i got stuck with NSSA behaviour. I might overlook manything. But i got usefull link from discussion groups

www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.sh...

Default Route in NSSA
There are two ways to have a default route in an NSSA. When you configure an area as NSSA, by default the NSSA ABR does not generate a default summary route. In the case of a stub area or an NSSA totally stub area, the NSSA ABR does generate a default summary route.

Default Summary Route
By defining an area as a NSSA totally stub area, the NSSA ABR generates a default summary route. As mentioned, if the NSSA area were not defined as totally stub, then a default summary route is not generated by NSSA ABR. This configuration generates a default summary route for a NSSA totally stub area.

router ospf 1
Area 1 nssa no-summary
Default Type 7
This configuration generates a type 7 default route. You can configure this command on any NSSA ASBR or NSSA ABR with these rules:

NSSA ASBR can generate a default only when it has a default route in its routing table.

The default route must be known through non-OSPF protocol

NSSA ABR can generate a default route with or without a default route in its own routing table.

This command is used in order to generate an NSSA default route:

router ospf 1
Area 1 nssa default-information-originate