5.6 Using and Configuring NSSA
5.6.2 How NSSA operates
In the graphic, the central site and branch office are interconnected through a slow WAN link. The branch office is not using OSPF, but the central site is. Rather than define a RIP domain to connect the sites, you can define an NSSA..

In this scenario, Router A is defined as an ASBR. It is configured to redistribute any routes within the RIP/EIGRP domain to the NSSA. The following is a description of what happens when the area between the connecting routers is defined as an NSSA:

  1. Router A receives RIP or EIGRP routes for networks 10.10.0.0/16, 10.11.0.0/16, and 20.0.0.0/8.
  2. Because Router A is also connected to an NSSA, it redistributes the RIP or EIGRP routers as Type 7 LSAs into the NSSA.
  3. Router B, an ABR between the NSSA and the backbone Area 0, receives the Type 7 LSAs.
  4. After the SPF calculation on the forwarding database, Router B translates the Type 7 LSAs into Type 5 LSAs and then floods them throughout Backbone Area 0.

It is at this point that router B could have summarized routes 10.10.0.0/16 and 10.11.0.0/16 as 10.0.0.0/8, or could have filtered one or more of the routes.