Network Working Group
Request for Comments: 1093
H.W. Braun
February 1989

The NSFNET Routing Architecture

Status of this Memo

This document describes the routing architecture for the NSFNET centered around the new NSFNET Backbone, with specific emphasis on the interface between the backbone and its attached networks. Distribution of this memo is unlimited.


This document describes the routing architecture for the NSFNET centered around the new NSFNET Backbone, with specific emphasis on the interface between the backbone and its attached networks. It reflects and augments thoughts described in [1], discussions during the Internet Engineering Task Force meeting at the San Diego Supercomputing Center in March 1988, discussions on mailing lists, especially on a backbone/regional network working group mailing list, and a final discussion held at the IBM T.J. Watson Research Center in Yorktown, NY, on the 21st of March 1988. The Yorktown meeting was attended by Hans-Werner Braun (Merit), Scott Brim (Cornell University), Mark Fedor (NYSERNet), Jeff Honig (Cornell University), and Jacob Rekhter (IBM). Thanks also to: Milo Medin (NASA), John Moy (Proteon) and Greg Satz (Cisco) for discussing this document by email and/or phone.

Understanding of [1] is highly recommended prior to reading this document.

1. Routing Overview

The new NSFNET backbone forms the core of the overall NSFNET, which connects to regional networks (or regional backbones) as well as to peer networks (other backbones like the NASA Science Network or the ARPANET). The NSFNET core uses a SPF based internal routing protocol, adapted from the IS-IS protocol submitted by ANSI for standardization to the ISO. The ANSI IS-IS protocol is based upon work done at Digital Equipment Corporation. Its adaptation to the Internet environment requires additional definitions, most notably to the addressing structure, which will be described in a later document. This adaptation was largely done by Jacob Rekhter of IBM Research in Yorktown, NY. The RCP/PSP routing architecture was largely implemented by Rick Boivie and his colleagues at IBM TCS in

Milford, CT. The adaptation of EGP to the NSS routing code and the new requirements was done jointly by Jeff Honig (who spent about a week to work on this at IBM Research) and Jacob Rekhter. Jeff is integrating the changes done for the new EGP requirements into the "gated" distributions.

The IGP derives routing tables from Internet address information. This information is flooded throughout the NSFNET core, and the individual NSS nodes create or update their routing information after running the SPF algorithm over the flooded information. A detailed description of the NSFNET backbone IGP will be documented in a future document.

The routing interface between the NSFNET core and regional backbones as well as peer networks utilizes the Exterior Gateway Protocol (EGP). The EGP/IGP consistency and integrity at the interface points is ensured by filtering mechanisms according to individual nodal routing policy data bases [1]. EGP is selected as the routing interface of choice between the NSFNET backbone and its regional attachments due to its widespread implementation as well its ability to utilize autonomous system designators and to allow for effective firewalls between systems. In the longer run the hope is to replace the EGP interface with a new inter Autonomous System protocol. Such a new protocol should also allow to move the filtering of network numbers or Autonomous Network number groups to the regional gateways in order for the regional gateways to decide as to what routing information they wish to receive.

A general model is to ensure consistent routing information between peer networks. This means that, e.g., the NSFNET core will have the same sets of Internet network numbers in its routing tables as are present in the ARPANET core. However, the redistribution of this routing information is tightly controlled and based on Autonomous System numbers. For example, ARPANET routes with the ARPANET Autonomous System number will not be redistributed into regional or other peer networks. If an NSFNET internal path exists to such a network known to the ARPANET it may be redistributed into regional networks, subject to further policy verification. Generally it may be necessary to have different trust models for peer and subordinate networks, while giving a greater level of trust to peer networks.

The described use of EGP, which is further elaborated on in [1] requires bidirectional translation of network information between the IGP in use and EGP.

2. Conclusions reached during the discussions

The following conclusions were reached during the meeting and in subsequent discussions:

No DDN-only routes (ARPANET/MILNET) shall be announced into the regional backbones. This is a specific case of the ability to suppress information from specific Autonomous Systems, as described later.

Regional backbones are required to use an unique Autonomous System number. Announcements from non-sanctioned autonomous systems, relative to a particular site, will not be believed and will instead trigger an alarm to the Network Operations Center.

Regional backbone attachments must not require routes to local subnets. This means that the locally attached network needs to use a flat space, without subnet bits, at least from the NSS point of view. The reason for this is that the EGP information exchanged between the regional gateway and the NSS cannot include subnet information. Therefore the NSS has no knowledge of remote subnets. The safest way to get around this limitation is to use a non-subnetted network (like a separate Class-C network) at the interface between a regional backbone and the NSFNET backbone. The other way is to use Proxy-ARP while having just the NSS think that the network is not subnetted. In the latter case care must be taken so that the E-PSP uses the proper local IP broadcast address.

Routing information received by the NSS from regional gateways will be verified on both network number and autonomous system number.

Metric reconstitution is done on a per-network basis. The NSS will construct the fixed metric it will use for a given network number from its internal data base. Network metrics given to the NSS via EGP will be ignored. The metrics used are a result of an ordered list of preferred paths as supplied by the regional backbones and the attached campuses. This metric is of relevance only to the NSFNET core itself. The mechanisms are further explained in [1].

Global metric reconstitution by Autonomous System numbers is necessary in specific cases, such as peer networks. An example is that ARPANET routes will be reconstituted to a global metric, as determined by the NSS.

      EGP announcements into regional networks will use a fixed metric.
      The metric used shall be "128."  The 128-metric is somewhat
      arbitrarily chosen to be high enough so that a regional backbone
      will get a metric high enough from the NSFNET Core AS to allow a

comparison against other (most likely internal) routes. "128" is also consistent with [2].

Peer network routes (e.g., ARPANET routes) are propagated through the NSS structure.

No DEFAULT routing information is distributed within the NSFNET backbone, as the NSFNET core has the combined routing knowledge of the attached regional and peer networks.

We do not expect the requirement for damping of routing update frequencies, at least initially. The frequency of net up/down changes combined with the available bandwidth and CPU capacity do not let the frequency of SPF floodings appear as being a major problem. Simple metric changes as heard by a NSS via EGP will not trigger updates.

An allowed list of Source Autonomous System information will be used to convert from the IGP to EGP, on a Destination Autonomous System number basis, to allow for specific exclusion of definable remote Autonomous System information.

EGP must only announce networks for which the preferred path is via the IGP. This means in particular that the EGP peer will never announce via EGP what it learned via EGP on the same interface, not even if the information was received from a third EGP peer. This will avoid the back-distribution of information learned via that same interface. The EGP peers of regional gateways must only announce information belonging to their own Autonomous System.

EGP will be used in interior mode only.

The regional backbones are responsible for generating DEFAULT routing information at their option. One possibility is to generate an IGP default on a peer base as long as the NSS EGP connection is working. The EGP information will not include a special indication for DEFAULT.

It is highly desirable to have direct peer-peer connections, to ease the implementation of a consistent routing data base.

A single Autonomous System number may not be used with two E-PSPs at the same time as long as the two E-PSP's belong to the same NSS. Otherwise the same Autonomous System number can be used from multiple points of attachment to the backbone and therefore can talk to more than one E-PSP. However, this may result in suboptimal routing unless multiple announcements are properly engineered according to [1].

The administrator of the regional networks should be warned that improper routing implementations within the region may create suboptimal regional routing by using this restriction if no care is taken in that:

Only networks belonging to their own Autonomous System get preferred over NSFNET backbone paths; this may extend to a larger virtual Autonomous System if backdoor paths are effectively implemented.

IGP implementations should not echo back routing information heard via the same path.

If two regional networks decide to implement a backdoor connection between themselves, then the backdoor must have a firewall in so that information about their own Autonomous System cannot flow in from the other Autonomous System. That is, a regional network must not allow information about networks that are interior to its Autonomous System to enter via exterior routes. Likewise, if a regional network is connected to the NSFNET via two NSS connections, the NSS cannot send back information about the Autonomous System into the Autonomous System where it originated. The end effect is that partitions within an Autonomous System will not be healed by using the NSS system. In addition, if three or more regionals connect to each other via multiple back-door paths, it is imperative that all back-door paths have firewalls that ensure that the above restrictions are imposed. These actions are necessary to prevent routing loops that involve the NSS system. Furthermore routing information should only be accepted from another regional backbone via backdoor paths for networks which are positively desired to be reached via this same backdoor path.

3. EGP requirements for attached gateways

The following EGP requirements are necessary for attached gateways; they may require changes in existing vendor products:

IGP to EGP routing exchanges need to be bidirectional. This feature should be selectable by the gateway administrator, and by default be configured OFF.

The metric used when translating from EGP to IGP should be configurable.

It must be possible for IGP information to override EGP information, so that the internal paths are preferred over external paths. Overriding EGP information on an absolute basis, where an external path would never be used as long as there is an internal one, is acceptable.

The ability to do route filtering in the regional gateways on a per net basis is highly desirable to allow the regional gateways to do a further selection as to what routes they would want to redistribute into their network.

The existence of an EGP connection should optionally lead to the generation of a DEFAULT announcement for propagation via the IGP. The DEFAULT metric should be independently configurable.

EGP routes with a metric of "128" should be acceptable. In most cases the regional backbone should ignore the EGP metric.

The regional gateways must only announce networks known to their own Autonomous System. At the very least they must not redistribute routing information via EGP for routes previously learned via EGP.

It would be beneficial if the regional IGPs would tag routes as being EGP derived.

If the EGP peer (e.g., a NSS) terminates the EGP exchange the previously learned routes should expire in a timely fashion.

4. References

   [1]  Rekhter, J., "EGP and Policy Based Routing in the New NSFNET
        Backbone", T.J. Watson Research Center, IBM Corporation, March
        1988.  Also as RFC 1092, February 1989.
   [2]  Mills, D., "Autonomous Confederations", RFC 975, M/A-COM
        Linkabit, February 1986.
   [3]  Mills, D., "Exterior Gateway Formal Specification", RFC 904,
        M/A-COM Linkabit, April 1984.

[4] "Exterior Gateway Protocol, Version 3, Revisions and Extensions,"

Working Notes of the IETF WG on EGP, Marianne L. Gardner and Mike Karels, February 1988.

   [5]  "Management and Operation of the NSFNET Backbone Network,"
        proposal to the National Science Foundation, Merit Computer
        Network, August 1987.

5. Appendix

The following are extensions implemented for the "gated" EGP implementation, as designed by Jeff Honig of the Cornell University Theory Center. These extensions are still in the design stage and may be changed over time. They are included here as an implementation example.

Changes to egpneighbor clause:

   egpneighbor <address>   metricin <metric>
                           egpmetricout <egpmetric>
                           ASin <as>
                           ASout <as>
                           defaultout <egpmetric>
   metricin <metric>

If specified, the metric of all nets received from this neighbor are set to <metric>.

   egpmetricout <egpmetric>

If specified, the metric of all nets sent to this neighbor, except default, are set to <egpmetric>.

   ASin <as>

If specified, EGP packets received from this neighbor must

specify this AS number of an EGP error packet is generated.

The AS number is only checked at neighbor acquisition time.

   ASout <as>

If specified, this AS number is used on all EGP packets sent to thiqs neighbor


If specified, this neighbor is not considered when generating a gateway default.


If specified, the default will be accepted from this

neighbor, otherwise it will be explicitly ignored.

   defaultout <egpmetric>

If specified, the internally generated default is send to

this neighbor in EGP updates. Default learned from other

gateways is not propogated.


If specifed, all nets learned from this EGP neighbor must

have a corresponding 'validAS' clause or they will be


Addition of a validAS clause:

validAS <net> AS <as> metric <metric>

This clause specifies which AS a network may be learned from and what internal metric to use when the net is learned. The specifies the 'validate' option. Note that more than one may be learned from more than one AS.

Addition of sendAS and donotsendAS clauses:

These clauses control the announcement of exterior (currently only EGP) routes. Normally, exterior routes are not considered for announcement. When the 'sendAS' or 'donotsendAS' clauses are used, the announce/donotannounce, egpnetsreachable and other restrictions still apply. The 'sendAS' and 'donotsendAS' clauses are mutually exclusive by autonomous system.

sendAS <as0> ASlist <as1> <as2> ...

This clause specifies that only nets learned from as1, as2, ... may be propogated to as0.

donotsendAS <as0> ASlist <as1> <as2> ...

This clause specifies that nets learned from as1, as2, ... may not be propogated to <as0>, all other nets are propogated.

An example of a "/etc/gated.conf" file could include the following:

RIP supplier
autonomousystem (regional AS)

egpneighbor (NSS address) ASin (NSS AS) nogendefault metricin (metric)
sendAS (NSS AS) ASlist (regional AS)


        Regional AS   Is the AS number of the regional network
        NSS address   Is the IP address of the local NSS
        NSS AS        Is the AS number the NSFNET backbone
        Metric        Is the gated internal (time delay) metric that
                      EGP learned routes should have.  This is the
                      metric used on output after conversion to a RIP
                      metric.  Some values are:
                                   HELLO   RIP
                                   100     1
                                   148     2
                                   219     3
                                   325     4
                                   481     5

Author's Address:

Hans-Werner Braun
University of Michigan
Computing Center
1075 Beal Avenue
Ann Arbor, MI 48109


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