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rfc:rfc2270

Network Working Group J. Stewart Request for Comments: 2270 ISI Category: Informational T. Bates

                                                             R. Chandra
                                                                E. Chen
                                                                  Cisco
                                                           January 1998
     Using a Dedicated AS for Sites  Homed to a Single Provider

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.

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 in RFC1930.

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 provider

Stewart, 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. Implications

3.1 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.com

Stewart, et. al. Informational [Page 5] RFC 2270 Dedicated AS January 1998

9. 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]

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