NSFNET backbone SPF based Interior Gateway Protocol
RFC - Unknown
(October 1988; No errata)
||RFC Editor Note
RFC 1074 (Unknown)
||Send notices to
Network Working Group J. Rekhter
Request for Comments 1074 T.J. Watson Research Center
The NSFNET Backbone SPF based Interior Gateway Protocol
Status of this Memo
This memo is an implementation description of the standard ANSI IS-IS
and ISO ES-IS routing protocols within the NSFNET backbone network.
Distribution of this memo is unlimited.
I would like to express my thanks to Hans-Werner Braun (MERIT) for
his contribution to this document.
This document provides an overview of the NSFNET Backbone routing
with specific emphasis on the intra-backbone routing.
By the end of 1987, the American National Standardization Institute
(ANSI) forwarded a specification for an Intermediate System to
Intermediate System routing protocol to the International
Standardization Organizations (ISO) for the adaptation as an
international standard. This ANSI IS-IS protocol is used as the
interior gateway protocol (IGP) of the NSFNET backbone. Documented
here is an implementation description which also includes further
definitions that were necessary for the integration into an Internet
Protocol (IP) environment. Therefore, it should be viewed as a
continuation of the specifications of the ANSI IS-IS protocol  and
the ISO standard End System to Intermediate System (ES-IS) protocol
. While the ANSI IS-IS protocol suffices as an IGP, additional
methods are used to orchestrate routing between the backbone and the
attached mid-level networks; most notably the Exterior Gateway
Protocol (EGP). Further information about the overall NSFNET routing
as well as some future aspects can be found in , ,  and .
2. A brief overview of the NSFNET backbone
The NSFNET backbone is a wide area network which currently connects
thirteen sites within the continental United States. All connections
are permanent point-to-point links at T1 speed (1.544Mbps). These T1
links may contain multiple logical links at sub-T1 and up to the full
T1 speed. The result is a hybrid circuit/packet switching network
able to contain a connectivity-richer logical topology than the
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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 . The routing protocol which is used between the
backbone and the attached mid-level networks is the Exterior Gateway
Protocol (EGP) . 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
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
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 . The ES-IS
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