OSPF Configuration Guidelines > Configuring Route Summarization

Area border routers send summary link advertisements to describe the routes to other areas. To minimize the number of these advertisements that are flooded, you can configure the router to coalesce, or summarize, a range of IP addresses and send reachability information about these addresses in a single link-state advertisement.

To summarize a range of IP addresses, include the area-range statement. To summarize multiple ranges, include multiple area-range statement.

(ospf | ospf3) { 

    area area-id {

        area-range network/mask-length <restrict > <exact> 

            <override-metric metric>; 

    }

}

For a list of hierarchy levels at which you can configure this statement, see the statement summary section for this statement.

All routes that match the specified area range are filtered at the area boundary, and the summary is advertised in their place. If you specify the restrict option, the routes are filtered but no summary is advertised. If you specify the exact option, summarization of a route is advertised only when an exact match is made with the configured summary range. To override the metric for the IP address range and configure a specific metric value, include the override-metric option. If you specify the override-metric option, the dynamically computed metric for the IP address range is overridden by the specified value.