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Versions: 00 RFC 2270

INTERNET-DRAFT                                John W. Stewart, III / ISI
<draft-ietf-idr-as-dedicated-00.txt>                  Tony Bates / Cisco
                                                    Ravi Chandra / Cisco
                                                       Enke Chen / Cisco
                                                               July 1997


         Using a Dedicated AS for Sites Homed to a Single Provider
                   <draft-ietf-idr-as-dedicated-00.txt>


Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working docu-
   ments of the Internet Engineering Task Force (IETF), its areas, and
   its working groups. Note that other groups may also distribute work-
   ing documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months.  Internet-Drafts may be updated, replaced or obsoleted by
   other documents at any time. It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress."

   Please check the abstract listing contained in each Internet-Draft
   directory to learn the current status of this or any other Internet-
   Draft.


Abstract

   With the increased growth of the Internet, the number of customers
   using BGP4 has grown significantly. RFC1930 outlines a set of guide-
   lines for when one needs and should use an AS. However, the customer
   and service provider (ISP) are left with a problem as a result of
   this in that while there is no need for an allocated AS under the
   guidelines, certain conditions make the use of BGP4 a very pragmatic
   and perhaps only way to connect a customer homed to a single ISP.
   This paper proposes a solution to this problem in line with recommen-
   dations set forth in RFC1930.











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1.  Problems

   With the increased growth of the Internet, the number of customers
   using BGP4 [1],[2] has grown significantly. RFC1930 [4] outlines a
   set of guidelines for when one needs and should use an AS. However,
   the customer and service provider (ISP) are left with a problem as a
   result of this in that while there is no need for an allocated AS
   under the guidelines, certain conditions make the use of BGP4 a very
   pragmatic and perhaps only way to connect a customer homed to a sin-
   gle ISP. These conditions are as follows:

   1) Customers multi-homed to single provider

      Consider the scenario outlined in Figure 1 below.

                                +-------+      +-------+
                           +----+       |      |       |
                +------+   |    | ISP A +------+ ISP B |
                | Cust.+---+    |       |      |       |
                |   X  +--------+       |      |       |
                +------+        ++-----++\     +-------+
                                 |     |  \
                                 |     |   \  +--------+
                                ++-----++   +-|        |
                                | Cust. |     |  ISP C |
                                |   Y   |     |        |
                                +-------+     +--------+

          Figure 1: Customers multi-home to a single provider

      Here both customer X and customer Y are multi-homed to a single
      provider, ISP A. Because these multiple connections are "local-
      ized" between the ISP A and its customers, the rest of the routing
      system (ISP B and ISP C in this case) doesn't need to see routing
      information for a single multi-homed customer any differently than
      a singly-homed customer as it has the same routing policy as ISP A
      relative to ISP B and ISP C.  In other words, with respect to the
      rest of the Internet routing system the organization is singly-
      homed, so the complexity of the multiple connections is not rele-
      vant in a global sense.  Autonomous System Numbers (AS) are iden-
      tifiers used in routing protocols and are needed by routing
      domains as part of the global routing system.  However, as [4]
      correctly outlines, organizations with the same routing policy as
      their upstream provider do not need an AS.

      Despite this fact, a problem exists in that many ISPs can only
      support the load-sharing and reliability requirements of a multi-
      homed customer if that customer exchanges routing information



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      using BGP-4 which does require an AS as part of the protocol.

   2) Singly-homed customers requiring dynamic advertisement of NLRI's

      While this is not a common case as static routing is generally
      used for this purpose, if a large amount of NLRI's need to be
      advertised from the customer to the ISP it is often administra-
      tively easier for these prefixes to be advertised using a dynamic
      routing protocol. Today, the only exterior gateway protocol (EGP)
      that is able to do this is BGP. This leads to the same problem
      outlined in condition 1 above.

   As can be seen there is clearly a problem with the recommendations
   set forth in [4] and the practice of using BGP4 in the scenarios
   above. Section 2 proposes a solution to this problem with following
   sections describing the implications and application of the proposed
   solution.

   It should also be noted that if a customer is multi-homed to more
   than one ISP then they are advised to obtain an official allocated AS
   from their allocation registry.


2.  Solution

   The solution we are proposing is that all BGP customers homed to the
   same single ISP use a single, dedicated AS specified by the ISP.

   Logically, this solution results in an ISP having many peers with the
   same AS, although that AS exists in "islands" completely disconnected
   from one another.

   Several practical implications of this solution are discussed in the
   next section.

3. Implications

3.1 Full Routing Table Announcement

   The solution precludes the ability for a BGP customer using the dedi-
   cated AS to receive 100% full routes.  Because of routing loop detec-
   tion of AS path, a BGP speaker rejects routes with its own AS number
   in the AS path.  Imagine Customer X and Customer Y maintain BGP peers
   with Provider A using AS number N. Then, Customer X will not be able
   to received routes of Customer Y.  We do not believe that this would
   cause a problem for Customer X, though, because Customer X and Cus-
   tomer Y are both stub networks so default routing is adequate, and
   the absence of a very small portion of the full routing table is



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   unlikely to have a noticeable impact on traffic patterns guided by
   MEDs received.

   A BGP customer using the dedicated AS must carry a default route
   (preferably receiving from its provider via BGP).


3.2  Change of External Connectivity

   The dedicated AS specified by a provider is purely for use in peering
   between its customers and the provider. When a customer using the
   dedicated AS changes its external connectivity, it may be necessary
   for the customer to reconfigure their network to use a different AS
   number (either a globally unique one if homed to multiple providers,
   or a dedicated AS of a different provider).


3.3  Aggregation

   As BGP customers using this dedicated AS are only homed to one ISP,
   their routes allocated from its providers CIDR block do not need to
   be announced upstream by its provider as the providers will already
   be originating the larger block. [6].


3.4  Routing Registries

   The Internet Routing Registry (IRR) [5] is used by providers to gen-
   erate route filtering lists.  Such lists are derived primarily from
   the "origin" attribute of the route objects.  The "origin" is the AS
   that originates the route.  With multiple customers using the same
   AS, finer granularity will be necessary to generate the correct route
   filtering.  For example, the "mntner" attribute or the "community"
   attribute of a route object can be used along with the "origin"
   attribute in generating the filtering lists.


4. Practice

   The AS number specified by a provider can either be an AS from the
   private AS space (64512 - 65535) [4], or be an AS previously allo-
   cated to the provider.  With the former, the dedicated AS like all
   other private AS's should be stripped from its AS path while the
   route is being propagated to the rest of the Internet routing system.







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5.  Security Considerations

   Security considerations are not discussed in this memo.


6.  Acknowledgments

   The authors would like to thank Roy Alcala of MCI and Arpakorn
   Boonkongchuen for  their input to this document.  The members of the
   IDR Working Group also provided helpful comments.

7.  References

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

   [2] Rekhter, Y., and Gross, P., "Application of the Border Gateway
   Protocol in the Internet", RFC1772, March 1995.

   [3] Rekhter, Y., "Routing in a Multi-provider Internet", RFC1787,
   April 1995.

   [4] Hawkinson, J., and Bates, T., "Guidelines for creation, selec-
   tion, and registration of an Autonomous System (AS)", RFC1930, March
   1996.

   [5] T. Bates, E. Gerich, L. Joncheray, J-M. Jouanigot, D. Karrenberg,
   M. Terpstra, & J. Yu., "Representation of IP Routing Policies in a
   Routing Registry (ripe-81++)", RFC1786, March 1995.

   [6] E. Chen, J. Stewart., "A Framework for Inter-Domain Route Aggre-
   gation", draft-ietf-idr-aggregation-framework-01.txt, July 1997.



















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8.  Author's Addresses


John Stewart
USC/ISI
4350 North Fairfax Drive
Suite 620
Arlington, VA  22203
email: jstewart@isi.edu

Tony Bates
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: tbates@cisco.com

Ravi Chandra
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: rchandra@cisco.com

Enke Chen
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: enkechen@cisco.com
























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