External routes cannot be imported to the stub area. To import external routes to the OSPF routing table and still keep other stub area features, the network administrator can configure the area as the NSSA area.
Stub areas cannot import external routes, thus leading to the concept of NSSA area. In NSSA area, Type7 LSA transmission is allowed. Originated by ASBR in NSSA area, Type7 LSA is transformed to AS-External LSA when it reaches the ABR of NSSA, and is further advertised to other areas.
You must use the nssa command to configure the NSSA area with NSSA attribute on the routers connected to the NSSA area.
The parameters in the nssa command are valid only when you configure the command on ABRs.
ospf [ process-id ]
area area-id
nssa [ { default-route-advertise | suppress-default-route } | flush-waiting-timer interval-value | no-import-route | no-summary | set-n-bit | suppress-forwarding-address | translator-always | translator-interval interval-value | zero-address-forwarding | translator-strict ] *
default-cost cost
The default-cost cost command can be configured on ABRs only.