



Internet Engineering Task Force (IETF)                    P. Psenak, Ed.
Request for Comments: 9929                                   C. Filsfils
Category: Standards Track                                       D. Voyer
ISSN: 2070-1721                                            Cisco Systems
                                                                S. Hegde
                                                                     HPE
                                                               G. Mishra
                                                            Verizon Inc.
                                                           February 2026


                  IGP Unreachable Prefix Announcement

Abstract

   Summarization is often used in multi-area or multi-domain networks to
   improve network efficiency and scalability.  With summarization in
   place, there is a need to signal loss of reachability to an
   individual prefix covered by the summary.  This enables fast
   convergence by steering traffic, when applicable, away from the node
   which owns the prefix and is no longer reachable.

   This document specifies protocol mechanisms in IS-IS and OSPF,
   together with two new flags, to advertise such prefix reachability
   loss.

   The term "OSPF" in this document is used to refer to both OSPFv2 and
   OSPFv3.

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/rfc9929.

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.  Generation of the UPA
   3.  Supporting UPA in IS-IS
     3.1.  Advertisement of UPA in IS-IS
     3.2.  Signaling UPA in IS-IS
     3.3.  Propagation of UPA in IS-IS
   4.  Supporting UPA in OSPF
     4.1.  Advertisement of UPA in OSPF
     4.2.  Signaling UPA in OSPF
       4.2.1.  Signaling UPA in OSPFv2
       4.2.2.  Signaling UPA in OSPFv3
     4.3.  Propagation of UPA in OSPF
   5.  Processing of the UPA
   6.  Area and Domain Partition
   7.  IANA Considerations
     7.1.  IS-IS Prefix Attribute Flags Sub-TLV
     7.2.  OSPFv2 and OSPFv3 Prefix Extended Flags
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Link-state Interior Gateway Protocols (IGPs) like Intermediate System
   to Intermediate System (IS-IS) [ISO10589], Open Shortest Path First
   version 2 (OSPFv2)) [RFC2328], and Open Shortest Path First version 3
   (OSPFv3) [RFC5340] are primarily used to distribute routing
   information between routers belonging to a single Autonomous System
   (AS) and to calculate the reachability for IPv4 or IPv6 prefixes
   advertised by the individual nodes inside the AS.  Each node
   advertises the state of its local adjacencies, connected prefixes,
   capabilities, etc.  The collection of these states from all the
   routers inside the area form a Link State Database (LSDB) that
   describes the topology of the area and holds additional state
   information about the prefixes, router capabilities, etc.

   The growth of networks running a link-state routing protocol results
   in the addition of more state, which leads to scalability and
   convergence challenges.  The organization of networks into levels/
   areas and IGP domains helps limit the scope of link-state information
   within certain boundaries.  However, the state related to prefix
   reachability often requires propagation across a multi-area/level
   and/or multi-domain IGP network.  IGP summarization is a network
   engineering technique for combining multiple smaller, contiguous IP
   networks into a single, larger summary route.  Techniques such as
   summarization have been used to address the scaling challenges
   associated with advertising prefix state outside of the local area/
   domain.  However, this results in suppression of the individual
   prefix state that is useful for triggering fast-convergence
   mechanisms outside of the IGPs -- e.g., Border Gateway Protocol (BGP)
   Prefix-Independent Convergence (PIC) [BGP-PIC].

   Similarly, when a router needs to be taken out of service for
   maintenance, the traffic is drained from the node before taking it
   down.  This is typically achieved by setting the OVERLOAD bit
   together with using a high metric for all prefixes advertised by the
   node in IS-IS.  The mechanisms available for that purpose are (in
   OSPFv2) using the cost of MaxLinkMetric for all non-stub links in the
   router-LSA [RFC6987] or using the H-bit [RFC8770], or (in OSPFv3
   [RFC5340]) using the R-bit.

   When prefixes from such nodes are summarized by an Area Border Router
   (ABR) or Autonomous System Boundary Router (ASBR), nodes outside of
   the area or domain are unaware of these summarized prefixes becoming
   unreachable.  This document proposes protocol extensions to carry
   information about such prefixes in a backward-compatible manner.

   This document does not define how to advertise a prefix that is not
   reachable for routing.  That has been defined for IS-IS in [RFC5305]
   and [RFC5308], for OSPFv2 in [RFC2328], and for OSPFv3 in [RFC5340].

   This document defines a method to signal a specific reason for which
   the prefix was advertised with the metric that excludes it from the
   route calculation.  This is done to distinguish it from any other
   possible cases, where such metric advertisement may be used.

   IGPs typically only advertise the reachability of the prefix.  A
   prefix that was previously advertised as reachable is made
   unreachable just by withdrawing the previous advertisement of the
   prefix.  Some of the use cases mentioned earlier in this section
   require that unreachability be signaled for a prefix for which the
   reachability was not explicitly signaled previously, because it was
   covered by the reachability of the summary prefix.

   This document defines two new flags in IS-IS, OSPFv2, and OSPFv3.
   These flags provide the support for advertising prefix
   unreachability, together with the reason for which the unreachability
   is advertised.  The functionality being described is called
   Unreachable Prefix Announcement (UPA).

   This document also defines how the UPA is propagated across IS-IS
   levels and OSPF areas.

   The term "OSPF" in this document is used to cover both OSPFv2 and
   OSPFv3 protocols.

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.  Generation of the UPA

   UPA MAY be generated by an ABR or ASBR for a prefix that is
   summarized by the summary prefix originated by an ABR or ASBR in the
   following cases:

   1.  A prefix that was reachable earlier becomes unreachable.

   2.  For any of the planned maintenance cases:

       *  the node originating the prefix is signaling the overload
          state in IS-IS, or the H-bit in OSPFv2 [RFC8770], or the R-bit
          in OSPFv3 [RFC5340], or

       *  the metric to reach the prefix from an ABR or ASBR crosses the
          configured threshold.

   Generation as well as propagation of the UPA at an ABR or ASBR is
   optional and SHOULD be controlled by a configuration knob.  It SHOULD
   be disabled by default.

   Implementations MAY limit the UPA generation as well as propagation
   to specific prefixes, e.g. host prefixes, Segment Routing over IPv6
   (SRv6) locators, or similar.  Such filtering is optional and SHOULD
   be controlled via configuration.

   The intent of UPA is to provide an event-driven signal of the
   transition of a destination from reachable to unreachable.  It is not
   intended to advertise a persistent state.

   ABR or ASBR MUST withdraw the previously advertised UPA when the
   reason for which the UPA was generated ceases, e.g., prefix
   reachability was restored or its metric has changed such that it is
   below a configured threshold value.

   Even if the reasons persist, UPA advertisements SHOULD be withdrawn
   after some amount of time, that would provide sufficient time for UPA
   to be flooded network-wide and acted upon by receiving nodes, but
   limits the presence of UPA in the network.  The time the UPA is kept
   in the network SHOULD also reflect the intended use case for which
   the UPA was advertised.  Not withdrawing the UPA would result in
   stale information being kept in the link state databases of all
   routers in the area.

   Implementations SHOULD provide a configuration option to specify the
   UPA lifetime at the originating ABR or ASBR.

   As UPA advertisements in IS-IS are advertised in existing Link State
   PDUs (LSPs) and the unit of flooding in IS-IS is an LSP, it is
   RECOMMENDED that, when possible, UPAs are advertised in LSPs
   dedicated to this type of advertisement.  This will minimize the
   number of LSPs that need to be updated when UPAs are advertised and
   withdrawn.

   In OSPFv2 and OSPFv3, each inter-area and external prefix is
   advertised in its own LSA, so the above consideration does not apply
   to OSPFv2 and OSPFv3.

   It is also RECOMMENDED that implementations limit the number of UPA
   advertisements that can be originated at a given time to limit the
   number of UPAs present in the network at any given point of time.
   UPA implementations SHOULD provide a configuration option to limit
   the number of such UPAs.

3.  Supporting UPA in IS-IS

   [RFC5305] defines the encoding for advertising IPv4 prefixes using 4
   octets of metric information, and Section 4 of [RFC5305] specifies:

   |  If a prefix is advertised with a metric larger than
   |  MAX_PATH_METRIC (0xFE000000, see paragraph 3.0), this prefix MUST
   |  NOT be considered during the normal SPF computation.  This allows
   |  advertisement of a prefix for purposes other than building the
   |  normal IP routing table.

   Similarly, [RFC5308] defines the encoding for advertising IPv6
   prefixes using 4 octets of metric information and Section 2 of
   [RFC5308] states:

   |  ...if a prefix is advertised with a metric larger than
   |  MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST NOT be
   |  considered during the normal Shortest Path First (SPF)
   |  computation.  This will allow advertisement of a prefix for
   |  purposes other than building the normal IPv6 routing table.

   This functionality can be used to advertise a prefix (IPv4 or IPv6)
   in a manner that indicates that reachability has been lost -- and to
   do so without requiring all nodes in the network to be upgraded to
   support the functionality.

3.1.  Advertisement of UPA in IS-IS

   Existing nodes in a network that do not support UPA will not use UPAs
   during the route calculation but will continue to flood them within
   the area.  This allows flooding of such advertisements to occur
   without the need to upgrade all nodes in a network to support this
   specification.

   Those ABRs or ASBRs that are responsible for propagating UPA
   advertisements into other areas or domains are also expected to
   recognize UPA advertisements.

   As per the definitions referenced in Section 3, any prefix
   advertisement with a metric value greater than 0xFE000000 can be used
   for purposes other than normal routing calculations.  Such a metric
   MUST be used when advertising UPA in IS-IS.

   [RFC7370] introduced the "IS-IS Sub-TLVs for TLVs Advertising Prefix
   Reachability" registry, which lists TLVs for advertising different
   types of prefix reachability.  (The list at the time of publication
   of this document is below.)  UPA in IS-IS is supported for prefixes
   advertised in all such TLVs identified by that registry, for example:

   *  SRv6 Locator [RFC9352]

   *  Extended IP reachability [RFC5305]

   *  Multi-Topology (MT) IP Reach [RFC5120]

   *  IPv6 IP Reach [RFC5308]

   *  MT IPv6 IP Reach [RFC5120]

   *  IPv4 Algorithm Prefix Reachability TLV [RFC9502]

   *  IPv6 Algorithm Prefix Reachability TLV [RFC9502]

3.2.  Signaling UPA in IS-IS

   In IS-IS, a prefix can be advertised with a metric higher than
   0xFE000000, for various reasons.  While IS-IS specifies the treatment
   of such metrics, explicit signaling is required to distinguish UPA
   from other high-metric advertisements.

   Two new bits in the IPv4/IPv6 Extended Reachability Attribute Flags
   [RFC7794] are defined:

   U-Flag:  Unreachable Prefix Flag (bit 5).  When set, it indicates
      that the prefix is unreachable.

   UP-Flag:  Unreachable Planned Prefix Flag (bit 6).  When set, this
      flag indicates that the prefix is unreachable due to a planned
      event (e.g., planned maintenance).

   The originating node MUST NOT set the UP-flag without setting the
   U-flag.

   The receiving node MUST ignore the UP-flag in the advertisement if
   the U-flag is not set.

   The prefix that is advertised with the U-flag MUST have the metric
   set to a value larger than 0xFE000000.  If the prefix metric is less
   than or equal 0xFE000000, both of these flags MUST be ignored.

3.3.  Propagation of UPA in IS-IS

   IS-IS L1/L2 routers, which would be responsible for propagating UPA
   advertisements between levels, need to recognize such advertisements.
   Failure to do so would prevent UPA from reaching the routers in the
   remote areas.

   IS-IS allows propagation of IP prefixes in both directions between
   level 1 and level 2.  Propagation is only done if the prefix is
   reachable in the source level, i.e., the prefix is only propagated
   from a level in which the prefix is reachable.  Such requirement of
   reachability MUST NOT be applied for UPAs, as they are propagating
   unreachability.

   IS-IS L1/L2 routers may wish to advertise received UPAs into other
   areas (upwards and/or downwards).  When propagating UPAs, the
   original metric value MUST be preserved.  The cost to reach the
   originator of the received UPA MUST NOT be considered when
   readvertising the UPA.

4.  Supporting UPA in OSPF

   Appendix B of [RFC2328] defines the following architectural constant
   for OSPFv2:

   |  LSInfinity
   |     The metric value indicating that the destination described by
   |     an LSA is unreachable.  Used in summary-LSAs and AS-external-
   |     LSAs as an alternative to premature aging (see Section 14.1).
   |     It is defined to be the 24-bit binary value of all ones:
   |     0xffffff.

   Appendix B of [RFC5340] states:

   |  Architectural constants for the OSPF protocol are defined in
   |  Appendix B of [OSPFV2].

   indicating that these same constants are applicable to OSPFv3.

   [RFC2328], Section 14.1 also describes the usage of LSInfinity as a
   way to indicate loss of prefix reachability:

   |  Premature aging can also be used when, for example, one of the
   |  router's previously advertised external routes is no longer
   |  reachable.  In this circumstance, the router can flush its AS-
   |  external-LSA from the routing domain via premature aging.  This
   |  procedure is preferable to the alternative, which is to originate
   |  a new LSA for the destination specifying a metric of LSInfinity.

   In addition, the NU-bit is defined for OSPFv3 [RFC5340].  Prefixes
   having the NU-bit set in their PrefixOptions field are not included
   in the routing calculation.

   UPA in OSPFv2 is supported for prefix reachability advertised via
   OSPFv2 Summary-LSA [RFC2328], AS-external-LSAs [RFC2328], Not-So-
   Stubby Area (NSSA) AS-external-LSA [RFC3101], and OSPFv2 IP Algorithm
   Prefix Reachability Sub-TLV [RFC9502].

   UPA in OSPFv3 is supported for prefix reachability advertised via
   OSPFv3 E-Inter-Area-Prefix-LSA [RFC8362], E-AS-External-LSA
   [RFC8362], E-Type-7-LSA [RFC8362], and SRv6 Locator LSA [RFC9513].

   For prefix reachability advertised via Inter-Area-Prefix-LSA
   [RFC5340], AS-External-LSA [RFC5340], NSSA-LSA [RFC5340], UPA is
   signaled using their corresponding extended LSAs.  This requires
   support of the OSPFv3 Extended LSAs in a sparse mode as specified in
   Section 6.2 of [RFC8362].

4.1.  Advertisement of UPA in OSPF

   If an ABR or ASBR advertises UPA in an advertisement of an inter-area
   or external prefix inside OSPFv2 or OSPFv3, then it MUST set the age
   to a value lower than MaxAge and set the metric to LSInfinity.

   UPA flooding inside the area follows the existing standard procedures
   defined by OSPFv2 [RFC2328] and OSPFv3 [RFC5340].

4.2.  Signaling UPA in OSPF

   A prefix can be advertised (in OSPFv2) with the LSInfinity metric or
   (in OSPFv3) with the NU-bit set in PrefixOptions, for various
   reasons.  While OSPFv2 and OSPFv3 specify the treatment of the
   LSInfinity metric and the NU-bit, explicit signaling is required to
   distinguish UPA from other scenarios using the LSInfinity metric or
   NU-bit.

   OSPFv2 and OSPFv3 Prefix Extended Flags Sub-TLVs been defined in
   [RFC9792] for advertising additional prefix attribute flags in OSPFv2
   and OSPFv3.

   Two new bits in the Prefix Attribute Flags Sub-TLV are defined:

   U-Flag:  Unreachable Prefix Flag (bit 0).  When set, it indicates
      that the prefix is unreachable.

   UP-Flag:  Unreachable Planned Prefix Flag (bit 1).  When set, this
      flag indicates that the prefix is unreachable due to a planned
      event (e.g., planned maintenance).

   The originating node MUST NOT set the UP-flag without setting the
   U-flag.

   The receiving node MUST ignore the UP-flag in the advertisement if
   the U-flag is not set.

4.2.1.  Signaling UPA in OSPFv2

   The OSPFv2 Prefix Extended Flags Sub-TLV [RFC9792] is a sub-TLV of
   the OSPFv2 Extended Prefix TLV [RFC7684].

   The prefix that is advertised with U-Flag or UP-Flag MUST have the
   metric set to a value LSInfinity.  If the prefix metric is not equal
   to LSInfinity, both of these flags MUST be ignored.  Therefore, for
   the prefixes in default algorithm 0 that are advertised with U-Flag,
   or Up-Flag, it is REQUIRED to advertise the unreachable prefix in the
   base OSPFv2 LSA, e.g., e.g., OSPFv2 Summary-LSA [RFC2328], or AS-
   external-LSAs [RFC2328], or NSSA AS-external LSA [RFC3101].

4.2.2.  Signaling UPA in OSPFv3

   OSPFv3 Prefix Extended Flags Sub-TLV is defined as a sub-TLV of the
   following OSPFv3 TLVs that are defined in [RFC8362]:

   *  Intra-Area-Prefix TLV

   *  Inter-Area-Prefix TLV

   *  External-Prefix TLV

   The prefix that is advertised with U-Flag or UP-flag MUST have the
   metric set to a value LSInfinity.  For default algorithm 0 prefixes,
   the LSInfinity MUST be set in the parent TLV.  For IP Algorithm
   Prefixes [RFC9502], the LSInfinity MUST be set in OSPFv3 IP Algorithm
   Prefix Reachability sub-TLV.  If the prefix metric is not equal to
   LSInfinity, both of these flags MUST be ignored.

   The prefix that is advertised with U-Flag or UP-Flag MUST have the
   NU-bit set in the PrefixOptions of the parent TLV.  If the NU-bit in
   PrefixOptions of the parent TLV is not set, both of these flags MUST
   be ignored.

4.3.  Propagation of UPA in OSPF

   OSPF ABRs, which would be responsible for propagating UPA
   advertisements into other areas, need to recognize such
   advertisements.  Failure to do so would prevent UPA from reaching the
   routers in the remote areas.

   Advertising prefix reachability between OSPF areas assumes prefix
   reachability in a source area.  Such a requirement of reachability
   MUST NOT be applied for UPAs, as they are propagating unreachability.

   OSPF ABRs or ASBRs MAY advertise received UPAs between connected
   areas or domains.  When doing so, the original LSInfinity metric
   value in UPA MUST be preserved.  The cost to reach the originator of
   the received UPA MUST NOT be considered when readvertising the UPA to
   connected areas.

5.  Processing of the UPA

   Processing of the received UPAs is optional and SHOULD be controlled
   by the configuration at the receiver.  The receiver itself, based on
   its configuration, decides what the UPA will be used for and what
   applications, if any, will be notified when UPA is received.  Usage
   of the UPA at the receiver is outside of the scope of this document.

   As an example, UPA may be used to trigger BGP PIC Edge at the
   receiving router [BGP-PIC].

   Applications using the UPA cannot use the absence of the UPA to infer
   that the reachability of the prefix is back.  They must rely on their
   own mechanisms to verify the reachability of the remote endpoints.

6.  Area and Domain Partition

   UPA is not meant to address an area/domain partition.  When an area
   or domain partitions, while multiple ABRs or ASBRs advertise the same
   summary, each of them can only reach a portion of the summarized
   prefix.  As a result, depending on which ABR or ASBR the traffic is
   using to enter a partitioned area, the traffic could be either
   dropped or delivered to its final destination.  UPA does not make the
   problem of an area partition any worse.  In case of an area
   partition, each ABR or ASBR will generate UPAs for the destinations
   for which the reachability was lost locally.  As the UPA propagates
   to the nodes outside a partitioned area, it may result in such nodes
   picking an alternative egress node for the traffic, if such a node
   exists.  If such an alternative egress node resides outside a
   partitioned area, traffic will be restored.  If such an alternative
   egress node resides in a partitioned area and is covered by the
   summary, the traffic will be dropped if it enters a partitioned area
   via an ABR or ASBR that cannot reach that node.  This will result in
   similar behavior as without the UPA.  The above statements are also
   applicable to a domain partition.

7.  IANA Considerations

7.1.  IS-IS Prefix Attribute Flags Sub-TLV

   This document adds two new bits in the "IS-IS Bit Values for Prefix
   Attribute Flags Sub-TLV" registry:

   Bit #:  5
   Name:  U-Flag
   Reference:  RFC 9929 (Section 3.2)

   Bit #:  6
   Name:  UP-Flag
   Reference:  RFC 9929 (Section 3.2)

7.2.  OSPFv2 and OSPFv3 Prefix Extended Flags

   This document adds two new bits in the "OSPFv2 Prefix Extended Flags"
   and "OSPFv3 Prefix Extended Flags" registries:

   Bit:  0
   Description:  U-Flag
   Reference:  RFC 9929 (Section 4.2)

   Bit:  1
   Description:  UP-Flag
   Reference:  RFC 9929 (Section 4.2)

8.  Security Considerations

   The use of UPAs introduces the possibility that an attacker could
   inject a false, but apparently valid, UPA.  However, the risk of this
   occurring is no greater than the risk today of an attacker injecting
   any other type of false advertisement.

   The risks can be reduced by the use of existing security extensions
   as described in:

   *  [RFC5304], [RFC5310], and [RFC7794] for IS-IS.

   *  [RFC2328], [RFC7474], and [RFC7684] for OSPFv2.

   *  [RFC5340], [RFC4552], and [RFC8362] for OSPFv3.

9.  References

9.1.  Normative References

   [ISO10589] ISO/IEC, "Information technology -- Telecommunications and
              information exchange between systems -- Intermediate
              System to Intermediate System intra-domain routeing
              information exchange protocol for use in conjunction with
              the protocol for providing the connectionless-mode network
              service (ISO 8473)", ISO/IEC 10589:2002, November 2002,
              <https://www.iso.org/en/contents/data/
              standard/03/09/30932.html>.

   [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>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC3101]  Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
              RFC 3101, DOI 10.17487/RFC3101, January 2003,
              <https://www.rfc-editor.org/info/rfc3101>.

   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <https://www.rfc-editor.org/info/rfc5308>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,
              <https://www.rfc-editor.org/info/rfc6987>.

   [RFC7370]  Ginsberg, L., "Updates to the IS-IS TLV Codepoints
              Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,
              <https://www.rfc-editor.org/info/rfc7370>.

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
              <https://www.rfc-editor.org/info/rfc7474>.

   [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>.

   [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
              U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
              and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
              March 2016, <https://www.rfc-editor.org/info/rfc7794>.

   [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>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.

   [RFC8770]  Patel, K., Pillay-Esnault, P., Bhardwaj, M., and S.
              Bayraktar, "Host Router Support for OSPFv2", RFC 8770,
              DOI 10.17487/RFC8770, April 2020,
              <https://www.rfc-editor.org/info/rfc8770>.

   [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>.

   [RFC9502]  Britto, W., Hegde, S., Kaneriya, P., Shetty, R., Bonica,
              R., and P. Psenak, "IGP Flexible Algorithm in IP
              Networks", RFC 9502, DOI 10.17487/RFC9502, November 2023,
              <https://www.rfc-editor.org/info/rfc9502>.

   [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>.

   [RFC9792]  Chen, R., Zhao, D., Psenak, P., Talaulikar, K., and L.
              Gong, "Prefix Flag Extension for OSPFv2 and OSPFv3",
              RFC 9792, DOI 10.17487/RFC9792, June 2025,
              <https://www.rfc-editor.org/info/rfc9792>.

9.2.  Informative References

   [BGP-PIC]  Bashandy, A., Ed., Filsfils, C., Mohapatra, P., and Y. Qu,
              "BGP Prefix Independent Convergence", Work in Progress,
              Internet-Draft, draft-ietf-rtgwg-bgp-pic-23, 15 February
              2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
              rtgwg-bgp-pic-23>.

Acknowledgements

   The authors would like to thank Kamran Raza, Michael MacKenzie, and
   Luay Jalil for their contributions and support of the overall
   solution proposed in this document.

Contributors

   The following people contributed to the content of this document and
   should be considered coauthors:

   Stephane Litkowski
   Email: slitkows@cisco.com


   Amit Dhamija
   Email: amitd@arrcus.com


   Gunter Van de Velde
   Email: gunter.van_de_velde@nokia.com


   The following people contributed to the problem statement and the
   solution requirement discussion:

   Aijun Wang
   Email: wangaj3@chinatelecom.cn


   Zhibo Hu
   Email: huzhibo@huawei.com


Authors' Addresses

   Peter Psenak (editor)
   Cisco Systems
   Pribinova Street 10
   Bratislava 81109
   Slovakia
   Email: ppsenak@cisco.com


   Clarence Filsfils
   Cisco Systems
   Brussels
   Belgium
   Email: cfilsfil@cisco.com


   Daniel Voyer
   Cisco Systems
   Email: davoyer@cisco.com


   Shraddha Hegde
   HPE
   Email: shraddha.hegde@hpe.com


   Gyan Mishra
   Verizon Inc.
   Email: hayabusagsm@gmail.com
