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Network Working Group                                          J. StewartRequest for Comments: 2270                                            ISICategory: Informational                                          T. Bates                                                               R. Chandra                                                                  E. Chen                                                                    Cisco                                                             January 1998Using a Dedicated AS for Sites  Homed to a Single ProviderStatus 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.Abstract   With the increased growth of the Internet, the number of customers   using BGP4 has grown significantly.RFC1930 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   single ISP.  This paper proposes a solution to this problem in line   with recommendations set forth inRFC1930.1.  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   single ISP. These conditions are as follows:   1) Customers multi-homed to single providerStewart, et. al.             Informational                      [Page 1]

RFC 2270                     Dedicated AS                   January 1998   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 "localized"   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 relevant in a   global sense.  Autonomous System Numbers (AS) are identifiers 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 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      administratively 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.Stewart, et. al.             Informational                      [Page 2]

RFC 2270                     Dedicated AS                   January 1998   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. Implications3.1 Full Routing Table Announcement   The solution precludes the ability for a BGP customer using the   dedicated AS to receive 100% full routes.  Because of routing loop   detection 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 Customer Y are both stub networks so default routing is adequate,   and the absence of a very small portion of the full routing table is   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,Stewart, et. al.             Informational                      [Page 3]

RFC 2270                     Dedicated AS                   January 1998   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   generate 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   allocated 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.5.  Security Considerations   The usage of AS numbers described in this document has no effective   security impact.  Acceptance and filtering of AS numbers from   customers is an issue dealt with in other documents.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 T. Li, "A Border Gateway Protocol 4 (BGP-4)",RFC 1771, March 1995.   [2] Rekhter, Y., and P. Gross, "Application of the Border Gateway   Protocol in the Internet",RFC 1772, March 1995.Stewart, et. al.             Informational                      [Page 4]

RFC 2270                     Dedicated AS                   January 1998   [3] Rekhter, Y., "Routing in a Multi-provider Internet",RFC 1787,   April 1995.   [4] Hawkinson, J., and T. Bates, "Guidelines for creation, selection,   and registration of an Autonomous System (AS)",RFC 1930, March 1996.   [5] Bates, T., Gerich, E., Joncheray, L., Jouanigot, J-M, Karrenberg,   D., Terpstra, M., and J. Yu., "Representation of IP Routing Policies   in a Routing Registry (ripe-81++)",RFC 1786, March 1995.   [6] Chen, E., and J. Stewart., "A Framework for Inter-Domain Route   Aggregation", Work in Progress.8.  Authors' 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.comStewart, et. al.             Informational                      [Page 5]

RFC 2270                     Dedicated AS                   January 19989.  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.Stewart, et. al.             Informational                      [Page 6]