OSPF can use two types of summarization:
- Interarea route summarization that can be done on the ABR.
- External route summarization that can be done on the ASBR.
Figure
shows a network
in which a router is not summarizing interarea routes.
Debugs and
Verification
Router R1 has the area range command
for summarization of area 3 routes.
The
syntax of the command is correct, but the problem is that R1 is not an ABR.
Router R1 is included in area 0, but it is not connected to area 3, and thus
cannot summarize area 3 routes.
Use the show ip ospf
command.to check if the router is summarizing the routes properly
The output of
show ip ospf shows that the area 3 range is passive.
Passive means that no addresses within this area fall within this range. In
fact, R1 does have the routes in this range; however, because R1 is not
connected to area 3, the range appears as passive.
Because summarization
is not happening, R1 is receiving four routes in the routing table instead of
one summarized route.

Solution
The solution to this problem is to configure the area
range command on the ABR, which is R2.
After changing
the configuration and removing the area range command on
R1, verify that the summarization is indeed active by using show ip
ospf on R2.
The summarized
route is now being installed in the routing table of R1.
