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Configuring a Not-So-Stubby Area

An OSPF stub area has no external routes, so you cannot redistribute from another protocol into a stub area. An NSSA allows external routes to be flooded within the area. These routes are then leaked into other areas. However, external routes from other areas still do not enter the NSSA.

You cannot configure an area to be both a stub area and an NSSA.

To configure an NSSA, include the nssa statement:




area area-id {



nssa {

area-range network/mask-length <restrict> <exact> <override-metric metric>;



default-lsa {

default-metric metric;

metric-type type;

type-7;
}


(summaries | no-summaries);
}


}

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

By default, a default route is not advertised. To advertise a default route with the specified metric within the area, include the default-metric statement. You can configure this option only on area border routers.

To prevent an ABR from advertising summary routes into an NSSA, include the no-summaries statement. If you include the default-metric option in addition to the no-summaries statement, only the default route is advertised. The default route is a Type 3 LSA injected into the NSSA. To flood summary LSAs into the NSSA area, include the summaries statement. When summaries is configured (which is the default if the no-summaries statement is not specified), a Type 7 LSA is sent. To define the type of metric, include the metric-type statement.

To aggregate external routes learned within the area when a route is advertised to other areas, include one or more area-range statements. If you also include the restrict option, the aggregate is not advertised, effectively creating a route filter. All external routes learned within the area that do not fall into the range of one of the prefixes are advertised individually to other areas. To restrict an exact area range, include the exact option. For an example, you can suppress the exact 0/0 prefix from being advertised from a NSSA area into the backbone area by including both the exact and restrict options. To override the metric for the IP address range and configure a specific metric value, include the override-metric option.

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