User's Manual

Configuring OSPF
Configuring OSPF NSSA
IPC-229
Cisco IOS IP Configuration Guide
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-summary keyword 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:
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-id authentication
Enables authentication for an OSPF area.
Router(config-router)# area area-id authentication
message-digest
Enables MD5 authentication for an OSPF area.
Router(config-router)# area area-id stub
[no-summary]
Defines an area to be a stub area.
Router(config-router)# area area-id default-cost
cost
Assigns a specific cost to the default summary route used for the
stub area.
Command Purpose
Router(config-router)# area area-id nssa
[no-redistribution] [default-information-originate]
Defines an area to be NSSA.