RFC 1074 (rfc1074) - Page 2 of 5

NSFNET backbone SPF based Interior Gateway Protocol

RFC 1074             NSFNET Backbone SPF based IGP          October 1988


   underlying physical topology would allow by itself.  Each site has a
   Nodal Switching Subsystem (NSS) which is responsible for packet
   switching.  Each NSS is a RISC technology based multiprocessor system
   using IBM RT/PC processors which operate a modified version of a
   4.3BSD kernel.  For the purpose of routing, each NSS is considered as
   a single entity which has connections to both other NSS (via the
   logical network infrastructure) and to regional networks (via local
   area network attachments; typically an Ethernet).

   The routing protocol which is used for the inter-NSS routing within
   the NSFNET backbone is an adaptation of the ANSI IS-IS routing
   protocol [1].  The routing protocol which is used between the
   backbone and the attached mid-level networks is the Exterior Gateway
   Protocol (EGP) [3].  The information exchange between the backbone
   and its connected EGP peers is subject to policy based routing
   restrictions which are maintained in the Policy Based Routing
   Database [4,5].

3.  An overview of the ANSI IS-IS routing document

   The ANSI IS-IS routing protocol specifies a two level hierarchical
   routing where Level 1 routing deals with routing within an area,
   while Level 2 routing deals with routing between different areas.

   This routing protocol belongs to a class of so called "Link State"
   protocols where each node maintains a complete topology of the whole
   network.  The route computation is based on a modified version of
   Dijkstra's Shortest Path First (SPF) algorithm.

   Both Level 1 and Level 2 routing use two types of Protocol Data Units
   (PDU):

        The Level 1 Router Link PDU lists IS neighbors.  The Level 1 End
        System PDU lists ES neighbors.

        The Level 2 Router Link PDU lists neighbor Level 2 routes.  The
        Level 2 End System PDU lists address prefixes for systems in
        other Routing Domains.

   The ANSI IS-IS document separates subnetwork independent functions
   from the subnetwork dependent functions.  Subnetwork independent
   functions include dissemination of Router Link and End System Link
   PDU's and the Routing Algorithm.  The subnetwork dependent functions
   cover different types of subnets such as X.25, permanent point-to-
   point links and LANs.

   The IS-IS Protocol is designed to interoperate with the End System to
   Intermediate System (ES-IS) routing exchange protocol [2].  The ES-IS



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