RFC 1092 (rfc1092) - Page 2 of 5

EGP and policy based routing in the new NSFNET backbone

RFC 1092            IP EGP and Policy Based Routing        February 1989


      however, the NSFNET (due to the presence of backdoor routes) does
      not fit into this model.  In the NSFNET the same network may be
      advertized as reachable by more than one regional network.
      Besides the fact that the overall NSFNET does not fit into a
      spanning tree model there are serious concerns with the concept
      of the "core" (central to the EGP) and its obvious deficiencies.

      While EGP is going to isolate intra-Regional routing from the
      intra-NSFNET-Backbone routing, it does not address the issue of
      false information which may be supplied by regional networks.
      EGP by itself does not protect a particular network from unwanted
      and unsolicited representation by some regional network.  As an
      example, if network N1 is reachable through regional network R1
      as well as through regional network R2, EGP has no provisions to
      specify one of these paths as a primary and one as a secondary,
      since there is not generally accepted interpretation of EGP
      metrics today.  Also, there is nothing in EGP which can prevent one
      or more regional networks from advertizing other networks (in
      particular, networks which belong to other regional networks) as
      reachable with zero distance.  This could result in the creation
      of a "black hole" or at least in suboptimal IP routing.

      EGP by itself has no provisions to guarantee that routes through
      the NSFNET Backbone will be preferred over routes through the
      backdoor routers or vice versa.

Policy Based Routing

   Looking at the problems listed above the appearance of the new
   factors like autonomy and mutual trust becomes obvious.  While trying
   to achieve the routing functionality required for the new NSFNET
   backbone we should realize that one of our primary concerns has to be
   the accommodation of those new factors.

   This means that some kind of a rudimentary Policy Based Routing
   method becomes imperative.  We would like to emphasize, however, that
   we are not talking about complete Policy Based Routing, but that we
   are rather concerned about supporting a minimum subset of a policy
   functionality to be an initial solution to the above mentioned
   problems.  This requires support and cooperation between the
   management of each of the networks connected to the NSFNET backbone.

   We need to support the ability of a particular network N, which
   belongs to one of the regional networks, to establish a bilateral
   agreement with one or more regional networks of the type "network N
   can be reached via one or more regional networks (RN1, RN2, ...
   RNx)".  This allows each network to select one or more
   representatives at the regional network level.  Once this agreement



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