



Inter-Domain Routing                                         D. Abraitis
Internet-Draft                                                    NetDef
Intended status: Standards Track                                 J. Haas
Expires: 5 December 2026                                             HPE
                                                             3 June 2026


          Maximum Number of Paths Reached Notification for BGP
              draft-abraitis-idr-maximum-paths-subcode-00

Abstract

   This document defines a new BGP Cease NOTIFICATION message subcode,
   "Maximum Number of Paths Reached", used when a BGP speaker terminates
   a peering because the number of paths received from a neighbor, as
   permitted by BGP ADD-PATH, exceeds a locally configured upper bound.

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
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   This Internet-Draft will expire on 5 December 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
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   provided without warranty as described in the Revised BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Maximum Number of Paths Reached Subcode . . . . . . . . . . .   2
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     Normative References  . . . . . . . . . . . . . . . . . . . . .   4
     Informative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The BGP Cease NOTIFICATION message [RFC4271] is used when a BGP
   speaker terminates its peering with a neighbor.  [RFC4486] defines
   subcodes for the Cease NOTIFICATION message to aid network operators
   in correlating network events and diagnosing BGP peering issues,
   including the "Maximum Number of Prefixes Reached" subcode.

   BGP ADD-PATH [RFC7911] permits a BGP speaker to advertise multiple
   paths for the same address prefix.  As a result, a BGP speaker may
   enforce a locally configured upper bound on the number of paths it
   accepts from a peer, distinct from any bound on the number of
   prefixes.  No existing Cease subcode precisely describes the
   termination of a peering for this reason.

   This document defines the "Maximum Number of Paths Reached" Cease
   NOTIFICATION subcode for this purpose.

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.  Maximum Number of Paths Reached Subcode

   The value TBD (suggested value 11, see Section 6) has been allocated
   by IANA for the "Maximum Number of Paths Reached" Cease NOTIFICATION
   message subcode.  When a BGP speaker terminates a peering because the
   number of paths [RFC7911] received from the neighbor for one or more
   <AFI, SAFI> exceeds a locally configured upper bound, the BGP speaker
   SHOULD send a NOTIFICATION message with the error code "Cease" and



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   the error subcode "Maximum Number of Paths Reached".

   Following the format defined in [RFC4486], Section 4, the message MAY
   optionally include the Address Family information [RFC4760] and the
   upper bound in the "Data" field, as shown in Figure 1.

              +------------------------------------------------+
              | Address Family Identifier (2 octets)           |
              +------------------------------------------------+
              | Subsequent Address Family Identifier (1 octet) |
              +------------------------------------------------+
              | Paths upper bound (4 octets)                   |
              +------------------------------------------------+

            Figure 1: Maximum Number of Paths Reached Data Field

   The "Paths upper bound" field indicates the locally configured
   maximum number of paths, for the indicated <AFI, SAFI>, that
   triggered the termination.  If a BGP speaker terminates a peering
   because the bound was exceeded for more than one <AFI, SAFI>, it MAY
   include the Data field for any one of them.

4.  Operational Considerations

   This subcode is meaningful only when BGP ADD-PATH [RFC7911] has been
   negotiated for at least one <AFI, SAFI> with the peer.  A speaker
   that bounds only the number of prefixes continues to use the "Maximum
   Number of Prefixes Reached" subcode [RFC4486].

   The PATHS-LIMIT Capability [I-D.abraitis-idr-addpath-paths-limit],
   when supported by both peers, lets a receiver negotiate with the
   sender the maximum number of paths it is willing to receive, per
   <AFI, SAFI>, so that the sender can avoid exceeding that bound in the
   first place.  This subcode is the complementary enforcement signal:
   it indicates termination when the number of received paths exceeds
   the receiver's locally configured upper bound regardless of whether
   the PATHS-LIMIT Capability was negotiated, for example because the
   sender does not support it, ignored it, or sent more paths than
   indicated.  Where feasible, graceful handling that preserves the
   session, as recommended in [I-D.ietf-idr-add-paths-guidelines], is
   preferred over termination.










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   When BGP Graceful Restart with NOTIFICATION support has been
   negotiated, a speaker that sends or receives this subcode follows the
   procedures of [RFC8538].  A speaker whose local policy requires the
   immediate discard of the routes received from the peer uses the Hard
   Reset mechanism of [RFC8538], carrying the "Cease" code and "Maximum
   Number of Paths Reached" subcode in the Data field of the Hard Reset
   message.

5.  Security Considerations

   This document defines a subcode for the BGP Cease NOTIFICATION
   message that provides information to aid network operators in
   correlating network events and diagnosing BGP peering issues.  This
   subcode is purely informational and has no impact on the BGP Finite
   State Machine beyond that already documented by [RFC4271], Sections
   6.6 and 6.7.

6.  IANA Considerations

   IANA is requested to assign a value (suggested value 11) from the
   "BGP Cease NOTIFICATION message subcodes" registry, with the name
   "Maximum Number of Paths Reached" and a reference to this document.

Acknowledgements

   TBD

References

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>.

   [RFC4486]  Chen, E. and V. Gillet, "Subcodes for BGP Cease
              Notification Message", RFC 4486, DOI 10.17487/RFC4486,
              April 2006, <https://www.rfc-editor.org/info/rfc4486>.







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   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <https://www.rfc-editor.org/info/rfc7911>.

   [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>.

   [RFC8538]  Patel, K., Fernando, R., Scudder, J., and J. Haas,
              "Notification Message Support for BGP Graceful Restart",
              RFC 8538, DOI 10.17487/RFC8538, March 2019,
              <https://www.rfc-editor.org/info/rfc8538>.

Informative References

   [I-D.abraitis-idr-addpath-paths-limit]
              Abraitis, D., Retana, A., and J. Haas, "Paths Limit for
              Multiple Paths in BGP", Work in Progress, Internet-Draft,
              draft-abraitis-idr-addpath-paths-limit-04, 14 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-abraitis-idr-
              addpath-paths-limit-04>.

   [I-D.ietf-idr-add-paths-guidelines]
              Uttaro, J., Francois, P., Patel, K., Haas, J., Simpson,
              A., and R. Fragassi, "Best Practices for Advertisement of
              Multiple Paths in IBGP", Work in Progress, Internet-Draft,
              draft-ietf-idr-add-paths-guidelines-08, 25 April 2016,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-add-
              paths-guidelines-08>.

Authors' Addresses

   Donatas Abraitis
   NetDef
   Email: donatas.abraitis@gmail.com


   Jeffrey Haas
   HPE
   Email: jeffrey.haas@hpe.com





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