3Com 5500-EI PWR Install Manual page 294

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 reaching the
NSSA ABR, Type-7 LSAs are translated into Type-5 LSAs by the ABR (the Type-7 LSAs translator state
must be enabled/elected) before being advertised 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 reach the NSSA ABR with the
Type-7 LSAs translator state as Enabled or Elected, 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.
Similar to stub areas, virtual links cannot transit NSSA areas.
Figure 4-5 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.
After an AS is divided into different areas that are interconnected through OSPF ABRs, The routing
information between areas can be reduced through route summarization. This reduces the size of
routing tables and improves the calculation speed of routers.
After an ABR in an area calculates the intra-area routes in the area, the ABR aggregates multiple OSPF
routes into one LSA (based on the summarization configuration) and sends the LSA outside the area.
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.
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