



Networking Working Group                                  P. Psenak, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                                D. Voyer
Expires: 3 January 2026                                    Cisco Systems
                                                                S. Hegde
                                                  Juniper Networks, Inc.
                                                               G. Mishra
                                                            Verizon Inc.
                                                             2 July 2025


                  IGP Unreachable Prefix Announcement
              draft-ietf-lsr-igp-ureach-prefix-announce-09

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 away from the node which owns the
   prefix and is no longer reachable.

   This document describes how to use the existing protocol mechanisms
   in IS-IS and OSPF, together with the two new flags, to advertise such
   prefix reachability loss.

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.

Status of This Memo

   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/.







Psenak, et al.           Expires 3 January 2026                 [Page 1]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   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 3 January 2026.

Copyright Notice

   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Generation of the UPA . . . . . . . . . . . . . . . . . . . .   4
   3.  Supporting UPA in IS-IS . . . . . . . . . . . . . . . . . . .   5
     3.1.  Advertisement of UPA in IS-IS . . . . . . . . . . . . . .   5
     3.2.  Propagation of UPA in IS-IS . . . . . . . . . . . . . . .   6
   4.  Supporting UPA in OSPF  . . . . . . . . . . . . . . . . . . .   6
     4.1.  Advertisement of UPA in OSPF  . . . . . . . . . . . . . .   7
     4.2.  Propagation of UPA in OSPF  . . . . . . . . . . . . . . .   8
   5.  Signaling UPA . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Signaling UPA in IS-IS  . . . . . . . . . . . . . . . . .   8
     5.2.  Signaling UPA in OSPF . . . . . . . . . . . . . . . . . .   9
       5.2.1.  Signaling UPA in OSPFv2 . . . . . . . . . . . . . . .   9
       5.2.2.  Signaling UPA in OSPFv3 . . . . . . . . . . . . . . .   9
   6.  Area and Domain Partition . . . . . . . . . . . . . . . . . .  10
   7.  Processing of the UPA . . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  IS-IS Prefix Attribute Flags Sub-TLV  . . . . . . . . . .  11
     8.2.  OSPFv2 and OSPFv3 OSPFv2 Prefix Extended Flags  . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15



Psenak, et al.           Expires 3 January 2026                 [Page 2]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


1.  Introduction

   Link-state IGP protocols like IS-IS [ISO10589], OSPF [RFC2328], and
   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.  Techniques such as summarization
   have been used traditionally to address the scale 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., BGP PIC Edge
   [I-D.ietf-rtgwg-bgp-pic].

   Similarly, when an egress 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 OVERLOAD bit together
   with using high metric for all prefixes advertised by the node in IS-
   IS, or by setting high metric on all-links and prefixes advertised by
   the node in OSPF.  When prefixes from such node are summarized by the
   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 prefix that is not
   reachable for routing.  That has been defined for IS-IS in [RFC5305]
   and [RFC5308], for OSPF in [RFC2328], and for OSPFv3 in [RFC5340].

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




Psenak, et al.           Expires 3 January 2026                 [Page 3]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   IGP protocols typically only advertise the reachability of the
   prefix.  Prefix that was previously advertised as reachable is made
   unreachable just by withdrawing the previous advertisement of the
   prefix.  In our use case, we want to signal unreachability for a
   prefix for which the reachability was not explicitly signaled
   previously, because it was covered by the reachability of the summary
   address.

   This document defines two new flags in IS-IS, OSPF, and OSPFv3.
   These flags, together with the existing protocol mechanisms, 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 ISIS
   levels and OSPF areas.

2.  Generation of the UPA

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

   1.  Reachability of a prefix that was reachable earlier was lost.

   2.  For any of the planned maintenance cases:

          - if the node originating the prefix is signalling the
          overload state in ISIS.

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

   Implementations MAY limit the UPA generation to specific prefixes,
   e.g.  host prefixes, SRv6 locators, or similar.  Such filtering is
   optional and MAY 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.  UPA advertisements SHOULD
   therefore be withdrawn after some amount of time, that would provides
   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.

   Implementation MAY provide a configuration option to specify the UPA
   lifetime at the originating ABR or ASBR.



Psenak, et al.           Expires 3 January 2026                 [Page 4]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   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 the configured threshold value.

   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 which need to be updated when UPAs are advertised and
   withdrawn.

   In OSPF and OSPFv3, each inter-area and external prefix is advertised
   in it's own LSA, so the above optimisation does not apply to OSPF.

   It is also RECOMMENDED that implementations limit the number of UPA
   advertisements which can be originated at a given time.

3.  Supporting UPA in IS-IS

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

   "If a prefix is advertised with a metric larger then 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.  Section 2 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 which 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 suport UPA will not use UPAs
   during the route calculation, but will continue to flood them.  This
   allows flooding of such advertisements to occur without the need to
   upgrade all nodes in a network.



Psenak, et al.           Expires 3 January 2026                 [Page 5]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   Recognition of the advertisement as UPA is only required on routers
   which have a valid use case for this information.  Those ABRs or
   ASBRs, which are responsible for propagating UPA advertisements into
   other areas or domains MUST also recognize UPA advertisements.

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

   UPA in IS-IS is supported for all IS-IS Sub-TLVs registered in the
   IS-IS Sub-TLVs Advertising Prefix Reachability registry, which was
   initially defined in [RFC7370], e.g.,:

      - SRv6 Locator [RFC9352]

      - Extended IP reachability [RFC5305]

      - 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.  Propagation of UPA in IS-IS

   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, e.g., 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

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





Psenak, et al.           Expires 3 January 2026                 [Page 6]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   "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."

   [RFC5340] Appendix B 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, NU-bit is defined for OSPFv3 [RFC5340].  Prefixes having
   the NU-bit set in their PrefixOptions field SHOULD NOT be included in
   the routing calculation.

   UPA in OSPFv2 is supported for OSPFv2 Summary-LSA [RFC2328], AS-
   external-LSAs [RFC2328], NSSA AS-external LSA [RFC3101], and OSPFv2
   IP Algorithm Prefix Reachability Sub-TLV [RFC9502].

   UPA in OSPFv3 is supported for Inter-Area-Prefix-LSA [RFC5340], AS-
   External-LSA [RFC5340], NSSA-LSA [RFC5340], E-Inter-Area-Prefix-LSA
   [RFC8362], E-AS-External-LSA [RFC8362], E-Type-7-LSA [RFC8362], and
   SRv6 Locator LSA [RFC9513].

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 OSPF [RFC2328] and OSPFv3 [RFC5340].







Psenak, et al.           Expires 3 January 2026                 [Page 7]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


4.2.  Propagation of UPA in OSPF

   OSPF ABRs or ASBRs, which would be responsible for propagating UPA
   advertisements into other areas MUST recognize such advertisements.

   Advertising prefix reachability between OSPF areas assumes prefix
   reachability in a source area.  Such 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.  Signaling UPA

   In IS-IS a prefix can be advertised with metric higher than
   0xFE000000, in OSPF with metric LSInfinity, or in OSPFv3 with NU-bit
   set in PrefixOptions, for various reasons.  Even though in all cases
   the treatment of such metric, or NU-bit, is specified for IS-IS, OSPF
   and OSPFv3, having an explicit way to signal that the prefix was
   advertised in order to signal unreachability is required to
   distinguish it from other cases where the prefix with such metric is
   advertised.

5.1.  Signaling UPA in IS-IS

   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).

      Originating node MUST NOT set the UP-flag without setting the
      U-fag.

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

   The prefix that is advertised with U-Flag or UP-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.



Psenak, et al.           Expires 3 January 2026                 [Page 8]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


5.2.  Signaling UPA in OSPF

   A new Prefix Attributes Sub-TLV has been defined in
   [I-D.ietf-lsr-ospf-prefix-extended-flags] for advertising additional
   prefix attribute flags in OSPFv2 and OSPFv3.

   Two new bits in Prefix Attributes 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).

      Originating node MUST NOT set the UP-flag without setting the
      U-fag.

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

5.2.1.  Signaling UPA in OSPFv2

   In OSPFv2 the Prefix Attributes Sub-TLV 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.  For default
   algorithm 0 prefixes with U-Flag or UP-Flag, it is therefore REQUIRED
   to advertise the unreachable prefix in the base OSPFv2 LSA - e.g.,
   OSPFv2 Summary-LSA [RFC2328], or AS-external-LSAs [RFC2328], or NSSA
   AS-external LSA [RFC3101].

5.2.2.  Signaling UPA in OSPFv3

   In OSPFv3 the Prefix Attribute 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



Psenak, et al.           Expires 3 January 2026                 [Page 9]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   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.

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 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 dropped or be
   delivered to its final destination.  UPA does not make the problem of
   an area partition any worse.  In case of an area partition each of
   the ABRs or ASBRs will generate UPAs for the destinations for which
   the reachability was lost locally.  As the UPA propagates to the
   nodes outside of a partitioned area, it may result in such nodes
   picking an alternative egress node for the traffic, if such alternate
   egress node exists.  If such alternate egress node resides outside of
   a partitioned area, traffic will be restored.  If such alternate
   egress node resides in a partitioned area and is covered by the
   summary, the trafic will be dropped if it enters a partitioned area
   via the ABR or ASBR that can not reach the alternate egress node -
   resulting in similar behavior as without the UPA.  Above is similarly
   applicable to a domain partition.

7.  Processing of the UPA

   The setting of the U-Flag signals that the prefix is unreachable.  If
   the U flag is set, the setting of the UP flag signals that the
   unreachability is due to a planned event.

   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 [I-D.ietf-rtgwg-bgp-pic].







Psenak, et al.           Expires 3 January 2026                [Page 10]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


8.  IANA Considerations

8.1.  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

      Description: U-Flag

      Reference: This document (Section 5.1).

      Bit #: 6

      Description: UP-Flag

      Reference: This document (Section 5.1).

8.2.  OSPFv2 and OSPFv3 OSPFv2 Prefix Extended Flags

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

      Bit #: 0

      Description: U-Flag

      Reference: This document (Section 5.2).

      Bit #: 1

      Description: UP-Flag

      Reference: This document (Section 5.2).

9.  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.




Psenak, et al.           Expires 3 January 2026                [Page 11]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


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

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

10.  Acknowledgements

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

11.  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


12.  References

12.1.  Normative References










Psenak, et al.           Expires 3 January 2026                [Page 12]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   [I-D.ietf-lsr-ospf-prefix-extended-flags]
              Chen, R., Zhao, D., Psenak, P., Talaulikar, K., and L.
              Gong, "Prefix Flag Extension for OSPFv2 and OSPFv3", Work
              in Progress, Internet-Draft, draft-ietf-lsr-ospf-prefix-
              extended-flags-07, 8 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lsr-
              ospf-prefix-extended-flags-07>.

   [ISO10589] ISO, "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)", November
              2002.

   [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>.



Psenak, et al.           Expires 3 January 2026                [Page 13]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   [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>.

   [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>.

   [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>.





Psenak, et al.           Expires 3 January 2026                [Page 14]

Internet-Draft     IGP Unreachable Prefix Announcement         July 2025


   [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>.

12.2.  Informative References

   [I-D.ietf-rtgwg-bgp-pic]
              Bashandy, A., Filsfils, C., and P. Mohapatra, "BGP Prefix
              Independent Convergence", Work in Progress, Internet-
              Draft, draft-ietf-rtgwg-bgp-pic-22, 20 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
              bgp-pic-22>.

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
   Juniper Networks, Inc.
   Embassy Business Park
   Bangalore, KA
   560093
   India
   Email: shraddha@juniper.net


   Gyan Mishra
   Verizon Inc.
   Email: gyan.s.mishra@verizon.com



Psenak, et al.           Expires 3 January 2026                [Page 15]
