OSPF area types Stub, Totally Stubby, Not so Stubby

I am starting with the pre-req that everyone understands basic OSPF, Areas, Area Border Routers (ABR), Autonomous Systems, and Autonomous System Border Routers (ASBR)

Alright, I know I'm jumping around a lot here, but things come up in real life, in forums, in books, and in all the blogs I follow that gets me headed off on another track, so today I am talking about OSPF stub area types.

First, what are the types of OSPF Link State Advertisements? These are how OSPF routers communicate routes and route changes etc.

Type1 - Router - router announcing itself and lists links to other routers with metrics in the same area
Type2 - Network - lists what routers are connected by the broadcast segment.
Type3 - Net Summary - An Area Border Router summarizes one area, and sends it to other areas
Type4 - ASBR Summary - An ASBR summarizes external routes that are injected from another routing protocol
Type5 - AS External - Full network routes imported from other routing protocols
Type6 - Group Membership - A multicast advertisement that is not used in OSPFv3
Type7 - NSSA External - Routers in a not so stubby area (NSSA) send external routing information to the ABR to be redistributed (as type 5s to other areas)

There are 11 types, but these are the ones that affect stub areas for this discussion.

First, why use stub areas? Stub areas greatly minimize the traffic and routing table sizes of routers in these stub areas, increasing performance. (And area 0 can't be a stub of any sort)

First we start out with a stub area. A stub area allows Type 2 and Type 3 LSAs. Type 5 LSAs are not passed into the area. A default route is also passed in from the ABR.
Now, what does that mean, Type 2 and Type 3 LSAs allow the area routers to get inter and intra area routes. Routes in the same area are seen as usual, and the routes on the other side of your ABR is visible. Routes that are injected from other protocols, like static or EIGRP, are not advertised. To get to these, the router must use the default route.
**Remember - Stub= No type 5 LSAs and default route

Next we'll look at a Totally Stubby network. Only Type 2 LSAs and a default route are passed into the area (although technically the default route is a type 3... but thats the only one).

This means the routes inside the area are advertised, but all other destinations will use the default route. In other words, to a router in a Totally Stubby area, the ABR is the end of the world and everything not in it's own little area gets passed to the ABR to deal with.
**Remember - Totally Stubby - ONLY Type 2 LSAs and default route

Next we have Not-So-Stubby Areas. NSSAs have Type 2, Type 3, and Type 7 LSAs.
Type 7 LSAs are ONLY found in NSSAs. A NSSA is when you want your area to have external routes and be able to pass them to other areas, but not have other area's external routes imported into this area. In other words, local external routes are allowed, but not external routes from other areas. The ABR changes the Type 7 into a Type 5 when it passes it to other areas.

Cisco says: "NSSAs are more flexible than stub areas in that an NSSA can import external routes into the OSPF routing domain, thereby providing transit service to small routing domains that are not part of the OSPF routing domain."
**Remember - NSSA - Type 2, 3, and 7 LSAs with default route

Finally, we get to Totally Not-So-Stubby Area or officially called Not-So-Stubby Totally Stubby Area (seriously). This combines the attributes of a Totally Stubby and a NSSA area. First you have the Totally Stubby in which Type 3 and 4 advertisements are not passed into the area (Everything outside of the area is found through a default gateway), but you can still connect an external route to the area by Type 7 LSA.
Why use this? Perhaps you have a remote site that you want to connect back by default route only, but it also has a backup DSL or Cable modem for local internet.
**Remember - Totally NSSA - Only Type 2 and 7 LSAs and default route.