rfc1268.txt

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   By definition, all transit AS's must be able to carry traffic which
   originates from and/or is destined to locations outside of that AS.
   This requires a certain degree of interaction and coordination
   between BGP and the Interior Gateway Protocol (IGP) used by that
   particular AS. In general, traffic originating outside of a given AS
   is going to pass through both interior gateways (gateways that
   support the IGP only) and border gateways (gateways that support both
   the IGP and BGP). All interior gateways receive information about
   external routes from one or more of the border gateways of the AS via
   the IGP.

   Depending on the mechanism used to propagate BGP information within a
   given AS, special care must be taken to ensure consistency between
   BGP and the IGP, since changes in state are likely to propagate at



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RFC 1268           Application of BGP in the Internet       October 1991


   different rates across the AS. There may be a time window between the
   moment when some border gateway (A) receives new BGP routing
   information which was originated from another border gateway (B)
   within the same AS, and the moment the IGP within this AS is capable
   of routing transit traffic to that border gateway (B). During that
   time window, either incorrect routing or "black holes" can occur.

   In order to minimize such routing problems, border gateway (A) should
   not advertise a route to some exterior network X via border gateway
   (B) to all of its BGP neighbors in other AS's until all the interior
   gateways within the AS are ready to route traffic destined to X via
   the correct exit border gateway (B). In other words, interior routing
   should converge on the proper exit gateway before/advertising routes
   via that exit gateway to other AS's.

A.2 Methods for Achieving Stable Interactions

   The following discussion outlines several techniques capable of
   achieving stable interactions between BGP and the IGP within an
   Autonomous System.

A.2.1 Propagation of BGP Information via the IGP

   While BGP can provide its own mechanism for carrying BGP information
   within an AS, one can also use an IGP to transport this information,
   as long as the IGP supports complete flooding of routing information
   (providing the mechanism to distribute the BGP information) and
   onepass convergence (making the mechanism effectively atomic). If an
   IGP is used to carry BGP information, then the period of
   desynchronization described earlier does not occur at all, since BGP
   information propagates within the AS synchronously with the IGP, and
   the IGP converges more or less simultaneously with the arrival of the
   new routing information. Note that the IGP only carries BGP
   information and should not interpret or process this information.

A.2.2  Tagged Interior Gateway Protocol

   Certain IGPs can tag routes exterior to an AS with the identity of
   their exit points while propagating them within the AS. Each border
   gateway should use identical tags for announcing exterior routing
   information (received via BGP) both into the IGP and into Internal
   BGP when propagating this information to other border gateways within
   the same AS. Tags generated by a border gateway must uniquely
   identify that particular border gateway--different border gateways
   must use different tags.

   All Border Gateways within a single AS must observe the following two
   rules:



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RFC 1268           Application of BGP in the Internet       October 1991


      1. Information received via Internal BGP by a border gateway A
         declaring a network to be unreachable must immediately be
         propagated to all of the External BGP neighbors of A.

      2. Information received via Internal BGP by a border gateway A
         about a reachable network X cannot be propagated to any of
         the External BGP neighbors of A unless/until A has an IGP
         route to X and both the IGP and the BGP routing information
         have identical tags.

   These rules guarantee that no routing information is announced
   externally unless the IGP is capable of correctly supporting it. It
   also avoids some causes of "black holes".

   One possible method for tagging BGP and IGP routes within an AS is to
   use the IP address of the exit border gateway announcing the exterior
   route into the AS. In this case the "gateway" field in the BGP UPDATE
   message is used as the tag.

A.2.3 Encapsulation

   Encapsulation provides the simplest (in terms of the interaction
   between the IGP and BGP) mechanism for carrying transit traffic
   across the AS. In this approach, transit traffic is encapsulated
   within an IP datagram addressed to the exit gateway. The only
   requirement imposed on the IGP by this approach is that it should be
   capable of supporting routing between border gateways within the same
   AS.

   The address of the exit gateway A for some exterior network X is
   specified in the BGP identifier field of the BGP OPEN message
   received from gateway A via Internal BGP by all other border gateways
   within the same AS. In order to route traffic to network X, each
   border gateway within the AS encapsulates it in datagrams addressed
   to gateway A. Gateway A then performs decapsulation and forwards the
   original packet to the proper gateway in another AS

   Since encapsulation does not rely on the IGP to carry exterior
   routing information, no synchronization between BGP and the IGP is
   required.

   Some means of identifying datagrams containing encapsulated IP, such
   as an IP protocol type code, must be defined if this method is to be
   used.

   Note, that if a packet to be encapsulated has length that is very
   close to the MTU, that packet would be fragmented at the gateway that
   performs encapsulation.



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RFC 1268           Application of BGP in the Internet       October 1991


A.2.4  Other Cases

   There may be AS's with IGPs which can neither carry BGP information
   nor tag exterior routes (e.g., RIP). In addition, encapsulation may
   be either infeasible or undesirable. In such situations, the
   following two rules must be observed:

      1. Information received via Internal BGP by a border gateway A
         declaring a network to be unreachable must immediately be
         propagated to all of the External BGP neighbors of A.

      2. Information received via Internal BGP by a border gateway A
         about a reachable network X cannot be propagated to any of
         the External BGP neighbors of A unless A has an IGP route to
         X and sufficient time (holddown) has passed for the IGP routes
         to have converged.

   The above rules present necessary (but not sufficient) conditions for
   propagating BGP routing information to other AS's. In contrast to
   tagged IGPs, these rules cannot ensure that interior routes to the
   proper exit gateways are in place before propagating the routes other
   AS's.

   If the convergence time of an IGP is less than some small value X,
   then the time window during which the IGP and BGP are unsynchronized
   is less than X as well, and the whole issue can be ignored at the
   cost of transient periods (of less than length X) of routing
   instability. A reasonable value for X is a matter for further study,
   but X should probably be less than one second.

   If the convergence time of an IGP cannot be ignored, a different
   approach is needed. Mechanisms and techniques which might be
   appropriate in this situation are subjects for further study.

References

   [1] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-
       3)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM
       Corp., October 1991.

   [2] Braun, H-W., "Models of Policy Based Routing", RFC 1104,
       Merit/NSFNET, June 1989.

Security Considerations

   Security issues are not discussed in this memo.





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RFC 1268           Application of BGP in the Internet       October 1991


Authors' Addresses

   Yakov Rekhter
   T.J. Watson Research Center IBM Corporation
   P.O. Box 218
   Yorktown Heights, NY 10598

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


   Phill Gross
   Advanced Network and Services (ANS)
   100 Clearbrook Road
   Elmsford, NY 10523

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

   IETF BGP WG mailing list: [e-mail]
   To be added: [e-mail]






























BGP Working Group                                              [Page 13]