rfc1093.txt

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         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.




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RFC 1093              NSFNET Routing Architecture          February 1989


      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.



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RFC 1093              NSFNET Routing Architecture          February 1989


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>
                           nogendefault
                           acceptdefault
                           defaultout <egpmetric>
                           validate

   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

   nogendefault

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

   acceptdefault

        If specified, the default will be accepted from this



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RFC 1093              NSFNET Routing Architecture          February 1989


        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.

   validate

        If specifed, all nets learned from this EGP neighbor must
        have a corresponding 'validAS' clause or they will be
        ignored.

   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)



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RFC 1093              NSFNET Routing Architecture          February 1989


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

   Where:

        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

   Phone: [phone]

   Email: [e-mail]
















Braun                                                           [Page 9]