RFC 3101 (rfc3101) - Page 2 of 33

The OSPF Not-So-Stubby Area (NSSA) Option

RFC 3101       The OSPF Not-So-Stubby Area (NSSA) Option    January 2003


Table Of Contents

   1.0 Overview .................................................  2
      1.1 Motivation - Transit Networks .........................  2
      1.2 Motivation - Corporate Networks .......................  4
      1.3 Proposed Solution .....................................  5
   2.0 NSSA Intra-Area Implementation Details ...................  7
      2.1 The N-bit .............................................  7
      2.2 Type-7 Address Ranges .................................  7
      2.3 Type-7 LSAs ...........................................  8
      2.4 Originating Type-7 LSAs ...............................  9
      2.5 Calculating Type-7 AS External Routes ................. 10
      2.6 Incremental Updates ................................... 14
      2.7 Optionally Importing Summary Routes ................... 14
   3.0 Intra-AS Implementation Details .......................... 15
      3.1 Type-7 Translator Election ............................ 15
      3.2 Translating Type-7 LSAs into Type-5 LSAs .............. 16
      3.3 Flushing Translated Type-7 LSAs ....................... 19
   4.0 Security Considerations .................................. 20
   5.0 Acknowledgements ......................................... 21
   6.0 Contributors ............................................. 22
   7.0 References ............................................... 22
   Appendix A: The Options Field ................................ 23
   Appendix B: Router-LSAs ...................................... 24
   Appendix C: Type-7 LSA Packet Format ......................... 26
   Appendix D: Configuration Parameters ......................... 27
   Appendix E: The P-bit Policy Paradox ......................... 28
   Appendix F: Differences from RFC 1587 ........................ 30
   Author's Addresses ........................................... 32
   Full Copyright Statement ..................................... 33

1.0  Overview

1.1  Motivation - Transit Networks

   Wide-area transit networks often have connections to moderately
   complex "leaf" sites.  A leaf site may have multiple IP network
   numbers assigned to it.  Typically, one of the leaf site's networks
   is directly connected to a router provided and administered by the
   transit network while the others are distributed throughout and
   administered by the site.  From the transit network's perspective,
   all of the network numbers associated with the site make up a single
   "stub" entity.  For example, BBN Planet has one site composed of a
   class-B network, 130.57.0.0, and a class-C network, 192.31.114.0.
   From BBN Planet's perspective, this configuration looks something
   like the diagram on the next page, where the "cloud" consists of the
   subnets of 130.57 and network 192.31.114, all of which are learned by
   RIP on router BR18.



Murphy                      Standards Track