Content

| 6.7 |  | Stub, Totally Stubby, and Not-So-Stubby Areas |
| 6.7.7 |  | How NSSA operates |

By configuring an area as an NSSA, routing tables can be minimized 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 flooded throughout a routing domain. When defining an NSSA, specific external routes can be imported as Type 7 LSAs into the NSSA. In addition, when translating Type 7 LSAs to be imported into nonstub areas, the LSAs can be summarized or filtered 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. 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, OSPF can be extended to handle the remote connection by defining the area between the corporate router and the remote router as an NSSA.

Figure shows the central site and branch offices interconnected through a slow WAN link. The branch office is not using OSPF, but the central site is. If a standard OSPF area between the two networks is configured, the slow WAN link could be overwhelmed by the ensuing flood of LSAs. This is especially true for Type 5 external LSAs. As an alternative, configure a RIP domain between the two networks, but that would mean running two routing protocols on the central site routers. A better 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 or 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.