rfc1745.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

                 imported from BGP/IDRP into the OSPF RD.  Hence, it is
                 assumed that the BGP/IDRP speaker will convey these
                 routes to other BGP/IDRP speakers within the same
                 autonomous system via BGP/IDRP.  An ASBR learning of
                 such a destination MUST wait for the BGP update from
                 its internal neighbours before advertising it to
                 external BGP/IDRP peers.

         export  These routes MUST not be exported into BGP/IDRP.








Varadhan, Hares & Rekhter                                      [Page 13]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


   4.4.  Miscellaneous tag settings

       0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|x|1|1|              Reserved  for  future  use               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The value of PathLength=11 is reserved during automatic tag
      generation.  Routers MUST NOT generate such a tag when importing
      reachable destinations into the OSPF routing domain.  ASBRs must
      ignore tags which indicate a PathLength=11.

5.  Setting OSPF Forwarding Address and BGP/IDRP NEXT_HOP attribute

   Forwarding addresses are used to avoid extra hops between multiple
   routers that share a common network and that speak different routing
   protocols with each other on the common network.

   Both BGP/IDRP and OSPF have equivalents of forwarding addresses.  In
   BGP and IDRP, the NEXT_HOP attribute is a well-known, mandatory
   attribute.  OSPF has a Forwarding address field.  We will discuss how
   these are to be filled in various situations.

   Consider the 4 router situation below:

   RT1 and RT2 are in one autonomous system, RT3 and RT4 are in another.
   RT1 and RT3 are talking BGP/IDRP with each other.  RT3 and RT4 are
   talking OSPF with each other.

            +-----+                 +-----+
            | RT1 |                 | RT2 |
            +-----+                 +-----+
               |                       |            common network
            ---+-----------------------+--------------------------
            <BGP/IDRP> |                       |
                    +-----+     <OSPF>      +-----+
                    | RT3 |                 | RT4 |
                    +-----+                 +-----+


     - Importing a reachable destination into OSPF:
          When importing a destination from BGP/IDRP into OSPF, RT3 MUST
          always fill the OSPF Forwarding Address with the BGP/IDRP
          NEXT_HOP attribute for the destination.






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RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


     - Exporting a reachable destination into BGP:
          When exporting set of reachable destinations internal to the
          OSPF routing domain from OSPF to BGP/IDRP, if all the
          destinations in the set of reachable destinations are through
          RT4, then RT3 MAY fill the NEXT_HOP attribute for the set of
          reachable destinations with the address of RT4.  This is to
          avoid requiring packets to take an extra hop through RT3 when
          traversing the AS boundary.  This is similar to the concept of
          indirect neighbour support in EGP [RFC888, RFC827].

6.  Changes from the BGP 3 - OSPF interactions document

     o    The use of the term "route" has attained a more complicated
          structure in BGP 4.  This document follows the constraint in
          the manner shown below:

          -    The term "set of reachable destinations" is called a NLRI
               in [RFC1654].

          -    The term "route" in the BGP context refers to a set of
               reachable destinations, and the associated attributes for
               the set.

          -    The term "route" in the OSPF context refers to the set of
               reachable destinations, and the cost and the type to
               reach destinations.  This is to keep the definitions
               consistent in the document.

     o    The notion of exchanging reachability information between BGP
          4 and OSPF has been updated to handle variable length net mask
          information.

     o    The previous term INTER_AS_METRIC in BGP 3 has now been
          changed to MULTI_EXIT_DISC.

     o    The default metric to be used for importing BGP information
          into the OSPF RD is now the LOCAL_PREF attribute, instead of a
          constant value.

     o    Section 3 which requires an ASBR to match the OSPF tag
          corresponding to a route to the BGP Identifier, can cause
          potential loops if the AS has equal cost multipath routing
          amongst the ASBRs.  This scenario is outlined in the Appendix
          below.  This is fixed in BGP4 by requiring the ASBR seeing
          equal cost multi-path routes to merge the PATHs through the
          various ASBRs into appropriate SETs.





Varadhan, Hares & Rekhter                                      [Page 15]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


     o    BGP 4 requires that the BGP identifier be an address assigned
          to the BGP speaker.  This is dealt with in section 3.

     o    Section 5 on setting NEXT_HOP attributes and Forwarding
          Address field has been updated to account for variable length
          net information.

     o    This section, 6, has been added.

7.  Security Considerations

   Security issues are not discussed in this memo.

8.  Acknowledgements

   We would like to thank Jeff Honig (Cornell University), John Moy
   (Cascade Communications Corp.), Tony Li (cisco Systems), Rob Coltun
   (Consultant), Dennis Ferguson (ANS, Inc.), Phil Almquist
   (Consultant), Scott Bradner (Harvard University), and Joel Halpern
   (Newbridge Networks Inc.) for their help and suggestions in writing
   this document.  Cengiz Aleattinoglu (USC/ISI) and Steve Hotz
   (USC/ISI) provided fresh insights into the packet looping problem
   described in the appendix.

   We would also like to thank the countless number of people from the
   OSPF and BGP working groups who have offered numerous suggestions and
   comments on the different stages of this document.

   Thanks also to Bob Braden (ISI), whose suggestions on the earlier
   BGP-OSPF document, [RFC1403] were useful even for this one, and have
   been carried through.

   We would also like to thank OARnet, where one of the authors did most
   of his work on this document, before moving to USC to resurrect his
   PhD.

9.  Bibliography

   [RFC827]  Rosen, E., "Exterior Gateway Protocol (EGP)", RFC 827,
             BBN, October 1982.

   [RFC888]  Seamonson, L., and E. Rosen, "`STUB' Exterior Gateway
             Protocol", RFC 888, BBN, January 1984.

   [RFC1058] Hedrick, C, "Routing Information Protocol", RFC 1058,
             Rutgers University, June 1988.





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RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


   [RFC1122] Braden, R., Editor, "Requirements for Internet Hosts -
             Communication Layers", STD 3, RFC 1122, USC/Information
             Sciences Institute, October 1989.

   [RFC1123] Braden, R., Editor, "Requirements for Internet Hosts -
             Application and Support", STD 3, RFC 1123,
             USC/Information Sciences Institute, October 1989.

   [RFC1247] Moy, J., "The OSPF Specification Version 2", RFC 1247,
             Proteon, January 1991.

   [RFC1403] Varadhan, K., "BGP OSPF Interaction", RFC 1403,
             OARnet, January 1993.

   [RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting:
             an Address Assignment and Aggregation Strategy", RFC 1519,
             BARRNet, cisco, Merit, OARnet, September 1993.

   [RFC1583] Moy, J., "The OSPF Specification Version 2", RFC 1583,
             (Obsoletes [RFC1247]), Proteon, March 1994.

   [RFC1654] Rekhter, Y., and T. Li, Editors, "A Border Gateway
             Protocol 4 (BGP-4)", RFC 1654, T.J. Watson Research Center,
             IBM Corp., cisco Systems, July 1994.

   [ROUTE-LEAKING] Almquist, P., "Ruminations on Route Leaking",
                   Work in Progress.

   [NEXT-HOP] Almquist, P., "Ruminations on the Next Hop,
              Work in Progress.

   [IDRP] Hares, S., "IDRP for IP", Work in Progress.

   [IS10747] ISO/IEC IS 10747 - Information Processing Systems -
             Telecommunications and Information Exchange between
             Systems - Protocol for Exchange of Inter-domain Routeing
             Information among Intermediate Systems to Support
             Forwarding of ISO 8473 PDUs, 1993.













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RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


10.  Appendix

   This is an example of how the two routing protocols, BGP/IDRP and
   OSPF, might both be consistent in their behaviour, and yet packets
   from a source domain, S, to a destination in domain X might be stuck
   in a forwarding loop.

                                       +--------+
                           X ----------| C1     |
                           |           |Domain C|
                           |           | C3  C2 |
                           |           +--------+
                           B             /   \
                            \           /     \
                             \         /      S
                              \       /      /
                               \     /      /
                             +--------+    /
                             | A1  A2 |   /
                             |Domain A|  /
                             |     A3 |-/
                             +--------+

   Given the domains, X, A, B, C and S, let domains A and C be running
   OSPF, and ASBRs A3 and C3 have equal cost multipath routes to A1, A2
   and C1, C2 respectively.  The picture above shows the internal
   structure of domains A and C only.

   During steady state, the following are the route advertisements:

     o    Domain B advertises to A path <B,X>

     o    ASBR A3 in domain A advertises path <A,B,X> to domain C, at
          ASBR C2.

     o    Domain C has two equal cost paths to X: one direct <C,X>, and
          another through A <C,A,B,X>

     o    BR C3 in domain C advertises to A2 path <C,X>

     o    Domain A has two equal cost paths to X: <A,C,X> and <A,B,X>

   Both C1 and C2 inject a route to X within the domain C, and likewise
   A1 and A2 inject a route to X within the domain A.  Since A3 and C3
   see equal cost routes to X through A1, A2 and C1, C2 respectively, a
   stable loop through ASBRs <A3, A2, C3, C2, A3> exists.





Varadhan, Hares & Rekhter                                      [Page 18]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


   Section 4 specifies that A3 and C3 that advertise a PATH to
   destination X, MUST aggregate all the PATHs through A1 and A2, and C1
   and C2 respectively.  This has the consequence of constraining the
   BGP/IDRP speaker in either domain A or domain C from choosing
   multiple routes to destination X, and importing only one route into
   OSPF.  This breaks the multiple paths seen in one domain.  The exact
   domain in which the multiple paths are broken is nondeterministic.

11.  Authors' Present Addresses

   Kannan Varadhan
   USC/Information Sciences Institute
   4676 Admiralty Way
   Marina Del Rey, CA 90292-6695

   Phone: [phone] x 402
   EMail: [e-mail]


   Susan Hares
   Merit, Inc.
   1071 Beal Avenue,
   Ann Arbor, MI 48109

   Phone: [phone]
   EMail: [e-mail]


   Yakov Rekhter
   T.J. Watson Research Center, IBM Corporation
   P.O. Box 704,
   Yorktown Heights, NY 10598.

   Phone: [phone]
   EMail: [e-mail]
















Varadhan, Hares & Rekhter                                      [Page 19]