Scalability of the routing protocol is an important factor for large networks such as service providers. By splitting up an OSPF network into multiple OSPF areas, it can reduce the size of the LSDB for that area. The number of routers and networks remain the same but Type 1 and Type 2 LSAs are exchanged for summarised Type 3 LSAs.
Type 3 LSAs give an opportunity for further summarisation with larger network prefixes; allowing SPF calculations to run faster and provide easier processing for the CPU and memory. It can do this easily simply by hiding the multiple smaller prefixes behind a larger prefix that is advertised to the rest of the OSPF network.
Interarea summarisation reduces the number of type 3 LSAs that the area border router (ABR) advertises into an area when it receives type 1 LSAs.
When the Type 1 LSA within the summarisation range reaches the area border router from the source area, the area border router creates a Type 3 LSA for the summarised network range with the more specific type 3 LSAs being suppressed.
The default metric for a summarised LSA is the lowest metric associated with it. For example if there were three more specific prefixes in the summarised prefix consisting of 15, 73, 109, 15 would be the chosen metric used in the summary route.
Configuring Interarea Summarisation
Under the OSPF process configuration mode, enter the command
area <area number> range <network> <subnet-mask>
There are optional keywords to the command,
not-advertise. Advertise is implicitly included so does not need to be specified explicitly. Adding a metric on the end with the keyword
metric <value> allows for a specific metric to be set.