OSPFv2 Anycast Property Advertisement

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. 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 and OSPFv3 , 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 ). The document also specifies a companion YANG module for managing this function.

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 when, and only when, they appear in all capitals, as shown here.

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 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 ) 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. 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.

BGP-LS Advertisement 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 . Thus the Flags field of the BGP-LS Prefix Attribute Flags TLV also conveys the anycast property introduced by this document.

YANG is a data definition language used to define the contents of a conceptual data store that allows networked devices to be managed using NETCONF or RESTCONF . 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 and the YANG Data Model for Routing Management .

This document uses the graphical representation of data models per . The following shows the tree diagram of the module:

The "ietf-ospf-anycast-flag" module defined in this document imports typedefs from and . WG List: Author: Ran Chen Author: Detao Zhao Author: Peter Psenak Author: Ketan Talaulikar Author: Changwang Lin "; 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 (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 "Indicates that the prefix is an anycast address, if set to 1 (true)."; } } } ]]>

IANA Considerations This document requests allocation for the following registry.

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).

OSPFv2 Anycast Flag YANG Module Registry IANA is requested to register the following URI in the "ns" registry within the "IETF XML Registry" group (): IANA is requested to register the following YANG module in the "YANG Module Names" registry () within the "YANG Parameters" registry group.

Security Considerations

Protocol Security Considerations Procedures and protocol extensions defined in this document do not affect the OSPFv2 security model. See the "Security Considerations"section of 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.

YANG Security Considerations This section is modeled after the template described in Section 3.7 of . 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 and RESTCONF . These protocols have to use a secure transport layer (e.g., SSH , TLS , and QUIC ) and have to use mutual authentication. The Network Configuration Access Control Model (NACM) 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:

/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.

References Normative References Informative References

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

Contributors This document has the following contributor: Futurewei Technologies

yingzhen.ietf@gmail.com