Cisco IOS Cookbook, 2nd Edition

Selecting the Appropriate Area Types

Problem

You want to limit the number of routes and entries in the Link State database to conserve router resources and ensure good convergence properties.

Solution

In the Introduction to this chapter, we talked about the various types of OSPF areas. You can configure these different types areas by using the appropriate keywords on the area command.

For a Stubby Area, use the stub keyword:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router ospf 55
Router1(config-router)#area 100 stub 
Router1(config-router)#exit
Router1(config)#end
Router1#

To configure a Totally Stubby Area, combine the stub and no-summary keywords on the ABR router:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router ospf 55
Router1(config-router)#area 100 stub no-summary
Router1(config-router)#exit
Router1(config)#end
Router1#

Only the ABRs needs the no-summary keyword, because they are the only routers that will be doing the route summarization. The other routers in a Totally Stubby Area need only be configured using the stub keyword, as in the previous example.

For Not So Stubby Areas (NSSA), you need to specify the nssa keyword. In this case we have also included the default-information-originate option so that the router can summarize external routes to a single default route:

Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. ...