPF Configuration
NSSA area. When traveling to the NSSA ABR, Type-7 LSAs are translated into Type-5
LSAs by the ABR for advertisement to other areas.
In the following figure, the OSPF AS contains three areas: Area 1, Area 2 and Area 0.
The other two ASs employ the RIP protocol. Area 1 is an NSSA area, and the ASBR in
it translates RIP routes into Type-7 LSAs and advertises them throughout Area 1. When
these LSAs travel to the NSSA ABR, the ABR translates Type-7 LSAs to Type-5 LSAs
for advertisement to Area 0 and Area 2.
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 3-5 NSSA area
VI. 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 3-6 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
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