Question: what is nssa in ospf?
Answer: OSPF NSSA. RFC3101 describes an OSPF area called not-so-stubby area (NSSA). The motivation behind NSSA is to allow OSPF Stub areas to carry External routes (routes learned using other routing protocols like RIP, EIGRP, BGP, etc). Stub areas are defined as areas that are not capable of importing routes external to OSPF.