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BGP BFD Strict-Mode

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Mercia Zheng , Acee Lindem , Jeffrey Haas , Albert Fu
Last updated 2023-07-03 (Latest revision 2023-01-05)
Replaces draft-merciaz-idr-bgp-bfd-strict-mode
RFC stream Internet Engineering Task Force (IETF)
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IDR Workgroup                                                   M. Zheng
Internet-Draft                                                     Ciena
Updates: RFC4271 (if approved)                                 A. Lindem
Intended status: Standards Track                    LabN Consulting, LLC
Expires: 4 January 2024                                          J. Haas
                                                  Juniper Networks, Inc.
                                                                   A. Fu
                                                          Bloomberg L.P.
                                                             3 July 2023

                          BGP BFD Strict-Mode


   This document specifies extensions to RFC4271 BGP-4 that enable a BGP
   speaker to negotiate additional Bidirectional Forwarding Detection
   (BFD) extensions using a BGP capability.  This BFD Strict-Mode
   Capability enables a BGP speaker to prevent a BGP session from being
   established until a BFD session is established.  It is referred to as
   BGP BFD "strict-mode".  BGP BFD strict-mode will be supported when
   both the local speaker and its remote peer are BFD strict-mode

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

   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 4 January 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  BFD Strict-Mode Capability  . . . . . . . . . . . . . . . . .   3
   4.  BGP BFD Strict-Mode Procedures  . . . . . . . . . . . . . . .   3
     4.1.  Closing BGP Sessions  . . . . . . . . . . . . . . . . . .   4
     4.2.  Stability Considerations  . . . . . . . . . . . . . . . .   5
   5.  Manageability Considerations  . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   5
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Bidirectional Forwarding Detection BFD [RFC5882] enables routers to
   monitor data plane connectivity and to detect faults in the
   bidirectional forwarding path between them.  This capability is
   leveraged by routing protocols such as BGP [RFC4271] to rapidly react
   to topology changes in the face of path failures.

   The BFD interaction with BGP is specified in Section 10.2 of
   [RFC5882].  When BFD is enabled for a BGP neighbor, faults in the
   bidirectional forwarding detected by BFD result in session
   termination.  It is possible in some failure scenarios for the
   network to be in a state such that a BGP session may be established
   but a BFD session cannot be established.  In some other scenarios, it
   may be possible to establish a BGP session, but a degraded or poor-
   quality link may result in the corresponding BFD session going up and
   down frequently.

   To avoid situations which result in routing churn and to minimize the
   impact of network interruptions, it will be beneficial to disallow
   BGP to establish a session until BFD session is successfully
   established and has stabilized.  We refer to this mode of operation
   as BGP BFD "strict-mode".  However, always using "strict-mode" would
   preclude BGP operation in an environment where not all routers

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   support BFD strict-mode or have BFD enabled.  This document defines
   BGP "strict-mode" operation as preventing BGP session establishment
   until both the local and remove speakers have a stable BFD session.
   The document also specifies the BGP protocol extensions for BGP
   capability [RFC5492] for announcing BFD parameters including a BGP
   speaker's support for "strict-mode", i.e., requiring a BFD session
   for BGP session establishment.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  BFD Strict-Mode Capability

   The BGP Strict-Mode Capability [RFC5492] will allow a BGP speaker's
   to advertise this capability.  The capability is defined as follows:

   Capability code: 74

   Capability length: 0 octets

4.  BGP BFD Strict-Mode Procedures

   A BGP speaker which supports capabilities advertisement and has BFD
   strict-mode enabled MUST include the BFD Strict-Mode Capability.

   A BGP speaker which supports the BFD Strict-Mode Capability, examines
   the list of capabilities present in the capabilities that the speaker
   receives from its peer.  If both the local and remote BGP speakers
   include the BFD Strict-Mode Capability, the BGP finite state machine
   does not transition to the Established state from OpenConfirm state
   [RFC4271] until the BFD session is in the Up state (see below for
   AdminDown state).  This means that a KEEPALIVE message is not sent
   nor is the KeepaliveTimer set.

   If the BFD session does not transition to the Up state, and the
   HoldTimer has been negotiated to a non-zero value, the BGP FSM will
   close the session appropriately.  If the HoldTimer has been
   negotiated to a zero value, the session should be closed after a time
   of X.  This time X is referred as "BGP BFD Hold time".  The proposed
   default BGP BFD Hold time value is 30 seconds.  The BGP BFD Hold time
   value is configurable.

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   If BFD session is in the AdminDown state, then the BGP finite state
   machine will proceed normally without input from BFD.  This means
   that BFD session "AdminDown" state WILL NOT prevent the BGP state
   transition from the OpenConfirm state to the Established state.

   Once the BFD session has transitioned to the Up state, the BGP FSM
   may proceed to transition from the OpenConfirm state to state
   Established state.  Once in the Established state, a KEEPALIVE
   message is sent and a KeepaliveTimer for the BGP peer is started.

   BGP strict-mode cannot be enabled unless BFD is configured for the
   BGP peer.  If BFD is removed for the BGP peer, then BGP strict-mode
   will also be disabled.

   Note that it is fully possible to have BFD enabled between the peers
   without BGP strict-mode.

   If either BGP peer has not advertised the BFD Strict-Mode Capability,
   then a BFD session WILL NOT be required for the BGP session to reach
   Established state.  This does not preclude usage of BFD after BGP
   session establishment [RFC5882].

   If BFD strict-mode is enabled or disabled for a BGP peer and the BGP
   session state is not Established state, then the BGP will close the

   If the BFD strict-mode is enabled or disabled for a BGP peer and the
   BGP session state is Established state, the local BFD strict-mode
   configuration will be modified but the session will remain in
   Established state.

   Since BFD strict-mode is only applicable during BGP session
   establishment, this inconsistency would not have an impact on the
   Established session unless the remote BGP peer is waiting in
   OpenConfirm state.  To avoid this situation, BFD strict-mode SHOULD
   be modified consistently on both the local and remote BGP peers.

4.1.  Closing BGP Sessions

   When BGP sessions are closed according to the procedures in this
   document, the session should be terminated with a NOTIFICATION
   message with the Cease Code and the "BFD Down" Subcode.
   ([I-D.ietf-idr-bfd-subcode]) This informs the operator that
   interaction with BFD is the root cause of the BGP session being
   unable to move to the Established state.

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4.2.  Stability Considerations

   The use of BGP BFD strict-mode along with mechanisms such as hold-
   down (a delay in the initial BGP Establishment state following BFD
   session establishment) and/or dampening (a delay in the BGP
   Establishment state following failure detected by BFD) may help
   reduce the frequency of BGP session flaps and therefore reduce the
   associated routing churn.  The details of these mechanisms are
   outside the scope of this document.

5.  Manageability Considerations

   Auto-configuration is possible for the enabling BGP BFD strict-mode.
   However, the configuration automation is out of the scope of this

   To simplify troubleshoorting and avoid inconsistencies, it is
   RECOMMENDED that BFD strict-mode configuration be consistent for both
   BGP peers.

6.  Security Considerations

   The mechanism defined in this document interacts with the BGP finite
   state machine when so configured.  The security considerations of BFD
   thus, become considerations for BGP-4 [RFC4271] so used.  Given that
   a BFD session is required for a BGP session, a Denial-of-Service
   (DoS) attack on BGP can now be mounted by preventing a BFD session
   between the BGP peers from being established or interrupting an
   existing BFD session.  The use of the BFD Authentication mechanism
   defined in [RFC5880] is thus RECOMMENDED when used to protect BGP-4

7.  IANA Considerations

   This document defines the BGP BFD Strict-Mode Capability.  The
   Capability Code 74 has been assigned from the First-Come-First-Served
   range (64-238) of the Capability Codes registry.

8.  Acknowledgement

   The authors would like to acknowledge the review and inputs from
   Shyam Sethuram, Mohammed Mirza, Bruno Decraene, Carlos Pignataro, and
   Enke Chen.

9.  Normative References

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              Haas, J., "A BGP Cease Notification Subcode For
              Bidirectional Forwarding Detection (BFD)", Work in
              Progress, Internet-Draft, draft-ietf-idr-bfd-subcode-05,
              27 December 2022, <

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC5882]  Katz, D. and D. Ward, "Generic Application of
              Bidirectional Forwarding Detection (BFD)", RFC 5882,
              DOI 10.17487/RFC5882, June 2010,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

Authors' Addresses

   Mercia Zheng
   3939 N. 1st Street
   San Jose, CA 95134
   United States

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   Acee Lindem
   LabN Consulting, LLC
   301 Midenhall Way
   Cary, NC 27513
   United States

   Jeffrey Haas
   Juniper Networks, Inc.
   1133 Innovation Way
   United States

   Albert Fu
   Bloomberg L.P.

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