OSPF Area types
Configuration of OSPF gives us a choice between 5 different areatypes:
- Totally Stub
- Not So Stubby Area
- Totally Not So Stubby Area (omg what a name?!)
To just list all the different types and behavior would not give the reader much help, it would just be confusing, so I decided to break it down the same way i did to learn it myself.
The normal area is not much to tell about, we already covered normal interarea behaviour.
Lets start with Stub area
The difference between a normal area and Stub Area is quite simple, the ABR filters away all external (and ASBR summary) and injects a default route to the area. Simplified: it filters routes that were redistributed into OSPF and the LSA info to the redistributing router and replaces those with one single LSA type 3 for the default route.
Totally Stubby areas, is easy to understand basically by looking at the configuration command. Its configured with the command “area 200 stub no summary” just as the command implies, its a stub area which also filters summary LSA’s. That means that it does the same filtering as Stub, and it also filters interarea prefixes.
Not So Stubby Area
The NSSA area is a bit different. This one is not just filtering inbound LSA’s like the Stub area, it also leaks redistributed routes from NSSA area. The NSSA ASBR router (meaning a router in NSSA area that redistributes routes) creates a type 7 LSA which floods the area, the NSSA ABR then translates those into normal type5 LSA’s as they are flooded to Area0.
Totally Not So Stubby Area
Totally NSSA is an area that acts as NSSA and also filters summary routes (like the difference between stub and totally stub) and is configured with “area 400 nssa no-summary”.