Configuring OSPF Route Summarization 117
■After you configure an area as a totally NSSA area, the ABR of the totally NSSA
area will automatically generate a Type-3 default LSA into the totally NSSA
area.
■For the ASBR of an NSSA area to generate a default Type-7 LSA, the default
route with the destination address 0.0.0.0/0 must exist in the routing table and
you need to execute the nssa default-route-advertise command.
■For the ABR of an NSSA area to generate a default Type-7 LSA, you only need
to execute the nssa default-route-advertise command on it.
Configuring OSPF Route Summarization
You can configure an ABR or ASBR to summarize routes with the same prefix into
a single route and distribute it to other areas.
An AS is divided into different areas that are interconnected through ABRs.
Through route summarization, routing information across areas and the size of
routing tables on routers will be reduced, improving the calculation speed of
routers.
After calculating the intra-area routes of an area, an ABR summarizes contiguous
networks into one route and advertises it to other areas according to the related
configuration.
For example, as shown in the following figure, in Area 1 are three intra-area 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.
Figure32 Route summarization
OSPF performs two types of route summarization:
1ABR route summarization
To distribute routing information to other areas, an ABR generates Type-3 LSAs on
a per network segment basis. If contiguous network segments are available in the
area, you can summarize them with a single network segment. In this way, the
ABR in the area distributes only the summary LSA to reduce the scale of LSDBs on
routers in other areas.
2ASBR 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.
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