| Internet-Draft | Anycast Property advertisement | August 2025 |
| Chen, et al. | Expires 2 March 2026 | [Page] |
An IPv4 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 identifier.¶
This document defines a new flag in the OSPFv2 Extended Prefix TLV Flags to advertise the anycast property.¶
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 2 March 2026.¶
Copyright (c) 2025 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.¶
An IPv4 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 identifier.¶
[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 an IPv4 prefix, but the definition of anycast flag to identify the IPv4 prefix as anycast has not yet been defined.¶
The flags field of the OSPFv2 Extended Prefix TLV (Section 2.1 of [RFC7684]) can be found in "OSPFv2 Extended Prefix TLV Flags" IANA registry [IANA-OSPFv2-EPF].¶
This document defines a new flag in the OSPFv2 Extended Prefix TLV Flags [RFC7684] to advertise the anycast property for an IPv4 prefix.¶
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.¶
In the absence of the N-flag, the node specific prefixes need to be identified from the anycast prefixs. A prefix that is advertised by a single node and without an Anycast Flag (AC-flag) MUST be considered node specific.¶
An IPv4 prefix may be configured as anycast and it is useful for other routers to know that the advertisement is for an anycast identifier.¶
[RFC7684] defines one-octet field contains flags applicable to the prefix, and it has been defined the below flags(see "OSPFv2 Extended Prefix TLV Flags" IANA registry [IANA-OSPFv2-EPF]):¶
| Value | Description | Reference |
|---|---|---|
| 0x80 | A | [RFC7684] |
| 0x40 | N | [RFC7684] |
| 0x02 | E-Flag | [RFC9089] |
A new bit in OSPFv2 Extended Prefix TLV Flags[RFC7684] is defined to advertise the anycast property:¶
Value: TBD¶
Description:Anycast Flag (AC-flag)¶
When the prefix is configured as anycast, the AC-flag SHOULD be set. Otherwise, this flag MUST be clear.¶
The AC-flag and the N-bit MUST NOT both be set. If both N-flag and AC-flag are set, the receiving routers MUST ignore the N-flag.¶
The AC-flag MUST be preserved when re-advertising the prefix across 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 SHOULD be considered as anycast.¶
A prefix that is advertised by a single node and without an AC-flag MUST be considered node specific prefix.¶
[RFC9085] defines the Prefix Attribute Flags TLV carries IPv4 prefix attribute flags information, and the Flags field of this TLV is interpreted according to OSPFv2 [RFC7684]. This section extends the interpretation of the Flags field of the Prefix Attribute Flags TLV.¶
Flags:¶
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 configure and 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].¶
This document uses the graphical representation of data models per [RFC8340].¶
The following show 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
¶
The following is the YANG module:¶
<CODE BEGINS> file "ietf-ospf-anycast-flag@2025-08-28.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:ketan.ietf@gmail.com>
Author: Changwang Lin
<mailto:linchangwang.04414@h3c.com>";
description
"This YANG module adds the support of configuring an OSPFv2
prefix as anycast.
This YANG module conforms to the Network Management
Datastore Architecture (NMDA) as described in RFC 8342.
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
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC XXXX;
see the RFC itself for full legal notices.";
reference
"RFC XXXX";
revision 2025-08-28 {
description
"Initial version";
reference
"RFC XXXX: OSPFv2 Anycast Property Advertisement";
}
identity AC-flag {
base ospf:ospfv2-extended-prefix-flag;
description
"Anycast flag. When set, it indicates that the prefix
is configured as anycast.";
}
/* Configuration */
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 configuration.";
}
description
"This augments OSPFv2 interface configuration with anycast
property advertisement.";
leaf anycast-flag {
type boolean;
default "false";
description
"Sets the prefix as an anycast address.";
}
}
}
<CODE ENDS>¶
The authors would like to thank Acee Lindem for aligning the terminology with existing OSPF documents and for editorial improvements. The author would also like to thank Yingzhen Qu for providing the YANG model and tree, as well as for valuable editorial comments.¶
This document requests allocation for the following registry.¶
This document adds a new bit in the "OSPFv2 Extended Prefix TLV Flags"registry:¶
AC-flag (Anycast Flag).¶
The IANA is requested to assign one new URI from the IETF XML registry ([RFC3688]). Authors are suggesting the following URI:¶
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¶
This document also requests one new YANG module name in the YANG Module Names registry ([RFC6020]) with the following suggestion :¶
name: ietf-ospf-anycast-flag namespace: urn:ietf:params:xml:ns:yang:ietf-ospf-anycast-flag prefix: ospf-anycast-flag reference: RFC XXXX¶
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 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 NETCONF Access Control Model (NACM) [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a pre-configured subset of all available NETCONF or RESTCONF protocol operations and content.¶
The following data nodes defined in the YANG module that are writable/creatable/deletable (i.e., config true). The modifications to these data nodes without proper protection could have prevent interpreting the IPv4 prefix as anycast.¶
Some of the readable data nodes in this YANG module may be considered sensitive or vulnerable in some network environments. Exposure of the OSPF link state database may be useful in mounting a Denial-of-Service (DoS) attacks. These are the readable data nodes:¶