RFC 1222 (rfc1222) - Page 2 of 6
Advancing the NSFNET routing architecture
Alternative Format: Original Text Document
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