



LSR                                                              R. Chen
Internet-Draft                                                   D. Zhao
Intended status: Standards Track                         ZTE Corporation
Expires: 23 July 2026                                          P. Psenak
                                                           K. Talaulikar
                                                           Cisco Systems
                                                                  C. Lin
                                                    New H3C Technologies
                                                         19 January 2026


                 OSPFv2 Anycast Property Advertisement
                     draft-ietf-lsr-anycast-flag-13

Abstract

   An IP prefix may be configured as anycast and as such the same value
   can be advertised by multiple routers.  It is useful for other
   routers to know that the advertisement is for an anycast prefix.

   This document defines a new flag in the OSPFv2 Extended Prefix TLV
   Flags to advertise the anycast property.  The document also specifies
   a companion YANG module for managing this function.

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 23 July 2026.

Copyright Notice

   Copyright (c) 2026 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 (https://trustee.ietf.org/
   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  OSPFv2 Anycast Property Advertisement . . . . . . . . . . . .   3
   3.  BGP-LS Advertisement  . . . . . . . . . . . . . . . . . . . .   4
   4.  YANG Data Model . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Tree for the YANG Data Model  . . . . . . . . . . . . . .   4
     4.2.  YANG Data Model for OSPFv2 Anycast Property
           Advertisement . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  OSPFv2 Extended Prefix TLV Flags Registry . . . . . . . .   7
     5.2.  OSPFv2 Anycast Flag YANG Module Registry  . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     6.1.  Protocol Security Considerations  . . . . . . . . . . . .   7
     6.2.  YANG Security Considerations  . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   An IP prefix may be configured as anycast and as such the same value
   can be advertised by multiple routers.  It is useful for other
   routers to know that the advertisement is for an anycast prefix.

   [RFC7684] defines OSPFv2 Opaque LSAs based on Type-Length-Value (TLV)
   tuples that can be used to associate additional attributes with
   prefixes or links.  The OSPFv2 Extended Prefix TLV that is contained
   in the OSPFv2 Extended Prefix Opaque LSA is used to advertise
   additional attributes associated with a prefix.

   Extensions related to the anycast property of prefixes have been
   specified for IS-IS [RFC9352] and OSPFv3 [RFC9513], even though those
   documents are related to Segment Routing over IPv6, the anycast
   property applies to any IP prefix advertisement.  This document



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   defines a flag to advertise the anycast property for a prefix
   advertisement in OSPFv2 in the Flags field of the OSPFv2 Extended
   Prefix TLV Flags (section 2.1 of [RFC7684]).  The document also
   specifies a companion YANG module for managing this function.

1.1.  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.

2.  OSPFv2 Anycast Property Advertisement

   An IP prefix may be configured as anycast and it is useful for other
   routers to know that the advertisement is for an anycast prefix.

   In the context of the flags defined in this document, the term 'set'
   means the bit is set to 1, and the term 'clear' means the bit is set
   to 0.

   A flag is introduced in OSPFv2 Extended Prefix TLV Flags [RFC7684] to
   advertise the anycast property:

   Value: TBD

   Description: Anycast Flag (AC-flag)

   The only meaning of the AC-flag is that the prefix is intended to be
   advertised by multiple nodes.

   When a prefix is configured as anycast, the AC-flag MUST be set.
   Otherwise, this flag MUST be clear.

   The AC-flag and the N-flag (section 2.1 of [RFC7684]) MUST NOT both
   be set.  The reception of an advertisement with both the N-flag and
   AC-flag set MUST be considered a configuration anomaly, and N-flag
   MUST be ignored.  Additionally, the detection of such a conflicting
   advertisement SHOULD be logged as an operational error(subject to
   rate-limiting).

   The AC-flag MUST be preserved when the OSPFv2 Extended Prefix Opaque
   LSA is re-advertised into other areas.

   The same prefix can be advertised by multiple routers, and that if at
   least one of them sets the AC-flag in its advertisement, the prefix
   is considered as anycast.



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   A prefix that is advertised by a single node and without an AC-flag
   is considered a node-specific prefix.

   Anycast prefixes SHOULD be consistently managed throughout the
   network.  Since an AC-flag set takes precedence in identifying
   anycast property, stale configurations should be strictly monitored.

3.  BGP-LS Advertisement

   [RFC9085] defines the Prefix Attribute Flags TLV for BGP-LS that
   carries prefix attribute flags information, and the Flags field of
   this TLV is interpreted according to OSPFv2 [RFC7684].  Thus the
   Flags field of the BGP-LS Prefix Attribute Flags TLV also conveys the
   anycast property introduced by this document.

4.  YANG Data Model

   YANG [RFC7950] is a data definition language used to define the
   contents of a conceptual data store that allows networked devices to
   be managed using NETCONF [RFC6241] or RESTCONF [RFC8040].

   This section defines a YANG data model that can be used to manage the
   usage of OSPFv2 Anycast Property as defined in this document, which
   augments the OSPF YANG data model [RFC9129] and the YANG Data Model
   for Routing Management [RFC8349].

4.1.  Tree for the YANG Data Model

   This document uses the graphical representation of data models per
   [RFC8340].

   The following shows the tree diagram of the module:

   module: ietf-ospf-anycast-flag

     augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:areas/ospf:area
           /ospf:interfaces/ospf:interface:
       +--rw anycast-flag?   boolean


4.2.  YANG Data Model for OSPFv2 Anycast Property Advertisement

   The "ietf-ospf-anycast-flag" module defined in this document imports
   typedefs from [RFC8349]and [RFC9129].






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   <CODE BEGINS> file "ietf-ospf-anycast-flag@2026-01-14.yang"
   module ietf-ospf-anycast-flag {
     yang-version 1.1;
     namespace
       "urn:ietf:params:xml:ns:yang:ietf-ospf-anycast-flag";
     prefix ospf-anycast-flag;

     import ietf-routing {
       prefix rt;
       reference
         "RFC 8349: A YANG Data Model for Routing
          Management (NMDA Version)";
     }
     import ietf-ospf {
       prefix ospf;
       reference
         "RFC 9129: YANG Data Model for the OSPF Protocol";
     }

     organization
       "IETF LSR - Link State Routing Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/lsr/>
        WG List:  <mailto:lsr@ietf.org>

        Author:   Ran Chen
                  <mailto:chen.ran@zte.com.cn>
        Author:   Detao Zhao
                  <mailto:zhao.detao@zte.com.cn>
        Author:   Peter Psenak
                  <mailto:ppsenak@cisco.com>
        Author:   Ketan Talaulikar
                  <mailto:ketant.ietf@gmail.com>
        Author:   Changwang Lin
                  <mailto:linchangwang.04414@h3c.com>";

     description
       "This YANG module adds the support of managing an OSPFv2
        prefix as anycast.

        Copyright (c) 2025 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject to
        the license terms contained in, the Revised BSD License set
        forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents



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        (https://trustee.ietf.org/license-info).

        All revisions of IETF and IANA published modules can
        be found at the YANG Parameters registry group
       (https://www.iana.org/assignments/yang-parameters);

        This version of this YANG module is part of RFC XXXX;
        see the RFC itself for full legal notices.";

     revision 2026-01-14 {
       description
         "Initial version";
       reference
         "RFC XXXX: OSPFv2 Anycast Property Advertisement";
     }

     identity ac-flag {
       base ospf:ospfv2-extended-prefix-flag;
       description
         "Indicates that the prefix is configured as anycast.";
     }

     augment "/rt:routing/rt:control-plane-protocols/"
           + "rt:control-plane-protocol/ospf:ospf/"
           + "ospf:areas/ospf:area/ospf:interfaces/ospf:interface" {
       when "derived-from(/rt:routing/rt:control-plane-protocols/"
          + "rt:control-plane-protocol/rt:type, 'ospf:ospfv2')" {
         description
           "This augments the OSPFv2 interface.";
       }
       description
         "This augments OSPFv2 interface with anycast
          property advertisement.";
       leaf anycast-flag {
         type boolean;
         must "not(../anycast-flag = 'true' and "
            + "/rt:routing/rt:control-plane-protocols/"
            + "rt:control-plane-protocol/ospf:ospf/"
            + "ospf:areas/ospf:area/ospf:interfaces/"
            + "ospf:interface/ospf:node-flag = 'true')" {
           error-message "The anycast-flag and the node-flag MUST "
                       + "NOT both be set to 1 (true).";
           description
             "Ensures architectural consistency by preventing a prefix
              from being marked as both anycast and node-specific.";
         }
         default "false";
         description



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           "Indicates that the prefix is an anycast address,
            if set to 1 (true).";
       }
     }
   }
   <CODE ENDS>

5.  IANA Considerations

   This document requests allocation for the following registry.

5.1.  OSPFv2 Extended Prefix TLV Flags Registry

   This document requests the allocation of new value in the "OSPFv2
   Extended Prefix TLV Flags" registry:

   TBD:AC-flag (Anycast Flag).

5.2.  OSPFv2 Anycast Flag YANG Module Registry

   IANA is requested to register the following URI in the "ns" registry
   within the "IETF XML Registry" group ([RFC3688]):

   URI: urn:ietf:params:xml:ns:yang:ietf-ospf-anycast-flag
   Registrant Contact: The IESG.
   XML: N/A, the requested URI is an XML namespace

   IANA is requested to register the following YANG module in the "YANG
   Module Names" registry ([RFC6020]) within the "YANG Parameters"
   registry group.

   name: ietf-ospf-anycast-flag
   Maintained by IANA?  N
   namespace: urn:ietf:params:xml:ns:yang:ietf-ospf-anycast-flag
   prefix: ospf-anycast-flag
   reference: RFC XXXX

6.  Security Considerations

6.1.  Protocol Security Considerations

   Procedures and protocol extensions defined in this document do not
   affect the OSPFv2 security model.  See the "Security
   Considerations"section of [RFC7684] for a discussion of OSPFv2
   security.






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   The newly introduced AC-flag, which MUST be either set or clear,
   introduces operational dependencies that impact the semantic validity
   of the advertised prefix.  The correct semantic interpretation of the
   AC-flag relies on both router implementation support for the flag and
   accurate operator configuration of the anycast route.  Consequently,
   receivers MUST consider the possibility of misconfiguration or
   inconsistent implementation when relying on the AC-flag for
   forwarding or security decisions.

6.2.  YANG Security Considerations

   This section is modeled after the template described in Section 3.7
   of [I-D.ietf-netmod-rfc8407bis].

   The "ietf-ospf-anycast-flag" YANG module defines a data model that is
   designed to be accessed via YANG-based management protocols, such as
   NETCONF [RFC6241] and RESTCONF [RFC8040].  These protocols have to
   use a secure transport layer (e.g., SSH [RFC4252], TLS [RFC8446], and
   QUIC [RFC9000]) and have to use mutual authentication.

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular NETCONF or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.

   There is a data node defined in this YANG module that is
   writable/creatable/deletable (i.e., config true, which is the
   default).  This data node can be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to this data node without proper protection can have a negative
   effect on network operations.  Specifically, the following subtree
   and data node have particular sensitivities/vulnerabilities:

      /ospf:ospf/ospf:areas/ospf:area/ospf:interfaces/ospf:interface/
      ospf-anycast-flag:anycast-flag

   As specified in Section 2, the AC-flag and the N-flag MUST NOT both
   be set to 1.  This rule is enforced by a "must" constraint in the
   YANG module to prevent configuration anomalies.  The handling of such
   anomalies is defined in Section 2.  Modifications to this data node
   without proper protection could prevent interpreting the IPv4 prefix
   as anycast or node-specific.

   The readable data node in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control read access (e.g., via get, get-config, or
   notification) to this data node.  Specifically, the following subtree
   and data node have particular sensitivities/vulnerabilities:



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      /ospf:ospf/ospf:areas/ospf:area/ospf:interfaces/ospf:interface/
      ospf-anycast-flag:anycast-flag

   Unauthorized access to the data node of this subtree can disclose
   specific anycast property information for OSPF prefixes on a device.

   There are no particularly sensitive RPC or action operations.

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>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [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>.

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

   [RFC8349]  Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for
              Routing Management (NMDA Version)", RFC 8349,
              DOI 10.17487/RFC8349, March 2018,
              <https://www.rfc-editor.org/info/rfc8349>.



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   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,
              <https://www.rfc-editor.org/info/rfc9085>.

   [RFC9129]  Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
              "YANG Data Model for the OSPF Protocol", RFC 9129,
              DOI 10.17487/RFC9129, October 2022,
              <https://www.rfc-editor.org/info/rfc9129>.

7.2.  Informative References

   [I-D.ietf-netmod-rfc8407bis]
              Bierman, A., Boucadair, M., and Q. Wu, "Guidelines for
              Authors and Reviewers of Documents Containing YANG Data
              Models", Work in Progress, Internet-Draft, draft-ietf-
              netmod-rfc8407bis-28, 5 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
              rfc8407bis-28>.

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.





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   [RFC9352]  Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
              and Z. Hu, "IS-IS Extensions to Support Segment Routing
              over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
              February 2023, <https://www.rfc-editor.org/info/rfc9352>.

   [RFC9513]  Li, Z., Hu, Z., Talaulikar, K., Ed., and P. Psenak,
              "OSPFv3 Extensions for Segment Routing over IPv6 (SRv6)",
              RFC 9513, DOI 10.17487/RFC9513, December 2023,
              <https://www.rfc-editor.org/info/rfc9513>.

Acknowledgements

   The authors would like to thank Acee Lindem for aligning the
   terminology with existing OSPF documents and for editorial
   improvements.

Contributors

   This document has the following contributor:

   Yingzhen Qu
   Futurewei Technologies
   Email: yingzhen.ietf@gmail.com


Authors' Addresses

   Ran Chen
   ZTE Corporation
   Nanjing
   China
   Email: chen.ran@zte.com.cn


   Detao Zhao
   ZTE Corporation
   Nanjing
   China
   Email: zhao.detao@zte.com.cn


   Peter Psenak
   Cisco Systems
   Email: ppsenak@cisco.com


   Ketan Talaulikar
   Cisco Systems



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   Email: ketant.ietf@gmail.com


   Changwang Lin
   New H3C Technologies
   Beijing
   China
   Email: linchangwang.04414@h3c.com











































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