RFC 1587: The OSPF NSSA Option
This RFC provides a description of a new type of optional OSPF area, the not-so-stubby area or NSSA. This optional stubby area is very similar in operation to the existing stubby areas, but it has the additional ability to import external OSPF routes from the Autonomous System to which it belongs.
This RFC is very good reading and its authors should be commended for bringing some of the real world into their discussion on the not-so-stubby area discussion. This RFC details a problem seen with the implementation of OSPF at the time of its writing. They provide a very good scenario and supporting documentation about the issue this RFC addresses.
Within this RFC, the authors propose adding a new option bit, referred to as the N bit and a new type of LSA area definition. This new N bit would assist in identifying routers that belong to a NSSA and allow them to agree upon the areas topology. The new LSA would allow for external route information to be exchanged within the area.
Discussion is provided on the new LSA and how it compares to existing LSAs and how the new LSA will operate. The need for NSSA area border routers to have a default route is also discussed and justified.
I would recommend reading more about this RFC because it provides a very good insight into how the OSPF protocol has matured and responded to the needs of its users. To assist the reader in clarifying that point, the following excerpt is provided from the RFC itself.
RFC 1745: BGP4/IDRP for IP-OSPF Interaction
This RFC has been included in this list in order to be as complete as possible, thereby helping the reader understand and be able to reference all the many sources of information available to them on OSPF.
This RFC provides the technical information necessary to design and deploy a network or implement an Autonomous System Border Router (ASBR) that will be running Border Gateway Protocol (BGP4) or Inter-Domain Routing Protocol (IDRP) for IP as your Exterior Gateway Protocol (EGP) with OSPF as your Interior Gateway Protocol (IGP). This document details the settings necessary between the fields and attributes of OSPF and the other protocols. BGP4 is referenced in RFC 1654.
RFC 1765: OSPF Database Overflow
This RFC deals with an undesirable occurrence known as OSPF Database Overflow. For OSPF to operate properly, a complete link-state database must be within each OSPF router in an area. The condition known as database overflow occurs when this link-state database becomes too large for the router to handle. This RFC allows for the handling on unanticipated overflows and gives some recommendations on how to configure your network if you are anticipating database overflow.
One way of handling database overflow is to encase routers having limited resources within OSPF stub areas or NSSAs. AS-external-LSAs are omitted from these areas link-state databases, thereby controlling database size.
However, unexpected database overflows cannot be handled in the above manner. This RFC describes a way of dynamically limiting database size under overflow conditions.
The method used to recover from unexpected database overflow is discussed in great detail and if you are interested or believe you are experiencing this condition, then consult the RFC.