OSPF summarization

OSPF summarization
Core
Router
Area 0
(Backbone)
Distribution
Router
Access
Router
Access
Router
Access
Router
Area 1
Distribution
Router
Access
Router
Access
Router
Access
Router
Area 2
10.12.0.0/16
10.13.0.0/16
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Chapter 11 
Design Considerations
This way, all the routes under each distribution router in Area 1 can be aggregated to a single
route—10.12.0.0/16—into Area 0 and from Area 2 using 10.13.0.0/16. Area 0 routers will see
only two aggregate routes and not the individual routes that make up Areas 1 and 2. This is
done by configuring the range of IP addresses that will be found in that area.
If you are also looking to aggregate routes between core routers, then OSPF will not be your
routing protocol of choice. This is because there is no border between two OSPF areas that corresponds
to the connection between the two core routers. You would need to have a third area
configured between the core routers, but this would break how OSPF operates and is not a good
design practice.
OSPF has a feature called a stub area, which allows a lower end router to operate in a larger
OSPF network without using a lot of system resources. No external routes are carried into the
stub area. A default route is the only way a router within a stub area can know how to get to
any external network. This router knows how to get to other networks within the same area and
knows the summary routes from other areas, but all external routes are filtered out at the ABR.
This makes it possible for a large number of devices to be supported within a network.
Cisco has added another feature to the concept of stub areas called the totally stubby area. This
area type filters out not only external routes at the ABR but also summary advertisements from
other areas. Only a default route and routes within that area are shown in the routing table.
These two features assist with your design considerations because if you have an area where
the routers are having problems running the SPF, or Dijkstra, algorithm because of insufficient
system resources, you can make that area a stub or totally stubby area. You can also increase
the number of routers within an area without affecting the entire network. This will also
decrease the time needed for the network to converge when a change occurs. Only those changes
needed are advertised into the area that needs that information.