Network Working Group
Internet Engineering Task Force (IETF) W. Kumari
Internet-Draft
Request for Comments: 9774 Google, Inc.
Obsoletes: 6472 (if approved) K. Sriram
Updates: 4271 4271, 5065 (if approved) L. Hannachi
Intended status:
Category: Standards Track USA NIST
Expires: 8 September 2025
ISSN: 2070-1721 J. Haas
Juniper Networks, Inc.
7 March
May 2025
Deprecation of AS_SET and AS_CONFED_SET in BGP
draft-ietf-idr-deprecate-as-set-confed-set-18
Abstract
BCP 172 (i.e., RFC 6472) recommends not using AS_SET and
AS_CONFED_SET AS_PATH segment types in the Border Gateway Protocol
(BGP). This document advances that recommendation to a standards
requirement in BGP; it prohibits the use of the AS_SET and
AS_CONFED_SET path segment types in the AS_PATH. This is done to
simplify the design and implementation of BGP and to make the
semantics of the originator of a BGP route clearer. This will also
simplify the design, implementation, and deployment of various BGP
security mechanisms. This document updates RFC 4271 by deprecating
the origination of BGP routes with AS_SET (Type 1 AS_PATH segment)
and updates RFC 5065 by deprecating the origination of BGP routes
with AS_CONFED_SET (Type 4 AS_PATH segment). Finally, it obsoletes
RFC 6472.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2025.
https://www.rfc-editor.org/info/rfc9774.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Updates to the Requirements of RFC RFCs 4271 and RFC 5065 . . . . 4
4. Treatment of Routes with AS_SET in RPKI-based RPKI-Based BGP Security . 4
5. BGP AS_PATH "Brief" Aggregation . . . . . . . . . . . . . . . 4
5.1. Issues with "Brief" AS_PATH Aggregation and RPKI-ROV . . 5
5.2. Recommendations to Mitigate Unpredictable AS_PATH Origins
for RPKI-ROV Purposes . . . . . . . . . . . . . . . . . . 6
6. Operational Considerations . . . . . . . . . . . . . . . . . 6
6.1. Implementing Consistent Brief Aggregation . . . . . . . . 6
6.2. Not Advertising Aggregate Routes to Contributing ASes . . 6
6.3. Mitigating Forwarding Loops . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1.
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2.
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Example of Route Filtering for Aggregate Routes and
their
Their Contributors . . . . . . . . . . . . . . . . . . . 10
Appendix B. Examples of Consistent and Inconsistent BGP Origin-AS Origin AS
Generated by Traditional Brief Aggregation . . . . . . . 11
B.1. Scenario 1: First one route, then another, each with a
fully disjoint AS_PATH . . . . . . . . . . . . . . . . . 12
B.2. Scenario 2: First one route, then another, and the AS_PATHs
overlap at the origin AS. . . . . . . . . . . . . . . . . 13 AS
B.3. Scenario 3: First one route, then another, and the AS_PATHs
overlap at the neighbor AS . . . . . . . . . . . . . . . 13
B.4. Achieving Consistent Origin-AS Origin AS During Aggregation . . . . 13
Appendix C. Discussion on Forwarding Loops and AS_SETs . . . . . 14
Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
BCP 172 [RFC6472] makes a recommendation for
[BCP172] recommends not using AS_SET (see
[RFC4271]) [RFC4271] and AS_CONFED_SET (see [RFC5065])
[RFC5065] AS_PATH path segment types in the Border Gateway Protocol
(BGP). This document advances the BCP recommendation to a standards
requirement in BGP; it prohibits the use of the AS_SET and
AS_CONFED_SET types of path segments in the AS_PATH. The purpose is
to simplify the design and implementation of BGP and to make the
semantics of the originator of a BGP route clearer. This will also
simplify the design, implementation, and deployment of various BGP
security mechanisms. In particular, the prohibition of AS_SETs and
AS_CONFED_SETs removes
the possibility of any ambiguity about the origin AS in RPKI-based route
origin validation RPKI-
based Route Origin Validation (RPKI-ROV) [RFC6811] [RFC6907]
[RFC9319].
The AS_SET path segment in the AS_PATH attribute (Sections 4.3 and
5.1.2 of [RFC4271]) is created by a router that is performing route
aggregation and contains an unordered set of Autonomous Systems
(ASes) that contributing prefixes in the aggregate have traversed.
The AS_CONFED_SET path segment (see [RFC5065]) [RFC5065] in the AS_PATH attribute is
created by a router that is performing route aggregation and contains
an unordered set of Member AS Numbers in the local confederation that
contributing prefixes in the aggregate have traversed. It is very
similar to an AS_SET but is used within a confederation.
By performing aggregation, a router is combining multiple BGP routes
for more specific destinations into a new route for a less specific
destination ([RFC4271], (see [RFC4271], Section 9.1.2.2.). 9.1.2.2). Aggregation may blur
the semantics of the origin AS for the prefix being announced by
producing an AS_SET or AS_CONFED_SET. Such sets can cause
operational issues, such as not being able to authenticate a route
origin for the aggregate prefix in new BGP security technologies such
as those that take advantage of X.509 extensions for IP addresses and
AS identifiers ([RFC6480], (see [RFC6480], [RFC6811], [RFC6907], [RFC8205], and
[RFC9319]). This could result in reachability problems for the
destinations covered by the aggregated prefix.
From analysis of historical Internet routing data, it is apparent
that aggregation that involves AS_SETs is very seldom used in
practice on the public Internet (see [Analysis]). When it is used,
it is often used incorrectly; only a single AS in the AS_SET is the
most common case [Analysis]. Also, very often the same AS appears in
the AS_SEQUENCE and the AS_SET in the BGP update. The occurrence of
reserved AS numbers ([IANA-SP-ASN]) [IANA-SP-ASN] is also somewhat frequent.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Updates to the Requirements of RFC RFCs 4271 and RFC 5065
Unless explicitly configured by a network operator to do otherwise
(e.g., during a transition phase), BGP speakers:
* MUST NOT advertise BGP UPDATE messages containing AS_SETs or
AS_CONFED_SETs,
AS_CONFED_SETs and
* Upon MUST use the "treat-as-withdraw" error handling behavior per
[RFC7606] upon reception of BGP UPDATE messages containing AS_SETs
or AS_CONFED_SETs in the AS_PATH or AS4_PATH [RFC6793], MUST use the
"treat-as-withdraw" error handling behavior as per [RFC7606]. [RFC6793].
Per the above specifications, this document updates RFC 4271 [RFC4271] and RFC 5065
[RFC5065] by deprecating AS_SET (see [RFC4271], Section 4.3) and
AS_CONFED_SET (see [RFC5065], Section 3), respectively.
4. Treatment of Routes with AS_SET in RPKI-based RPKI-Based BGP Security
Resource Public Key Infrastructure (RPKI) [RFC6480] uses X.509
extensions for IP addresses and AS identifiers [RFC3779]. RPKI-based
Route Origin Validation (ROV) RPKI-ROV
[RFC6811] [RFC6907] is a BGP security technology that never allows a
route with AS_SET to be considered Valid. BGPsec [RFC8205] and
Autonomous System Provider Authorization (ASPA) [I-D.ietf-sidrops-aspa-verification] [ASPA-VERIFICATION]
are also BGP security technologies based on RPKI. BGPsec does not
support AS_SETs. In ASPA-based AS_PATH verification, a route with
AS_SET is always considered Invalid and hence ineligible for route
selection.
5. BGP AS_PATH "Brief" Aggregation
Sections 9.1.4 and 9.2.2.2 of [RFC4271] describe BGP aggregation
procedures. Appendix F.6 in of [RFC4271] describes a generally less
utilized "Complex AS_PATH Aggregation" procedure.
[RFC4271], Section 5.1.6, describing 5.1.6 describes the ATOMIC_AGGREGATE Path
Attribute,
Attribute and notes that:
| When a BGP speaker aggregates several routes for the purpose of
| advertisement to a particular peer, the AS_PATH of the aggregated
| route normally includes an AS_SET formed from the set of ASes from
| which the aggregate was formed. In many cases, the network
| administrator can determine if the aggregate can safely be
| advertised without the AS_SET, and without forming route loops.
|
| If an aggregate excludes at least some of the AS numbers present
| in the AS_PATH of the routes that are aggregated as a result of
| dropping the AS_SET, the aggregated route, when advertised to the
| peer, SHOULD include the ATOMIC_AGGREGATE attribute.
When BGP AS_PATH aggregation is done according to the procedures in
[RFC4271], Section 9.2.2.2 procedures, 9.2.2.2, and any resulting AS_SETs are discarded,
this
it is typically referred to as "brief" aggregation in
implementations. Brief aggregation results in an AS_PATH that has
the following property (from [RFC4271], Section 9.2.2.2):
| determine [D]etermine the longest leading sequence of tuples (as defined
| above) common to all the AS_PATH attributes of the routes to be
| aggregated. Make this sequence the leading sequence of the
| aggregated AS_PATH attribute.
The ATOMIC_AGGREGATE Path Attribute is subsequently attached to the
BGP route, if AS_SETs are dropped.
5.1. Issues with "Brief" AS_PATH Aggregation and RPKI-ROV
While brief AS_PATH aggregation has the desirable property of not
containing AS_SETs, the resulting aggregated AS_PATH may contain an
unpredictable origin AS. This is because the aggregating AS may be
different from the purported origin AS (for the aggregate), which may
vary as explained below. Such an unpredictable origin ASes may result
in RPKI-ROV validation issues:
* Depending on the contributing routes to the aggregate route, the
resulting origin AS may vary.
* The presence of expected contributing routes may be unpredictable
due to route availability from BGP neighbors.
* In the presence of such varying origin ASes, it would be necessary
for the resource holder to register Route Origin Authorizations
(ROAs) ROAs [RFC9582] for each
potential origin AS that may result from the expected aggregated
AS_PATHs.
5.2. Recommendations to Mitigate Unpredictable AS_PATH Origins for
RPKI-ROV Purposes
To ensure a consistent BGP origin AS is announced for aggregate BGP
routes for implementations of "brief" BGP aggregation, the
implementation MUST be configured to truncate the AS_PATH after the
right-most instance of the desired origin AS for the aggregate. The
desired origin AS could be the aggregating AS itself. A ROA would be
necessary for the aggregate prefix with the desired origin AS.
This form of brief aggregation is referred to as "consistent brief"
BGP aggregation.
If the resulting AS_PATH would be truncated from the otherwise
expected result of BGP AS_PATH aggregation (an AS_SET would not be
generated and possibly some ASes are removed from the "longest
leading sequence" of ASes), the ATOMIC_AGGREGATE Path Attribute
SHOULD be attached. This is consistent with the intent of [RFC4271],
Section 5.1.6.
6. Operational Considerations
This section provides advice to operators regarding deployment and
configuration.
6.1. Implementing Consistent Brief Aggregation
When aggregating prefixes, network operators MUST use consistent
brief aggregation as described in Section 5.2. In consistent brief
aggregation, the AGGREGATOR and ATOMIC_AGGREGATE Path Attributes are
included, but the AS_PATH does not have AS_SET or AS_CONFED_SET path
segment types. See Appendix B for examples of brief aggregation
while keeping the origin AS unambiguous and generating appropriate
ROAs.
6.2. Not Advertising Aggregate Routes to Contributing ASes
An aggregate prefix SHOULD NOT be announced to the contributing ASes.
Instead, more specific prefixes (from the aggregate) SHOULD be
announced to each contributing AS, excluding any that were learned
from the contributing AS in consideration. See Appendix A for an
example of this filtering policy.
6.3. Mitigating Forwarding Loops
As discussed in Section 5.1 of [RFC4632], the presence of
When both less specific and more specific destinations has the possibility are present,
it's possible to create forwarding loops between networks. networks, as
discussed in Section 5.1 of [RFC4632].
As a reminder, Rule #2 of in Section 5.1 of [RFC4632] requires that BGP
implementations performing aggregation discard packets that match the
aggregate route but do not match any of the more-specific more specific routes.
Further discussion of forwarding loops and their relationship to
AS_SETs can be found in Appendix C.
7. Security Considerations
This document deprecates the use of aggregation techniques that
create AS_SETs or AS_CONFED_SETs. Obsoleting these path segment
types from BGP and the removal of the related code from
implementations would potentially decrease the attack surface for
BGP. Deployments of new BGP security technologies (e.g., [RFC6480],
[RFC6811], and [RFC8205]) benefit greatly if AS_SETs and
AS_CONFED_SETs are not used in BGP.
8. IANA Considerations
This document requires has no IANA actions.
9. Acknowledgements
The authors would like to thank Alvaro Retana, John Scudder, Ketan
Talaulikar, Keyur Patel, Susan Hares, Claudio Jeker, Nick Hilliard,
Robert Raszuk, John Heasley, Job Snijders, Jared Mauch, Jakob Heitz,
Tony Przygienda, Douglas Montgomery, Randy Bush, Curtis Villamizar,
Danny McPherson, Chris Morrow, Tom Petch, Ilya Varlashkin, Enke Chen,
Tony Li, Florian Weimer, John Leslie, Paul Jakma, Rob Austein, Russ
Housley, Sandra Murphy, Steve Bellovin, Steve Kent, Steve Padgett,
and Alfred Hoenes for comments and suggestions. The comments and
suggestions received from the IESG reviewers are also much
appreciated.
10. References
10.1.
9.1. Normative References
[BCP172] Best Current Practice 172,
<https://www.rfc-editor.org/info/bcp172>.
At the time of writing, this BCP comprises the following:
Kumari, W. and K. Sriram, "Recommendation for Not Using
AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
DOI 10.17487/RFC6472, December 2011,
<https://www.rfc-editor.org/info/rfc6472>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August
2006, <https://www.rfc-editor.org/info/rfc4632>.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065,
DOI 10.17487/RFC5065, August 2007,
<https://www.rfc-editor.org/info/rfc5065>.
[RFC6472] Kumari, W. and K. Sriram, "Recommendation for Not Using
AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
DOI 10.17487/RFC6472, December 2011,
<https://www.rfc-editor.org/info/rfc6472>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2.
9.2. Informative References
[Analysis] Hannachi, L. and K. Sriram, "Detailed analysis of AS_SETs in BGP updates", NIST Robust Inter-domain Routing Project
Website , October 2019, commit
eb0fc22, March 2022,
<https://github.com/ksriram25/IETF/blob/main/Detailed-
AS_SET-analysis.txt>.
[I-D.ietf-sidrops-aspa-verification]
[ASPA-VERIFICATION]
Azimov, A., Bogomazov, E., Bush, R., Patel, K., Snijders,
J., and K. Sriram, "BGP AS_PATH Verification Based on
Autonomous System Provider Authorization (ASPA) Objects",
Work in Progress, Internet-Draft, draft-ietf-sidrops-aspa-
verification-20, 4 January
verification-22, 23 March 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-sidrops-
aspa-verification-20>.
aspa-verification-22>.
[IANA-SP-ASN]
IANA, "Special-Purpose Autonomous System (AS) Numbers",
<https://www.iana.org/assignments/iana-as-numbers-special-
registry/iana-as-numbers-special-registry.xhtml>.
registry>.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779,
DOI 10.17487/RFC3779, June 2004,
<https://www.rfc-editor.org/info/rfc3779>.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
February 2012, <https://www.rfc-editor.org/info/rfc6480>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC6907] Manderson, T., Sriram, K., and R. White, "Use Cases and
Interpretations of Resource Public Key Infrastructure
(RPKI) Objects for Issuers and Relying Parties", RFC 6907,
DOI 10.17487/RFC6907, March 2013,
<https://www.rfc-editor.org/info/rfc6907>.
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
[RFC9319] Gilad, Y., Goldberg, S., Sriram, K., Snijders, J., and B.
Maddison, "The Use of maxLength in the Resource Public Key
Infrastructure (RPKI)", BCP 185, RFC 9319,
DOI 10.17487/RFC9319, October 2022,
<https://www.rfc-editor.org/info/rfc9319>.
[RFC9582] Snijders, J., Maddison, B., Lepinski, M., Kong, D., and S.
Kent, "A Profile for Route Origin Authorizations (ROAs)",
RFC 9582, DOI 10.17487/RFC9582, May 2024,
<https://www.rfc-editor.org/info/rfc9582>.
Appendix A. Example of Route Filtering for Aggregate Routes and their Their
Contributors
Presented here is an
The illustration of presented below shows how an AS_SET is not used when
aggregating and still data-plane how data plane route loops are avoided. Consider
that p1/24 (from AS 64501), p2/24 (from AS 64502), p3/24 (from AS
64503), and p4/24 (from AS 64504) are aggregated by AS 64505 to p/22.
AS_SET is not used with the aggregate p/22 but AGGREGATOR and ATOMIC
AGGREGATE are used. Data-plane Data plane route loops are avoided by not
announcing the aggregate p/22 to the contributing ASes, i.e., AS
64501, AS 64502, AS 64503, and AS 64504. Instead, as further
illustration,
illustrated, p1/24, p2/24, and p4/24 are announced to AS 64503. The
routing tables (post aggregation) of each of the ASes are depicted in
the diagram below.
( ) ( ) ( ) ( )
( AS64501 ) ( AS64502 ) ( AS64503 ) ( AS64504 )
( ) ( ) ( ) ( )
p1/24 p2/24 p3/24 p4/24
| | | |
| +--> ( ) <--+ |
| ( AS64505 ) |
+----------------> ( ) <----------------+
p/22
|
V
AS 64501 AS 64502
========================== ==========================
p1/24 AS_PATH "" p1/24 AS_PATH "64505 64501"
p2/24 AS_PATH "64505 64502" p2/24 AS_PATH ""
p3/24 AS_PATH "64505 64503" p3/24 AS_PATH "64505 64503"
p4/24 AS_PATH "64505 64504" p4/24 AS_PATH "64505 64504"
AS 64503 AS 64504
========================== ==========================
p1/24 AS_PATH "64505 64501" p1/24 AS_PATH "64505 64501"
p2/24 AS_PATH "64505 64502" p2/24 AS_PATH "64505 64502"
p3/24 AS_PATH "" p3/24 AS_PATH "64505 64503"
p4/24 AS_PATH "64505 64504" p4/24 AS_PATH ""
AS 64505
==========================
p/22 AS_PATH "" AGGREGATOR 64505 ATOMIC_AGGREGATE
p1/24 AS_PATH "64501"
p2/24 AS_PATH "64502"
p3/24 AS_PATH "64503"
p4/24 AS_PATH "64504"
Appendix B. Examples of Consistent and Inconsistent BGP Origin-AS Origin AS
Generated by Traditional Brief Aggregation
In the
The examples below, it is illustrated below illustrate how traditional brief aggregation may
result in an inconsistent origin AS.
AS 64500 aggregates more specific routes into 192.0.2.0/24.
Consider the following scenarios where brief aggregation is done by
AS 64500 and what the resultant origin ASes would be.
Routes:
R1 - 192.0.2.0/26 AS_PATH "64501"
R2 - 192.0.2.64/26 AS_PATH "64502"
R3 - 192.0.2.128/26 AS_PATH "64504 64502"
R4 - 192.0.2.192/26 AS_PATH "64504 64501"
( ) ( )
( AS64501 ) ( AS64502 )
( ) ( )
192.0.2.0/26 192.0.2.192/26 192.0.2.128/26 192.0.2.64/26
| | | |
| | | |
| \/ \/ |
| ( ) |
| ( AS64504 ) |
| ( ) |
| | | |
| R4 | | R3 |
| | | |
| \/ \/ |
| R1 ( ) R2 |
+------------------->( AS64500 )<-------------+
( )
|
| (announcing
| aggregate 192.0.2.0/24)
\/
B.1. Scenario 1: First one route, then another, each with a fully
disjoint AS_PATH
Receive R1. Aggregate 192.0.2.0/24 AS_PATH "64501"
Alternate "bug?": Aggregate 192.0.2.0/24 AS_PATH "[ 64501 ]"
(Note: AS numbers within square brackets represent an AS_SET.)
Receive R2. Aggregate 192.0.2.0/24 AS_PATH "[ 64501 64502 ]"
If brief aggregation is in use, the AS_PATH would be truncated to the
empty AS_PATH, "".
The resulting AS_PATH is thus not stable and depends on the presence
of specific routes.
B.2. Scenario 2: First one route, then another, and the AS_PATHs
overlap at the origin AS. AS
Receive R1. Aggregate 192.0.2.0/24 AS_PATH "64501"
Receive R4. Aggregate 192.0.2.0/24 AS_PATH "[ 64504 64501 ]"
If brief aggregation is in use, the AS_PATH is truncated to "".
The resulting AS_PATH is thus not stable and depends on the presence
of specific routes.
B.3. Scenario 3: First one route, then another, and the AS_PATHs
overlap at the neighbor AS
Receive R3. Aggregate 192.0.2.0/24 AS_PATH "64504 64501". 64501"
Receive R4. Aggregate 192.0.2.0/24 AS_PATH "64504 [ 64501 64502 ]"
If brief aggregation is in use, the AS_PATH is truncated to "64504".
The resulting AS_PATH is thus not stable and depends on the presence
of specific routes.
B.4. Achieving Consistent Origin-AS Origin AS During Aggregation
In the three scenarios above, the aggregating AS 64500 is using
traditional brief aggregation. This results in inconsistent origin
ASes as the contributing routes are learned. This motivates the
"consistent brief" BGP aggregation mentioned in Section 5.2 and
discussed further with examples below.
The trivial solution to addressing the issue is to simply discard all
of the ASes for the contributing routes. In simple BGP aggregation
topologies, this is likely the correct thing to do. The AS
originating the aggregate, 192.0.2.0/24 in this example, is likely
the resource holder for the route in question. In such a case,
simply originating the route to its BGP upstream neighbors in the
Internet with its own AS, 64500, means that a consistent Route Origin
Authorization (ROA) ROA could be
registered in the RPKI for this prefix. This satisfies the need for
a consistent (unambiguous) origin AS.
If the contributing ASes are themselves multihomed to the Internet
outside of their connections to AS 64500, then additional ROAs would
need to be created for each of the more specific prefixes.
In more complex proxy aggregation scenarios, there may be a desire to
permit some stable (i.e., common) portion of the contributing
AS_PATHs to be kept in the aggregate route. Consider the case for
Scenario 3, where the neighbor AS is the same for both R3 and R4 - --
AS 64504. In such a case, an implementation may permit the
aggregate's brief AS_PATH to be "64504", and a ROA would be created
for the aggregate prefix with 64504 as the origin AS.
Appendix C. Discussion on Forwarding Loops and AS_SETs
Although BGP-4 was designed to carry CIDR Classless Inter-Domain Routing
(CIDR) routes, [RFC4271] does not discuss the installation of
"discard" or "null" routes when implementing its aggregation
procedures. Implementations could originate an aggregate prefix
without a covering route for a more- more specific prefix (subsumed by the
aggregate prefix) present in the local routing table.
When aggregating more specific routes according to [RFC4271] the aggregation procedures,
procedures of [RFC4271], the aggregating BGP speaker will place
contributing routes into the generated AS_PATH, perhaps using
AS_SETs. As a result, a contributing AS will not install the
aggregated route into its RIB since the route is an AS_PATH loop.
This provides a form of protection against forwarding loops created
by BGP aggregation.
When brief aggregation methods are used, a BGP speaker may receive a
route containing such a less specific destination covering a local more
specific destination and install it in its routing table since it is
not prevented from doing so by BGP AS_PATH loop detection. This
gives rise to the possibility of forwarding loops. To help prevent
forwarding loops, it is critical to adhere to the following:
1. Rule #2 of in Section 5.1 of [RFC4632]: "A
| A router that generates an aggregate route for multiple, more-specific more-
| specific routes must discard packets that match the aggregate
| route, but not any of the more-
specific more-specific routes. In other words,
| the "next hop" for the aggregate route should be the null destination."
| destination.
2. Not advertising aggregate routes to contributing ASes as
specified in Section 6.2 of this document (also see Appendix A).
Acknowledgements
The authors would like to thank Alvaro Retana, John Scudder, Ketan
Talaulikar, Keyur Patel, Susan Hares, Claudio Jeker, Nick Hilliard,
Robert Raszuk, John Heasley, Job Snijders, Jared Mauch, Jakob Heitz,
Tony Przygienda, Douglas Montgomery, Randy Bush, Curtis Villamizar,
Danny McPherson, Chris Morrow, Tom Petch, Ilya Varlashkin, Enke Chen,
Tony Li, Florian Weimer, John Leslie, Paul Jakma, Rob Austein, Russ
Housley, Sandra Murphy, Steven M. Bellovin, Steve Kent, Steve
Padgett, and Alfred Hoenes for comments and suggestions. The
comments and suggestions received from the IESG reviewers are also
much appreciated.
Authors' Addresses
Warren Kumari
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Phone: +1 571 748 4373
Email: warren@kumari.net
Kotikalapudi Sriram
USA NIST
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Phone: +1 301 975 3973
Email: ksriram@nist.gov
Lilia Hannachi
USA NIST
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Phone: +1 301 975 3259
Email: lilia.hannachi@nist.gov
Jeffrey Haas
Juniper Networks, Inc.
1133 Innovation Way
Sunnyvale, CA 94089
United States of America
Email: jhaas@juniper.net