RFC 1222 (rfc1222) - Page 2 of 6

Advancing the NSFNET routing architecture

RFC 1222       Advancing the NSFNET Routing Architecture        May 1991


   Network metrics were translated among the three network levels
   (backbone, mid-level networks, and campuses).

   With the development of the gatedaemon, sites were able to introduce
   filtering based on IP network numbers.  This process was controlled
   by the staff at each individual site.

   Once specific network routes were learned, the infrastructure
   forwarded metric changes throughout the interconnected network. The
   end-result was that a metric fluctuation on one end of the
   interconnected network could permeate all the way to the other end,
   crossing multiple network administrations.  The frequency of metric
   fluctuations within the Backbone itself was further increased when
   event-driven updates (e.g., metric changes) were introduced.  Later,
   damping of the event driven updates lessened their frequency, but the
   overall routing environment still appeared to be quite unstable.

   Given that only limited tools and protocols were available to
   engineer the flow of dynamic routing information, it was fairly easy
   for routing loops to form.  This was amplified as the topology became
   more fully connected without insulation of routing components from
   each other.

   All six nodes of the Phase 1 Backbone were located at client sites,
   specifically NSF funded supercomputer centers.


2. NSFNET Phase 2 Routing Architecture

   The routing architecture for the second phase of the NSFNET Backbone,
   implemented on T1 (1.5Mbps) lines, focused on the lessons learned in
   the first NSFNET phase.  This resulted in a strong decoupling of the
   IGP environments of the backbone network and its attached clients
   [5].  Specifically, each of the administrative entities was able to
   use its own IGP in any way appropriate for the specific network.  The
   interface between the backbone network and its attached client was
   built by means of exterior routing, initially via the Exterior
   Gateway Protocol (EGP) [1,4].

   EGP improved provided routing isolation in two ways.  First, EGP
   signals only up/down transitions for individual network numbers, not
   the fluctuations of metrics (with the exception of metric acceptance
   of local relevance to a single Nodal Switching System (NSS) only for
   inbound routing information, in the case of multiple EGP peers at a
   NSS).  Second, it allowed engineering of the dynamic distribution of
   routing information.  That is, primary, secondary, etc., paths can be
   determined, as long as dynamic externally learned routing information
   is available.  This allows creation of a spanning tree routing



Braun & Rekhter