Cisco Systems 78-11741-02 manual Configuring Route Summarization Between Ospf Areas, IPC-230

Configuring OSPF

Configuring Route Summarization Between OSPF Areas

To control summarization and filtering of type 7 LSAs into type 5 LSAs, use the following command in router configuration mode on the ABR:

Command	Purpose
Router(config-router)# summary address {ip-address mask	Controls the summarization and filtering during the
prefix mask} [not advertise] [tag tag]	translation.

Implementation Considerations

Evaluate the following considerations before you implement this feature:

•You can set a type 7 default route that can be used to reach external destinations. When configured, the router generates a type 7 default into the NSSA or the NSSA ABR.

•Every router within the same area must agree that the area is NSSA; otherwise, the routers will not be able to communicate.

Configuring Route Summarization Between OSPF Areas

Route summarization is the consolidation of advertised addresses. This feature causes a single summary route to be advertised to other areas by an ABR. In OSPF, an ABR will advertise networks in one area into another area. If the network numbers in an area are assigned in a way such that they are contiguous, you can configure the ABR to advertise a summary route that covers all the individual networks within the area that fall into the specified range.

To specify an address range, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# area area-idrange ip-address mask	Specifies an address range for which a single route
[advertise not-advertise][cost cost]	will be advertised.

Configuring Route Summarization When Redistributing Routes into OSPF

When routes from other protocols are redistributed into OSPF (as described in the chapter “Configuring IP Routing Protocol-Independent Features”), each route is advertised individually in an external LSA. However, you can configure the Cisco IOS software to advertise a single route for all the redistributed routes that are covered by a specified network address and mask. Doing so helps decrease the size of the OSPF link-state database.

Cisco IOS IP Configuration Guide

IPC-230