Open Shortest Path First (OSPF) - Router Alley

OSPF v1.31 – Aaron Balchunas
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Inter-Area OSPF Summarization
Consider the above example. OSPF is a classless routing protocol, thus all of
the listed networks would be advertised individually. This increases the size
of the topology databases and routing tables on routers in the domain, and
may be undesirable. Advertising only a summary route for inter-area
communication can reduce the load on router CPUs.
For example, all of the networks in Area 1 can be summarized as
10.1.0.0/21. Similarly, all of the networks in Area 2 can be summarized as
10.1.8.0/21.
Inter-area summarization is configured on Area Border Routers (ABRs).
Configuration on Router A would be as follows:
RouterA(config)# router ospf 1
RouterA(config-router)# network 10.1.0.0 0.0.7.255 area 1
RouterA(config-router)# area 1 range 10.1.0.0 255.255.248.0
The network statement includes all of the 10.1.x.0 networks into Area 1. The
area 1 range command creates a summary route for those networks, which
is then advertised into Area 0, as opposed to each route individually.
Proper design dictates that a static route be created for the summarized
network, pointing to the Null interface. This sends any traffic destined
specifically to the summarized address to the bit-bucket in the sky, in order
to prevent routing loops:
RouterA(config)# ip route 10.1.0.0 255.255.248.0 null0
In IOS versions 12.1(6) and later, this static route is created automatically.
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