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

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.

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,

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)",
RFC1771, March 1995.

[2] Rekhter, Y., and P. Gross, "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 T. Bates, "Guidelines for creation, selection,
and registration of an Autonomous System (AS)", RFC1930, 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++)", RFC1786, 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

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