AIS AI2524 user manual Configure Ospf Not So Stubby Area Nssa, Define an area to be a stub area

Chapter 8: AI2524 Protocol Configuration Steps

zDefine an area to be a stub area.

area area-idstub [no-summary]

zAssign a specific cost to the default summary route used for th stub area.

area area-iddefault-cost cost

Configure OSPF Not So Stubby Area (NSSA)

NSSA area is similar to OSPF stub area. NSSA does not flood Type 5 external link state advertisements (LSAs) from the core into the area, but it can import AS external routes in a limited fashion within the area.

NSSA allows importing of Type 7 AS external routes within NSSA area by redistribution. These Type 7 LSAs are translated into Type 5 LSAs by NSSA Area Border Router (ABR), 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 proto- col.

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. A simple protocol like RIP was usually run to handle the redistribution. This meant maintaining two routing protocols. With NSSA, you can extend OSPF to cover the remote connection by defining the area be- tween the corporate router and the remote router as an NSSA.

In router configuration mode, specify these area parameters as needed to configure OSPF NSSA:

area area-idnssa [no-redistribution] [default-information-originate]

In router configuration mode on the ABR, specify this command to control summarization and filtering of Type 7 LSA into Type 5 LSA (optional):

summary address prefix mask [not advertise] [tag tag]

August 1997	Page 8-5
2524UM