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 identifier. 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 ).

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

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 configure and 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 show the tree diagram of the module:

The following is the YANG module: 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 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 2025-11-18 { 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."; } } } ]]>

IANA Considerations This document requests allocation for the following registry.

OSPFv2 Extended Prefix TLV Flags Registry This document adds a new bit in the "OSPFv2 Extended Prefix TLV Flags"registry: 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 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 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 NETCONF Access Control Model (NACM) 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.

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

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:

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

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