When you start with OSPF, everything seems to be simple. But later on you learn about many differnent types of OSPF areas.
- Stub area – has no connection to external networks, it does not contain any external routes, relieys mostly on default routes, does not have AS-external routes (Type 5 LSAs)
- Totaly Stub area – is an extension to the stub area and includes external routes and routes to other area over a default route, get these by Typ 3LSAs, does not support summary routes
- Backbone area – is always area zero
- NSSA, Not so stubby area – the same as SA, but can import external routes and send them to the backbone or other areas, type 3 and type 4 summary routes are not advertised into NSSA
Source:
http://en.wikipedia.org/wiki/Open_Shortest_Path_First