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RFC2270 - Using a Dedicated AS for Sites Homed to a Single Provider

王朝other·作者佚名  2008-05-31
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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

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.

 
 
 
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