RFC 2966 (rfc2966) - Page 2 of 14

Domain-wide Prefix Distribution with Two-Level IS-IS

RFC 2966            Domain-wide Prefix Distribution         October 2000


   RFC 1195 [2] defines the Type, Length and Value (TLV) tuples that are
   used to transport IPv4 routing information in IS-IS.  RFC 1195 also
   specifies the semantics and procedures for interactions between
   levels.  Specifically, routers in a L1 area will exchange information
   within the L1 area.  For IP destinations not found in the prefixes in
   the L1 database, the L1 router should forward packets to the nearest
   router that is in both L1 and L2 (i.e., an L1L2 router) with the
   "attached bit" set in its L1 Link State Protocol Data Unit (LSP).

   Also per RFC 1195, an L1L2 router should be manually configured with
   a set of prefixes that summarizes the IP prefixes reachable in that
   L1 area.  These summaries are injected into L2.  RFC 1195 specifies
   no further interactions between L1 and L2 for IPv4 prefixes.

1.1 Motivations for domain-wide prefix distribution

   The mechanisms specified in RFC 1195 are appropriate in many
   situations, and lead to excellent scalability properties.  However,
   in certain circumstances, the domain administrator may wish to
   sacrifice some amount of scalability and distribute more specific
   information than is described by RFC 1195.  This section discusses
   the various reasons why the domain administrator may wish to make
   such a tradeoff.

   One major reason for distributing more prefix information is to
   improve the quality of the resulting routes.  A well know property of
   prefix summarization or any abstraction mechanism is that it
   necessarily results in a loss of information.  This loss of
   information in turn results in the computation of a route based upon
   less information, which will frequently result in routes that are not
   optimal.

   A simple example can serve to demonstrate this adequately.  Suppose
   that a L1 area has two L1L2 routers that both advertise a single
   summary of all prefixes within the L1 area.  To reach a destination
   inside the L1 area, any other L2 router is going to compute the
   shortest path to one of the two L1L2 routers for that area.  Suppose,
   for example, that both of the L1L2 routers are equidistant from the
   L2 source, and that the L2 source arbitrarily selects one L1L2
   router.  This router may not be the optimal router when viewed from
   the L1 topology.  In fact, it may be the case that the path from the
   selected L1L2 router to the destination router may traverse the L1L2
   router that was not selected.  If more detailed topological
   information or more detailed metric information was available to the
   L2 source router, it could make a more optimal route computation.






                             Informational