





Network Working Group                                           T. Bates
Request for Comments: 2260                                 Cisco Systems
Category: Informational                                       Y. Rekhter
                                                           Cisco Systems
                                                            January 1998


      Scalable Support for Multi-homed Multi-provider Connectivity

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

2. Abstract

   This document describes addressing and routing strategies for multi-
   homed enterprises attached to multiple Internet Service Providers
   (ISPs) that are intended to reduce the routing overhead due to these
   enterprises in the global Internet routing system.

3. Motivations

   An enterprise may acquire its Internet connectivity from more than
   one Internet Service Provider (ISP) for some of the following
   reasons.  Maintaining connectivity via more than one ISP could be
   viewed as a way to make connectivity to the Internet more reliable.
   This way when connectivity through one of the ISPs fails,
   connectivity via the other ISP(s) would enable the enterprise to
   preserve its connectivity to the Internet. In addition to providing
   more reliable connectivity, maintaining connectivity via more than
   one ISP could also allow the enterprise to distribute load among
   multiple connections. For enterprises that span wide geographical
   area this could also enable better (more optimal) routing.

   The above considerations, combined with the decreasing prices for the
   Internet connectivity, motivate more and more enterprises to become
   multi-homed to multiple ISPs. At the same time, the routing overhead
   that such enterprises impose on the Internet routing system becomes
   more and more significant. Scaling the Internet, and being able to
   support a growing number of such enterprises demands mechanism(s) to
   contain this overhead. This document assumes that an approach where
   routers in the "default-free" zone of the Internet would be required



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   to maintain a route for every multi-homed enterprise that is
   connected to multiple ISPs does not provide an adequate scaling.
   Moreover, given the nature of the Internet, this document assumes
   that any approach to handle routing for such enterprises should
   minimize the amount of coordination among ISPs, and especially the
   ISPs that are not directly connected to these enterprises.

   There is a difference of opinions on whether the driving factors
   behind multi-homing to multiple ISPs could be adequately addressed by
   multi-homing just to a single ISP, which would in turn eliminate the
   negative impact of multi-homing on the Internet routing system.
   Discussion of this topic is beyond the scope of this document.

   The focus of this document is on the routing and addressing
   strategies that could reduce the routing overhead due to multi-homed
   enterprises connected to multiple ISPs in the Internet routing
   system.

   The strategies described in this document are equally applicable to
   both IPv4 and IPv6.

4. Address allocation and assignment

   A multi-homed enterprise connected to a set of ISPs would be
   allocated a block of addresses (address prefix) by each of these ISPs
   (an enterprise connected to N ISPs would get N different blocks).
   The address allocation from the ISPs to the enterprise would be based
   on the "address-lending" policy [RFC2008]. The allocated addresses
   then would be used for address assignment within the enterprise.

   One possible address assignment plan that the enterprise could employ
   is to use the topological proximity of a node (host) to a particular
   ISP (to the interconnect between the enterprise and the ISP) as a
   criteria for selecting which of the address prefixes to use for
   address assignment to the node. A particular node (host) may be
   assigned address(es) out of a single prefix, or may have addresses
   from different prefixes.

5. Routing information exchange

   The issue of routing information exchange between an enterprise and
   its ISPs is decomposed into the following components:

      a) reachability information that an enterprise border router
      advertises to a border router within an ISP

      b) reachability information that a border router within an ISP
      advertises to an enterprise border router



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   The primary focus of this document is on (a); (b) is covered only as
   needed by this document.

5.1. Advertising reachability information by enterprise border routers

   When an enterprise border router connected to a particular ISP
   determines that the connectivity between the enterprise and the
   Internet is up through all of its ISPs, the router advertises (to the
   border router of that ISP) reachability to only the address prefix
   that the ISP allocated to the enterprise. This way in a steady state
   routes injected by the enterprise into its ISPs are aggregated by
   these ISPs, and are not propagated into the "default-free" zone of
   the Internet.

   When an enterprise border router connected to a particular ISP
   detemrines that the connectivity between the enterprise and the
   Internet through one or more of its other ISPs is down, the router
   starts advertising reachability to the address prefixes that was
   allocated by these ISPs to the enterprise. This would result in
   injecting additional routing information into the "default-free" zone
   of the Internet. However, one could observe that the probability of
   all multi-homed enterprises in the Internet concurrently losing
   connectivity to the Internet through one or more of their ISPs is
   fairly small.  Thus on average the number of additional routes in the
   "default-free" zone of the Internet due to multi-homed enterprises is
   expected to be a small fraction of the total number of such
   enterprises.

   The approach described above is predicated on the assumption that an
   enterprise border router has a mechanism(s) by which it could
   determine (a) whether the connectivity to the Internet through some
   other border router of that enterprise is up or down, and (b) the
   address prefix that was allocated to the enterprise by the ISP
   connected to the other border router. One such possible mechanism
   could be provided by BGP [RFC1771]. In this case border routers
   within the enterprise would have an IBGP peering with each other.
   Whenever one border router determines that the intersection between
   the set of reachable destinations it receives via its EBGP (from its
   directly connected ISP) peerings and the set of reachable
   destinations it receives from another border router (in the same
   enterprise) via IBGP is empty, the border router would start
   advertising to its external peer reachability to the address prefix
   that was allocated to the enterprise by the ISP connected to the
   other border router. The other border router would advertise (via
   IBGP) the address prefix that was allocated to the enterprise by the
   ISP connected to that router. This approach is known as "auto route
   injection".




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   As an illustration consider an enterprise connected to two ISPs,
   ISP-A and ISP-B. Denote the enterprise border router that connects
   the enterprise to ISP-A as BR-A; denote the enterprise border router
   that connects the enterprise to ISP-B as BR-B. Denote the address
   prefix that ISP-A allocated to the enterprise as Pref-A; denote the
   address prefix that ISP-B allocated to the enterprise as Pref-B.
   When the set of routes BR-A receives from ISP-A (via EBGP) has a
   non-empty intersection with the set of routes BR-A receives from BR-B
   (via IBGP), BR-A advertises to ISP-A only the reachability to Pref-A.
   When the intersection becomes empty, BR-A would advertise to ISP-A
   reachability to both Pref-A and Pref-B. This would continue for as
   long as the intersection remains empty. Once the intersection becomes
   non-empty, BR-A would stop advertising reachability to Pref-B to
   ISP-A (but would still continue to advertise reachability to Pref-A
   to ISP-A). Figure 1 below describes this method graphically.

        +-------+    +-------+         +-------+    +-------+
        (       )    (       )         (       )    (       )
        ( ISP-A )    ( ISP-B )         ( ISP-A )    ( ISP-B )
        (       )    (       )         (       )    (       )
        +-------+    +-------+         +-------+    +-------+
            |   /\       |   /\            |   /\       |
            |   ||       |   ||            | Pref-A  (connection
            | Pref-A     | Pref-B          | Pref-B    broken)
            |   ||       |   ||            |   ||       |
         +-----+      +-----+           +-----+      +-----+
         | BR-A|------|BR-B |           | BR-A|------|BR-B |
         +-----+ IBGP +-----+           +-----+ IBGP +-----+

          non-empty intersection         empty intersection


             Figure 1: Reachability information advertised

   Although strictly an implementation detail, calculating the
   intersection could potentially be a costly operation for a large set
   of routes. An alternate solution to this is to make use of a selected
   single (or more) address prefix received from an ISP (the ISP's
   backbone route for example) and configure the enterprise border
   router to perform auto route injection if the selected prefix is not
   present via IBGP. Let's suppose ISP-B has a well known address
   prefix, ISP-Pref-B for its backbone. ISP-B advertises this to BR-B
   and BR-B in turn advertises this via IBGP to BR-A. If BR-A sees a
   withdraw for ISP-Pref-B it advertises Pref-B to ISP-A.







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   The approach described in this section may produce less than the full
   Internet-wide connectivity in the presence of ISPs that filter out
   routes based on the length of their address prefixes. One could
   observe however, that this would be a problem regardless of how the
   enterprise would set up its routing and addressing.

5.2. Further improvements

   The approach described in the previous section allows to
   significantly reduce the routing overhead in the "default-free" zone
   of the Internet due to multi-homed enterprises. The approach
   described in this section allows to completely eliminate this
   overhead.

   An enterprise border router would maintain EBGP peering not just with
   the directly connected border router of an ISP, but with the border
   router(s) in one or more ISPs that have their border routers directly
   connected to the other border routers within the enterprise.  We
   refer to such peering as "non-direct" EBGP.

   An ISP that maintains both direct and non-direct EBGP peering with a
   particular enterprise would advertise the same set of routes over
   both of these peerings. An enterprise border router that maintains
   either direct or non-direct peering with an ISP advertises to that
   ISP reachability to the address prefix that was allocated by that ISP
   to the enterprise.  Within the ISP routes received over direct
   peering should be preferred over routes received over non-direct
   peering.  Likewise, within the enterprise routes received over direct
   peering should be preferred over routes received over non-direct
   peering.

   Forwarding along a route received over non-direct peering should be
   accomplished via encapsulation [RFC1773].

   As an illustration consider an enterprise connected to two ISPs,
   ISP-A and ISP-B. Denote the enterprise border router that connects
   the enterprise to ISP-A as E-BR-A, and the ISP-A border router that
   is connected to E-BR-A as ISP-BR-A; denote the enterprise border
   router that connects the enterprise to ISP-B as E-BR-B, and the ISP-B
   border router that is connected to E-BR-B as ISP-BR-B. Denote the
   address prefix that ISP-A allocated to the enterprise as Pref-A;
   denote the address prefix that ISP-B allocated to the enterprise as
   Pref-B.  E-BR-A maintains direct EBGP peering with ISP-BR-A and
   advertises reachability to Pref-A over that peering. E-BR-A also
   maintain a non-direct EBGP peering with ISP-BR-B and advertises
   reachability to Pref-B over that peering. E-BR-B maintains direct
   EBGP peering with ISP-BR-B, and advertises reachability to Pref-B
   over that peering.  E-BR-B also maintains a non-direct EBGP peering



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   with ISP-BR-A, and advertises reachability to Pref-A over that
   peering.

   When connectivity between the enterprise and both of its ISPs (ISP-A
   and ISP-B is up, traffic destined to hosts whose addresses were
   assigned out of Pref-A would flow through ISP-A to ISP-BR-A to E-BR-
   A, and then into the enterprise. Likewise, traffic destined to hosts
   whose addresses were assigned out of Pref-B would flow through ISP-B
   to ISP-BR-B to E-BR-B, and then into the enterprise. Now consider
   what would happen when connectivity between ISP-BR-B and E-BR-B goes
   down.  In this case traffic to hosts whose addresses were assigned
   out of Pref-A would be handled as before. But traffic to hosts whose
   addresses were assigned out of Pref-B would flow through ISP-B to
   ISP-BR-B, ISP-BR-B would encapsulate this traffic and send it to E-
   BR-A, where the traffic will get decapsulated and then be sent into
   the enterprise. Figure 2 below describes this approach graphically.

                    +---------+         +---------+
                    (         )         (         )
                    (  ISP-A  )         (  ISP-B  )
                    (         )         (         )
                    +---------+         +---------+
                         |                   |
                     +--------+          +--------+
                     |ISP-BR-A|          |ISP-BR-B|
                     +--------+          +--------+
                          |            /+/   |
                     /\   |  Pref-B  /+/     |
                     ||   |        /+/      \./
                    Pref-A|      /+/ non-   /.\
                     ||   |    /+/  direct   |
                          |  /+/     EBGP    |
                      +------+           +-------+
                      |E-BR-A|-----------|E-BR-B |
                      +------+    IBGP   +-------+


   Figure 2: Reachability information advertised via non-direct EBGP

   Observe that with this scheme there is no additional routing
   information due to multi-homed enterprises that has to be carried in
   the "default-free" zone of the Internet. In addition this scheme
   doesn't degrade in the presence of ISPs that filter out routes based
   on the length of their address prefixes.

   Note that the set of routers within an ISP that maintain non-direct
   peering with the border routers within an enterprise doesn't have to
   be restricted to the ISP's border routers that have direct peering



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   with the enterprise's border routers. The non-direct peering could be
   maintained with any router within the ISP. Doing this could improve
   the overall robustness in the presence of failures within the ISP.

5.3. Combining the two

   One could observe that while the approach described in Section 5.2
   allows to completely eliminate the routing overhead due to multi-
   homed enterprises in the "default-free" zone of the Internet, it may
   result in a suboptimal routing in the presence of link failures. The
   sub-optimality could be reduced by combining the approach described
   in Section 5.2 with a slightly modified version of the approach
   described in Section 5.1. The modification consists of constraining
   the scope of propagation of additional routes that are advertised by
   an enterprise border router when the router detects problems with the
   Internet connectivity through its other border routers. A way to
   constrain the scope is by using the BGP Community attribute
   [RFC1997].

5.4. Better (more optimal) routing in steady state

   The approach described in this document assumes that in a steady
   state an enterprise border router would advertise to a directly
   connected ISP border router only the reachability to the address
   prefix that this ISP allocated to the enterprise. As a result,
   traffic originated by other enterprises connected to that ISP and
   destined to the parts of the enterprise numbered out of other address
   prefixes would not enter the enterprise at this border router,
   resulting in potentially suboptimal paths. To improve the situation
   the border router may (in steady state) advertise reachability not
   only to the address prefix that was allocated by the ISP that the
   router is directly connected to, but to the address prefixes
   allocated by some other ISPs (directly connected to some other border
   routers within the enterprise).  Distribution of such advertisements
   should be carefully constrained, or otherwise this may result in
   significant additional routing information that would need to be
   maintained in the "default-free" part of the Internet. A way to
   constrain the distribution of such advertisements is by using the BGP
   Community attribute [RFC1997].

6. Comparison with other approaches

   CIDR [RFC1518] proposes several possible address allocation
   strategies for multi-homed enterprises that are connected to multiple
   ISPs.  The following briefly reviews the alternatives being used
   today, and compares them with the approaches described above.





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6.1. Solution 1

   One possible solution suggested in [RFC1518] is for each multi-homed
   enterprise to obtain its IP address space independently from the ISPs
   to which it is attached.  This allows each multi-homed enterprise to
   base its IP assignments on a single prefix, and to thereby summarize
   the set of all IP addresses reachable within that enterprise via a
   single prefix.  The disadvantage of this approach is that since the
   IP address for that enterprise has no relationship to the addresses
   of any particular ISPs, the reachability information advertised by
   the enterprise is not aggregatable with any, but default route.
   results in the routing overhead in the "default-free" zone of the
   Internet of O(N), where N is the total number of multi-homed
   enterprises across the whole Internet that are connected to multiple
   ISPs.

   As a result, this approach can't be viewed as a viable alternative
   for all, but the enterprises that provide high enough degree of
   addressing information aggregation. Since by definition the number of
   such enterprises is likely to be fairly small, this approach isn't
   viable for most of the multi-homed enterprises connected to multiple
   ISPs.

6.2. Solution 2

   Another possible solution suggested in [RFC1518] is to assign each
   multi-homed enterprise a single address prefix, based on one of its
   connections to one of its ISPs.  Other ISPs to which the multi-homed
   enterprise is attached maintain a routing table entry for the
   organization, but are extremely selective in terms of which other
   ISPs are told of this route and would need to perform "proxy"
   aggregation.  Most of the complexity associated with this approach is
   due to the need to perform "proxy" aggregation, which in turn
   requires t addiional inter-ISP coordination and more complex router
   configuration.

7. Discussion

   The approach described in this document assumes that addresses that
   an enterprise would use are allocated based on the "address lending"
   policy. Consequently, whenever an enterprise changes its ISP, the
   enterprise would need to renumber part of its network that was
   numbered out of the address block that the ISP allocated to the
   enterprise.  However, these issues are not specific to multihoming
   and should be considered accepted practice in todays internet. The
   approach described in this document effectively eliminates any
   distinction between single-home and multi-homed enterprise with
   respect to the impact of changing ISPs on renumbering.



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   The approach described in this document also requires careful address
   assignment within an enterprise, as address assignment impacts
   traffic distribution among multiple connections between an enterprise
   and its ISPs.

   Both the issue of address assignment and renumbering could be
   addressed by the appropriate use of network address translation
   (NAT). The use of NAT for multi-homed enterprises is the beyond the
   scope of this document.

   Use of auto route injection (as described in Section 5.1) increases
   the number of routers in the default-free zone of the Internet that
   could be affected by changes in the connectivity of multi-homed
   enterprises, as compared to the use of provider-independed addresses
   (as described in Section 6.1).  Specifically, with auto route
   injection when a multi-homed enterprise loses its connectivity
   through one of its ISPs, the auto injected route has to be propagated
   to all the routers in the default-free zone of the Internet. In
   contrast, when an enterprise uses provider-independent addresses,
   only some (but not all) of the routers in the default-free zone would
   see changes in routing when the enterprise loses its connectivity
   through one of its ISPs.

   To supress excessive routing load due to link flapping the auto
   injected route has to be advertised until the connectivity via the
   other connection (that was previously down and that triggered auto
   route injection) becomes stable.

   Use of the non-direct EBGP approach (as described in Section 5.2)
   allows to eliminate route flapping due to multi-homed enterprises in
   the default-free zone of the Internet. That is the non-direct EBGP
   approach has better properties with respect to routing stability than
   the use of provider-independent addresses (as described in Section
   6.1).

8. Applications to multi-homed ISPs

   The approach described in this document could be applicable to a
   small to medium size ISP that is connected to several upstream ISPs.
   The ISP would acquire blocks of addresses (address prefixes) from its
   upstream ISPs, and would use these addresses for allocations to its
   customers.  Either auto route injection, or the non-direct EBGP
   approach, or a combination of both could be used by the ISP when
   peering with its upstream ISPs. Doing this would provide routability
   for the customers of such ISP, without advertsely affecting the
   overall scalability of the Internet routing system.





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9. Security Considerations

   Since the non-direct EBGP approach (as described in Section 5.2)
   requires EBGP sessions between routers that are more than one IP hop
   from each other, routers that maintain these sessions should use an
   appropriate authentication mechanism(s) for BGP peer authentication.

   Security issues related to the IBGP peering, as well as the EBGP
   peering between routers that are one IP hop from each other are
   outside the scope of this document.

10. Acknowledgments

   The authors of this document do not make any claims on the
   originality of the ideas described in this document. Anyone who
   thought about these ideas before should be given all due credit.

11. References

   [RFC1518]
        Rekhter, Y., and T. Li, "An Architecture for IP Address
        Allocation with CIDR", RFC 1518, September 1993.

   [RFC1771]
        Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
        RFC 1771, March 1995.

   [RFC1773]
        Hanks, S., Li, T., Farinacci, T., and P. Traina, "Generic
        Routing Encapsulation over IPv4 networks", RFC 1773, October
        1994.

   [RFC1918]
        Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot G.J., and
        E. Lear, "Address Allocation for Private Internets", RFC 1918,
        February 1996.

   [RFC1997]
        Chandra, R., Traina, P., and T. Li, "BGP Communities Attribute",
        RFC 1997, August 1996.

   [RFC2008]
        Rekhter, Y., and T. Li, "Implications of Various Address
        Allocation Policies for Internet Routing", BCP 7, RFC 2008,
        October 1996.






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12. Authors' Addresses

   Tony Bates
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: tbates@cisco.com


   Yakov Rekhter
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   EMail: yakov@cisco.com




































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13.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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