3Com MSR 50 Series Configuration Manual page 923

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 (type5 LSA) but
can import type7 LSAs that are generated by the ASBR and distributed throughout
the NSSA area. When traveling to the NSSA ABR, type7 LSAs are translated into
type5 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 type7 LSAs and advertises them throughout
Area 1. When these LSAs travel to the NSSA ABR, the ABR translates type7 LSAs to
type5 LSAs for advertisement to Area 0 and Area 2.
On the left of the figure, RIP routes are translated into type5 LSAs by the ASBR of
Area 2 and distributed into the OSPF AS. However, Area 1 is an NSSA area, so
these type5 LSAs cannot travel to Area 1.
Similar to stub areas, virtual links cannot transit NSSA areas.
Figure 273 NSSA area
RIP
Type 5
Type 5
ASBR
Area 2
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 274 Route summarization
Router A
19.1.0.0/16
Area 0
ABR
Type 5
Type 5
NSSA
ABR
Area 0
ABR
19.1.1.0/24
19.1.2.0/24
Router B
19.1.3.0/24
ABR
Area 1
Introduction to OSPF 923
RIP
NSSA
Type 7
NSSA
Area 1
ASBR