Introduction to OSPF 279
■ A (totally) stub area cannot have an ASBR because AS external routes cannot
be distributed into the stub area.
■ Virtual links cannot transit (totally) stub areas.
NSSA area
Similar to a stub area, an NSSA area imports no AS external LSA (Type-5 LSA) but
can import Type-7 LSAs that are generated by the ASBR and distributed
throughout the 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 77 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 78 Route summarization
NSSA
Area 2
Type 7Type 5
Type 5
Type 5
Type 5
Area 0 Area 1
NSSA
ABR
ABR
NSSA
ASBR
ASBR
RIP
RIP
Router A
ABR
Router B
ABR
Area 0
19.1.1.0/24
19.1.2.0/24
19.1.3.0/24
ĂĂ
Area 1
19.1.0.0/16
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