Internet Engineering Task Force (IETF) R. Chen
Request for Comments: 9983 D. Zhao
Category: Standards Track ZTE Corporation
ISSN: 2070-1721 P. Psenak
K. Talaulikar
Cisco Systems
C. Lin
New H3C Technologies
May 2026
OSPFv2 Anycast Property Advertisement
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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9983.
Copyright Notice
Copyright (c) 2026 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/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
1.1. Requirements Language
2. OSPFv2 Anycast Property Advertisement
3. BGP-LS Advertisement
4. YANG Data Model
4.1. Tree for the YANG Data Model
4.2. YANG Data Model for OSPFv2 Anycast Property Advertisement
5. IANA Considerations
5.1. OSPFv2 Extended Prefix TLV Flags Registry
5.2. OSPFv2 Anycast Flag YANG Module Registration
6. Security Considerations
6.1. Protocol Security Considerations
6.2. YANG Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
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 Link State Advertisements (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
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; 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; "clear" means the bit is set to 0.
A flag is introduced in the "OSPFv2 Extended Prefix TLV Flags" IANA
registry (see [RFC7684]) to advertise the anycast property:
Value: 0x10
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 the
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, if at
least one of them sets the AC-Flag in its advertisement, the prefix
is considered to be anycast.
A prefix that is advertised by a single node and without an AC-Flag
is considered to be a node-specific prefix.
Anycast prefixes SHOULD be consistently managed throughout the
network. Since an AC-Flag set takes precedence in identifying the
anycast property, stale configurations should be strictly monitored.
3. BGP-LS Advertisement
[RFC9085] defines the Prefix Attribute Flags TLV for Border Gateway
Protocol - Link State (BGP-LS) that carries prefix attribute flags
information. 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 Network Configuration Protocol (NETCONF) [RFC6241]
or RESTCONF [RFC8040].
This section defines a YANG data model that can be used to manage the
usage of the 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].
<CODE BEGINS> file "ietf-ospf-anycast-flag@2026-05-12.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.
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) (RFC 8174) when, and only when,
they appear in all capitals, as shown here.
Copyright (c) 2026 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
(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 9983;
see the RFC itself for full legal notices.";
revision 2026-05-12 {
description
"Initial version";
reference
"RFC 9983: 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
"Indicates that the prefix is an anycast address,
if set to 1 (true).";
}
}
}
<CODE ENDS>
5. IANA Considerations
IANA has allocated and/or registered the following values in their
respective registries.
5.1. OSPFv2 Extended Prefix TLV Flags Registry
IANA has allocated the following value in the "OSPFv2 Extended Prefix
TLV Flags" registry:
0x10: AC-Flag (Anycast Flag)
5.2. OSPFv2 Anycast Flag YANG Module Registration
IANA has registered the following URI in the "ns" registry within the
"IETF XML Registry" registry group (see [RFC3688]):
ID: yang:ietf-ospf-anycast-flag
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 has registered 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 9983
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.
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 [RFC9907].
The "ietf-ospf-anycast-flag" YANG module defines a data model that is
designed to be accessed via YANG-based management protocols, such as
Network Configuration Protocol (NETCONF) [RFC6241] and RESTCONF
[RFC8040]. These YANG-based management protocols (1) have to use a
secure transport layer (e.g., SSH [RFC4252], TLS [RFC8446], and QUIC
[RFC9000]) and (2) 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)
and delete operations to this data node without proper protection or
authentication 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:
/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>.
[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
[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>.
[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>.
[RFC9907] Bierman, A., Boucadair, M., Ed., and Q. Wu, "Guidelines
for Authors and Reviewers of Documents Containing YANG
Data Models", BCP 216, RFC 9907, DOI 10.17487/RFC9907,
March 2026, <https://www.rfc-editor.org/info/rfc9907>.
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
Email: ketant.ietf@gmail.com
Changwang Lin
New H3C Technologies
Beijing
China
Email: linchangwang.04414@h3c.com