5.6 Not-So-Stubby Areas
5.6.2 How NSSA operates
By configuring an area as an NSSA, you can minimize routing tables within the area but still import external routing information into OSPF.

Figure illustrates the example network, including an NSSA implementation. RTA can import external routes as Type 7 LSAs, and ABRs will translate Type 7 LSAs into Type 5 LSAs as they leave the NSSA. A benefit of Type 7 LSAs is that they can be summarized. The OSPF specification prohibits the summarizing or filtering of Type 5 LSAs. It is an OSPF requirement that Type 5 LSAs always be flooding throughout a routing domain. When you define an NSSA, you can import specific external routes as Type 7 LSAs into the NSSA. In addition, when translating Type 7 LSAs to be imported into nonstub areas, you can summarize or filter the LSAs before importing them as Type 5 LSAs.

NSSAs are often used when a remote site (which uses RIP or IGRP) must be connected to a central site using OSPF. You can use NSSA to simplify the administration of this kind of topology. Before NSSA, the connection between the corporate site ABR and the remote router used RIP or EIGRP. This meant maintaining two routing protocols. Now, with NSSA, you can extend OSPF to handle the remote connection by defining the area between the corporate router and the remote router as an NSSA.

In Figure , 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. If you configure a standard OSPF area between the two networks, the slow WAN link could be overwhelmed by the ensuing flood of LSAs, especially Type 5 external LSAs. As an alternative, you could configure a RIP domain between the two networks, but that would mean running two routing protocols on the central site's routers. A more attractive solution is to configure an OSPF area and define it as a NSSA.

In this scenario, RTA 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:

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

It is at this point that RTB 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.