Cisco Systems 78-11741-02 manual Configuring Ospf Nssa, IPC-229

Configuring OSPF

Configuring OSPF NSSA

Stub areas are areas into which information on external routes is not sent. Instead, there is a default external route generated by the ABR, into the stub area for destinations outside the autonomous system. To take advantage of the OSPF stub area support, default routing must be used in the stub area. To further reduce the number of LSAs sent into a stub area, you can configure the no-summarykeyword of the area stub router configuration command on the ABR to prevent it from sending summary link advertisement (LSAs type 3) into the stub area.

To specify an area parameter for your network, use the following commands in router configuration mode as needed:

Command	Purpose
Router(config-router)# area area-idauthentication	Enables authentication for an OSPF area.
Router(config-router)# area area-idauthentication	Enables MD5 authentication for an OSPF area.
message-digest
Router(config-router)# area area-idstub	Defines an area to be a stub area.
[no-summary]
Router(config-router)# area area-iddefault-cost	Assigns a specific cost to the default summary route used for the
cost	stub area.

Configuring OSPF NSSA

The OSPF implementation of NSSA is similar to OSPF stub area. NSSA does not flood type 5 external LSAs from the core into the area, but it can import autonomous system external routes in a limited fashion within the area.

NSSA allows importing of type 7 autonomous system external routes within NSSA area by redistribution. These type 7 LSAs are translated into type 5 LSAs by NSSA ABRs, which are flooded throughout the whole routing domain. Summarization and filtering are supported during the translation.

Use NSSA to simplify administration if you are an Internet service provider (ISP) or a network administrator that must connect a central site using OSPF to a remote site that is using a different routing protocol.

Prior to NSSA, the connection between the corporate site border router and the remote router could not be run as OSPF stub area because routes for the remote site could not be redistributed into stub area, and two routing protocols needed to be maintained. A simple protocol like RIP was usually run and handled the redistribution. With NSSA, you can extend OSPF to cover the remote connection by defining the area between the corporate router and the remote router as an NSSA.

To specify area parameters as needed to configure OSPF NSSA, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# area area-idnssa	Defines an area to be NSSA.
[no-redistribution][default-information-originate]

Cisco IOS IP Configuration Guide

IPC-229