Stub, Totally Stubby, NSSA what's the difference?

Stub Area

A stub area is an area in which you do not allow advertisements of external routes (LSA Type 5), which thus reduces the size of the database even more. Instead, a default summary route (0.0.0.0) is inserted into the stub area in order to reach these external routes.

Totally Stubby Area

A stub area that only allows routes internal to the area and restricts Type 3 LSAs from entering the stub area is often called a totally stubby area. You can convert an area to a totally stubby area by configuring the ABR to only advertise and allow the default route to enter into the area. In short only intra-area routes are allowed within a totally stubby area, no type 3 LSA (inter-area LSA) allowed, only a default route in injected to send traffic out of the area.

Not So Stubby Area (NSSA)

RFC3101 describes an OSPF area called not-so-stubby area (NSSA). The motive 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. Once an external route converted to type 7 passes the NSSA it is converted to type 5 by the ABR is advertised as type 5 in rest of the network. NSSA does not allow types 5 routes from other areas.

Stub vs NSSA

No redistributed routes (External) appear in the routing table of a stub area. All inter-area routes appear in the stub network along with a default route is injected. With the totally stubby (area xx stub no-summary) all inter-area routes are removed and only a default route 0.0.0.0/0 appears in the routing table for OSPF learned routes.

In the case of NSSA when we redistribute from another routing protocol into OSPF and configure NSSA we see the routes shown as N2 on the ABR router of NSSA. Which means N2 - OSPF NSSA external type 2

Gateway of last resort is not set
Output Omitted
O N2 10.10.125.0/30 [110/20] via 10.10.35.5, 00:19:54, Ethernet1/0
O N2 10.10.125.125/32 [110/20] via 10.10.35.5, 00:16:40, Ethernet1/0

N2 (OSPF NSSA) routes and E2 (OSPF External type 2) routes do not appear in stub areas.

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