wdiff rfc9929.original rfc9929.txt

Networking Working Group

Internet Engineering Task Force (IETF) P. Psenak, Ed.
Internet-Draft
Request for Comments: 9929 C. Filsfils
Intended status:
Category: Standards Track D. Voyer
Expires: 29 March 2026
ISSN: 2070-1721 Cisco Systems
S. Hegde
Juniper Networks, Inc.
G. Mishra
Verizon Inc.
25 September 2025
February 2026

IGP Unreachable Prefix Announcement
draft-ietf-lsr-igp-ureach-prefix-announce-11

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 aplicable, 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 "OSPF" in this document is used to refer to both OSPFv2 and
OSPFv3.

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 an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum 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 six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 29 March 2026.
https://www.rfc-editor.org/info/rfc9929.

This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language
2. Generation of the UPA . . . . . . . . . . . . . . . . . . . . 4
3. Supporting UPA in IS-IS . . . . . . . . . . . . . . . . . . . 6
3.1. Advertisement of UPA in IS-IS . . . . . . . . . . . . . . 6
3.2. Signaling UPA in IS-IS . . . . . . . . . . . . . . . . . 7
3.3. Propagation of UPA in IS-IS . . . . . . . . . . . . . . . 8
4. Supporting UPA in OSPF . . . . . . . . . . . . . . . . . . . 8
4.1. Advertisement of UPA in OSPF . . . . . . . . . . . . . . 9
4.2. Signaling UPA in OSPF . . . . . . . . . . . . . . . . . . 9
4.2.1. Signaling UPA in OSPFv2 . . . . . . . . . . . . . . . 10
4.2.2. Signaling UPA in OSPFv3 . . . . . . . . . . . . . . . 10
4.3. Propagation of UPA in OSPF . . . . . . . . . . . . . . . 11
5. Processing of the UPA . . . . . . . . . . . . . . . . . . . . 11
6. Area and Domain Partition . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7.1. IS-IS Prefix Attribute Flags Sub-TLV . . . . . . . . . . 12
7.2. OSPFv2 and OSPFv3 OSPFv2 Prefix Extended Flags . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1.
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2.
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Acknowledgements
Contributors
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16

1. Introduction

Link-state Interior Gateway Protocols (IGPs) protocols 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 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 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 traditionally to address the scale 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 Prefix-Independent Convergence (PIC)
[I-D.ietf-rtgwg-bgp-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. In OSPFv2 using the cost of MaxLinkMetric for all
non-stub links in the router-LSA [RFC6987], or H-bit [RFC8770], and
R-bit for OSPFv3 [RFC5340] are mechanisms available for that purpose.

When prefixes from such node 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 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.

IGP protocols

IGPs typically only advertise the reachability of the prefix. 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 to signal 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 "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. 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 signaling the overload
state in IS-IS, or or H-bit in OSPFv2 [RFC8770], or R-bit in
OSPFv3 [RFC5340] .

- [RFC5340].

* 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, SRv6 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 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. 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 use case for which
the UPA was advertised. Not withdrawing the UPA would result in
stale information being kept in the link state database 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 which 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 which 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 information, and its section Section 4 of [RFC5305] specifies:

"If

| 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." table.

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

"...if

| ...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." table.

This functionality can be used to advertise a prefix (IPv4 or IPv6)
in a manner which 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 suport support UPA will not use UPAs
during the route calculation, 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 which that are responsible for propagating UPA
advertisements into other areas or domains, domains are also expected to
recognise
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 a
metric MUST be used when advertising UPA in IS-IS.

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

- for example:

* SRv6 Locator [RFC9352]

* Extended IP reachability [RFC5305]

- MT

* 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 IS-IS, a prefix can be advertised with a metric higher than
0xFE000000, for various reasons. Even though in all cases the
treatment of such metric is specified for IS-IS, having an explicit
way to signal that the prefix was advertised in order to signal UPA
is required to distinguish it from other cases where the prefix with
such a metric is advertised.

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

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

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

Originating

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

Receiving
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 U-Flag 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 levels, need to recognize such advertisements.
Failure to do so would prevent UPA to reach 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 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 of [RFC2328] defines the following architectural constant
for OSPFv2:

"LSInfinity

| LSInfinity
| The metric value indicating that the destination described by
| an LSA is unreachable. Used in summary-LSAs and AS-
external-LSAs 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]
| 0xffffff.

Appendix B of [RFC5340] states:

"Architectural

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

indicating that these same constants are applicable to OSPFv3.

[RFC2328] section 14.1.

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

"Premature

| 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." 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], NSSA AS-
external LSA 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
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 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

In OSPFv2 a prefix can be advertised with metric LSInfinity, or in
OSPFv3 with the NU-bit set in PrefixOptions, for various reasons.
Even though in all cases the treatment of such a metric, or NU-bit,
is specified for OSPFv2 and OSPFv3, having an explicit way to signal
that the prefix was advertised in order to signal UPA is required to
distinguish it from other cases where the prefix with such a metric
is advertised.

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 Attributes Attribute Flags Sub-TLV are defined:

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

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

Originating

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

Receiving
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 sub-TLV of
the OSPFv2 Extended Prefix TLV [RFC7684].

The prefix that is advertised with U-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 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].

4.2.2. Signaling UPA in OSPFv3

OSPFv3 Prefix Extended Flags Sub-TLV is defined as a Sub-TLV 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 areas, need to recognize such
advertisements. Failure to do so would prevent UPA to reach 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 [I-D.ietf-rtgwg-bgp-pic]. [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 end-points. 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
partition, each of an ABRs ABR or ASBRs ASBR 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 a node
exists. If such alternate an alternative egress node resides outside of a
partitioned area, traffic will be restored. If such alternate an alternative
egress node resides in a partitioned area and is covered by the
summary, the trafic traffic will be dropped if it enters a partitioned area
via an ABR or ASBR that can not cannot reach the alternate egress node -
resulting that node. This will result in
similar behavior as without the UPA. Above is similarly 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

Description:
Name: U-Flag
Reference: This document RFC 9929 (Section 3.2). 3.2)

Bit #: 6

Description:
Name: UP-Flag
Reference: This document RFC 9929 (Section 3.2). 3.2)

7.2. OSPFv2 and OSPFv3 OSPFv2 Prefix Extended Flags

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

Bit #:

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

Bit #: 4.2)

Bit: 1
Description: UP-Flag
Reference: This document RFC 9929 (Section 4.2). 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] [RFC7474], and [RFC7684] for OSPFv2.

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

9. 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.

10. 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

11. References

11.1.

9.1. Normative References

[ISO10589] ISO, "Intermediate system ISO/IEC, "Information technology -- Telecommunications and
information exchange between systems -- Intermediate
System to Intermediate system intra-
domain System intra-domain routeing
information exchange protocol for use in conjunction with
the protocol for providing the connectionless-mode Network Service network
service (ISO 8473)", ISO/IEC 10589:2002, November
2002. 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>.

11.2.

9.2. Informative References

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

[BGP-PIC] Bashandy, A., Ed., Filsfils, C., and P. Mohapatra, P., and Y. Qu,
"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>.
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
Juniper Networks, Inc.
Embassy Business Park
Bangalore, KA
560093
India
Email: shraddha@juniper.net

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