Internet DRAFT - draft-ietf-idr-bgp-fwd-rr
draft-ietf-idr-bgp-fwd-rr
Network Working Group K. Vairavakkalai, Ed.
Internet-Draft N. Venkataraman, Ed.
Updates: 4456 (if approved) Juniper Networks, Inc.
Intended status: Experimental 16 February 2024
Expires: 19 August 2024
BGP Route Reflector in Forwarding Path
draft-ietf-idr-bgp-fwd-rr-01
Abstract
The procedures in BGP Route Reflection (RR) spec [RFC4456] primarily
deal with scenarios where the RR is reflecting BGP routes with next
hop unchanged.
These procedures can sometimes result in traffic forwarding loops in
deployments where the RR is in forwarding path, because of reflecting
BGP routes with next hop set to self.
This document specifies approaches to minimize possiblity of such
traffic forwarding loops. One of those approaches updates path
selection procedures specified in Section 9 of BGP RR. [RFC4456]
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 RFC 2119 [RFC2119] RFC 8174 [RFC8174] when, and only when, they
appear in all capitals, as shown here.
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 of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents 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 19 August 2024.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Avoiding Loops Between Route Reflectors in Forwarding Path . 3
3.1. Path selection change . . . . . . . . . . . . . . . . . . 4
3.2. Other mechanisms . . . . . . . . . . . . . . . . . . . . 4
4. Managabeality Considerations . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Appendix . . . . . . . . . . . . . . . . . . . . . . 6
A.1. Document History . . . . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The procedures in BGP Route Reflection (RR) spec [RFC4456] primarily
deal with scenarios where the RR is reflecting BGP routes with next
hop unchanged.
These procedures can sometimes result in traffic forwarding loops in
deployments where the RR is in forwarding path, and is reflecting BGP
routes with next hop set to self. RR with next hop self is used at
ABR nodes in Inter-AS Option C (Section 10, [RFC4364]) deployments.
This document specifies approaches to minimize possiblity of such
traffic forwarding loops. One of those approaches updates path
selection procedures specified in Section 9 of BGP RR. [RFC4456]
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2. Terminology
ABR: Area Border Router
AS: Autonomous System
AFI: Address Family Identifier
BN: Border Node
EP: Endpoint, e.g. a loopback address in the network
MPLS: Multi Protocol Label Switching
PE: Provider Edge
SAFI: Subsequent Address Family Identifier
3. Avoiding Loops Between Route Reflectors in Forwarding Path
[RR26] [RR27] [RR16]
| | |
| | |
|+-[ABR23]--+|+--[ASBR21]---[ASBR13]-+|+--[PE11]--+
|| ||| ` / ||| |
[CE41]--[PE25]--[P28] [P29] `/ [P15] [CE31]
| | | /` | | |
| | | / ` | | |
| | | / ` | | |
+--[ABR24]--+ +--[ASBR22]---[ASBR14]-+ +--[PE12]--+
| AS2 | AS2 | |
AS4 + region-1 + region-2 + AS1 + AS3
| | | |
203.0.113.41 ------------ Traffic Direction ----------> 203.0.113.31
Figure 1: Reference Topology: Inter-domain BGP Transport Network
A pair of redundant ABRs (ABR23, ABR24 in Figure 1), each acting
as an RR with next hop self, may choose each other as best path
towards egress PE11, instead of the upstream ASBR (ASBR21 or
ASBR22), causing a traffic forwarding loop.
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This happens because of following the path selection rule
specified in Section 9 of BGP RR [RFC4456] that tie-breaks on
ORIGINATOR_ID before CLUSTER_LIST. RFC4456 considers pure RR
functionality which leaves next hop unchanged.
When a RR inserts itself in forwarding path because of reflecting
routes with next hop self, as is the case for ABR BNs in an Inter-
AS Option C (Section 10 [RFC4364]) BGP transport network, this
rule may cause loops.
This problem can happen for routes of any BGP address family,
including BGP LU (1/4 or 2/4) and BGP CT (AFI/SAFIs: 1/76 or
2/76).
Using one or more of the following approaches softens the
possibility of such loops in a network with redundant ABRs.
3.1. Path selection change
Implementations SHOULD provide a way to alter the tie-breaking
rule specified in Section 9 of BGP RR [RFC4456] so as to tie-break
on CLUSTER_LIST step before ORIGINATOR_ID step, when performing
path selection for BGP routes.
This document suggests the following modification to the BGP
Decision Process Tie Breaking rules (Section 9.1.2.2 of [RFC4271])
that can be applied to path selection of BGP routes:
The following rule SHOULD be inserted between Steps e) and f): a
BGP Speaker SHOULD prefer a route with the shorter CLUSTER_LIST
length. The CLUSTER_LIST length is zero if a route does not carry
the CLUSTER_LIST attribute.
3.2. Other mechanisms
Some other deployment considerations, if feasible, can also help
in avoiding this problem:
- Configuring the same CLUSTER_ID at the redundant ABR nodes.
CLUSTER_ID Loop check will make routes reflected by an ABR
unusable at redundant ABRs.
- IGP metric assignment, such that "ABR to redundant ABR" cost is
inferior to "ABR to upstream ASBR" cost.
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- Using procedures described in [BGP-CT] , tunnels belonging to
non 'best effort' Transport Classes not be provisioned between
ABRs. This will ensure that the BGP CT route received from an
ABR with next hop self will be unusable at a redundant ABR.
4. Managabeality Considerations
The path selection change mentioned in Section 3.1 can be deployed
incrementally at the redundant ABRs, since the forwarding loop
would break when one of the ABRs stops chosing the other as best
path. Neverthless, it is recommended to consistently provision
the path selection change on all redundant ABR/RR nodes in a
domain. This provides consistent route selection at the transport
layer ABRs in the IBGP domain.
The operator should carefully consider the overall impact of any
of these options on a specific network deployment.
5. IANA Considerations
This document makes no new requests of IANA.
6. Security Considerations
This document does not change the underlying security issues inherent
in the existing BGP protocol, such as those described in [RFC4271],
[RFC4272] and [RFC4456].
Mehanisms described in this document reduce possibility of loops
within an IBGP domain. They do not affect routing across EBGP
sessions.
7. References
7.1. Normative References
[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>.
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[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
[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>.
7.2. Informative References
[BGP-CT] Vairavakkalai, Ed. and Venkataraman, Ed., "BGP Classful
Transport Planes", 22 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
ct-16>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
Appendix A. Appendix
A.1. Document History
The content in this document was introduced as part of [BGP-CT]. But
because the described problem is not specific to BGP CT and is useful
for other BGP families also, it is being extracted out to this
separate document.
Acknowledgements
The authors thank Jeff Haas, Jon Hardwick, Keyur Patel, Robert
Raszuk, Susan Hares for the discussions and review comments.
Authors' Addresses
Kaliraj Vairavakkalai (editor)
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: kaliraj@juniper.net
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Natrajan Venkataraman (editor)
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: natv@juniper.net
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