rfc9929xml2.original.xml   rfc9929.xml 
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<?rfc inline="yes"?>
<?rfc compact="yes"?> <rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std" docName="draft-ie
<?rfc subcompact="no"?> tf-lsr-igp-ureach-prefix-announce-11" number="9929" ipr="trust200902" obsoletes=
<rfc category="std" docName="draft-ietf-lsr-igp-ureach-prefix-announce-11" "" updates="" consensus="true" xml:lang="en" submissionType="IETF" tocInclude="t
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<front> <front>
<title abbrev="IGP Unreachable Prefix Announcement">IGP Unreachable Prefix <title abbrev="IGP Unreachable Prefix Announcement">IGP Unreachable Prefix
Announcement</title> Announcement</title>
<seriesInfo name="RFC" value="9929"/>
<author fullname="Peter Psenak" initials="P" role="editor" <author fullname="Peter Psenak" initials="P" role="editor" surname="Psenak">
surname="Psenak">
<organization>Cisco Systems</organization> <organization>Cisco Systems</organization>
<address> <address>
<postal> <postal>
<street>Pribinova Street 10</street> <street>Pribinova Street 10</street>
<city>Bratislava 81109</city> <city>Bratislava 81109</city>
<region/>
<code/>
<country>Slovakia</country> <country>Slovakia</country>
</postal> </postal>
<phone/>
<facsimile/>
<email>ppsenak@cisco.com</email> <email>ppsenak@cisco.com</email>
<uri/>
</address> </address>
</author> </author>
<author fullname="Clarence Filsfils" initials="C" surname="Filsfils"> <author fullname="Clarence Filsfils" initials="C" surname="Filsfils">
<organization>Cisco Systems</organization> <organization>Cisco Systems</organization>
<address> <address>
<postal> <postal>
<street/>
<city>Brussels</city> <city>Brussels</city>
<code/>
<region/>
<country>Belgium</country> <country>Belgium</country>
</postal> </postal>
<email>cfilsfil@cisco.com</email> <email>cfilsfil@cisco.com</email>
</address> </address>
</author> </author>
<author fullname="Daniel Voyer" initials="D" surname="Voyer"> <author fullname="Daniel Voyer" initials="D" surname="Voyer">
<organization>Cisco Systems</organization> <organization>Cisco Systems</organization>
<address> <address>
<postal>
<street/>
</postal>
<email>davoyer@cisco.com</email> <email>davoyer@cisco.com</email>
</address> </address>
</author> </author>
<author fullname="Shraddha Hegde" initials="S" surname="Hegde"> <author fullname="Shraddha Hegde" initials="S" surname="Hegde">
<organization>Juniper Networks, Inc.</organization> <organization>Juniper Networks, Inc.</organization>
<address> <address>
<postal> <postal>
<street>Embassy Business Park</street> <street>Embassy Business Park</street>
<street/> <street/>
<city>Bangalore, KA</city> <city>Bangalore, KA</city>
<region>560093</region> <region>560093</region>
<country>India</country> <country>India</country>
<code/>
</postal> </postal>
<email>shraddha@juniper.net</email> <email>shraddha@juniper.net</email>
</address> </address>
</author> </author>
<author fullname="Gyan Mishra" initials="G. " surname="Mishra"> <author fullname="Gyan Mishra" initials="G. " surname="Mishra">
<organization>Verizon Inc.</organization> <organization>Verizon Inc.</organization>
<address> <address>
<email>gyan.s.mishra@verizon.com</email> <email>gyan.s.mishra@verizon.com</email>
</address> </address>
</author> </author>
<date year="2026" month="February"/>
<area>RTG</area>
<workgroup>lsr</workgroup>
<date year="2025"/> <!-- [rfced] Please insert any keywords (beyond those that appear in
the title) for use on https://www.rfc-editor.org/search. -->
<area>Routing Area</area>
<workgroup>Networking Working Group</workgroup>
<keyword/> <keyword>example</keyword>
<abstract> <abstract>
<t>Summarization is often used in multi-area or multi-domain networks to i mprove <t>Summarization is often used in multi-area or multi-domain networks to i mprove
network efficiency and scalability. With summarization in place, there is a need network efficiency and scalability. With summarization in place, there is a need
to signal loss of reachability to an individual prefix covered by the summ ary. to signal loss of reachability to an individual prefix covered by the summ ary.
This enables fast convergence by steering traffic, when aplicable, away fr om the This enables fast convergence by steering traffic, when applicable, away f rom the
node which owns the prefix and is no longer reachable.</t> node which owns the prefix and is no longer reachable.</t>
<t>This document specifies protocol mechanisms in IS-IS and OSPF, together with <t>This document specifies protocol mechanisms in IS-IS and OSPF, together with
two new flags, to advertise such prefix reachability loss.</t> two new flags, to advertise such prefix reachability loss.</t>
<t>The term "OSPF" in this document is used to refer to both OSPFv2 and OS
<t>The term OSPF in this document is used to refer to both OSPFv2 and OSPF PFv3.</t>
v3.</t>
</abstract> </abstract>
<note title="Requirements Language">
<t>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
<xref target="RFC2119"/><xref target="RFC8174"/> when, and only when,
they appear in all capitals, as shown here.</t>
</note>
</front> </front>
<middle> <middle>
<section title="Introduction"> <section>
<t>Link-state Interior Gateway Protocols (IGPs) protocols like Intermediat <name>Introduction</name>
e <t>Link-state Interior Gateway Protocols (IGPs) like Intermediate
System to Intermediate System (IS-IS) <xref target="ISO10589"/>, Open Shor test Path System to Intermediate System (IS-IS) <xref target="ISO10589"/>, Open Shor test Path
First version 2 (OSPFv2)) <xref target="RFC2328"/>, and Open Shortest Path First version 2 (OSPFv2)) <xref target="RFC2328"/>, and Open Shortest Path
First version 3 (OSPFv3) <xref target="RFC5340"/> are primarily used to First version 3 (OSPFv3) <xref target="RFC5340"/> are primarily used to
distribute routing information between routers belonging to a single distribute routing information between routers belonging to a single
Autonomous System (AS) and to calculate the reachability for IPv4 or Autonomous System (AS) and to calculate the reachability for IPv4 or
IPv6 prefixes advertised by the individual nodes inside the AS. Each IPv6 prefixes advertised by the individual nodes inside the AS. Each
node advertises the state of its local adjacencies, connected prefixes, node advertises the state of its local adjacencies, connected prefixes,
capabilities, etc. The collection of these states from all the routers capabilities, etc. The collection of these states from all the routers
inside the area form a link-state database (LSDB) that describes the inside the area form a Link State Database (LSDB) that describes the
topology of the area and holds additional state information about the topology of the area and holds additional state information about the
prefixes, router capabilities, etc.</t> prefixes, router capabilities, etc.</t>
<t>The growth of networks running a link-state routing protocol results <t>The growth of networks running a link-state routing protocol results
in the addition of more state which leads to scalability and convergence in the addition of more state, which leads to scalability and convergence
challenges. The organization of networks into levels/areas and IGP challenges. The organization of networks into levels/areas and IGP
domains helps limit the scope of link-state information within certain domains helps limit the scope of link-state information within certain
boundaries. However, the state related to prefix reachability often boundaries. However, the state related to prefix reachability often
requires propagation across a multi-area/level and/or multi-domain IGP requires propagation across a multi-area/level and/or multi-domain IGP
network. IGP summarization is a network engineering technique for combinin g network. IGP summarization is a network engineering technique for combinin g
multiple smaller, contiguous IP networks into a single, larger summary rou te. multiple smaller, contiguous IP networks into a single, larger summary rou te.
Techniques such as summarization have been used traditionally to address t he scale Techniques such as summarization have been used traditionally to address t he scaling
challenges associated with advertising prefix state outside of the local a rea/domain. challenges associated with advertising prefix state outside of the local a rea/domain.
However, this results in suppression of the individual prefix state that i s useful However, this results in suppression of the individual prefix state that i s useful
for triggering fast-convergence mechanisms outside of the IGPs - e.g., for triggering fast-convergence mechanisms outside of the IGPs -- e.g.,
Border Gateway Protocol (BGP) Prefix Independent Convergence (PIC) Border Gateway Protocol (BGP) Prefix-Independent Convergence (PIC)
<xref target="I-D.ietf-rtgwg-bgp-pic"/>.</t> <xref target="I-D.ietf-rtgwg-bgp-pic"/>.</t>
<!--[rfced] Please clarify this sentence. Specifically, please make the
statements about OSPFv2 and OSPFv3 parallel. Is is accurate that the first
two mechanisms are for OSPFv2, and the last one is for OSPFv3?
Original:
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.
Perhaps:
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.
-->
<t>Similarly, when a router needs to be taken out of service for maintenan ce, <t>Similarly, when a router needs to be taken out of service for maintenan ce,
the traffic is drained from the node before taking it down. This is typica lly the traffic is drained from the node before taking it down. This is typica lly
achieved by setting the OVERLOAD bit together with using a high metric for all prefixes 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 advertised by the node in IS-IS. In OSPFv2 using the cost of MaxLinkMetric for all
non-stub links in the router-LSA <xref target="RFC6987"/>, or H-bit non-stub links in the router-LSA <xref target="RFC6987"/>, or H-bit
<xref target="RFC8770"/>, and R-bit for OSPFv3 <xref target="RFC5340"/> ar e mechanisms <xref target="RFC8770"/>, and R-bit for OSPFv3 <xref target="RFC5340"/> ar e mechanisms
available for that purpose.</t> available for that purpose.</t>
<t>When prefixes from such nodes are summarized
<t>When prefixes from such node are summarized
by an Area Border Router (ABR) or Autonomous System Boundary Router (ASBR) , by an Area Border Router (ABR) or Autonomous System Boundary Router (ASBR) ,
nodes outside of the area or domain are unaware of these summarized nodes outside of the area or domain are unaware of these summarized
prefixes becoming unreachable. This document proposes protocol extensions to carry prefixes becoming unreachable. This document proposes protocol extensions to carry
information about such prefixes in a backward compatible manner.</t> information about such prefixes in a backward-compatible manner.</t>
<t>This document does not define how to advertise a prefix that is not rea chable for <t>This document does not define how to advertise a prefix that is not rea chable for
routing. That has been defined for IS-IS in <xref target="RFC5305"/> and routing. That has been defined for IS-IS in <xref target="RFC5305"/> and
<xref target="RFC5308"/>, for OSPFv2 in <xref target="RFC2328"/>, and for OSPFv3 <xref target="RFC5308"/>, for OSPFv2 in <xref target="RFC2328"/>, and for OSPFv3
in <xref target="RFC5340"/>.</t> in <xref target="RFC5340"/>.</t>
<t>This document defines a method to signal a specific reason for which th e <t>This document defines a method to signal a specific reason for which th e
prefix was advertised with the metric that excludes it from the route calc ulation. prefix was advertised with the metric that excludes it from the route calc ulation.
This is done to distinguish it from any other possible cases, where such This is done to distinguish it from any other possible cases, where such
metric advertisement may be used.</t> metric advertisement may be used.</t>
<t>IGPs typically only advertise the reachability of the prefix. A prefix
<t>IGP protocols typically only advertise the reachability of the prefix. that was
Prefix that was
previously advertised as reachable is made unreachable just by withdrawing the previously advertised as reachable is made unreachable just by withdrawing the
previous advertisement of the prefix. Some of the use cases mentioned earl ier in this previous advertisement of the prefix. Some of the use cases mentioned earl ier in this
section require to signal unreachability for a prefix for which the reacha bility section require that unreachability be signaled for a prefix for which the reachability
was not explicitly signaled previously, because it was covered by the was not explicitly signaled previously, because it was covered by the
reachability of the summary prefix.</t> reachability of the summary prefix.</t>
<t>This document defines two new flags in IS-IS, OSPFv2, and OSPFv3. <t>This document defines two new flags in IS-IS, OSPFv2, and OSPFv3.
These flags provide the support for advertising prefix unreachability, These flags provide the support for advertising prefix unreachability,
together with the reason for which the unreachability is advertised. The together with the reason for which the unreachability is advertised. The
functionality being described is called Unreachable Prefix Announcement (U PA).</t> functionality being described is called Unreachable Prefix Announcement (U PA).</t>
<t>This document also defines how the UPA is propagated across IS-IS level s and <t>This document also defines how the UPA is propagated across IS-IS level s and
OSPF areas.</t> OSPF areas.</t>
<t>The term "OSPF" in this document is used to cover both OSPFv2 and OSPFv 3 protocols.</t>
<t>The term OSPF in this document is used to cover both OSPFv2 and OSPFv3 <section>
protocols.</t> <name>Requirements Language</name>
<t>
The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
"<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>
",
"<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
"<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to
be
interpreted as described in BCP&nbsp;14 <xref target="RFC2119"/> <xref
target="RFC8174"/> when, and only when, they appear in all capitals, as
shown here.
</t>
</section> </section>
</section>
<section anchor="UPA-GEN" title="Generation of the UPA"> <section anchor="UPA-GEN">
<name>Generation of the UPA</name>
<t>UPA MAY be generated by an ABR or ASBR for a prefix that is summarized <t>UPA <bcp14>MAY</bcp14> be generated by an ABR or ASBR for a prefix that
by the is summarized by the
summary prefix originated by an ABR or ASBR in the following cases:</t> summary prefix originated by an ABR or ASBR in the following cases:</t>
<ol spacing="normal" type="1">
<!--[rfced] Please clarify item 1.
Original:
1. Reachability of a prefix that was reachable earlier was lost.
<ol spacing="normal" type="1" indent="adaptive" start="1"> Perhaps:
<li derivedCounter="1."> 1. A prefix that was reachable earlier is now lost (i.e., no longer
<t>Reachability of a prefix that was reachable earlier was lost.</t> reachable).
</li> -->
<li derivedCounter="2."> <!--[rfced] Please clarify item 2. Why is the first item "if"?
<t>For any of the planned maintenance cases: Is the second item dependent on the first "if" clause? If so, we
<list style="hanging"> suggest putting them together.
<t>- if the node originating the prefix is signalling the overload st Original:
ate in 2. For any of the planned maintenance cases:
IS-IS, or or H-bit in OSPFv2 <xref target="RFC8770"/>, or R-bit in OS
PFv3
<xref target="RFC5340"/> .</t>
<t>- the metric to reach the prefix from an ABR or ASBR crosses the c - if the node originating the prefix is signalling the
onfigured overload state in IS-IS, or or H-bit in OSPFv2 [RFC8770], or
threshold.</t> R-bit in OSPFv3 [RFC5340] .
</list></t>
</li> - the metric to reach the prefix from an ABR or ASBR crosses
</ol> the configured threshold.
Perhaps:
For any of the planned maintenance cases, if the node originating the
prefix is signaling the overload state in IS-IS, or is using the H-bit
in OSPFv2 [RFC8770] or the R-bit in OSPFv3 [RFC5340], then the metric
to reach the prefix from an ABR or ASBR crosses the configured
threshold.
-->
<li>Reachability of a prefix that was reachable earlier was lost.</li>
<li><t>For any of the planned maintenance cases:</t>
<ul spacing="normal">
<li>if the node originating the prefix is signaling the overload
state in IS-IS, or H-bit in OSPFv2 <xref target="RFC8770"/>, or
R-bit in OSPFv3 <xref target="RFC5340"/>.</li>
<li>the metric to reach the prefix from an ABR or ASBR crosses the
configured threshold.</li>
</ul></li>
</ol>
<t>Generation as well as propagation of the UPA at an ABR or ASBR is optio nal <t>Generation as well as propagation of the UPA at an ABR or ASBR is optio nal
and SHOULD be controlled by a configuration knob. It SHOULD be disabled by and <bcp14>SHOULD</bcp14> be controlled by a configuration knob. It <bcp14
default.</t> >SHOULD</bcp14> be disabled by default.</t>
<t>Implementations <bcp14>MAY</bcp14> limit the UPA generation as well as
<t>Implementations MAY limit the UPA generation as well as propagation to propagation to specific
specific prefixes, e.g. host prefixes, Segment Routing over IPv6 (SRv6) locators, o
prefixes, e.g. host prefixes, SRv6 locators, or similar. Such filtering is r similar. Such filtering is optional
optional and <bcp14>SHOULD</bcp14> be controlled via configuration.</t>
and SHOULD be controlled via configuration.</t> <t>The intent of UPA is to provide an event-driven signal of the transitio
n of
<t>The intent of UPA is to provide an event driven signal of the transitio
n of
a destination from reachable to unreachable. It is not intended to adverti se a destination from reachable to unreachable. It is not intended to adverti se
a persistent state.</t> a persistent state.</t>
<t>ABR or ASBR <bcp14>MUST</bcp14> withdraw the previously advertised UPA
<t>ABR or ASBR MUST withdraw the previously advertised UPA when the reason when the reason for which
for which the UPA was generated ceases, e.g., prefix reachability was restored or it
the UPA was generated ceases - e.g. prefix reachability was restored or it s metric
s metric
has changed such that it is below a configured threshold value.</t> has changed such that it is below a configured threshold value.</t>
<t>Even if the reasons persist, UPA advertisements <bcp14>SHOULD</bcp14> b
<t>Even if the reasons persist, UPA advertisements SHOULD be withdrawn aft e withdrawn after some
er some
amount of time, that would provide sufficient time for UPA to be flooded 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 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 refle in the network. The time the UPA is kept in the network <bcp14>SHOULD</bc
ct the p14> also reflect the
intended use-case for which the UPA was advertised. Not withdrawing the UP intended use case for which the UPA was advertised. Not withdrawing the UP
A A
would result in stale information being kept in the link state database of all would result in stale information being kept in the link state database of all
routers in the area.</t> routers in the area.</t>
<!--[rfced] Should this be plural "databases"?
<t>Implementations SHOULD provide a configuration option to specify the UP Original:
A lifetime at Not withdrawing the UPA would result in
the originating ABR or ASBR.</t> stale information being kept in the link state database of all
routers in the area.
Perhaps:
Not withdrawing the UPA would result in
stale information being kept in the link state databases of all
routers in the area.
Or rephrase:
Withdrawing the UPA prevents the LSDB from holding stale
information about all routers in the area.
-->
<t>Implementations <bcp14>SHOULD</bcp14> provide a configuration option to
specify the UPA lifetime at
the originating ABR or ASBR.</t>
<t>As UPA advertisements in IS-IS are advertised in existing Link State PD Us <t>As UPA advertisements in IS-IS are advertised in existing Link State PD Us
(LSPs) and the unit of flooding in IS-IS is an LSP, it is RECOMMENDED that , when (LSPs) and the unit of flooding in IS-IS is an LSP, it is <bcp14>RECOMMEND ED</bcp14> that, when
possible, UPAs are advertised in LSPs dedicated to this type of advertisem ent. possible, UPAs are advertised in LSPs dedicated to this type of advertisem ent.
This will minimize the number of LSPs which need to be updated when UPAs a re This will minimize the number of LSPs that need to be updated when UPAs ar e
advertised and withdrawn.</t> advertised and withdrawn.</t>
<t>In OSPFv2 and OSPFv3, each inter-area and external prefix is advertised in its <t>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.</ t> own LSA, so the above consideration does not apply to OSPFv2 and OSPFv3.</ t>
<t>It is also <bcp14>RECOMMENDED</bcp14> that implementations limit the nu
<t>It is also RECOMMENDED that implementations limit the number of UPA adv mber of UPA advertisements
ertisements that can be originated at a given time to limit the number of UPAs present
which can be originated at a given time to limit the number of UPAs presen in the network at any given point of time. UPA implementations <bcp14>SHOU
t LD</bcp14> provide a
in the network at any given point of time. UPA implementations SHOULD prov
ide a
configuration option to limit the number of such UPAs.</t> configuration option to limit the number of such UPAs.</t>
</section> </section>
<section anchor="UPA-IS-IS">
<name>Supporting UPA in IS-IS</name>
<t><xref target="RFC5305"/> defines the encoding for advertising IPv4
prefixes using 4 octets of metric information, and <xref
target="RFC5305" section="4"/> specifies:</t>
<section anchor="UPA-IS-IS" title="Supporting UPA in IS-IS"> <blockquote>
<t>If a prefix is advertised with a metric larger than MAX_PATH_METRIC
<t><xref target="RFC5305"/> defines the encoding for advertising IPv4 pref (0xFE000000, see paragraph 3.0), this prefix <bcp14>MUST NOT</bcp14> be co
ixes nsidered
using 4 octets of metric information and its section 4 specifies:</t>
<t>"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 during the normal SPF computation. This allows advertisement of a prefix
for purposes other than building the normal IP routing table."</t> for purposes other than building the normal IP routing table.</t>
</blockquote>
<t>Similarly, <xref target="RFC5308"/> defines the encoding for <t>Similarly, <xref target="RFC5308"/> defines the encoding for
advertising IPv6 prefixes using 4 octets of metric information and its sec advertising IPv6 prefixes using 4 octets of metric information and
tion <xref target="RFC5308" section="2"/> states:</t>
2 states:</t>
<t>"...if a prefix is advertised with a metric larger than <blockquote>
MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST NOT be considered <t>...if a prefix is advertised with a metric larger than
MAX_V6_PATH_METRIC (0xFE000000), this prefix <bcp14>MUST NOT</bcp14> be co
nsidered
during the normal Shortest Path First (SPF) computation. This will allow during the normal Shortest Path First (SPF) computation. This will allow
advertisement of a prefix for purposes other than building the normal advertisement of a prefix for purposes other than building the normal
IPv6 routing table."</t> IPv6 routing table.</t>
</blockquote>
<t>This functionality can be used to advertise a prefix (IPv4 or IPv6) <t>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 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 so without requiring all nodes in the network to be upgraded to support
the functionality.</t> the functionality.</t>
<section anchor="UPA-IS-IS-ADV">
<section anchor="UPA-IS-IS-ADV" title="Advertisement of UPA in IS-IS"> <name>Advertisement of UPA in IS-IS</name>
<t>Existing nodes in a network that do not support UPA will not use UPAs
<t>Existing nodes in a network that do not suport UPA will not use UPAs during the route calculation but will continue to flood them within the
during the route calculation, but will continue to flood them within the area.
area.
This allows flooding of such advertisements to occur without the need to This allows flooding of such advertisements to occur without the need to
upgrade all nodes in a network to support this specification.</t> upgrade all nodes in a network to support this specification.</t>
<t> Those ABRs or ASBRs that are responsible for propagating UPA adverti
sements
into other areas or domains are also expected to recognize UPA advertise
ments.</t>
<!--[rfced] Does "preceding section" refer to Section 2 or Section 3? We
suggest updating the text to use the section number.
<t> Those ABRs or ASBRs which are responsible for propagating UPA advert Original (Section 3.1):
isements As per the definitions referenced in the preceding section, any
into other areas or domains, are also expected to recognise UPA advertis prefix advertisement with a metric value ...
ements.</t>
Perhaps:
As per the definitions referenced in Section 3, any
prefix advertisement with a metric value ...
-->
<t>As per the definitions referenced in the preceding section, any <t>As per the definitions referenced in the preceding section, any
prefix advertisement with a metric value greater than 0xFE000000 can prefix advertisement with a metric value greater than 0xFE000000 can
be used for purposes other than normal routing calculations. Such metric be used for purposes other than normal routing calculations. Such a metr
MUST be used when advertising UPA in IS-IS.</t> ic
<bcp14>MUST</bcp14> be used when advertising UPA in IS-IS.</t>
<t><xref target="RFC7370"/> introduced the IS-IS Sub-TLVs for TLVs Adver <t><xref target="RFC7370"/> introduced the "IS-IS Sub-TLVs for TLVs Adve
tising Prefix rtising Prefix
Reachability registry which lists TLVs for advertising different types o Reachability" registry, which lists TLVs for advertising different types
f prefix of prefix
reachability (that list at the time of publication of this document is b reachability. (The list at the time of publication of this document is b
elow). elow.)
UPA in IS-IS is supported for prefixes advertised in all such TLVs ident ified UPA in IS-IS is supported for prefixes advertised in all such TLVs ident ified
by that registry, e.g.: by that registry, for example:</t>
<ul spacing="normal">
<list style="hanging"> <li>SRv6 Locator <xref target="RFC9352"/></li>
<t>- SRv6 Locator <xref target="RFC9352"/></t> <li>Extended IP reachability <xref target="RFC5305"/></li>
<t>- Extended IP reachability <xref target="RFC5305"/></t> <li>Multi-Topology (MT) IP Reach <xref target="RFC5120"/></li>
<t>- MT IP Reach <xref target="RFC5120"/></t> <li>IPv6 IP Reach <xref target="RFC5308"/></li>
<t>- IPv6 IP Reach <xref target="RFC5308"/></t> <li>MT IPv6 IP Reach <xref target="RFC5120"/></li>
<t>- MT IPv6 IP Reach <xref target="RFC5120"/></t> <li>IPv4 Algorithm Prefix Reachability TLV <xref target="RFC9502"/></l
<t>- IPv4 Algorithm Prefix Reachability TLV <xref target="RFC9502"/></t> i>
<t>- IPv6 Algorithm Prefix Reachability TLV <xref target="RFC9502"/></t <li>IPv6 Algorithm Prefix Reachability TLV <xref target="RFC9502"/></
> li>
</ul>
</list></t>
</section> </section>
<section anchor="UPA-SIGN-IS-IS">
<section anchor="UPA-SIGN-IS-IS" title="Signaling UPA in IS-IS"> <name>Signaling UPA in IS-IS</name>
<t>In IS-IS, a prefix can be advertised with a metric higher than 0xFE00
<t>In IS-IS a prefix can be advertised with metric higher than 0xFE000000, 0000, for various
for various
reasons. Even though in all cases the treatment of such metric is specifie d for IS-IS, reasons. Even though in all cases the treatment of such metric is specifie d for IS-IS,
having an explicit way to signal that the prefix was advertised in order t o signal having an explicit way to signal that the prefix was advertised in order t o signal
UPA is required to distinguish it from other cases where the prefix with UPA is required to distinguish it from other cases where the prefix with
such metric is advertised.</t> such a metric is advertised.</t>
<t>Two new bits in the IPv4/IPv6 Extended Reachability Attribute Flags
<t>Two new bits in the IPv4/IPv6 Extended Reachability Attribute Flags <xref target="RFC7794"/> are defined:</t>
<xref target="RFC7794 "/> are defined: <dl newline="false" spacing="normal">
<list style="hanging"> <dt>U-Flag:</dt><dd>Unreachable Prefix Flag (bit 5). When set, it indi
cates that the
<t>U-Flag: - Unreachable Prefix Flag (Bit 5). When set, it indicates that prefix is unreachable.</dd>
the <dt>UP-Flag:</dt><dd>Unreachable Planned Prefix Flag (bit 6). When set
prefix is unreachable.</t> , this flag
<t>UP-Flag: - Unreachable Planned Prefix Flag (Bit 6). When set, this flag
indicates that the prefix is unreachable due to a planned event indicates that the prefix is unreachable due to a planned event
(e.g., planned maintenance).</t> (e.g., planned maintenance).</dd>
</dl>
<t>Originating node MUST NOT set the UP-flag without setting the U-fag.</t <t>The originating node <bcp14>MUST NOT</bcp14> set the UP-flag withou
> t setting the U-flag.</t>
<t>The receiving node <bcp14>MUST</bcp14> ignore the UP-flag in the ad
<t>Receiving node MUST ignore the UP-flag in the advertisement if the U-fl vertisement if the U-flag is not set.</t>
ag is not set.</t>
</list></t>
<t>The prefix that is advertised with U-Flag MUST have the metric <t>The prefix that is advertised with the U-flag <bcp14>MUST</bcp14> hav e the metric
set to a value larger than 0xFE000000. If the prefix metric is less set to a value larger than 0xFE000000. If the prefix metric is less
than or equal 0xFE000000, both of these flags MUST be ignored.</t> than or equal 0xFE000000, both of these flags <bcp14>MUST</bcp14> be ignor
ed.</t>
</section> </section>
<section anchor="UPA-IS-IS-PROP">
<section anchor="UPA-IS-IS-PROP" title="Propagation of UPA in IS-IS"> <name>Propagation of UPA in IS-IS</name>
<t>IS-IS L1/L2 routers, which would be responsible for propagating UPA <t>IS-IS L1/L2 routers, which would be responsible for propagating UPA
advertisements between levels need to recognize such advertisements. advertisements between levels, need to recognize such advertisements.
Failure to do so would prevent UPA to reach the routers in the remote ar Failure to do so would prevent UPA from reaching the routers in the remo
eas.</t> te areas.</t>
<t>IS-IS allows propagation of IP prefixes in both directions between le
<t>IS-IS allows propagation of IP prefixes in both directions between le vel 1 and level 2. Propagation is only done if the prefix is reachable in the so
vel 1 and urce level,
level 2. Propagation is only done if the prefix is reachable in the sour i.e., the prefix is only propagated from a level in which the prefix is
ce level, reachable.
i.e., prefix is only propagated from a level in which the prefix is reac Such requirement of reachability <bcp14>MUST NOT</bcp14> be applied for
hable. UPAs, as they are
Such requirement of reachability MUST NOT be applied for UPAs, as they a
re
propagating unreachability.</t> propagating unreachability.</t>
<t>IS-IS L1/L2 routers may wish to advertise received UPAs into other ar eas (upwards <t>IS-IS L1/L2 routers may wish to advertise received UPAs into other ar eas (upwards
and/or downwards). When propagating UPAs the original metric value and/or downwards). When propagating UPAs, the original metric value
MUST be preserved. The cost to reach the originator of the received <bcp14>MUST</bcp14> be preserved. The cost to reach the originator of th
UPA MUST NOT be considered when readvertising the UPA.</t> e received
UPA <bcp14>MUST NOT</bcp14> be considered when readvertising the UPA.</t
</section> >
</section> </section>
</section>
<section anchor="UPA-OSPF" title="Supporting UPA in OSPF"> <section anchor="UPA-OSPF">
<name>Supporting UPA in OSPF</name>
<t><xref target="RFC2328"/> Appendix B defines the following <t><xref section="B" target="RFC2328"/> defines the following
architectural constant for OSPFv2:</t> architectural constant for OSPFv2:</t>
<blockquote>
<dl newline="true"><dt>LSInfinity</dt>
<dd>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.</dd></dl>
</blockquote>
<t>"LSInfinity The metric value indicating that the destination <t><xref section="B" target="RFC5340"/> states:</t>
described by an LSA is unreachable. Used in summary-LSAs and <blockquote>
AS-external-LSAs as an alternative to premature aging (see Section <t>Architectural constants for the OSPF protocol are defined in
14.1). It is defined to be the 24-bit binary value of all ones: Appendix B of [OSPFV2].</t>
0xffffff."</t> </blockquote>
<t><xref target="RFC5340"/> Appendix B states:</t>
<t>"Architectural constants for the OSPF protocol are defined in
Appendix B of [OSPFV2]."</t>
<t>indicating that these same constants are applicable to OSPFv3.</t> <t>indicating that these same constants are applicable to OSPFv3.</t>
<t><xref target="RFC2328" section="14.1" sectionFormat="comma"/> also
describes the usage of LSInfinity as a way to indicate loss of prefix
reachability:</t>
<t><xref target="RFC2328"/> section 14.1. also describes the usage of <blockquote>
LSInfinity as a way to indicate loss of prefix reachability:</t> <t>Premature aging can also be used when, for example, one of the
<t>"Premature aging can also be used when, for example, one of the
router's previously advertised external routes is no longer router's previously advertised external routes is no longer
reachable. In this circumstance, the router can flush its AS- reachable. In this circumstance, the router can flush its AS-
external-LSA from the routing domain via premature aging. This external-LSA from the routing domain via premature aging. This
procedure is preferable to the alternative, which is to procedure is preferable to the alternative, which is to
originate a new LSA for the destination specifying a metric of originate a new LSA for the destination specifying a metric of
LSInfinity."</t> LSInfinity.</t>
</blockquote>
<t>In addition, NU-bit is defined for OSPFv3 <xref target="RFC5340"/>. Pref ixes having <t>In addition, the NU-bit is defined for OSPFv3 <xref target="RFC5340"/>. Prefixes having
the NU-bit set in their PrefixOptions field are not included in the routing the NU-bit set in their PrefixOptions field are not included in the routing
calculation.</t> calculation.</t>
<t>UPA in OSPFv2 is supported for prefix reachability advertised via <t>UPA in OSPFv2 is supported for prefix reachability advertised via
OSPFv2 Summary-LSA <xref target="RFC2328"/>, OSPFv2 Summary-LSA <xref target="RFC2328"/>,
AS-external-LSAs <xref target="RFC2328"/>, NSSA AS-external LSA <xref targ et="RFC3101"/>, AS-external-LSAs <xref target="RFC2328"/>, Not-So-Stubby Area (NSSA) AS-ex ternal-LSA <xref target="RFC3101"/>,
and OSPFv2 IP Algorithm Prefix Reachability Sub-TLV <xref target="RFC9502" />.</t> and OSPFv2 IP Algorithm Prefix Reachability Sub-TLV <xref target="RFC9502" />.</t>
<t>UPA in OSPFv3 is supported for prefix reachability advertised via OSPFv 3 <t>UPA in OSPFv3 is supported for prefix reachability advertised via OSPFv 3
E-Inter-Area-Prefix-LSA <xref target="RFC8362"/>, E-Inter-Area-Prefix-LSA <xref target="RFC8362"/>,
E-AS-External-LSA <xref target="RFC8362"/>, E-Type-7-LSA <xref target="RFC 8362"/>, E-AS-External-LSA <xref target="RFC8362"/>, E-Type-7-LSA <xref target="RFC 8362"/>,
and SRv6 Locator LSA <xref target="RFC9513"/>.</t> and SRv6 Locator LSA <xref target="RFC9513"/>.</t>
<t>For prefix reachability advertised via Inter-Area-Prefix-LSA <xref targ et="RFC5340"/>, <t>For prefix reachability advertised via Inter-Area-Prefix-LSA <xref targ et="RFC5340"/>,
AS-External-LSA <xref target="RFC5340"/>, NSSA-LSA <xref target="RFC5340"/ >, AS-External-LSA <xref target="RFC5340"/>, NSSA-LSA <xref target="RFC5340"/ >,
UPA is signaled using their corresponding extended LSAs. This requires sup port of the UPA is signaled using their corresponding extended LSAs. This requires sup port of the
OSPFv3 Extended LSAs in a sparse mode as specified in section 6.2 of <xref OSPFv3 Extended LSAs in a sparse mode as specified in <xref target="RFC836
target="RFC8362"/>.</t> 2" section="6.2"/>.</t>
<section anchor="UPA-OSPF-ADV">
<section anchor="UPA-OSPF-ADV" title="Advertisement of UPA in OSPF"> <name>Advertisement of UPA in OSPF</name>
<t>If an ABR or ASBR advertises UPA in an advertisement of an inter-area
<t>If an ABR or ASBR advertises UPA in an advertisement of an inter-area o or
r external prefix inside OSPFv2 or OSPFv3, then it <bcp14>MUST</bcp14> set t
external prefix inside OSPFv2 or OSPFv3 then it MUST set the age he age
to a value lower than MaxAge and set the metric to LSInfinity.</t> to a value lower than MaxAge and set the metric to LSInfinity.</t>
<t>UPA flooding inside the area follows the existing standard procedures
<t>UPA flooding inside the area follows the existing standard procedures d defined
efined
by OSPFv2 <xref target="RFC2328"/> and OSPFv3 <xref target="RFC5340"/>.</t > by OSPFv2 <xref target="RFC2328"/> and OSPFv3 <xref target="RFC5340"/>.</t >
</section> </section>
<section anchor="UPA-SIGN-OSPF">
<name>Signaling UPA in OSPF</name>
<section anchor="UPA-SIGN-OSPF" title="Signaling UPA in OSPF"> <!-- [rfced] How may this paragraph be rephrased for clarity?
<t>In OSPFv2 a prefix can be advertised with metric LSInfinity, or in OSP Original (Section 3.2):
Fv3 with In IS-IS a prefix can be advertised with metric higher than
NU-bit set in PrefixOptions, for various reasons. Even though in all case 0xFE000000, for various reasons. Even though in all cases the
s the treatment of such metric is specified for IS-IS, having an explicit
treatment of such metric, or NU-bit, is specified for OSPFv2 and OSPFv3, way to signal that the prefix was advertised in order to signal UPA
having is required to distinguish it from other cases where the prefix with
such metric is advertised.
Perhaps:
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.
-->
<!--[rfced] Similarly, how may this paragraph be rephrased for clarity?
Original (Section 4.2):
In OSPFv2 a prefix can be advertised 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 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 metric is
advertised.
Perhaps:
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.
-->
<t>In OSPFv2 a prefix can be advertised with metric LSInfinity, or in OS
PFv3 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 sign al UPA an explicit way to signal that the prefix was advertised in order to sign al UPA
is required to distinguish it from other cases where the prefix with such is required to distinguish it from other cases where the prefix with such a
metric is advertised.</t> metric is advertised.</t>
<t>OSPFv2 and OSPFv3 Prefix Extended Flags Sub-TLVs been defined in
<t>OSPFv2 and OSPFv3 Prefix Extended Flags Sub-TLVs been defined in
<xref target="RFC9792"/> for advertising additional <xref target="RFC9792"/> for advertising additional
prefix attribute flags in OSPFv2 and OSPFv3.</t> prefix attribute flags in OSPFv2 and OSPFv3.</t>
<t>Two new bits in Prefix Attributes Sub-TLV are defined: <!--[rfced] FYI, we updated this TLV name to match the name used in the
<list style="hanging"> IANA Considerations. Please let us know if this is not accurate.
<t>U-Flag: - Unreachable Prefix Flag (Bit 0). When set, it indicates that Original: Prefix Attributes Sub-TLV
the Current: Prefix Attribute Flags Sub-TLV
prefix is unreachable.</t> -->
<t>UP-Flag: - Unreachable Planned Prefix Flag (Bit 1). When set, this flag <t>Two new bits in the Prefix Attribute Flags Sub-TLV are defined:</t>
<dl newline="false" spacing="normal">
<dt>U-Flag:</dt><dd>Unreachable Prefix Flag (bit 0). When set, it indi
cates that the
prefix is unreachable.</dd>
<dt>UP-Flag:</dt><dd>Unreachable Planned Prefix Flag (bit 1). When set
, this flag
indicates that the prefix is unreachable due to a planned event indicates that the prefix is unreachable due to a planned event
(e.g., planned maintenance).</t> (e.g., planned maintenance).</dd>
</dl>
<t>The originating node <bcp14>MUST NOT</bcp14> set the UP-flag withou
t setting the U-flag.</t>
<t>The receiving node <bcp14>MUST</bcp14> ignore the UP-flag in the ad
vertisement if the U-flag is not set.</t>
<t>Originating node MUST NOT set the UP-flag without setting the U-fag.</t <section anchor="UPA-SIGN-OSPFv2">
> <name>Signaling UPA in OSPFv2</name>
<t>The OSPFv2 Prefix Extended Flags Sub-TLV <xref target="RFC9792"/> i
s a sub-TLV of
the OSPFv2 Extended Prefix TLV <xref target="RFC7684"/>.</t>
<t>The prefix that is advertised with U-Flag <bcp14>MUST</bcp14> have
the metric set to
a value LSInfinity. If the prefix metric is not equal to LSInfinity, both o
f these
flags <bcp14>MUST</bcp14> be ignored.
<!--[rfced] Please clarify this sentence, in particular the opening phrase.
<t>Receiving node MUST ignore the UP-flag in the advertisement if the U-fl Original:
ag is not set.</t> 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].
</list> Perhaps:
</t> Therefore, for the default algorithm 0 with prefixes advertised with U-Flag,
it is REQUIRED to advertise the
unreachable prefix in the base OSPFv2 LSA - e.g., OSPFv2 Summary-LSA
[RFC2328], AS-external-LSAs [RFC2328], or NSSA AS-external LSA
[RFC3101].
<section anchor="UPA-SIGN-OSPFv2" title="Signaling UPA in OSPFv2"> Regarding a similar phrase in Section 4.2.2, may it be updated as follows?
<t>OSPFv2 Prefix Extended Flags Sub-TLV <xref target="RFC9792"/> is a Sub-T Original:
LV of For default algorithm 0 prefixes,
the OSPFv2 Extended Prefix TLV <xref target="RFC7684"/>.</t> the LSInfinity MUST be set in the parent TLV.
<t>The prefix that is advertised with U-Flag MUST have the metric set to Perhaps:
a value LSInfinity. If the prefix metric is not equal to LSInfinity, both o For prefixes in default algorithm 0,
f these the LSInfinity MUST be set in the parent TLV.
flags MUST be ignored. For default algorithm 0 prefixes with U-Flag it
is therefore REQUIRED to advertise the unreachable prefix in the base OSPFv Or:
2 LSA - When advertising prefix reachability in default algorithm 0,
the LSInfinity MUST be set in the parent TLV.
-->
For default algorithm 0 prefixes with U-Flag it
is therefore <bcp14>REQUIRED</bcp14> to advertise the unreachable prefix in
the base OSPFv2 LSA -
e.g., OSPFv2 Summary-LSA <xref target="RFC2328"/>, or AS-external-LSAs e.g., OSPFv2 Summary-LSA <xref target="RFC2328"/>, or AS-external-LSAs
<xref target="RFC2328"/>, or NSSA AS-external LSA <xref target="RFC3101"/>. </t> <xref target="RFC2328"/>, or NSSA AS-external LSA <xref target="RFC3101"/>. </t>
</section>
</section> <section anchor="UPA-SIGN-OSPFv3">
<name>Signaling UPA in OSPFv3</name>
<section anchor="UPA-SIGN-OSPFv3" title="Signaling UPA in OSPFv3"> <t>OSPFv3 Prefix Extended Flags Sub-TLV is defined as a sub-TLV of the
<t>OSPFv3 Prefix Extended Flags Sub-TLV is defined as a Sub-TLV of the
following OSPFv3 TLVs that are defined in <xref target="RFC8362"/>: following OSPFv3 TLVs that are defined in <xref target="RFC8362"/>:
<list style="hanging"> </t>
<t>Intra-Area Prefix TLV</t>
<t>Inter-Area Prefix TLV</t> <!-- [rfced] Do you want to keep these TLV names as they are, or change to
match how they appear in RFC 8362?
<t>External Prefix TLV</t> This document:
Intra-Area Prefix TLV
Inter-Area Prefix TLV
External Prefix TLV
</list></t> [RFC8362] and the corresponding IANA registry (https://www.iana.org/assignments/ ospfv3-parameters/ospfv3-parameters.xhtml#extended-lsa-tlvs) use the following ( with an additional hyphen in each):
<t>The prefix that is advertised with U-Flag or UP-flag MUST have the metric Intra-Area-Prefix TLV
set to Inter-Area-Prefix TLV
a value LSInfinity. For default algorithm 0 prefixes, the LSInfinity MUST be External-Prefix TLV
set -->
<ul spacing="normal">
<li>Intra-Area Prefix TLV</li>
<li>Inter-Area Prefix TLV</li>
<li>External Prefix TLV</li>
</ul>
<t>The prefix that is advertised with U-Flag or UP-flag <bcp14>MUST</b
cp14> have the metric set to
a value LSInfinity. For default algorithm 0 prefixes, the LSInfinity <bcp14>
MUST</bcp14> be set
in the parent TLV. For IP Algorithm Prefixes <xref target="RFC9502"/>, in the parent TLV. For IP Algorithm Prefixes <xref target="RFC9502"/>,
the LSInfinity MUST be set in OSPFv3 IP Algorithm Prefix Reachability sub-TL the LSInfinity <bcp14>MUST</bcp14> be set in OSPFv3 IP Algorithm Prefix Reac
V. hability sub-TLV.
If the prefix metric is not equal to LSInfinity, both of these flags MUST be If the prefix metric is not equal to LSInfinity, both of these flags <bcp14>
ignored.</t> MUST</bcp14> be ignored.</t>
<t>The prefix that is advertised with U-Flag or UP-Flag <bcp14>MUST</b
<t>The prefix that is advertised with U-Flag or UP-Flag MUST have the NU-bit cp14> have the NU-bit set
set
in the PrefixOptions of the parent TLV. If the NU-bit in PrefixOptions of th e parent in the PrefixOptions of the parent TLV. If the NU-bit in PrefixOptions of th e parent
TLV is not set, both of these flags MUST be ignored.</t> TLV is not set, both of these flags <bcp14>MUST</bcp14> be ignored.</t>
</section>
</section> </section>
<section anchor="UPA-OSPF-PROP">
</section> <name>Propagation of UPA in OSPF</name>
<t>OSPF ABRs, which would be responsible for propagating
<section anchor="UPA-OSPF-PROP" title="Propagation of UPA in OSPF"> UPA advertisements into other areas, need to recognize such advertisement
s.
<t>OSPF ABRs, which would be responsible for propagating Failure to do so would prevent UPA from reaching the routers in the remot
UPA advertisements into other areas need to recognize such advertisements e areas.</t>
. <t>Advertising prefix reachability between OSPF areas assumes prefix rea
Failure to do so would prevent UPA to reach the routers in the remote are chability
as.</t> in a source area. Such a requirement of reachability <bcp14>MUST NOT</bcp
14> be applied
<t>Advertising prefix reachability between OSPF areas assumes prefix reac
hability
in a source area. Such requirement of reachability MUST NOT be applied
for UPAs, as they are propagating unreachability.</t> for UPAs, as they are propagating unreachability.</t>
<t>OSPF ABRs or ASBRs <bcp14>MAY</bcp14> advertise received UPAs between
<t>OSPF ABRs or ASBRs MAY advertise received UPAs between connected connected
areas or domains. When doing so, the original LSInfinity metric value in 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 <bcp14>MUST</bcp14> be preserved. The cost to reach the originator of
UPA MUST NOT be considered when readvertising the UPA to connected areas. the received
</t> UPA <bcp14>MUST NOT</bcp14> be considered when readvertising the UPA to c
onnected areas.</t>
</section>
</section> </section>
</section>
<section anchor="UPA-RXPROCESS" title="Processing of the UPA"> <section anchor="UPA-RXPROCESS">
<name>Processing of the UPA</name>
<t>Processing of the received UPAs is optional and SHOULD be controlled b <t>Processing of the received UPAs is optional and <bcp14>SHOULD</bcp14> b
y the e controlled by the
configuration at the receiver. The receiver itself, based on its configur ation, configuration at the receiver. The receiver itself, based on its configur ation,
decides what the UPA will be used for and what applications, if any, will be notified 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 when UPA is received. Usage of the UPA at the receiver is outside of the scope of this
document.</t> document.</t>
<t>As an example, UPA may be used to trigger BGP PIC Edge at the receiving
<t>As an example, UPA may be used to trigger BGP PIC Edge at the receivin router
g router
<xref target="I-D.ietf-rtgwg-bgp-pic"/>.</t> <xref target="I-D.ietf-rtgwg-bgp-pic"/>.</t>
<t>Applications using the UPA cannot use the absence of the UPA to infer t
<t>Applications using the UPA cannot use the absence of the UPA to infer hat the
that the
reachability of the prefix is back. They must rely on their own mechanism s to verify reachability of the prefix is back. They must rely on their own mechanism s to verify
the reachability of the remote end-points.</t> the reachability of the remote endpoints.</t>
</section> </section>
<section anchor="UPA-PARTITION">
<section anchor="UPA-PARTITION" <name>Area and Domain Partition</name>
title="Area and Domain Partition"> <t>UPA is not meant to address an area/domain partition. When an area or d
omain partitions,
<t>UPA is not meant to address an area/domain partition. When an area or while multiple ABRs or ASBRs advertise the same summary, each of them ca
domain partitions, n only reach a portion
while multiple ABRs or ASBRs advertise the same summary, each of them ca
n only reach portion
of the summarized prefix. As a result, depending on which ABR or ASBR th e traffic is using of the summarized prefix. As a result, depending on which ABR or ASBR th e traffic is using
to enter a partitioned area, the traffic could be either dropped or deli vered to its to enter a partitioned area, the traffic could be either dropped or deli vered to its
final destination. UPA does not make the problem of an area partition an y worse. final destination. UPA does not make the problem of an area partition an y worse.
In case of an area partition each of an ABRs or ASBRs will generate UPAs for the In case of an area partition, each ABR or ASBR will generate UPAs for th e
destinations for which the reachability was lost locally. As the UPA pro pagates to the destinations for which the reachability was lost locally. As the UPA pro pagates to the
nodes outside of a partitioned area, it may result in such nodes picking nodes outside a partitioned area, it may result in such nodes picking
an alternative egress node for the traffic, if such alternate egress nod an alternative egress node for the traffic, if such a node exists.
e exists. If such an alternative egress node resides outside a partitioned area, t
If such alternate egress node resides outside of a partitioned area, tra raffic will
ffic will be restored. If such an alternative egress node resides in a partitioned
be restored. If such alternate egress node resides in a partitioned area area and is
and is covered by the summary, the traffic will be dropped if it enters a parti
covered by the summary, the trafic will be dropped if it enters a partit tioned
ioned area via an ABR or ASBR that cannot reach that node. This will result in
area via an ABR or ASBR that can not reach the alternate egress node - r similar behavior as without the UPA. The above statements are also appli
esulting in cable to a
similar behavior as without the UPA. Above is similarly applicable to a
domain partition.</t> domain partition.</t>
</section> </section>
<section anchor="IANA">
<section anchor="IANA" title="IANA Considerations"> <name>IANA Considerations</name>
<section anchor="UBIT-IS-IS">
<section anchor="UBIT-IS-IS" title="IS-IS Prefix Attribute Flags Sub-TLV"> <name>IS-IS Prefix Attribute Flags Sub-TLV</name>
<t>This document adds two new bits in the "IS-IS Bit Values for Prefix A
<t>This document adds two new bits in the "IS-IS Bit Values for Prefix At ttribute
tribute Flags Sub-TLV" registry:</t>
Flags Sub-TLV" registry: <dl newline="false" spacing="compact">
<dt>Bit #:</dt><dd>5</dd>
<list style="hanging"> <dt>Name:</dt><dd>U-Flag</dd>
<dt>Reference:</dt><dd>RFC 9929 (<xref target="UPA-SIGN-IS-IS"/>)</dd>
<t>Bit #: 5</t> </dl>
<dl newline="false" spacing="compact">
<t>Description: U-Flag</t> <dt>Bit #:</dt><dd>6</dd>
<dt>Name:</dt><dd>UP-Flag</dd>
<t>Reference: This document (<xref target="UPA-SIGN-IS-IS"/>).</t> <dt>Reference:</dt><dd>RFC 9929 (<xref target="UPA-SIGN-IS-IS"/>)</dd>
</dl>
<t>Bit #: 6</t> </section>
<section anchor="UBIT-OSPF">
<t>Description: UP-Flag</t> <name>OSPFv2 and OSPFv3 Prefix Extended Flags</name>
<t>This document adds two new bits in the "OSPFv2 Prefix Extended Flags"
<t>Reference: This document (<xref target="UPA-SIGN-IS-IS"/>).</t> and "OSPFv3 Prefix Extended Flags" registries:</t>
<dl newline="false" spacing="compact">
</list></t> <dt>Bit:</dt><dd>0</dd>
<dt>Description:</dt><dd>U-Flag</dd>
</section> <dt>Reference:</dt><dd>RFC 9929 (<xref target="UPA-SIGN-OSPF"/>)</dd>
</dl>
<section anchor="UBIT-OSPF" title="OSPFv2 and OSPFv3 OSPFv2 Prefix Extended <dl newline="false" spacing="compact">
Flags"> <dt>Bit:</dt><dd>1</dd>
<dt>Description:</dt><dd>UP-Flag</dd>
<t>This document adds two new bits in the "OSPFv2 Prefix Extended Flags" <dt>Reference:</dt><dd>RFC 9929 (<xref target="UPA-SIGN-OSPF"/>)</dd>
and "OSPFv3 Prefix Extended Flags" registries: </dl>
<list style="hanging">
<t>Bit #: 0</t>
<t>Description: U-Flag</t>
<t>Reference: This document (<xref target="UPA-SIGN-OSPF"/>).</t>
<t>Bit #: 1</t>
<t>Description: UP-Flag</t>
<t>Reference: This document (<xref target="UPA-SIGN-OSPF"/>).</t>
</list></t>
</section> </section>
</section> </section>
<section anchor="SEC_ISIS">
<section anchor="SEC_ISIS" title="Security Considerations"> <name>Security Considerations</name>
<t>The use of UPAs introduces the possibility that an attacker could <t>The use of UPAs introduces the possibility that an attacker could
inject a false, but apparently valid, UPA. However, the risk of this 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 occurring is no greater than the risk today of an attacker injecting any
other type of false advertisement.</t> other type of false advertisement.</t>
<t>The risks can be reduced by the use of existing security extensions <t>The risks can be reduced by the use of existing security extensions
as described in: as described in:</t>
<list style="hanging">
<t>- <xref target="RFC5304"/>, <xref target="RFC5310"/>, and <xref target=
"RFC7794"/>
for IS-IS.</t>
<t>- <xref target="RFC2328"/>, <xref target="RFC7474"/> and <xref target=" <ul spacing="normal">
RFC7684"/> <li><xref target="RFC5304"/>, <xref target="RFC5310"/>, and <xref
for OSPFv2.</t> target="RFC7794"/> for IS-IS.</li>
<li><xref target="RFC2328"/>, <xref target="RFC7474"/>, and <xref
target="RFC7684"/> for OSPFv2.</li>
<li><xref target="RFC5340"/>, <xref target="RFC4552"/>, and <xref
target="RFC8362"/> for OSPFv3.</li>
</ul>
</section>
<t>- <xref target="RFC5340"/>, <xref target="RFC4552"/> and <xref target=" </middle>
RFC8362"/> <back>
for OSPFv3.</t>
</list></t> <displayreference target="I-D.ietf-rtgwg-bgp-pic" to="BGP-PIC"/>
<references>
<name>References</name>
<references>
<name>Normative References</name>
</section> <reference anchor="ISO10589" target="https://www.iso.org/en/contents/dat
a/standard/03/09/30932.html">
<front>
<title>Information technology -- Telecommunications and information
exchange between systems -- Intermediate System to Intermediate System intra-dom
ain routeing information exchange protocol for use in conjunction with the proto
col for providing the connectionless-mode network service (ISO 8473)</title>
<author>
<organization abbrev="ISO/IEC">International Organization for
Standardization/International Electrotechnical Commission </organiza
tion>
</author>
<date month="Nov" year="2002"/>
</front>
<seriesInfo name="ISO/IEC" value="10589:2002"/>
</reference>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2
119.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2
328.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4
552.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
305.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
304.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
308.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
310.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
340.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
370.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
474.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
684.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
174.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
120.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3
101.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6
987.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
362.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
770.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
794.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
502.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
513.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
352.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
792.xml"/>
</references>
<references>
<name>Informative References</name>
<!-- [I-D.ietf-rtgwg-bgp-pic] long form to include editor information -->
<reference anchor="I-D.ietf-rtgwg-bgp-pic" target="https://datatracker.ietf.org/
doc/html/draft-ietf-rtgwg-bgp-pic-23">
<front>
<title>BGP Prefix Independent Convergence</title>
<author fullname="Ahmed Bashandy" initials="A." surname="Bashandy" role="edi
tor">
<organization>HPE</organization>
</author>
<author fullname="Clarence Filsfils" initials="C." surname="Filsfils">
<organization>Cisco Systems</organization>
</author>
<author fullname="Prodosh Mohapatra" initials="P." surname="Mohapatra">
<organization>Sproute Networks</organization>
</author>
<author fullname="Yingzhen Qu" initials="Y." surname="Qu">
<organization>Futurewei Technologies</organization>
</author>
<date day="15" month="February" year="2026"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-rtgwg-bgp-pic-23"/>
</reference>
</references>
</references>
<section anchor="Acknowledgements" title="Acknowledgements"> <section anchor="Acknowledgements" numbered="false">
<t>The authors would like to thank Kamran Raza, Michael MacKenzie <name>Acknowledgements</name>
and Luay Jalil for their contribution and support of the overall solution <t>The authors would like to thank <contact fullname="Kamran Raza"/>, <con
tact fullname="Michael MacKenzie"/>,
and <contact fullname="Luay Jalil"/> for their contributions and support o
f the overall solution
proposed in this document.</t> proposed in this document.</t>
</section> </section>
<section anchor="CONTR" title="Contributors"> <section anchor="CONTR" numbered="false">
<name>Contributors</name>
<t>The following people contributed to the content of this document and shou <t>The following people contributed to the content of this document and sh
ld be ould be
considered coauthors:</t> considered coauthors:</t>
<contact fullname="Stephane Litkowski" initials="S." surname="Litkowski">
<contact fullname="Stephane Litkowski" initials="S." surname="Litkowski"> <address>
<address> <email>slitkows@cisco.com</email>
<email>slitkows@cisco.com</email> </address>
</address> </contact>
</contact> <contact fullname="Amit Dhamija" initials="A." surname="Dhamija">
<address>
<contact fullname="Amit Dhamija" initials="A." surname="Dhamija"> <email>amitd@arrcus.com</email>
<address> </address>
<email>amitd@arrcus.com</email> </contact>
</address> <contact fullname="Gunter Van de Velde" initials="G." surname="Van de Veld
</contact> e">
<address>
<contact fullname="Gunter Van de Velde" initials="G." surname="Van de Velde" <email>gunter.van_de_velde@nokia.com</email>
> </address>
<address> </contact>
<email>gunter.van_de_velde@nokia.com</email> <t>The following people contributed to the problem statement and the solut
</address> ion
</contact>
<t>The following people contributed to the problem statement and the solutio
n
requirement discussion:</t> requirement discussion:</t>
<contact fullname="Aijun Wang" initials="A." surname="Wang">
<contact fullname="Aijun Wang" initials="A." surname="Wang"> <address>
<address> <email>wangaj3@chinatelecom.cn</email>
<email>wangaj3@chinatelecom.cn</email> </address>
</address> </contact>
</contact> <contact fullname="Zhibo Hu" initials="Z." surname="Hu">
<address>
<contact fullname="Zhibo Hu" initials="Z." surname="Hu"> <email>huzhibo@huawei.com</email>
<address> </address>
<email>huzhibo@huawei.com</email> </contact>
</address>
</contact>
</section> </section>
</middle> </back>
<back>
<references title="Normative References">
<reference anchor="ISO10589">
<front>
<title>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)</title>
<author>
<organization abbrev="ISO">International Organization for
Standardization</organization>
</author>
<date month="Nov" year="2002"/>
</front>
</reference>
<?rfc include='reference.RFC.2119'?>
<?rfc include='reference.RFC.2328'?>
<?rfc include='reference.RFC.4552'?>
<?rfc include='reference.RFC.5305'?>
<?rfc include='reference.RFC.5304'?>
<?rfc include='reference.RFC.5308'?>
<?rfc include='reference.RFC.5310'?>
<?rfc include='reference.RFC.5340'?>
<?rfc include='reference.RFC.7370'?>
<?rfc include='reference.RFC.7474'?>
<?rfc include='reference.RFC.7684'?>
<?rfc include='reference.RFC.8174'?>
<?rfc include='reference.RFC.5120'?>
<?rfc include='reference.RFC.3101'?>
<?rfc include='reference.RFC.6987'?>
<?rfc include='reference.RFC.8362'?>
<?rfc include='reference.RFC.8770'?>
<?rfc include='reference.RFC.7794'?>
<?rfc include='reference.RFC.9502'?>
<?rfc include='reference.RFC.9513'?>
<?rfc include='reference.RFC.9352'?>
<?rfc include='reference.RFC.9792'?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.ietf-rtgwg-bgp-pic'?> <!-- [rfced] Please review the "Inclusive Language" portion of the online
Style Guide <https://www.rfc-editor.org/styleguide/part2/#inclusive_language>
and let us know if any changes are needed. Updates of this nature typically
result in more precise language, which is helpful for readers.
</references> Please consider whether "traditionally" should be updated for clarity.
</back> While the NIST website
<https://web.archive.org/web/20250214092458/https://www.nist.gov/nist-research-l
ibrary/nist-technical-series-publications-author-instructions#table1>
indicates that this term is potentially biased, it is also ambiguous.
"Traditionally" is a subjective term, as it is not the same for everyone.
-->
</rfc> </rfc>
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