This blog will discuss the benefits derived by configuring OSPF area type as Totally NSSA
OSPF Area type NSSA and Totally NSSA both allow redistribution within the NSSA area.
Such redistribution creates Type 7 LSAs which are converted to Type 5 LSA by the ABR.
Referencing the topology above, R1 or R2 which serve as ABRs would do the Type 7 to Type 5 translation as routes move from Area 1 to Area 0
Router with higher router ID is elected to do this translation where there are multiple ABRs interfacing backbone area 0
As compared to NSSA, a Totally NSSA will inject a default route automatically in the NSSA area.
Also, a Totally NSSA will allow only a single Type 3 LSA corresponding to the default route.
Compared to Totally NSSA, area configured as NSSA will not filter Type 3 LSAs and will only filter Type 4/Type 5 LSAs
Above output shows OSPF learned routes on router R1 which is the ABR
Network 10.13.13.0 has been redistributed on routers R3 and R4
Routers R3 and R4 have static route for DMZ1 towards next hop IP of firewall.
Notice a similar output is obtained on router R2
Notice in the output above that R3 is able to get a default route from both the ABRs upstream.
And this default route is a Type 3 LSA in case of Totally NSSA area.