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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std" docName="draft-ietf-lsvr-bgp-spf-51" number="0000" ipr="trust200902" obsoletes="" updates="" submissionType="IETF" xml:lang="en" tocInclude="true" tocDepth="4" symRefs="true" sortRefs="true" version="3" consensus="true">
  <!-- xml2rfc v2v3 conversion 3.12.1 -->
  <!-- category values: std, bcp, info, exp, and historic
       ipr values: full3667, noModification3667, noDerivatives3667

<!--[rfced] The Abstract/Introduction mentions that this document
provides extensions for use with BGP Link-State distribution and
the SPF algorithm. Would you can add like to include "extensions" in the attributes updates="NNNN" and obsoletes="NNNN"
       they will automatically be output
document title for consistency with "(if approved)" -->

  <!-- ***** FRONT MATTER ***** the Abstract/Introduction?

Original:
   BGP Link-State Shortest Path First (SPF) Routing

Perhaps:
   Extensions for BGP Link-State Shortest Path First (SPF) Routing
-->

  <front>
    <title abbrev="BGP Link-State SPF Routing">
    BGP Routing">BGP Link-State Shortest Path First (SPF) Routing</title>
    <!-- add 'role="editor"' below for the editors if appropriate -->

    <!-- Another author who claims to be an editor -->
    <seriesInfo name="RFC" value="0000"/>
    <author fullname="Keyur Patel" initials="K" surname="Patel">
      <organization>Arrcus, Inc.</organization>
      <address>
        <email>keyur@arrcus.com</email>
      </address>
    </author>
    <author fullname="Acee Lindem" initials="A" surname="Lindem">
      <organization>LabN Consulting, LLC</organization>
      <address>
        <postal>
          <street>301 Midenhall Way</street>
          <city>Cary</city>
          <region>NC</region>
          <code>27513</code>
          <country>USA</country>
          <country>United States of America</country>
        </postal>
        <email>acee.ietf@gmail.com</email>
      </address>
    </author>
    <author fullname="Shawn Zandi" initials="S" surname="Zandi">
      <organization>LinkedIn</organization>
      <address>
        <postal>
          <street>222 2nd Street</street>
          <city>San Francisco</city>
          <region>CA</region>
          <code>94105</code>
          <country>USA</country>
          <country>United States of America</country>
        </postal>
        <email>szandi@linkedin.com</email>
      </address>
    </author>
    <author fullname="Wim Henderickx" initials="W" surname="Henderickx">
      <organization>Nokia</organization>
      <address>
        <postal>
          <street>copernicuslaan 50</street>
          <city>Antwerp</city>
          <code>2018</code>
          <country>Belgium</country>
        </postal>
        <email>wim.henderickx@nokia.com</email>
      </address>
    </author>
    <date/>
    <!-- Meta-data Declarations -->

    <area>Routing</area>
    <workgroup>Link State Vector Routing (LSVR) Working Group</workgroup>
    <date month="June" year="2025"/>

    <area>RTG</area>
    <workgroup>lsvr</workgroup>

    <keyword>IDR</keyword>
    <!-- Keywords will be incorporated into HTML output
         files in a meta tag but they have no effect on text or nroff
         output. If you submit your draft to the RFC Editor, the
         keywords will be used for the search engine. -->

    <abstract>
      <t>
        Many Massively Scaled Data Centers (MSDCs) have converged on simplified
        L3 (Layer 3)
        Layer 3 (L3) routing. Furthermore, requirements for operational simplicity
        has led many of these MSDCs to converge on BGP as their single routing
        protocol for both their fabric routing and their Data Center Interconnect
        (DCI) routing. This document describes extensions to BGP to for use with BGP -
        Link-State (BGP-LS) distribution and the Shortest Path First (SPF) algorithm.
        In doing this, it allows
        BGP to be efficiently used as both the underlay protocol and the overlay protocol in
        MSDCs.
      </t>
    </abstract>
  </front>
  <middle>
    <section anchor="introduction" numbered="true" toc="default">
      <name>Introduction</name>
      <t>
        Many Massively Scaled Data Centers (MSDCs) have converged on simplified
        L3 (Layer 3)
        Layer 3 (L3) routing. Furthermore, requirements for operational simplicity
        has led many of these MSDCs to converge on BGP <xref target="RFC4271" format="default"/>
        as their single routing protocol for both their fabric routing and
        their
        Data Center Interconnect (DCI) routing <xref target="RFC7938" format="default"/>.
        This document describes an alternative solution which that leverages
        BGP-LS <xref target="RFC9552" format="default"/>
        and the Shortest Path First (SPF) algorithm used by
        Internal Gateway Protocols (IGPs).
      </t>
      <t>
        This document leverages both the BGP protocol <xref target="RFC4271" format="default"/> and
        the
        BGP-LS extensions <xref target="RFC9552" format="default"/> extensions. format="default"/>. The relationship, relationship as well as
        the scope of changes is are described respectively in Sections <xref target="BGP-base" format="default"/> format="counter"/>
        and <xref target="BGP-LS" format="default"/>. format="counter"/>, respectively. The modifications to
        <xref target="RFC4271" format="default"/>
        for BGP SPF described herein only apply to IPv4 and IPv6 as underlay unicast
        Subsequent Address Families Family Identifiers (SAFIs).
        Operations for any other BGP SAFIs are outside the scope of this document.
      </t>
      <t>
        This solution avails the benefits of both BGP and SPF-based IGPs.
        These include TCP-based flow-control, no periodic link-state refresh, and
        completely incremental NLRI Network Layer Reachability Information (NLRI) advertisement.
	These advantages can reduce the
        overhead in MSDCs where there is a high degree of Equal Cost Multi-Path Equal-Cost Multipath
        (ECMP) load-balancing. load balancing.
        Additionally, using an SPF-based computation can support fast convergence and
        the computation of Loop-Free Alternatives (LFAs). The SPF LFA extensions defined
        in <xref target="RFC5286" format="default"/> can be similarly applied to BGP SPF calculations.

<!--[rfced] May we rephrase "are a matter of implementation detail" to
"are specific to implementation" or "are specific to the
implementation process" for clarity?

Original:
   However, the details are a matter of implementation detail
   and out of scope for this document.

Perhaps:
   However, the details are specific to implementation and are
   out of scope for this document.
-->

        However, the details are a matter of implementation detail and out of scope for this
        document.
        Furthermore, a BGP-based solution lends itself to multiple peering models
        including those incorporating route-reflectors route reflectors <xref target="RFC4456" format="default"/>
        or controllers.
      </t>
      <section anchor="terms" numbered="true" toc="default">
        <name>Terminology</name>
        <t>
          This specification reuses terms defined in section 1.1 of <xref section="1.1" target="RFC4271" format="default"/>.
        </t>
        <t>Additionally, this document introduces the following terms:
        </t>
        <dl newline="false" spacing="normal">
          <dt>BGP SPF Routing Domain:</dt>
          <dd> A set of BGP routers that are under a single
          administrative domain and that exchange link-state information using the BGP-LS-SPF SAFI
          and compute routes using that use BGP SPF SPF, as described herein.</dd>
          <dt>BGP-LS-SPF NLRI:</dt>
          <dd> This refers to
          <dd>The BGP-LS Network Layer Reachability
          Information (NLRI) that is being advertised in the BGP-LS-SPF SAFI (<xref target="SAFI" format="default"/>)
          and is being used for BGP SPF route computation.</dd>
          <dt>Dijkstra Algorithm:</dt>
          <dd>
            An algorithm for computing the shortest path from a given node in a graph
            to every other node in the graph.
          </dd>
          <dt>Prefix NLRI:</dt>
          <dd>
            In the context of BGP SPF, this term refers to both or either the IPv4 Topology Prefix NLRI
            and/or the IPv6 Topology Prefix NLRI.
          </dd>
        </dl>
      </section>
      <section numbered="true" toc="default">
        <name>BGP Shortest Path First (SPF) Motivation</name>
        <t>
          Given that <xref target="RFC7938" format="default"/> already describes how BGP could be used
          as the sole routing protocol in an MSDC, one might question the motivation for
          defining an alternative BGP deployment model when a mature solution exists.
          For both alternatives, BGP offers the operational benefits of a single
          routing protocol as opposed to the combination of an IGP for the underlay
          and BGP as an the overlay. However, BGP SPF offers some unique advantages above
          and beyond standard BGP path-vector routing. With BGP SPF, the
          simple single-hop peering model recommended in section 5.2.1 of <xref section="5.2.1" target="RFC7938"/>
          is augmented with peering models requiring fewer BGP sessions.
        </t>
        <t>
          A primary advantage is that all BGP speakers in the BGP SPF routing domain
          have a complete view of the topology. This allows support for ECMP,
          IP fast-reroute (e.g., Loop-Free Alternatives) Alternatives (LFAs) <xref target="RFC5286" format="default"/>,
          Shared Risk Link Groups (SRLGs) <xref target="RFC4202"/>,
          and other routing enhancements without advertisement of additional
          BGP paths <xref target="RFC7911" format="default"/>  or other extensions.
        </t>
        <t>
          With the BGP SPF decision process as defined in
          <xref target="bgp-decision" format="default"/>, NLRI changes can be disseminated throughout the BGP
          routing domain much more rapidly. The added advantage of BGP using TCP for reliable
          transport leverages TCP's inherent flow-control and guaranteed in-order delivery.
          </t>
          <t>
            Another primary advantage is a potential reduction in NLRI advertisement.

<!--[rfced] In the RFC Series, "100s or 1000s" is typically spelled
out. Would you like to spell it out here?

Original:
   With standard BGP path-vector routing, a single link
   failure may impact 100s or 1000s of prefixes and result in the
   withdrawal or re-advertisement of the attendant NLRI.

Perhaps:
   With standard BGP path-vector routing, a single link
   failure may impact hundreds or thousands of prefixes
   and result in the withdrawal or re-advertisement of
   the attendant NLRI.
-->

            With standard BGP path-vector routing, a single link failure may impact
            100s or 1000s of prefixes and result in the withdrawal or readvertisement of
            the attendant NLRI. With BGP SPF, only the BGP speakers originating
            the link NLRI need to withdraw the corresponding BGP-LS-SPF Link NLRI. Additionally,
            the changed NLRI is advertised immediately as opposed to normal BGP where it
            is only advertised after the best route selection. These advantages provide
            NLRI dissemination throughout the BGP SPF routing domain with efficiencies similar
            to link-state protocols.
          </t>
          <t>
            With controller and route-reflector peering models, BGP SPF advertisement
            and distributed computation require a minimal number of sessions and
            copies of the NLRI since as only the latest version of the NLRI from the
            originator is required (see <xref target="peering-models"/>).
            Given that verification of whether or not to advertise a link (with a
            BGP-LS-SPF Link NLRI) is done outside of BGP, each BGP
            speaker only needs as many sessions and copies of the NLRI as required for
            redundancy. Additionally, a controller could inject topology (i.e., BGP-LS-SPF NLRI)
            that is learned outside the BGP SPF routing domain.
          </t>
          <t>
            Given that BGP-LS NLRI is already defined
            <xref target="RFC9552" format="default"/>, this functionality
            can be reused for BGP-LS-SPF NLRI.
          </t>
          <t>
            Another advantage of BGP SPF is that both IPv6 and IPv4 can
            be supported using the BGP-LS-SPF SAFI with the same BGP-LS-SPF Link NLRIs. In many
            MSDC fabrics, the IPv4 and IPv6 topologies are congruent (refer to
            <xref target="Link-NLRI" format="default"/>).
            Although
            However, beyond the scope of this document, BGP-LS-SPF NLRI multi-topology extensions could
            be defined to support separate IPv4, IPv6, unicast, and multicast topologies
            while sharing the same NLRI.
          </t>
          <t>

<!--[rfced] How may we rephrase "and realize all the above advantages"
for clarity? Is the intended meaning that the BGF SPF topology
"offers" the above advantages, as shown below?

Original:
   Finally, the BGP SPF topology can be used as an underlay for other
   BGP SAFIs (using the existing model) and realize all the above
   advantages.

Perhaps:
   Finally, the BGP SPF topology can be used as an underlay for other
   BGP SAFIs (using the existing model), and it offers all the above
   advantages.
-->

            Finally, the BGP SPF topology can be used as an underlay for other BGP
            SAFIs (using the existing model) and realize all the above
            advantages.
          </t>
          </section>
          <section numbered="true" toc="default">
          <name>Document Overview</name>

          <t>

<!--[rfced] FYI - We rephrased this sentence as shown below to avoid
any confusion with "[RFC4271] (Section 2)" and "[RFC9552] (Section 3)".

Original:
   The document begins with sections defining the precise relationship
   that BGP SPF has with both the base BGP protocol [RFC4271]
   (Section 2) and the BGP Link-State (BGP-LS) extensions [RFC9552]
   (Section 3).

Current:
   This document begins with Section 2 defining the precise relationship
   that BGP SPF has with the base BGP protocol [RFC4271] and Section 3
   defining the BGP - Link-State (BGP-LS) extensions [RFC9552].
-->

            This document begins with <xref target="BGP-base" format="default"/> defining the
            precise relationship that BGP SPF has
            with the base BGP protocol <xref target="RFC4271" format="default"/>  (<xref target="BGP-base" format="default"/>) and <xref target="BGP-LS" format="default"/> defining the
            BGP - Link-State (BGP-LS) extensions <xref target="RFC9552" format="default"/>
            (<xref target="BGP-LS" format="default"/>). format="default"/>.
            The BGP peering models, models as well as
            their respective trade-offs are then discussed in
            <xref target="peering-models" format="default"/>. The remaining sections, which make up the bulk of the
            document, define the protocol enhancements necessary to support BGP SPF including BGP-LS Extensions extensions
            (<xref target="protocol-extend" format="default"/>), replacement of the base BGP decision process
            with the SPF computation (<xref target="bgp-decision" format="default"/>), and BGP SPF error
            handling (<xref target="error-handling" format="default"/>).
          </t>
          </section>
          <section numbered="true" toc="default">
            <name>Requirements Language</name>
            <t>The
        <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
            NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
            "MAY", "<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 "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
    described in BCP 14 BCP&nbsp;14 <xref target="RFC2119" format="default"/> target="RFC2119"/> <xref target="RFC8174" format="default"/> target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.</t> here.
        </t>

</section>
</section>
<!-- for Introductions section -->

<section anchor="BGP-base" numbered="true" toc="default">
<name>Base BGP Protocol Relationship</name>
<t>
  With the exception of the decision process, the BGP SPF extensions leverage the BGP
  protocol <xref target="RFC4271" format="default"/>  without change. This includes the BGP protocol
  Finite State Machine, BGP messages and their encodings, the processing of BGP messages,
  BGP attributes and path attributes, BGP NLRI encodings, and any error handling
  defined in <xref target="RFC4271" format="default"/>, <xref target="RFC4760" format="default"/>,
  and <xref target="RFC7606" format="default"/>.
</t>
<t>
  Due to the changes to in the decision
  process, there are mechanisms and encodings that are no longer applicable.
  Unless explicitly specified in the context of BGP SPF, all optional path
  attributes SHOULD NOT <bcp14>SHOULD NOT</bcp14> be advertised.  If received, all path attributes MUST <bcp14>MUST</bcp14>
  be accepted, validated, and propagated consistent consistently with the BGP protocol
  <xref target="RFC4271"/>, even if not needed by BGP SPF.
</t>
<t>
  Section 9.1 of
  <xref section="9.1" target="RFC4271" format="default"/> defines the decision process that
  is used to select routes for subsequent advertisement
  by applying the policies in the local Policy Information Base (PIB) to the
  routes stored in its Adj-RIBs-In. The output of the Decision Process is the
  set of routes that are announced by a BGP speaker to its peers. These
  selected routes are stored by a BGP speaker in the speaker's Adj-RIBs-Out Adj-RIBs-Out,
  according to policy.
</t>
<t>
  The BGP SPF extension fundamentally changes the decision process, as described
  herein. Specifically:
</t>
<ol spacing="normal" type="1">
  <li>
    BGP advertisements are readvertised to neighbors immediately without waiting
    or dependence on the route computation computation, as specified in phase 3 of the base BGP
    decision process. Multiple peering models are supported supported, as specified in
    <xref target="peering-models" format="default"/>.
</li>
<li>

<!--[rfced] May we rephrase this sentence as follows so that it parses
and is parallel with the third entry in the list?

Original:
   Determining the degree of preference for BGP routes for the SPF
   calculation as described in phase 1 of the base BGP decision
   process is replaced with the mechanisms in Section 6.1.

Perhaps:
   Phase 1 of the base BGP decision process, which determines the
   degreee of preferencce for BGP routes for the SPF calculation,
   is replaced with the mechanisms in Section 6.1.
-->

  Determining the degree of preference for BGP routes for the SPF calculation as
  described in phase 1 of the base BGP decision process is replaced with the mechanisms
  in <xref target="Phase-1" format="default"/>.
</li>
<li>
  Phase 2 of the base BGP protocol decision process is replaced with the
  Shortest Path First (SPF)
  SPF algorithm, also known as the Dijkstra algorithm.
</li>
</ol>
</section>
<!-- for BGP relationship section -->

<section anchor="BGP-LS" numbered="true" toc="default">
<name>BGP - Link-State (BGP-LS) Relationship</name>
<t>
  <xref target="RFC9552" format="default"/> describes a mechanism by
  which link-state and Traffic Engineering (TE) information can be collected from networks and shared with external
  entities using BGP.
  This is achieved by defining NLRI NLRIs that are advertised using the BGP-LS AFI. The BGP-LS extensions defined in
  <xref target="RFC9552" format="default"/> make use of the decision process defined in
  <xref target="RFC4271" format="default"/>.  Rather than reusing the BGP-LS SAFI, the BGP-LS-SPF SAFI
  (<xref target="SAFI" format="default"/>) is introduced to ensure backward compatibility
  for the BGP-LS SAFI usage.
</t>
<t>
  The "BGP-LS NLRI and Attribute TLVs" registry <xref target="RFC9552"/> is shared between
  the BGP-LS SAFI and the BGP-LS-SPF SAFI.
  However, the TLVs defined in this document may not be applicable to
  the BGP-LS SAFI.  As specified in Section 5.1 of <xref section="5.1" target="RFC9552"/>, the presence
  of unknown or unexpected TLVs is required to not result in so that the NLRI or
  the
  BGP-LS Attribute being will not be considered
  malformed (section 5.2 of <xref (<xref section="5.2" target="RFC9552"/>).  The list of BGP-LS TLVs applicable
  to the BGP-LS-SPF SAFI are described in
  <xref target="NLRI-Use"/>. By default, the usage of other BGP-LS TLVs or
  extensions are ignored for the BGP-LS-SPF SAFI. However, this doesn't preclude the usage
  specification of these TLVs for the BGP-LS-SPF SAFI in future documents.
</t>
</section>
<!-- for BGP-LS relationship section -->

<section anchor="peering-models" numbered="true" toc="default">
<name>BGP SPF Peering Models</name>
<t>
  Depending on the topology, scaling, capabilities of the BGP speakers, and redundancy
  requirements, various peering models are supported. The only requirement is that all BGP
  speakers in the BGP SPF routing domain adhere to this specification.
</t>
<t>
  The choice of the deployment model is up to the operator and their requirements and policies.
  Deployment model choice is out of scope for this document and is discussed in
  <xref target="I-D.ietf-lsvr-applicability" format="default"/>. The sub-sections below
  describe several BGP SPF deployment models. However, this doesn't preclude other
  deployment models.
</t>
<section anchor="single-hop-peering" numbered="true" toc="default">
<name>BGP Single-Hop Peering on Network Node Connections</name>
<t>
  The simplest peering model is the one where
  EBGP
  External BGP (EBGP) single-hop sessions are established over direct point-to-point links
  interconnecting the nodes in the BGP SPF routing domain.

<!--[rfced] In RFC 4760, the term "Multiprotocol Extensions
capabilities" is used rather than "Multi-Protocol Extensions
Capability". We have updated the text below to reflect
this. Note that there is another instance in Section 5.1.
Please let us know if these changes are not correct.

Original:
   Once the single-hop BGP session has been established and the
   Multi-Protocol Extensions Capability with the BGP-LS-SPF AFI/SAFI
   has been exchanged [RFC4760] for the corresponding session...

Current:
   Once the single-hop BGP session has been established and the
   Multiprotocol Extensions capabilities have been exchanged with
   the BGP-LS-SPF AFI/SAFI [RFC4760] for the corresponding session...
-->

  Once the single-hop BGP session has been
  established and the Multiprotocol Extensions capabilities have been exchanged with the BGP-LS-SPF AFI/SAFI
  <xref target="RFC4760" format="default"/> for the corresponding session, then the link is considered up and available from
  a BGP SPF perspective perspective, and the corresponding BGP-LS-SPF Link NLRI is advertised.
</t>
<t>
  An End-of-RIB (EoR) Marker marker (<xref target="BGP-LS-SPF-EOR"/>) for the BGP-LS-SPF
  SAFI MAY <bcp14>MAY</bcp14> be required from a peer prior to advertising the BGP-LS-SPF Link NLRI
  for the corresponding link to that peer.

<!--[rfced] The following sentence does not parse - are some words
perhaps missing after "default"? Please let us know how we may
rephrase for clarity.

Original:
   When required, the default wait indefinitely for the EoR Marker prior
   to advertising the BGP-LS-SPF Link NLRI.  Refer to Section 10.4.
-->

  When required, the default
  wait indefinitely for the EoR marker prior to advertising the BGP-LS-SPF Link NLRI.
  Refer to <xref target="Adjacency-EoR-Required"/>.
</t>
<t>
  A failure to consistently configure the use of the EoR marker can
  result in transient micro-loops and dropped traffic due to incomplete
  forwarding state.
</t>
<t>
  If the session goes down, the corresponding Link NLRI NLRIs are withdrawn. Topologically,
  this would be equivalent to the peering model in <xref target="RFC7938" format="default"/> where there
  is a BGP session on every link in the data center switch fabric.  The content of the Link NLRI
  is described in <xref target="Link-NLRI" format="default"/>.
</t>
</section>
<section numbered="true" toc="default">
<name>BGP Peering Between Directly-Connected Directly Connected Nodes</name>
<t>
  In this model, BGP speakers peer with all directly-connected directly connected
  nodes but the sessions may be between loopback addresses (i.e.,
  two-hop sessions) sessions), and the direct connection
  discovery and liveness detection for the interconnecting links are
  independent of the BGP protocol. The BFD Bidirectional Forwarding Detection (BFD) protocol <xref target="RFC5880" format="default"/>
  is RECOMMENDED <bcp14>RECOMMENDED</bcp14> for liveness detection. Usage of other liveness connection mechanisms
  is outside the scope of this document.
  Consequently, there is a single BGP session even if there are multiple
  direct connections between BGP speakers. The BGP-LS-SPF Link NLRI is advertised
  as long as a BGP session has been established, the BGP-LS-SPF AFI/SAFI
  capability has been exchanged <xref target="RFC4760" format="default"/>,
  the link is operational as determined using liveness detection mechanisms,
  and, optionally, the EoR Marker marker has been received as described in the
  <xref target="BGP-LS-SPF-EOR"/>.
  This is much like the previous peering model only model, except peering is between
  loopback addresses and the interconnecting links can be unnumbered. However,
  since there are BGP sessions between every directly-connected directly connected node in the
  BGP SPF routing domain, there is a reduction in BGP sessions when there
  are parallel links between nodes. Hence, this peering model is RECOMMENDED <bcp14>RECOMMENDED</bcp14>
  over the single-hop peering model <xref target="single-hop-peering"/>.
</t>
</section>
<section numbered="true" toc="default">
<name>BGP Peering in Route-Reflector or Controller Topology</name>
<t>
  In this model, BGP speakers peer solely with one or more Route Reflectors route reflectors
  <xref target="RFC4456" format="default"/> or controllers. As in the previous model, direct
  connection discovery and liveness detection for those links in the BGP
  SPF routing domain are done outside of the BGP protocol.
  BGP-LS-SPF Link NLRI is advertised as long as the corresponding link is
  considered up and available as per the chosen liveness detection mechanism (The (thus, the BFD protocol
  <xref target="RFC5880" format="default"/> is RECOMMENDED). <bcp14>RECOMMENDED</bcp14>).
</t>
<t>
  This peering model, known as sparse peering, "sparse peering", allows for fewer BGP sessions
  and, consequently, fewer instances of the same NLRI received from multiple peers.
  Ideally, the route-reflectors route reflectors or controller BGP sessions would be on directly-connected directly connected
  links to avoid dependence on another routing protocol for session connectivity. However,
  multi-hop peering is not precluded. The number of BGP sessions is dependent
  on the redundancy requirements and the stability of the BGP sessions.
</t>
<t>
  The controller may use constraints to determine
  when to advertise BGP-LS-SPF NLRI for BGP-LS peers. For example, a controller
  may delay advertisement of a link between two peers the until the EoR
   marker <xref target="BGP-LS-SPF-EOR"/> has been
  received from both BGP peers and the BGP-LS Link NLRI for the link(s) between the two nodes
  have
  has been received from both BGP peers.
</t>
</section>
</section>

<!--[rfced] May we update the title of Section 5 to reflect "Shortest
Path Forward (SPF)" instead of "Shortest Path Routing (SPF)" for
consistency? And may we remove the SPF expansion in the title of
Section 5.1 since it was expanded in the title of Section 5?

Original:
   5.  BGP Shortest Path Routing (SPF) Protocol Extensions . . .  9
   5.1.  BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . .  10

Perhaps:
   5.  BGP Shortest Path First (SPF) Protocol Extensions . . . . . 9
   5.1.  BGP-LS SPF SAFI . . . . . . . . . . . . . . . . . . . . . 10
-->

<section anchor="protocol-extend" numbered="true" toc="default">
<name>BGP Shortest Path Routing (SPF) Protocol Extensions</name>
<section anchor="SAFI" numbered="true" toc="default">
<name>BGP-LS Shortest Path Routing (SPF) SAFI</name>
<t>
  This document introduces the BGP-LS-SPF SAFI with a value of 80.
  The SPF-based decision process (Section 6) (<xref target="bgp-decision"/>) applies only to the
  BGP-LS-SPF SAFI and MUST NOT <bcp14>MUST NOT</bcp14> be used with other combinations of
  the BGP-LS AFI (16388).
  In order for two BGP speakers to
  exchange BGP-LS-SPF NLRI, they MUST <bcp14>MUST</bcp14> exchange the Multiprotocol
  Extensions Capability capabilities <xref target="RFC4760" format="default"/>
  to ensure that they are both capable of properly processing such an
  NLRI.  This is done with AFI 16388 / SAFI 80.  The BGP-LS-SPF SAFI
  is used to advertise IPv4 and IPv6 prefix information in a
  format facilitating an SPF-based decision process.
</t>
<section anchor="BGP-LS-TLV" numbered="true" toc="default">
<name>BGP-LS-SPF NLRI TLVs</name>
<t>
  All the TLVs defined for BGP-LS <xref target="RFC9552" format="default"/>
  are applicable and can be used with the BGP-LS-SPF SAFI to describe links, nodes,
  and prefixes comprising BGP-SPF LSDB BGP SPF Link-State Database (LSDB) information.
</t>
<t>
  The NLRI and comprising TLVs MUST <bcp14>MUST</bcp14> be encoded as specified in
  section 5.1
  <xref section="5.1" target="RFC9552" format="default"/>. TLVs specified as
  mandatory in <xref target="RFC9552" format="default"/> are
  considered mandatory for the BGP-LS-SPF SAFI as
  well. If a mandatory TLV is not present, the NLRI MUST NOT <bcp14>MUST NOT</bcp14> be used in the
  BGP SPF route calculation. All the other TLVs are considered as optional TLVs. Documents
  specifying usage of optional TLV TLVs for BGP SPF MUST <bcp14>MUST</bcp14> address backward compatibility.
</t>
</section>
<section numbered="true" toc="default">
<name>BGP-LS Attribute</name>
<t>
  The BGP-LS attribute of the BGP-LS-SPF SAFI uses exactly the exact same format of as the BGP-LS AFI
  <xref target="RFC9552" format="default"/>. In
  other words, all the TLVs used in the BGP-LS attribute of the BGP-LS AFI are applicable
  and are used for the BGP-LS attribute of the BGP-LS-SPF SAFI. This attribute is an optional,
  non-transitive BGP attribute that is used to carry link, node, and prefix
  properties and attributes. The BGP-LS attribute is a set of TLVs.
</t>
<t>
  All the TLVs defined for the BGP-LS Attribute  <xref target="RFC9552" format="default"/>
  are applicable and can be used with the BGP-LS-SPF SAFI to carry link, node, and prefix
  properties and attributes.
</t>
<t>
  The BGP-LS attribute may potentially be quite large depending on
  the amount of link-state information associated with a single BGP-LS-SPF NLRI.
  The BGP specification <xref target="RFC4271" format="default"/> mandates a maximum BGP
  message size of 4096 octets.  It is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that an
  implementation support <xref target="RFC8654" format="default"/> in order to accommodate a greater
  amount of information within the BGP-LS Attribute.  BGP speakers MUST <bcp14>MUST</bcp14>
  ensure that they limit the TLVs included in the BGP-LS Attribute to
  ensure that a BGP update message for a single BGP-LS-SPF NLRI does
  not cross the maximum limit for a BGP message.  The determination of
  the types of TLVs to be included by the BGP speaker
  originating the attribute is outside the scope of this document.
  If, due to the limits on the maximum size of an UPDATE message, a single
  route doesn't fit into the message, the BGP speaker MUST NOT <bcp14>MUST NOT</bcp14> advertise the
  route to its peer and MAY <bcp14>MAY</bcp14> choose to log an error locally <xref target="RFC4271"/>.
</t>
</section>
</section>
<section anchor="NLRI-Use" numbered="true" toc="default">
<name>Extensions to BGP-LS</name>
<t>
  <xref target="RFC9552" format="default"/> describes a mechanism
  by which link-state and TE
  information can be collected from IGPs and shared with external components
  using the BGP protocol. It describes both the definition of the BGP-LS NLRI
  that advertise links, nodes, and prefixes comprising IGP link-state
  information and the definition of a BGP path attribute (BGP-LS
  attribute) that carries link, node, and prefix properties and
  attributes, such as the link and prefix metric or auxiliary
  Router-IDs of nodes, etc. This document extends the usage of BGP-LS NLRI for
  the purpose of BGP SPF calculation via advertisement in the BGP-LS-SPF SAFI.
</t>
<t>
  The protocol identifier specified in the Protocol-ID field
  <xref target="RFC9552" format="default"/>
  represents the origin of the advertised NLRI. For Node NLRI and Link NLRI,
  the specified Protocol-ID MUST <bcp14>MUST</bcp14> be the direct protocol (4). Node or Link NLRI with a
  Protocol-ID other than the direct protocol is considered malformed.
  For Prefix NLRI, the specified Protocol-ID
  MUST
  <bcp14>MUST</bcp14> be the origin of the prefix. The local Local and remote node descriptors Remote Node Descriptors for all NLRI MUST <bcp14>MUST</bcp14>
  include the BGP Router-ID (TLV 516) <xref target="RFC9086"/>
  and the AS Number Autonomous System (TLV 512) number
  <xref target="RFC9552" format="default"/>.
  The BGP Confederation Member (TLV 517)
  <xref target="RFC9086" format="default"/> is not applicable.
</t>
<section numbered="true" toc="default">
<name>Node NLRI Usage</name>
<t>
  The Node NLRI MUST <bcp14>MUST</bcp14> be advertised unconditionally by all routers in
  the BGP SPF routing domain.
</t>
<section anchor="node-status-tlv" numbered="true" toc="default">
<name>BGP-LS-SPF Node NLRI Attribute SPF Status TLV</name>
<t>
  A BGP-LS Attribute SPF Status TLV of the BGP-LS-SPF Node NLRI is defined to indicate the status of
  the node with respect to the BGP SPF calculation. This is used to rapidly take a
  node out of service (refer to <xref target="node-failure" format="default"/>)
  or to indicate that the node is not to be
  used for transit (i.e., non-local) traffic (refer to <xref target="BGP-SPF" format="default"/>).
  If the SPF Status TLV is not included with the Node NLRI, the node is considered to be up
  and is available for transit traffic. A single TLV type is shared by the Node, Link, and
   Prefix NLRI.  The TLV type is 1184.
</t>
<artwork align="left" name="" type="" alt=""><![CDATA[
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Type (1184)                 |       Length (1 Octet)        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status    |
+-+-+-+-+-+-+-+-+

SPF
]]></artwork>

<table>
  <name>SPF Status Values: 0 - Reserved
                   1 - Node Values</name>
  <thead>
    <tr><th>Value</th><th>Description</th></tr>
  </thead>
  <tbody>
    <tr><td>0</td><td>Reserved</td></tr>
    <tr><td>1</td><td>Node unreachable with respect to BGP SPF
                   2 - Node SPF</td></tr>
    <tr><td>2</td><td>Node does not support transit with respect to BGP SPF
                   3-254 - Undefined
                   255 - Reserved

]]></artwork> SPF</td></tr>
    <tr><td>3-254</td><td>Unassigned</td></tr>
    <tr><td>255</td><td>Reserved</td></tr>
  </tbody>
</table>

<t>
  If a BGP speaker received the Node NLRI but
  the SPF Status TLV is not received, then any previously received SPF status information is
  considered as implicitly withdrawn withdrawn, and the NLRI is propagated to other BGP speakers.
  A BGP speaker receiving a BGP Update containing
  an SPF Status TLV in the BGP-LS attribute <xref target="RFC9552" format="default"/>
  with an unknown value SHOULD <bcp14>SHOULD</bcp14> be advertised to other
  BGP speakers and MUST <bcp14>MUST</bcp14> ignore the Status TLV with an unknown value in
  the SPF computation.
  An implementation MAY <bcp14>MAY</bcp14> log this condition for further analysis.
  If the SPF Status TLV contains a reserved value (0 or 255) 255), the TLV is considered malformed and
  is handled as described in <xref target="new-TLVs"/>.
</t>
</section>
</section>
<section anchor="Link-NLRI" numbered="true" toc="default">
<name>Link NLRI Usage</name>
<t>
  The criteria for advertisement of Link NLRI NLRIs are discussed in
  <xref target="peering-models" format="default"/>.
</t>
<t>
  Link NLRI is advertised with unique local Local and remote node descriptors Remote Node Descriptors
  dependent on the IP addressing.

<!--[rfced] FYI - We updated the following text to reflect the TLV
names per RFC 9552.

Original:
   For IPv4 links, the link's local IPv4 (TLV 259) and remote IPv4
   (TLV 260) addresses are used.  For IPv6 links, the local IPv6
   (TLV 261) and remote IPv6 (TLV 262) addresses are used (Section
   5.2.2 of <xref [RFC9552]).

Current:
   For IPv4 links, the link's local IPv4 interface address (TLV 259)
   and remote IPv4 neighbor address (TLV 260) are used.  For IPv6
   links, the local IPv6 interface address (TLV 261) and remote IPv6
   neighbor address (TLV 262) are used (Section 5.2.2 of [RFC9552]).
-->

  For IPv4 links, the
  link's local IPv4 interface address (TLV 259) and remote IPv4 neighbor address (TLV 260) are used.
  For IPv6 links, the local IPv6 interface address (TLV 261) and remote IPv6 neighbor address (TLV 262)
  are used (<xref section="5.2.2" target="RFC9552"/>). IPv6 links without
  global IPv6 addresses are considered unnumbered links and are handled as
  described below.
  For links supporting having both IPv4 and IPv6 addresses, both sets
  of descriptors MAY <bcp14>MAY</bcp14> be included in the same Link NLRI.
</t>
<t>
  For unnumbered links, the Link Local/Remote Identifiers (TLV 258)
  are used. The Link Remote Identifier isn't normally exchanged in BGP BGP,
  and discovering the Link Remote Identifier is beyond the scope of this
  document. If the Link Remote Identifier is unknown, a Link Remote Identifier of
  0 MUST <bcp14>MUST</bcp14> be advertised. When 0 is advertised and there are parallel unnumbered links
  between a pair of BGP speakers, there may be transient intervals where the
  BGP speakers don't agree on which of the parallel unnumbered links are operational.
  For this reason, it is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that the Link Remote Identifiers be
  known (e.g., discovered using alternate mechanisms or configured) in the presence
  of parallel unnumbered links.
</t>
<t>
  The link descriptors are
  described in table Table 4 of <xref target="RFC9552" format="default"/>.
  Additionally, the Address Family Link Descriptor TLV is defined to determine whether an
  unnumbered link can be used in the IPv4 SPF, the IPv6, or both (refer to
  <xref target="af-link-descriptor-tlv"/>).
</t>
<t>

  For a link to be used in SPF computation for a given address family,
  i.e., IPv4 or IPv6, both routers connecting the link MUST <bcp14>MUST</bcp14> have matching addresses (i.e.,
  router interface addresses must be on the same subnet for numbered interfaces interfaces, and the
  local/remote link identifiers (<xref target="BGP-SPF"/>) must match for unnumbered
  interfaces).
</t>
<t>
  The IGP metric attribute TLV Metric (TLV 1095) MUST <bcp14>MUST</bcp14> be advertised. If a BGP speaker
  receives a Link NLRI without an IGP metric Metric attribute TLV, then it MUST <bcp14>MUST</bcp14> consider
  the received NLRI as a malformed (refer to <xref target="error-handling"/>).
  The BGP SPF metric length is 4 octets. A metric is associated with the output side of each
  router interface.  This metric is configurable by the system administrator.  The
  lower the metric, the more likely the interface is to be used to forward data traffic.
  One possible default for the  metric would be to give each interface a metric of 1
  making it effectively a hop count.
</t>
<t>
  The usage of other link attribute TLVs is beyond the scope of this document.
</t>
<section anchor="af-link-descriptor-tlv" numbered="true" toc="default">
  <name>BGP-LS Link NLRI Address Family Link Descriptor TLV</name>
  <t>

<!--[rfced] Section 5.2.2.1. For consistency, should instances of
"Address Family Link Descriptor" include "TLV" (i.e., "Address
Family Link Descriptor TLV") in the following paragraph (as the
latter part of the sentence (not shown) includes it)?

Original:
   For unnumbered links, the address family cannot be ascertained from
   the endpoint link descriptors.  Hence, the Address Family (AF) Link
   Descriptor SHOULD be included with the Link Local/Remote Identifiers
   TLV for unnumbered links, so that the link can be used in the
   respective address family SPF.  If the Address Family Link Descriptor
   is not present for an unnumbered link, the link will not be used in
   the SPF computation for either address family.  If the Address Family
   Link Descriptor is present for a numbered link, the link descriptor
   will be ignored.
-->

    For unnumbered links, the address family cannot be ascertained from the
    endpoint link descriptors. Hence, the Address Family Link Descriptor <bcp14>SHOULD</bcp14>
    be included with the Link Local/Remote Identifiers TLV for unnumbered links,
    so that the link can be used in the respective address family SPF. If the
    Address Family Link
    Descriptor is not present for an unnumbered link, the link will not be
    used in the SPF computation for either address family. If the Address Family Link
    Descriptor is present for a numbered link, the link descriptor
    will be ignored. If the Address Family Link Descriptor TLV contains an
    undefined value (3-254), the link descriptor will be ignored.
    If the Address Family Link Descriptor TLV contains a
    reserved value (0 or 255) 255), the TLV is considered malformed and
    is handled as described in <xref target="new-TLVs"/>.
  </t>
  <t>
    Note that an unnumbered link can be used for both the IPv4 and IPv6
    SPF computation by advertising separate Address Family Link Descriptor
    TLVs for IPv4 and IPv6.
  </t>
  <artwork align="left" name="" type="" alt=""><![CDATA[
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Type (1185)                 |      Length (1 Octet)         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Address Family|
  +-+-+-+-+-+-+-+-+

  Address
]]></artwork>

  <table>
    <name>Address Family Values: 0 - Reserved
                         1 - IPv4 Values</name>
    <thead>
      <tr><th>Value</th><th>Description</th></tr>
    </thead>
    <tbody>
      <tr><td>0</td><td>Reserved</td></tr>
      <tr><td>1</td><td>IPv4 Address Family
                         2 - IPv6 Family</td></tr>
      <tr><td>2</td><td>IPv6 Address Family
                         3-254 - Undefined
                         255 - Reserved

  ]]></artwork> Family</td></tr>
      <tr><td>3-254</td><td>Undefined</td></tr>
      <tr><td>255</td><td>Reserved</td></tr>
    </tbody>
  </table>

</section>
<section anchor="link-status-tlv" numbered="true" toc="default">
  <name>BGP-LS-SPF Link NLRI Attribute SPF Status TLV</name>
  <t>
    This
    The BGP-LS-SPF Attribute TLV of the BGP-LS-SPF Link NLRI is defined to
    indicate the status of the link with respect to the BGP SPF calculation. This is used to expedite
    convergence for link failures as discussed in <xref target="failure-converge" format="default"/>. If the
    SPF Status TLV is not included with the Link NLRI, the link is considered
    up and available. The SPF status is acted upon with the execution of the
    next SPF calculation <xref (<xref target="BGP-SPF" format="default"/>. format="default"/>).
    A single TLV type is shared by the Node, Link, and Prefix NLRI.
    The TLV type is 1184.
  </t>
  <artwork align="left" name="" type="" alt=""><![CDATA[
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Type (1184)                 |      Length (1 Octet)         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | SPF Status    |
  +-+-+-+-+-+-+-+-+

  BGP
]]></artwork>

  <table>
    <name>BGP Status Values: 0 - Reserved
                     1 - Link Unreachable Values</name>
    <thead>
      <tr><th>Value</th><th>Description</th></tr>
    </thead>
    <tbody>
      <tr><td>0</td><td>Reserved</td></tr>
      <tr><td>1</td><td>Link unreachable with respect to BGP SPF
                     2-254 - Undefined
                     255 - Reserved

  ]]></artwork> SPF</td></tr>
      <tr><td>2-254</td><td>Unassigned</td></tr>
      <tr><td>255</td><td>Reserved</td></tr>
    </tbody>
  </table>
  <t>
    If a BGP speaker received the Link NLRI but
    the SPF Status TLV is not received, then any previously received SPF status information is
    considered as implicitly withdrawn withdrawn, and the NLRI is propagated to other BGP speakers.
    A BGP speaker receiving a BGP Update containing
    an SPF Status TLV in the BGP-LS attribute <xref target="RFC9552" format="default"/>
    with an unknown value SHOULD <bcp14>SHOULD</bcp14> be advertised to other
    BGP speakers and MUST <bcp14>MUST</bcp14> ignore the SPF Status TLV with an unknown value in
    the SPF computation.
    An implementation MAY <bcp14>MAY</bcp14> log this information for further analysis.
    If the SPF Status TLV contains a reserved value (0 or 255) 255), the TLV is considered malformed and
    is handled as described in <xref target="new-TLVs"/>.
  </t>
</section>
</section>
<section anchor="Prefix-NLRI" numbered="true" toc="default">
  <name>IPv4/IPv6 Prefix NLRI Usage</name>
  <t>
    A IPv4/IPv6 Prefix NLRI is advertised with a Local Node Descriptor and
    the prefix and length. The Prefix Descriptors Descriptor field includes the IP Reachability
    Information TLV (TLV 265) as described in <xref target="RFC9552" format="default"/>.
    The Prefix Metric TLV (TLV 1155) MUST <bcp14>MUST</bcp14> be advertised to be considered for route calculation. The IGP Route Tag TLV (TLV 1153) MAY <bcp14>MAY</bcp14> be advertised. The usage of other BGP-LS
    attribute TLVs is beyond the scope of this document.
  </t>
  <section anchor="prefix-status-tlv" numbered="true" toc="default">
    <name>BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV</name>
    <t>
      A BGP-LS Attribute SPF Status TLV of the BGP-LS-SPF Prefix NLRI is defined to indicate the status of
      the prefix with respect to the BGP SPF calculation. This is used to expedite
      convergence for prefix unreachability unreachability, as discussed in <xref target="failure-converge" format="default"/>.
      If the SPF Status TLV is not included with the Prefix NLRI, the prefix is considered
      reachable.
      A single TLV type is shared by the Node, Link, and Prefix NLRI.
      The TLV type is 1184.
    </t>
    <artwork align="left" name="" type="" alt=""><![CDATA[
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type (1184)                 |      Length (1 Octet)         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | SPF Status    |
    +-+-+-+-+-+-+-+-+
    ]]></artwork>

<!--[rfced] We updated the description for BGP status value "1" (in
Section 5.2.3.1) for consistency with IANA's "BGP-LS-SPF Prefix
NLRI Attribute SPF Status TLV Status" registry
<https://www.iana.org/assignments/bgp-spf/>, as shown below.

We also placed the information in a table to match the formatting of
similar text in Section 5.2.2.2. Tables 3 and 4 are both titled
"BGP Status Values". Would you like to update one of the titles to
differentiate the tables?

Original:
 BGP Status Values:
   0 -  Reserved
   1 -  Prefix Unreachable with respect to SPF
   2-254 -  Undefined
   255 -  Reserved
    ]]></artwork>

Current:
   +=======+============================================+
   | Value | Description                                |
   +=======+============================================+
   | 0     | Reserved                                   |
   + - - - + - - - - - - - - - - - - - - - - - - - -  - +
   | 1     | Prefix unreachable with respect to BGP SPF |
   + - - - + - - - - - - - - - - - - - - - - - - - - - -+
   | 2-254 | Unassigned                                 |
   + - - - + - - - - - - - - - - - - - - - - - - - - - -+
   | 255   | Reserved                                   |
   + - - - + - - - - - - - - - - - - - - - - - - - - - -+

              Table 4: BGP Status Values
-->

 <table>
    <name>BGP Status Values</name>
    <thead>
      <tr><th>Value</th><th>Description</th></tr>
    </thead>
    <tbody>
      <tr><td>0</td><td>Reserved</td></tr>
      <tr><td>1</td><td>Prefix unreachable with respect to BGP SPF</td></tr>
      <tr><td>2-254</td><td>Unassigned</td></tr>
      <tr><td>255</td><td>Reserved</td></tr>
    </tbody>
  </table>
    <t>
      If a BGP speaker received the Prefix NLRI but
      the SPF Status TLV is not received, then any previously received SPF status information is
      considered as implicitly withdrawn withdrawn, and the NLRI is propagated to other BGP speakers.
      A BGP speaker receiving a BGP Update containing
      an SPF Status TLV in the BGP-LS attribute <xref target="RFC9552" format="default"/>
      with an unknown value SHOULD <bcp14>SHOULD</bcp14> be advertised to other
      BGP speakers and MUST <bcp14>MUST</bcp14> ignore the Status TLV with an unknown value in
      the SPF computation.
      An implementation MAY <bcp14>MAY</bcp14> log this information for further analysis.
      If the SPF Status TLV contains a reserved value (0 or 255) 255), the TLV is considered malformed and
      is handled as described in <xref target="new-TLVs"/>.
    </t>
  </section>
</section>
<section anchor="sequence-number-tlv" numbered="true" toc="default">
  <name>BGP-LS Attribute Sequence Number TLV</name>
  <t>
    A BGP-LS Attribute Sequence Number TLV of the BGP-LS-SPF NLRI types is defined to assure the most
    recent version of a given NLRI is used in the SPF computation. The Sequence Number TLV is
    mandatory for BGP-LS-SPF NLRI.
    The TLV type 1181 has been assigned by IANA. The BGP-LS
    Attribute Sequence Number TLV contains an 8-octet sequence number. The usage of the
    Sequence Number TLV is described in <xref target="Phase-1" format="default"/>.
  </t>
  <artwork align="left" name="" type="" alt=""><![CDATA[
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Type (1181)                 |      Length (8 Octets)        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                Sequence Number (High-Order 32 Bits)           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                Sequence Number (Low-Order 32 Bits)            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ]]></artwork>
  <t>
    Sequence Number:
    The 64-bit strictly-increasing strictly increasing sequence number MUST <bcp14>MUST</bcp14> be incremented for every
    self-originated version of a BGP-LS-SPF NLRI. BGP speakers implementing this specification
    MUST
    <bcp14>MUST</bcp14> use available mechanisms to preserve the sequence number's strictly increasing property
    for the deployed life of the BGP speaker (including cold restarts).
    One mechanism for accomplishing this would be to use the high-order 32 bits of the
    sequence number as a wrap/boot count that is incremented any time the BGP router
    loses its sequence number state or the low-order 32 bits wrap.
  </t>
  <t>
    When incrementing the sequence number for each self-originated NLRI,
    the sequence number should be treated as an unsigned 64-bit
    value. If the lower-order 32-bit value wraps, the higher-order 32-bit value should
    be incremented and saved in non-volatile storage. If a BGP speaker completely
    loses its sequence number state (e.g., the BGP speaker hardware
    is replaced or experiences a cold-start), cold start), the BGP NLRI selection rules
    (see <xref target="Phase-1" format="default"/>) ensure convergence, albeit not immediately.
  </t>
  <t>
    If the Sequence Number TLV
    is not received, then the corresponding NLRI is considered as malformed and
    MUST
    <bcp14>MUST</bcp14> be handled as 'Treat-as-withdraw'. 'treat-as-withdraw'. An implementation SHOULD <bcp14>SHOULD</bcp14> log an error for
    further analysis.
  </t>
</section>
</section>
 <section anchor="BGP-LS-SPF-EOR" numbered="true" toc="default">
 <name>BGP-LS-SPF End of RIB (EoR) Marker</name>
 <t>
   The usage of the End-of-RIB (EoR) Marker EoR marker <xref target="RFC4724"/> with the BGP-LS-SPF
   SAFI is somewhat different than the other BGP SAFIs. Reception of the EoR
   marker MAY <bcp14>MAY</bcp14> optionally be expected prior to advertising an LINK-NLRI a Link NLRI for a given peer.
 </t>
</section>
<section anchor="NEXT-HOP" numbered="true" toc="default">
  <name>BGP Next-Hop Information</name>
  <t>
    The rules for setting the BGP Next-Hop in the MP_REACH_NLRI attribute <xref target="RFC4760"/>
    for the BGP-LS-SPF SAFI follow the rules in section 5.5 of <xref section="5.5" target="RFC9552" format="default"/>.
    All BGP peers that support SPF extensions will locally compute the Local-RIB Next-Hop
    as a result of the SPF process. Hence, the use of the MP_REACH_NLRI Next-Hop as a tiebreaker in the
    standard BGP path decision processing is not applicable.
  </t>
</section>
</section>
<section anchor="bgp-decision" numbered="true" toc="default">
  <name>Decision Process with the SPF Algorithm</name>
  <t>
    The Decision Process described in <xref target="RFC4271" format="default"/> takes place in
    three distinct phases. The Phase 1 decision function of the Decision Process is
    responsible for calculating the degree
    of preference for each route received from a BGP speaker's peer. The Phase 2 decision
    function is invoked on completion of the Phase 1 decision function and is responsible
    for choosing the best route out of all those available for each
    distinct destination, destination and for installing each chosen route into the Local-RIB.
    The combination of the Phase 1 and 2 decision functions is characterized as
    a Path Vector algorithm.
  </t>
  <t>
    The SPF-based Decision process Process replaces the BGP Decision process Process described in
    <xref target="RFC4271" format="default"/>.
    Since BGP-LS-SPF NLRI always contains the local node descriptor Local Node Descriptor as described in
    <xref target="NLRI-Use" format="default"/>, each NLRI is uniquely originated by a single
    BGP speaker in the BGP SPF routing domain (the BGP node matching the NLRI's Node
    Descriptors). Instances of the same NLRI originated by multiple BGP speakers would be
    indicative of a configuration error or a masquerading attack
    (refer to <xref target="Security" format="default"/>).
    These selected Node NLRI NLRIs and their Link/Prefix NLRI NLRIs are used to build a directed
    graph during the SPF computation as described below. The best routes for BGP prefixes
    are installed in the RIB as a result of the SPF process.
</t>
<t>
  When BGP-LS-SPF NLRI is received, all that is required is to determine
  whether it is the most recent by examining the Node-ID and sequence number as described
  in <xref target="Phase-1" format="default"/>. If the received NLRI has changed, it is advertised
  to other BGP-LS-SPF peers. If the attributes have changed (other than the sequence number),
  a BGP SPF calculation is triggered. However, a changed NLRI MAY <bcp14>MAY</bcp14> be advertised immediately
  to other peers and prior to any SPF calculation. Note that the BGP
  MinASOriginationIntervalTimer <xref target="RFC4271" format="default"/> timer is not applicable
  to the BGP-LS-SPF SAFI. The MinRouteAdvertisementIntervalTimer is applicable with a suggested default
  of 5 seconds consistent with Internal BGP (IBGP) (refer to section 10 of <xref section="10" target="RFC4271"/>).

<!--[rfced] We note that "SPF Back-Off algorithm" is called the "SPF
Back-Off Delay algorithm" in RFC 8405. We updated the text below
for consistency. Please let us know of any objections.

Original:
   The scheduling of the SPF calculation, as described in Section 6.3, is an
   implementation and/or configuration matter.  Scheduling MAY be dampened
   consistent with the SPF back-off algorithm specified in [RFC8405].

Current:
   The scheduling of the SPF calculation, as described in Section 6.3, is an
   implementation and/or configuration matter.  Scheduling MAY be dampened
   consistent with the SPF Back-Off Delay algorithm specified in [RFC8405].
-->

  The scheduling of the SPF calculation, as described in
  <xref target="BGP-SPF" format="default"/>, is an implementation and/or configuration matter.
  Scheduling MAY <bcp14>MAY</bcp14> be dampened consistent with the SPF back-off Back-Off Delay algorithm
  specified in <xref target="RFC8405" format="default"/>.
</t>
<t>
  The Phase 3 decision function
  of the Decision Process <xref target="RFC4271" format="default"/> is also simplified since because under
  normal SPF operation, a BGP speaker MUST <bcp14>MUST</bcp14> advertise the changed NLRIs
  to all BGP peers with the BGP-LS-SPF AFI/SAFI and install the changed routes in
  the GLOBAL-RIB. The only exception exceptions are unchanged
  NLRIs or stale NLRIs, i.e., an NLRI received with a less recent (numerically smaller)
  sequence number.
</t>
<section anchor="Phase-1" numbered="true" toc="default">
<name>BGP SPF NLRI Selection</name>
<t>
   For all BGP-LS-SPF NLRIs, the selection rules for phase Phase 1 of the BGP
   decision process, section 9.1.1 process (see <xref section="9.1.1" target="RFC4271" format="default"/>, format="default"/>) no longer apply.
</t>
<ol spacing="normal" type="1"><li>
  NLRI
  NLRIs self-originated from directly-connected directly connected BGP SPF peers are preferred.
  This condition can be determined by comparing the BGP Identifiers in
  the received Local Node Descriptor and the BGP OPEN message for an active
  BGP session. This rule assures that a stale NLRI is updated even if a BGP SPF router
  loses its sequence number state due to a cold-start. cold start. Note that once the BGP session
  goes down, the NLRI received is no longer considered as being from a directly
  connected BGP SPF peer.
</li>
<li>
  Consistent with base BGP <xref target="RFC4271"/>, an NLRI received from a peer will always
  replace the same NLRI received from that peer. Coupled with rule #1, this will ensure that
  any stale NLRI in the BGP SPF routing domain will be updated.
</li>
<li>
  The NLRI with the most recent Sequence Number TLV, i.e., the highest sequence number is selected.
</li>
<li>
  The NLRI received from the BGP speaker with the numerically larger BGP
  Identifier is preferred.
</li>
</ol>
<t>
  When a BGP speaker completely loses its sequence number state, e.g., due to a cold start, or
  in the unlikely possibility that a 64-bit sequence number wraps, the BGP routing domain will
  still converge.

<!--[rfced] How may we clarify "as more recent" in the following
text. Have BGP speakers been accepting the self-originated NLRIs
recently (rather than "always"), as shown below?

Original:
   This is due to the fact that BGP speakers adjacent to the router
   always accept self-originated NLRI from the associated speaker as
   more recent (rule #1).

Perhaps:
   This is due to the fact that BGP speakers adjacent to the router
   have been recently accepting self-originated NLRIs from the
   associated speaker (per rule #1).
-->

  This is due to the fact that BGP speakers adjacent to the router
  always accept self-originated NLRIs from the associated speaker as more recent (rule #1). When a
  BGP speaker reestablishes a connection with its peers, any existing sessions are taken
  down and stale NLRI NLRIs are replaced. The adjacent BGP speakers update their NLRI
  advertisements and advertise to their neighbors until the BGP routing domain has converged.
</t>
<t>
  The modified SPF Decision Process performs an SPF calculation rooted at the local BGP
  speaker using the metrics from the Link Attribute IGP Metric TLV (1095) (TLV 1095) and
  the Prefix Attribute Prefix Metric TLV (1155) (TLV 1155) <xref target="RFC9552" format="default"/>.
  These metrics are considered consistently across the BGP SPF domain.
  As a result, any other BGP attributes that
  would influence the BGP decision process defined in <xref target="RFC4271" format="default"/> including
  ORIGIN, MULTI_EXIT_DISC, and
  LOCAL_PREF attributes are ignored by the SPF algorithm. The Next Hop in the MP_REACH_NLRI attribute
  <xref target="RFC4760"/> is discussed in <xref target="NEXT-HOP" format="default"/>.
  The AS_PATH and AS4_PATH attributes <xref target="RFC6793" format="default"/> attributes
  are preserved and used for loop detection <xref target="RFC4271" format="default"/>. They are ignored
  during the SPF computation for BGP-LS-SPF NLRIs.
</t>
<section anchor="Self-Origin" numbered="true" toc="default">
<name>BGP Self-Originated NLRI</name>
<t>
  Node, Link,
  Nodes, Links, or Prefix NLRI NLRIs with Node Descriptors matching the local BGP speaker are
  considered self-originated. When a self-originated NLRI is received and it doesn't match the
  local node's NLRI content (including the sequence number), special processing is required.
</t>
<ul spacing="normal">
  <li>
    If a self-originated NLRI is received and the sequence number is more recent (i.e., greater than
    the local node's sequence number for the NLRI), the NLRI sequence number is advanced to
    one greater than the received sequence number number, and the NLRI is readvertised to all peers.
  </li>
  <li>
    If a self-originated NLRI is received and the sequence number is the same as the local node's
    sequence number but the attributes differ, the NLRI sequence number is advanced to
    one greater than the received sequence number number, and the NLRI is readvertised to all peers.
  </li>
<!--
  <li>
    If self-originated Link or Prefix NLRI is received and the Link or Prefix NLRI is no longer
    being advertised by the local node, the NLRI is considered stale and is withdrawn using the
    standard BGP Update message Withdrawn Routes encodings <xref target="RFC4760"/>.
   </li>
-->
</ul>
<t>
  The above actions are performed immediately when the first instance of a newer self-originated NLRI is
  received. In this case, the newer instance is considered to be a stale instance that was advertised by
  the local node prior to a restart where the NLRI state was lost.
  However, if subsequent newer self-originated
  NLRI is received for the same Node, Link, or Prefix NLRI, the readvertisement
  or withdrawal is delayed by BGP_LS_SPF_SELF_READVERTISEMENT_DELAY (default 5) seconds
  since it is likely being advertised by a misconfigured or rogue BGP speaker
  (refer to <xref target="Security" format="default"/>).
</t>
</section>
</section>
<section anchor="dual-stack" numbered="true" toc="default">
<name>Dual Stack Support</name>
<t>
  The SPF-based decision process operates on Node, Link, and Prefix NLRIs that support
  both IPv4 and IPv6 addresses. Whether to run a single SPF computation or multiple
  SPF computations for separate AFs is an implementation and/or policy matter. Normally, IPv4
  next-hops are calculated for IPv4 prefixes prefixes, and IPv6 next-hops are calculated for IPv6
  prefixes.
</t>
</section>
<section anchor="BGP-SPF" numbered="true" toc="default">
<name>SPF Calculation based Based on BGP-LS-SPF NLRI</name>
<t>
  This section details the BGP-LS-SPF local routing information base Routing Information Base (RIB) calculation.
  The router uses BGP-LS-SPF Node, Link, and Prefix NLRI NLRIs to compute routes using the
  following algorithm. This calculation yields the set of routes associated
  with the BGP SPF Routing Domain.  A router calculates the shortest-path tree using itself
  as the root. Optimizations to the BGP-LS-SPF algorithm are possible but MUST <bcp14>MUST</bcp14> yield
  the same set of routes. The algorithm below supports Equal Cost Multi-Path (ECMP) ECMP
  routes. Weighted Unequal Cost Multi-Path Unequal-Cost Multipath (UCMP) routes are out of scope.
</t>
<t>
  The following abstract data structures are defined in order to specify the algorithm.
</t>
<ul spacing="normal">
 <li>
<dl spacing="normal" newline="false">
   <dt>Local Route Information Base (Local-RIB):</dt><dd>A routing table
   that contains reachability information (i.e., next hops) for all prefixes (both
   IPv4 and IPv6) as well as BGP-LS-SPF node reachability.

<!--[rfced] Please clarify "Prefix versus Node reachability" in the
last sentence. Does "versus" mean "or" in this context?

Also, we see "BGP-LS-SPF node reachability" in the first sentence.
Should "node" be "Node" for consistency with "Node reachability"
(e.g., "BGP-LS-SPF Node reachability")?

Original:
   Local Route Information Base (Local-RIB) - This routing table
      contains reachability information (i.e., next hops) for all
      prefixes (both IPv4 and IPv6) as well as BGP-LS-SPF node
      reachability. Implementations may choose to implement this with
      separate RIBs for each address family and/or Prefix versus Node
      reachability.
 </li>
 <li>
   Global Routing

Perhaps:
   Local Route Information Base (GLOBAL-RIB) - This is the (Local-RIB):  A routing table that
      contains reachability information (i.e., next hops) for all
      prefixes (both IPv4 and IPv6) as well as BGP-LS-SPF Node
      reachability. Implementations may choose to implement this with
      separate RIBs for each address family and/or Prefix or Node
      reachability.
-->

   Implementations may
   choose to implement this with separate RIBs for each address family and/or
   Prefix versus Node reachability.</dd>

   <dt>Global Routing Information Base (RIB) (GLOBAL-RIB):</dt><dd>The
   RIB containing the current routes that are
   installed in the router's forwarding plane.  This is commonly referred to
   in networking parlance as "the RIB".
 </li>
 <li>
   Link State RIB".</dd>

   <dt>Link-State NLRI Database (LSNDB) - Database (LSNDB):</dt><dd>A database of BGP-LS-SPF NLRI NLRIs
   that facilitates facilitate access to all Node, Link, and Prefix NLRI.
 </li>
 <li>
  Candidate NLRIs.</dd>

   <dt>Candidate List (CAN-LIST) - This is a (CAN-LIST):</dt><dd>A list of candidate Node
   NLRIs used during the BGP SPF calculation. The list is sorted by the cost
   to reach the Node NLRI NLRI, with the Node NLRI with that has the lowest reachability cost
   at the head of the list. This facilitates the execution of the Dijkstra algorithm
   algorithm, where the shortest paths between the local node and other nodes
   in the graph are computed.  The CAN-LIST is typically implemented as a heap but
   other data structures have been used.
</li>
</ul> used.</dd>
</dl>

<t>The Dijkstra algorithm consists of the steps below:
</t> below:</t>
<ol spacing="normal" type="1"><li>
  The current Local-RIB is invalidated, and the CAN-LIST is initialized to be empty.
  The Local-RIB is rebuilt during the course of the SPF computation.  The existing routing entries
  are preserved for comparison to determine changes that need to be made to the GLOBAL-RIB in
  step
  Step 6. These routes are referred to as stale routes. "stale routes".
</li>
<li>
   The cost of the Local-RIB Node route entry for the computing router is set to 0.
   The computing router's Node NLRI is added to the CAN-LIST (which was previously initialized
   to be empty in step Step 1). The next-hop list is set to the internal loopback next-hop.
</li>
<li>
  The Node NLRI with the lowest cost is removed from the CAN-LIST for processing.
  If the BGP-LS Node attribute includes an SPF Status TLV
  (refer to <xref target="node-status-tlv" format="default"/>)
  indicating the node is unreachable, the Node NLRI is ignored and the next lowest cost lowest-cost
  Node NLRI is selected from the CAN-LIST. The
  Node corresponding to this NLRI is referred to as the Current-Node. "Current-Node". If the CAN-LIST
  list is empty, the SPF calculation has completed and the algorithm proceeds to step Step 6.
</li>
<li>
  <t>
    All the Prefix NLRI NLRIs with the same Local Node Descriptors as the Current-Node are considered
    for installation. The next-hop(s) for these Prefix NLRI NLRIs are inherited from the Current-Node.
    If the Current-Node is for the local BGP Router, the next-hop for the prefix is a direct
    next-hop.  The cost for each prefix is the metric advertised in the Prefix Attribute
    Prefix Metric TLV (1155) (TLV 1155) added to the cost to reach the Current-Node. The following
    is done for each Prefix NLRI (referred to as the Current-Prefix): "Current-Prefix"):
</t>
<ul spacing="normal">
  <li>
    If the BGP-LS Prefix attribute includes an SPF Status TLV indicating the prefix is
    unreachable, the Current-Prefix is considered unreachable unreachable, and the next Prefix
    NLRI is examined in Step 4.
  </li>
  <li>

 <!--[rfced] Please clarify what "less than" refers to - is it the
 metric's cost, length, or other?

Original:
   If the Current-Prefix's corresponding prefix is in the Local-RIB
   and the Local-RIB metric is less than the Current-Prefix's metric,
   the Current-Prefix does not contribute to the route and the next
   Prefix NLRI is examined in Step 4.
-->

  <li>
    If the Current-Prefix's corresponding prefix is in the Local-RIB and the
    Local-RIB metric is less than the Current-Prefix's metric,
    the Current-Prefix does not contribute to the route, and the next Prefix NLRI is
    examined in Step 4.
  </li>

<!--[rfced] Please clarify "and the metric being updated". Is the
intended meaning perhaps "the metric is updated" (option A) or
that the next-hops are installed as the "Local-RIB route and
updated metric's next-hops" (option B)?

Original:
   If the Current-Prefix's corresponding prefix is not in the
   Local-RIB, the prefix is installed with the Current-Node's
   next-hops installed as the Local-RIB route's next-hops and
   the metric being updated.

Perhaps A:
   If the Current-Prefix's corresponding prefix is not in the
   Local-RIB, the prefix is installed with the Current-Node's
   next-hops installed as the Local-RIB route's next-hops, and
   the metric is updated.

Perhaps B:
   If the Current-Prefix's corresponding prefix is not in the
   Local-RIB, the prefix is installed with the Current-Node's
   next-hops installed as the Local-RIB route and updated
   metric's next-hops.
-->

  <li>
    If the Current-Prefix's corresponding prefix is not in the Local-RIB,
    the prefix is installed with the Current-Node's next-hops
    installed as the Local-RIB route's next-hops and the metric being updated. If the
    IGP Route Tag TLV (1153) (TLV 1153) is
    included in the Current-Prefix's NLRI Attribute, the tag(s) are is installed in the
    current Local-RIB route's tag(s).
  </li>
  <li>
    If the Current-Prefix's corresponding prefix is in the Local-RIB and the cost is less
    than the Local-RIB route's metric, the prefix is installed with the Current-Node's next-hops
    replacing next-hops, which
    replace the Local-RIB route's next-hops and the metric being updated updated, and any route tags
    are removed. If the IGP Route Tag TLV (1153) (TLV 1153) is
    included in the Current-Prefix's NLRI Attribute, the tag(s) are is installed in the
    current Local-RIB route's tag(s).
  </li>
  <li>
    If the Current-Prefix's corresponding prefix is in the Local-RIB and the cost
    is the same as the Local-RIB route's metric, the Current-Node's next-hops are merged
    with the Local-RIB route's next-hops.
    The algorithm below supports Equal Cost Multi-Path (ECMP) ECMP routes.
    Some platforms or implementations may have limits on the number of
    ECMP routes that can be supported.  The setting or identification
    of any limitations is outside the scope if this document.
    Weighted Unequal Cost Multi-Path UCMP routes are out of scope as well.
  </li>
</ul>
</li>
<li>
  <t>
    All the Link NLRI NLRIs with the same Node Identifiers as the Current-Node are considered
    for installation. Each link is examined and is referred to as the "Current-Link" in the following text
    as the Current-Link. text. The cost of the Current-Link is the advertised IGP Metric TLV (1095) (TLV 1095)
    from the Link NLRI BGP-LS attribute added to the cost to reach the Current-Node.
    If the Current-Node is for the local BGP Router,
    the next-hop for the link is a direct next-hop pointing to the corresponding local
    interface. For any other Current-Node, the next-hop(s) for the Current-Link are is inherited
    from the Current-Node. The following is done for each link:
</t>
<ol spacing="normal" type="a">
<li>
  If the Current-Link's NLRI attribute includes an SPF Status TLV indicating the link is
  down, the BGP-LS-SPF Link NLRI is considered down down, and the next link
  for the Current-Node is examined in Step 5.
</li>
<li>
  If the Current-Node NLRI attributes includes include the SPF Status TLV
  (refer to <xref target="node-status-tlv" format="default"/>) and the status
  indicates that the Node doesn't support transit, the next link for the Current-Node is
  processed in Step 5.
</li>
<li>
  <t>
    The Current-Link's Remote Node NLRI is accessed (i.e., the Node NLRI
    with the same Node identifiers Identifiers as the Current-Link's Remote Node Descriptors). If it exists,
    it is referred to as the Remote-Node "Remote-Node" and the algorithm proceeds as follows:
  </t>
<ul spacing="normal">
  <li>
    If the Remote-Node's NLRI attribute includes an SPF Status TLV indicating the node is
    unreachable, the next link for the Current-Node is examined in Step 5.
</li>
<li>
  <t>
  All the Link NLRI NLRIs corresponding to the Remote-Node are searched for a Link
  NLRI pointing to the Current-Node. Each Remote-Node's Link NLRI (referred to as the
  Remote-Link) is examined for Remote Node Descriptors matching the Current-Node and
  Link Descriptors matching the Current-Link.
  </t>
  <ul spacing = "normal">
    <li>
      For IPv4/IPv6 numbered Link Descriptors Decriptors to match during the IPv4 SPF computation, the
      Current-Link's IP4/IPv6 interface address link descriptor MUST <bcp14>MUST</bcp14> match the Remote-Link
      IPv4/IPv6 neighbor address link descriptor descriptor, and the Current-Link's IPv4/IPv6 neighbor
      address MUST <bcp14>MUST</bcp14> match the Remote-Link's IPv4/IPv6 interface address.
    </li>
    <li>
      For unnumbered links to match during the IPv4 or IPv6 SPF computation,
      the Current-Link and Remote-Link's Address Family Link Descriptor TLV must match
      the address family of the IPv4 or IPv6 SPF computation,
      the Current-Link's Remote Identifier MUST <bcp14>MUST</bcp14> match the Remote-Link's Local
      Identifier
      Identifier, and the Current-Link's Remote Identifier MUST <bcp14>MUST</bcp14> match the
      Remote-Link's Local Identifier.

<!--[rfced] Section 6.3. We note the following variations - are these
terms different? Please let us know if/how we may update these
for consistency.

  Link Local/Remote Identifiers (TLV 258)
  Current or Remote Link's Local Identifier
  Current-Link's Remote Identifier
  Link Remote Identifier
  Link's Remote Identifier
  Remote Link Identifiers

As an example, how may we update this sentence for consistency? Should
reference to TLV 258 be "Link Local/Remote Identifiers" per RFC 9552?

Original:
   Since the Link's Remote Identifier may not be known, a value of 0
   is considered a wildcard and will match any Current or Remote
   Link's Local Identifier (see TLV 258 [RFC9552]).

Perhaps:
   Since the Link Remote Identifier may not be known, a value of 0
   is considered a wildcard and will match any Link Local/Remote
   Identifiers (see TLV 258 [RFC9552]).
-->
      Since the Link's Remote Identifier may not be known, a value of 0
      is considered a wildcard and will match any Current or Remote Link's Local
      Identifier (see TLV 258 <xref target="RFC9552" format="default"/>).
      Address Family Link Descriptor TLVs for multiple address families may be
      advertised so that an unnumbered link can be used in the SPF computation for
      multiple address families.
    </li>
  </ul>
  <t>
    If these conditions are satisfied for one of the Remote-Node's links,
    the bi-directional bidirectional connectivity
    check succeeds and the Remote-Node may be processed further. The
    Remote-Node's Link NLRI providing bi-directional bidirectional connectivity
    is referred to as the Remote-Link. If no Remote-Link is found, the next
    link for the Current-Node is examined in Step 5.
  </t>
 </li>
<li>
  If the Remote-Link NLRI attribute includes an SPF Status TLV indicating
  the link is down, the Remote-Link NLRI is considered down down, and the next link
  for the Current-Node is examined in Step 5.
</li>
<li>
  If the Remote-Node is not on the CAN-LIST, it is inserted based
  on the cost. The Remote Node's cost is the cost of the Current-Node added
  to the Current-Link's IGP Metric TLV (1095). (TLV 1095). The next-hop(s) for the Remote-Node
  are
  is inherited from the Current-Link.
</li>
<li>
  If the Remote-Node NLRI is already on the CAN-LIST with a higher cost, it
  must be removed and reinserted with the Remote-Node cost based on the
  Current-Link (as calculated in the previous step). The
  next-hop(s) for the Remote-Node are is inherited from the Current-Link.
</li>
<li>
  If the Remote-Node NLRI is already on the CAN-LIST with the same cost, it need
  not be reinserted on the CAN-LIST. However, the Current-Link's next-hop(s)
  must be merged into the current set of next-hops for the Remote-Node.
</li>
<li>
  If the Remote-Node NLRI is already on the CAN-LIST with a lower cost, it need
  not be reinserted on the CAN-LIST.
</li>
</ul>
</li>
<li>
  Return to step Step 3 to process the next lowest cost lowest-cost Node NLRI on the CAN-LIST.
</li>
</ol>
</li>
<li>
  <t>
    The Local-RIB is examined and changes (adds, deletes, and modifications) are installed into
    the GLOBAL-RIB. For each route in the Local-RIB:
</t>
<ul spacing="normal">
  <li>
    If the route was added during the current BGP SPF computation, install the route into
    the GLOBAL-RIB.
</li>
<li>
  If the route was modified during the current BGP SPF computation (e.g., metric, tags,
  or next-hops), update the route in the GLOBAL-RIB.
</li>
<li>
  If the route was not installed during the current BGP SPF computation, remove the route
  from the GLOBAL-RIB.
</li>
</ul>
</li>
</ol>
</section>
<section numbered="true" toc="default">
<name>IPv4/IPv6 Unicast Address Family Interaction</name>
<t>
  While the BGP-LS-SPF address family and the BGP unicast address families may install routes
  into the same device routing tables, tables of the same device, they operate independently (i.e., "Ships-in-the-Night" "ships-in-the-night" mode).
  There is no implicit route redistribution between the BGP-LS-SPF address family and the BGP
  unicast address families.
</t>
<t>
  It is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that BGP-LS-SPF IPv4/IPv6 route computation and
  installation be given scheduling priority by default over other BGP address families
  as these address families are considered as underlay SAFIs.
</t>
</section>
<section anchor="NLRI-Advertise" numbered="true" toc="default">
<name>NLRI Advertisement</name>
<section anchor="failure-converge" numbered="true" toc="default">
<name>Link/Prefix Failure Convergence</name>
<t>
  A local failure prevents a link from being used in the SPF calculation
  due to the IGP bi-directional bidirectional connectivity requirement. Consequently, local link
  failures SHOULD <bcp14>SHOULD</bcp14> always be communicated as quickly as possible and given priority
  over other categories of changes to ensure expeditious propagation
  and optimal convergence.
</t>
<t>
  According to standard BGP procedures, the link would continue
  to be used until the last copy of the BGP-LS-SPF Link NLRI is
  withdrawn. In order to avoid this delay, the originator of the Link NLRI SHOULD <bcp14>SHOULD</bcp14>
  advertise a more recent version with an increased Sequence Number TLV for
  the BGP-LS-SPF Link NLRI including the SPF Status TLV
  (refer to <xref target="link-status-tlv" format="default"/>) indicating the link
  is down with respect to BGP SPF.

<!--[rfced] The following sentences do not parse, for example, "that
the BGP-LS-LINK NLRI is advertised with SPF Status". How may we
rephrase this text for clarity?

Also, should "BGP-LS-LINK NLRI" be updated as "BGP-LS-SPF Link NLRI"
in the first sentence and "BGP-LS-Prefix NLRI" be updated as
"BGP-LS-SPF Prefix NLRI" in the second sentence for consistency?

Original:
   The configurable LinkStatusDownAdvertise timer controls the interval
   that the BGP-LS-LINK NLRI is advertised with SPF Status indicating
   the link is down prior to withdrawal.

   The configurable PrefixStatusDownAdvertise timer controls the
   interval that the BGP-LS-Prefix NLRI is advertised with SPF
   Status indicating the prefix is unreachable prior to withdrawal.

Perhaps:
   The configurable PrefixStatusDownAdvertise timer controls the
   interval when a BGP-LS-SPF Link NLRI has been advertised with the
   SPF Status TLV and indicates that the prefix is unreachable prior
   to withdrawal.

   The configurable PrefixStatusDownAdvertise timer controls the
   interval when a BGP-LS-SPF Prefix NLRI is advertised with the
   SPF Status TLV and indicates that the prefix is unreachable prior
   to withdrawal.
-->

  The configurable LinkStatusDownAdvertise timer
  controls the interval that the BGP-LS-LINK NLRI is advertised with SPF Status indicating
  the link is down prior to withdrawal.
  If a BGP-LS-SPF Link NLRI has been advertised with the SPF Status
  TLV and the link becomes available
  in that period, the originator of the BGP-LS-SPF LINK Link NLRI MUST <bcp14>MUST</bcp14> advertise a more recent
  version of the BGP-LS-SPF Link NLRI without the SPF Status TLV in the BGP-LS Link Attributes.
  The suggested default value for the LinkStatusDownAdvertise timer is 2 seconds.
</t>
<t>
  Similarly, when a prefix becomes unreachable, a more recent version of the BGP-LS-SPF
  Prefix NLRI SHOULD <bcp14>SHOULD</bcp14> be advertised with the SPF Status TLV
  (refer to <xref target="prefix-status-tlv" format="default"/>)
  indicating
  to indicate that the prefix is unreachable in the BGP-LS Prefix Attributes Attributes, and the prefix will be
  considered unreachable with respect to BGP SPF.

  The configurable PrefixStatusDownAdvertise timer
  controls the interval that the BGP-LS-Prefix NLRI is advertised with SPF Status indicating
  the prefix is unreachable prior to withdrawal.
  If the BGP-LS-SPF Prefix has been advertised with the SPF Status TLV and the prefix
  becomes reachable in that period, the originator of the BGP-LS-SPF Prefix NLRI
  MUST
  <bcp14>MUST</bcp14> advertise a more recent version of the BGP-LS-SPF Prefix NLRI without the
  SPF Status TLV in the BGP-LS Prefix Attributes.
  The suggested default value for the PrefixStatusDownAdvertise timer is 2 seconds.
</t>
</section>
<section anchor="node-failure" numbered="true" toc="default">
<name>Node Failure Convergence</name>
<t>
  By default, all the NLRI NLRIs advertised by a node are withdrawn when a session
  failure is detected <xref target="RFC4271"/>. If fast failure detection such as BFD
  <xref target="RFC5880"/> is utilized, and the node is
  on the fastest converging path, the most recent versions of BGP-LS-SPF NLRI will be
  withdrawn. This may result in older versions of NLRI NLRIs received from peer(s) one or more peers on
  a different path(s) being in the LSNDB until they are withdrawn.
  These stale NLRI NLRIs will not delay convergence since the adjacent nodes detect the
  link failure and advertise a more recent NLRI indicating the link is down with respect to
  BGP SPF (refer to <xref target="failure-converge" format="default"/>) and the
  bi-directional
  bidirectional connectivity check fails during the BGP SPF calculation
  (refer to <xref target="BGP-SPF" format="default"/>).
</t>
</section>
</section>
</section>
<section anchor="error-handling" numbered="true" toc="default">
<name>Error Handling</name>
<t>
  This section describes the Error Handling error-handling actions, as described in
  <xref target="RFC7606" format="default"/>, that are specific to SAFI BGP-LS-SPF SAFI BGP Update
  message processing.
</t>
<section anchor="new-TLVs" numbered="true" toc="default">
<name>Processing of BGP-LS-SPF TLVs</name>
<t>
  When a BGP speaker receives a BGP Update containing a malformed Node NLRI
  SPF Status TLV in the BGP-LS Attribute <xref target="RFC9552" format="default"/>,
  the corresponding Node NLRI is
  considered as malformed and MUST <bcp14>MUST</bcp14> be handled as 'Treat-as-withdraw'. 'treat-as-withdraw'. An
  implementation SHOULD <bcp14>SHOULD</bcp14> log an error (subject to rate-limiting) rate limiting) for further analysis.
</t>
<t>
  When a BGP speaker receives a BGP Update containing a malformed Link NLRI
  SPF Status TLV in the BGP-LS Attribute <xref target="RFC9552" format="default"/>,
  the corresponding Link NLRI is
  considered as malformed and MUST <bcp14>MUST</bcp14> be handled as 'Treat-as-withdraw'. 'treat-as-withdraw'. An
  implementation SHOULD <bcp14>SHOULD</bcp14> log an error (subject to rate-limiting) rate limiting) for further analysis.
</t>
<t>
  When a BGP speaker receives a BGP Update containing a malformed Address Family
  Link Descriptor TLV in the BGP-LS Attribute <xref target="RFC9552" format="default"/>,
  the corresponding Link NLRI is
  considered as malformed and MUST <bcp14>MUST</bcp14> be handled as 'Treat-as-withdraw'. 'treat-as-withdraw'. An
  implementation SHOULD <bcp14>SHOULD</bcp14> log an error (subject to rate-limiting) rate limiting) for further analysis.
</t>
<t>
  When a BGP speaker receives a BGP Update containing a malformed Prefix NLRI
  SPF Status TLV in the BGP-LS Attribute <xref target="RFC9552" format="default"/>,
  the corresponding Prefix NLRI is
  considered as malformed and MUST <bcp14>MUST</bcp14> be handled as 'Treat-as-withdraw'. 'treat-as-withdraw'. An
  implementation SHOULD <bcp14>SHOULD</bcp14> log an error (subject to rate-limiting) rate limiting) for further analysis.
</t>
<t>
  When a BGP speaker receives a BGP Update containing any a malformed BGP-LS Attribute TE
  and IGP Metric TLV, the corresponding NLRI is considered as malformed
  and MUST <bcp14>MUST</bcp14> be handled as 'Treat-as-withdraw' 'treat-as-withdraw' <xref target="RFC7606" format="default"/>.
  An implementation SHOULD <bcp14>SHOULD</bcp14> log an error (subject to rate-limiting) rate limiting) for further analysis.
</t>
<t>
  The BGP-LS Attribute consists of Node attribute TLVs, Link attribute TLVs, and the Prefix
  attribute TLVs. Node attribute TLVs and their error handling error-handling rules are either defined in
  <xref target="RFC9552" format="default"/>
  or derived from <xref target="RFC5305" format="default"/> and <xref target="RFC6119" format="default"/>.
  If a BGP speaker receives a BGP-LS Attribute which that is considered malformed based on these
  error handling
  error-handling rules, then it MUST <bcp14>MUST</bcp14> consider the received NLRI as malformed malformed, and the receiving
  BGP speaker MUST <bcp14>MUST</bcp14> handle such a malformed NLRI as 'Treat-as-withdraw' 'treat-as-withdraw' <xref target="RFC7606" format="default"/>.
</t>
<t>
  Node Descriptor TLVs and their error handling error-handling rules are defined in
  section 5.2.1 of
  <xref section="5.2.1" target="RFC9552" format="default"/>.
  Node Attribute TLVs and their error handling error-handling rules are either defined in
  <xref target="RFC9552" format="default"/>
  or derived from <xref target="RFC5305" format="default"/> and <xref target="RFC6119" format="default"/>.
</t>
<t>
  Link Descriptor TLVs and their error handling error-handling rules are defined in
  section 5.2.2 of
  <xref section="5.2.2" target="RFC9552" format="default"/>.
  Link Attribute TLVs and their error handling error-handling rules are either defined in
  <xref target="RFC9552" format="default"/>
  or derived from <xref target="RFC5305" format="default"/> and <xref target="RFC6119" format="default"/>.
</t>
<t>
  Prefix Descriptor TLVs and their error handling error-handling rules are defined in
  section 5.2.3 of
  <xref section="5.2.3" target="RFC9552" format="default"/>.
  Prefix Attribute TLVs and their error handling error-handling rules are either defined in
  <xref target="RFC9552" format="default"/>
  or derived from <xref target="RFC5130" format="default"/> and
  <xref target="RFC2328" format="default"/>.
</t>
<t>
  If a BGP speaker receives NLRI with a Node Descriptor TLV, Link Descriptor TLV, or Prefix Descriptor
  TLV that is considered malformed based on error handling rules defined in the above references,
  then it MUST <bcp14>MUST</bcp14> consider the received NLRI as malformed malformed, and the receiving
  BGP speaker MUST <bcp14>MUST</bcp14> handle such a malformed NLRI as 'Treat-as-withdraw' 'treat-as-withdraw' <xref target="RFC7606" format="default"/>.
</t>
<t>
  When a BGP speaker receives a BGP Update that does not contain any BGP-LS Attribute, Attributes,
  it is most likely an indication of 'Attribute Discard' fault handling handling, and the
  BGP speaker SHOULD <bcp14>SHOULD</bcp14> preserve and propagate the BGP-LS-SPF NLRI as described in
  Section 8.2.2 of
  <xref section="8.2.2" target="RFC9552"/>. However, NLRI NLRIs without the BGP-LS attribute
  MUST NOT
  <bcp14>MUST NOT</bcp14> be used in the SPF Calculation <xref target="BGP-SPF"/>. calculation (<xref target="BGP-SPF"/>). How this is accomplished
  is an implementation matter matter, but one way would be for these NLRI NLRIs not to be returned in
  LSNDB lookups.
</t>
</section>
<section anchor="bgpspf-nlri" numbered="true" toc="default">
<name>Processing of BGP-LS-SPF NLRIs</name>
<t>
  A BGP speaker supporting the BGP-LS-SPF SAFI MUST <bcp14>MUST</bcp14> perform the syntactic validation checks of the
  BGP-LS-SPF NLRI listed in Section 8.2.2 of <xref section="8.2.2" target="RFC9552" format="default"/> to determine if
  it is malformed.
</t>
</section>
<section anchor="bgpspf-attribute" numbered="true" toc="default">
<name>Processing of BGP-LS Attribute</name> Attributes</name>
<t>
 A BGP speaker supporting the BGP-LS-SPF SAFI MUST <bcp14>MUST</bcp14> perform the syntactic validation checks of the
 BGP-LS Attribute listed in Section 8.2.2 of <xref section="8.2.2" target="RFC9552" format="default"/> to determine if
 it is malformed.
 </t>
 <t>
  An implementation SHOULD <bcp14>SHOULD</bcp14> log an error for further analysis for problems
  detected during syntax validation.
</t>
</section>
<section anchor="bgp-sync" numbered="true" toc="default">
  <name>BGP-LS-SPF Link State Link-State NLRI Database Synchronization</name>
  <t>
    While uncommon, there may be situations where the LSNDBs of two
    BGP speakers supporting support the BGP-LS-SPF SAFI lose synchronization.
    In these situations, the BGP
    session MUST <bcp14>MUST</bcp14> be reset unless other means of resynchronization are used
    (beyond the scope of this document).
    When the session is reset, the BGP speaker MUST <bcp14>MUST</bcp14> send a NOTIFICATION
    message with the BGP error code "Loss of LSDB Synchronization" as described
    in section 3 of <xref section="3" target="RFC4271"/>. The mechanisms to detect loss of
    synchronization are beyond the scope of this document.
  </t>
</section>
</section>
<section anchor="IANA" numbered="true" toc="default">
<name>IANA Considerations</name>
<section numbered="true" toc="default">
  <name>BGP-LS-SPF Allocation in the SAFI Parameters Values Registry</name>
<t>
  IANA has assigned value 80 for BGP-LS-SPF from the First Come First
  Served range <xref target="RFC8126"/> and listed this document as a reference in the "SAFI Values" registry within the "Subsequent Address Family Identifiers (SAFI) Parameters" registry.  IANA is requested to update the registration
  to reference only to this document. registry group.
</t>
</section>
<section numbered="true" toc="default">
  <name>BGP-LS-SPF Assignments to in the BGP-LS NLRI and Attribute TLV TLVs Registry</name>
<t>
  IANA has assigned six TLVs for BGP-LS-SPF NLRI in the "BGP-LS NLRI and Attribute
  TLV"
  TLVs" registry. Supported TLV types include the Sequence Number, SPF Status TLV type, Status, and Address Family Link
  Descriptor TLV type, and Sequence Number TLV type.
  Descriptor.  Deprecated TLV types include the SPF Capability TLV type, Capability, IPv4 Link Prefix Length TLV type, Length, and IPv6
  Link Prefix Length TLV type. Length.
</t>

<!--[rfced] FYI - We placed the information in Table 5 in ascending
order to match the "BGP-LS NLRI and Attribute TLVs" registry at
<https://www.iana.org/assignments/bgp-ls-parameters/>
-->

<table anchor="tab.iana-attr" align="center">
<name>NLRI Attribute TLVs</name>
<thead>
  <tr>
<th align="left">TLV Code Point</th>
<th align="left">Description</th>
<th align="left">Reference</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">1185</td>
<td align="left">Address Family Link Descriptor</td>
<td align="left"><xref target="af-link-descriptor-tlv"/>, RFCXXXX ([this document]).</td>
</tr>
<tr>
<td align="left">1181</td>
<td align="left">Sequence Number</td>
<td align="left">RFCXXXX ([this document]), <xref target="sequence-number-tlv"/></td> align="left"><xref target="sequence-number-tlv"/> of RFC XXXX</td>
</tr>
<tr>
<td align="left">1184</td>
<td align="left">SPF Status</td>
<td align="left"><xref target="node-status-tlv"/>, RFCXXXX ([this document]), align="left">Sections <xref target="node-status-tlv" format="counter"/>,
<xref target="link-status-tlv"/> target="link-status-tlv" format="counter"/>, and <xref target="prefix-status-tlv"/></td> target="prefix-status-tlv" format="counter"/> of RFC XXXX</td>
</tr>
<tr>
<td align="left">1185</td>
<td align="left">Address Family Link Descriptor</td>
<td align="left"><xref target="af-link-descriptor-tlv"/> of RFC XXXX</td>
</tr>
</tbody>
</table>
<t>
  The early allocation assignments for the TLV types SPF Capability (1180),
  IPv4 Link Prefix Length (1182), and IPv6 Link Prefix Length (1183) are no longer required
  and are to be have been deprecated.
</t>
</section>
<section numbered="true" toc="default">
  <name>BGP-LS-SPF Node NLRI Attribute SPF Status TLV Status Registry</name>
  <t>
    IANA is requested to create has created the "BGP-LS-SPF Node NLRI Attribute SPF
    Status TLV Status" Registry registry for status values in a new BGP SPF within the "BGP Shortest Path First (BGP SPF)" registry group.
    Initial values for this registry are provided below.  Future assignments
    are to be made using the Expert Review registration policy <xref target="RFC8126"/>
    with guidance for Designated Experts designated experts as per section 7.2 of <xref section="7.2" target="RFC9552"/>.
  </t>
<table anchor="tab.iana-node-status" align="center">
   <name>BGP-LS-SPF Node NLRI Attribute SPF Status TLV Status Registry Assignments</name>
   <thead>
     <tr>
       <th align="left">Values</th>
       <th align="left">Description</th>
      </tr>
     </thead>
    <tbody>
     <tr>
       <td align="left">0</td>
       <td align="left">Reserved</td>
     </tr>
     <tr>
       <td align="left">1</td>
       <td align="left">Node unreachable with respect to BGP SPF</td>
     </tr>
     <tr>
       <td align="left">2</td>
       <td align="left">Node does not support transit traffic with respect to BGP SPF</td>
     </tr>
     <tr>
       <td align="left">3-254</td>
       <td align="left">Unassigned</td>
     </tr>
     <tr>
       <td align="left">255</td>
       <td align="left">Reserved</td>
     </tr>
   </tbody>
</table>
</section>
<section numbered="true" toc="default">
  <name>BGP-LS-SPF Link NLRI Attribute SPF Status TLV Status Registry</name>
  <t>
    IANA is requested to create has created the "BGP-LS-SPF Link NLRI Attribute SPF
    Status TLV Status" Registry registry for status values in a new within the BGP SPF Shortest Path First (BGP SPF)" registry group.
    Initial values for this registry are provided below.  Future assignments
    are to be made using the IETF Review registration policy <xref target="RFC8126"/>.
  </t>
<table anchor="tab.iana-link-status" align="center">
   <name>BGP-LS-SPF Link NLRI Attribute SPF Status TLV Status Registry Assignments</name>
   <thead>
     <tr>
       <th align="left">Value</th>
       <th align="left">Description</th>
      </tr>
     </thead>
    <tbody>
     <tr>
       <td align="left">0</td>
       <td align="left">Reserved</td>
     </tr>
     <tr>
       <td align="left">1</td>
       <td align="left">Link unreachable with respect to BGP SPF</td>
     </tr>
     <tr>
       <td align="left">2-254</td>
       <td align="left">Unassigned</td>
     </tr>
     <tr>
       <td align="left">255</td>
       <td align="left">Reserved</td>
     </tr>
   </tbody>
</table>
</section>
<section numbered="true" toc="default">
  <name>BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV Status Registry</name>
  <t>
    IANA is requested to create has created the "BGP-LS-SPF Prefix NLRI Attribute SPF
    Status TLV Status" Registry registry for status values in a new BGP SPF within the "BGP Shortest Path First (BGP SPF)" registry group.
    Initial values for this registry are provided below.  Future assignments
    are to be made using the IETF Review registration policy <xref target="RFC8126"/>.
  </t>
<table anchor="tab.iana-prefix-status" align="center">
   <name>BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV Status Registry Assignments</name>
   <thead>
     <tr>
       <th align="left">Value</th>
       <th align="left">Description</th>
      </tr>
     </thead>
    <tbody>
     <tr>
       <td align="left">0</td>
       <td align="left">Reserved</td>
     </tr>
     <tr>
       <td align="left">1</td>
       <td align="left">Prefix unreachable with respect to BGP SPF</td>
     </tr>
     <tr>
       <td align="left">2-254</td>
       <td align="left">Unassigned</td>
     </tr>
     <tr>
       <td align="left">255</td>
       <td align="left">Reserved</td>
     </tr>
   </tbody>
</table>
</section>
<section numbered="true" toc="default">
  <name>BGP
  <name>Assignment in the BGP Error (Notification) Code Assignment</name> Codes Registry</name>
  <t>
    IANA is requested to assign a TBD code has assigned value 9 for "Loss Loss of LSDB Synchronization" Synchronization in the
    BGP
    "BGP Error (Notification) Codes" registry in within the
    "Border Gateway Protocol (BGP) Parameters" registry group.
 </t>
</section>
</section>
<section anchor="Security" numbered="true" toc="default">
<name>Security Considerations</name>
<t>
  This document defines a BGP SAFI, i.e., the BGP-LS-SPF SAFI. This document
  does not change the underlying security issues inherent in the BGP protocol
  <xref target="RFC4271" format="default"/>. The Security Considerations security considerations
  discussed in <xref target="RFC4271" format="default"/> apply to the BGP SPF functionality as well.
  The analysis of the security issues for BGP mentioned
  in <xref target="RFC4272" format="default"/> and <xref target="RFC6952" format="default"/> also applies to this document.
  The threats and security considerations are the similar to the BGP IPv4 Unicast SAFI and
  IPv6 Unicast SAFI when utilized in similar deployments, e.g., <xref target="RFC7938"/>.
  The analysis of Generic Threats generic threats to Routing Protocols done routing protocols in <xref target="RFC4593" format="default"/>
  is also worth noting.
</t>
<t>
  As the modifications for
  BGP SPF described in this document for
  BGP SPF apply to IPv4 Unicast and IPv6 Unicast as underlay SAFIs in a single
  BGP SPF Routing Domain, the BGP
  security solutions described in <xref target="RFC6811" format="default"/> and <xref target="RFC8205" format="default"/>
  are out of scope as they are meant to apply for inter-domain BGP BGP, where
  multiple BGP Routing Domains are typically involved. The BGP-LS-SPF SAFI NLRI NLRIs described
  in this document are typically advertised between External BGP (EBGP) EBGP or
  Internal BGP (IBGP)
  IBGP speakers under a single administrative domain.
</t>
<t>

<!--[rfced] We having trouble parsing this sentence. Does the
processing of the BGP SPF and BGP-LS-SPF SAFI cause the encoding
to be inherited from BGP-LS (option A)? Or do BGP-LS-SPF SAFIs
and processed BGP SPFs inherit the encoding (option B)? Please
clarify.

Original:
   The BGP SPF processing and the BGP-LS-SPF SAFI inherit the encoding
   from BGP-LS [RFC9552], and consequently, inherit the security
   considerations for BGP-LS associated with encoding.

Perhaps A:
   When BGP SPF and BGP-LS-SPF SAFI are processed, they inherit
   encoding from BGP-LS [RFC9552] and, consequently, inherit the
   security considerations for the BGP-LS associated with encoding.

Perhaps B:
   BGP-LS-SPF SAFIs and processed BGP SPFs inherit the encoding
   from BGP-LS [RFC9552] and, consequently, inherit the security
   considerations for BGP-LS associated with encoding.
-->

  The BGP SPF processing and the BGP-LS-SPF SAFI inherit the encoding from BGP-LS
  <xref target="RFC9552" format="default"/>, and consequently, inherit
  the security considerations for BGP-LS associated with encoding. Additionally,
  given that the BGP SPF processing
  is used to install IPv4 and IPv6 Unicast unicast routes, the BGP SPF processing is vulnerable to
  attacks to the routing control plane that aren't applicable to BGP-LS. One notable
  Denial-of-Service attack, attack would be to include malformed BGP attributes in a replicated
  BGP Update, causing the receiving peer to treat the advertised BGP-LS-SPF to a
  withdrawal <xref target="RFC7606" format="default"/>.
</t>
<t>
  In order to mitigate the risk of peering with BGP speakers masquerading
  as legitimate authorized BGP speakers, it is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that
  the TCP Authentication Option (TCP-AO) <xref target="RFC5925" format="default"/> be used to
  authenticate BGP sessions. If an authorized BGP peer is compromised, that
  BGP peer could advertise a modified Node, Link, or Prefix NLRI which result that results
  in misrouting, repeating origination of NLRI, and/or excessive SPF
  calculations. When a BGP speaker detects that its self-originated NLRI
  is being originated by another BGP speaker, an appropriate error SHOULD <bcp14>SHOULD</bcp14>
  be logged so that the operator can take corrective action. This exposure is similar
  to other BGP AFI/SAFIs.
</t>
</section>
<section anchor="Management" numbered="true" toc="default">
<name>Management Considerations</name>
<t>
  This section includes unique management considerations for the BGP-LS-SPF address family.
</t>
<section anchor="Config" numbered="true" toc="default">
<name>Configuration</name>
<t>
  All routers in the BGP SPF Routing Domain are under a single administrative domain
  allowing for consistent configuration.
</t>
</section>
<section anchor="link-metric-config" numbered="true" toc="default">
  <name>Link Metric Configuration</name>
  <t>
    For loopback prefixes, it is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that the metric be 0.
    For non-loopback prefixes, the setting of the
    metric is a local matter and beyond the scope of this document.
  </t>
  <t>

<!--[rfced] How may we update this sentence for clarity?

Original:
   Algorithms such as setting the metric inversely to the link speed as
   supported in some IGP implementations MAY be supported.

Perhaps:
   Algorithms that set the metric inversely to the link speed
   in some IGP implementations MAY be supported.
-->

    Algorithms such as setting the metric inversely to the link speed as
    supported in some IGP implementations <bcp14>MAY</bcp14> be supported. However, the
    details of how the metric is computed are beyond the scope of this document.
  </t>
  <t>
   Within a BGP SPF Routing Domain, the IGP metrics for all advertised links SHOULD <bcp14>SHOULD</bcp14> be configured or
   defaulted consistently. For example, if a default metric is used for one router's links, then a
   similar metric should be used for all router's links. Similarly, if the link metric is
   derived from using the inverse of the link bandwidth on one router, then this SHOULD <bcp14>SHOULD</bcp14>
   be done for all routers routers, and the same reference bandwidth SHOULD <bcp14>SHOULD</bcp14> be used to derive the
   inversely proportional metric. Failure to do so will result in incorrect routing based on
   the link metric.
 </t>
</section>
<section anchor="neighbor-config" numbered="true" toc="default">
  <name>Unnumbered Link Configuration</name>
  <t>
    When parallel unnumbered links between BGP-SPF BGP and SPF routers are included in the BGP SPF routing
    domain and the Remote Link Identifiers aren't readily discovered, it is RECOMMENDED <bcp14>RECOMMENDED</bcp14> that these
    the Remote Link Identifiers be configured so that precise NLRI Link NLRI matching can be done.
  </t>
</section>
<section anchor="Adjacency-EoR-Required" numbered="true" toc="default">
<name>Adjacency End-of-RIB (EOR) Marker Requirement</name>
<t>
  Depending on the peering model, topology, and convergence requirements, an
  End-of-RIB (EoR) Marker <xref target="BGP-LS-SPF-EOR"/>
  EoR marker (<xref target="BGP-LS-SPF-EOR"/>) for the BGP-LS-SPF
  SAFI MAY <bcp14>MAY</bcp14> be required from the peer prior to advertising a BGP-LS Link NLRI
  for the peer. If configuration is supported, this MUST <bcp14>MUST</bcp14> be configurable at
  the BGP SPF instance level and MUST <bcp14>MUST</bcp14> be configured consistently throughout
  the BGP SPF routing domain.
</t>
<t>
  When this configuration is provided, the default MUST <bcp14>MUST</bcp14> be to wait
  indefinitely prior to advertising a BGP-LS link Link NLRI. Configuration of
  a timer specifying the maximum time to wait prior to advertisement
  MAY
  <bcp14>MAY</bcp14> be provided.
</t>
</section>
<section anchor="spf-backoff-config" numbered="true" toc="default">
<name>backoff-config</name>
<t>
  In addition to the configuration of the BGP-LS-SPF address family, implementations SHOULD <bcp14>SHOULD</bcp14>
  support the "Shortest Path First (SPF) Back-Off Delay Algorithm for Link-State IGPs"
  <xref target="RFC8405" format="default"/>. If supported, configuration of the INITIAL_SPF_DELAY, SHORT_SPF_DELAY,
  LONG_SPF_DELAY, TIME_TO_LEARN, and HOLDDOWN_INTERVAL MUST <bcp14>MUST</bcp14> be supported <xref target="RFC8405" format="default"/>.
  Section 6 of
  <xref section="6" target="RFC8405" format="default"/> recommends consistent configuration of these values
  throughout the IGP routing domain domain, and this also applies to the BGP SPF Routing Domain.
</t>
</section>
<section anchor="bgp-ls-spf-readvertisement-delay" numbered="true" toc="default">
<name>BGP-LS-SPF NLRI Readvertisement Delay</name>
<t>
  The configuration parameter that specifies the delay for readvertising a more recent
  instance of a self-originated NLRI when received more than once in succession
  is BGP_LS_SPF_SELF_READVERTISEMENT_DELAY. The default is 5 seconds.
</t>
</section>
<section anchor="Operation" numbered="true" toc="default">
<name>Operational Data</name>
<t>
  In order to troubleshoot SPF issues, implementations SHOULD <bcp14>SHOULD</bcp14> support an SPF log including
  entries for previous SPF computations. Each SPF log entry would include the BGP-LS-SPF NLRI SPF
  triggering the SPF, SPF scheduled time, SPF start time time, and SPF end time.
  Since the size of the log is finite, implementations
  SHOULD
  <bcp14>SHOULD</bcp14> also maintain counters for the total number of SPF computations and the
  total number of SPF triggering events. Additionally, to troubleshoot troubleshooting should be available for SPF scheduling and
  back-off <xref target="RFC8405" format="default"/>, the current SPF back-off state, the remaining time-to-learn,
  the remaining hold-down interval, the last trigger event time, the last SPF time, and the next SPF time should be
  available. time.
</t>
</section>
<section anchor="bgp-ls-spf-isolation" numbered="true" toc="default">
<name>BGP-LS-SPF Address Family Session Isolation</name>
<t>
  In common deployment scenarios, the unicast routes installed during
  BGP-LS-SPF AFI/SAFI SPF computation serve as the
  underlay for other BGP AFI/SAFIs.
  To avoid errors encountered in other AFI/SAFIs from impacting
  the BGP-LS-SPF AFI/SAFI or vice versa, isolation mechanisms such as
  separate BGP instances or separate BGP sessions (e.g., using different
  addresses for peering) for BGP SPF Link-State information distribution
  SHOULD be used.
</t>
</section>
</section>
<section anchor="implementation" numbered="true" toc="default">
<name>Implementation Status</name>
<t>
 Note RFC Editor: Please remove this section and the associated references
 prior to publication.
</t>
<t>
  This section records the status of known implementations of the
  protocol defined by this specification at the time of posting of
  this Internet-Draft and is based on a proposal described in
  <xref target="RFC7942" format="default"/>.  The description of implementations in this section is
  intended to assist the IETF in its decision processes in
  progressing drafts to RFCs.  Please note that the listing of any
  individual implementation here does not imply endorsement by the
  IETF.  Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors.
  This is not intended as, and must not
  <bcp14>SHOULD</bcp14> be construed to be, a
  catalog of available implementations or their features.  Readers
  are advised to note that other implementations may exist.
</t>
<t>
  According to RFC 7942, "this will allow reviewers and working
  groups to assign due consideration to documents that have the
  benefit of running code, which may serve as evidence of valuable
  experimentation and feedback that have made the implemented
  protocols more mature.  It is up to the individual working groups
  to use this information as they see fit".
</t>
<t>
  The document <xref target="I-D.psarkar-lsvr-bgp-spf-impl" format="default"/>
  contains an implementation report documenting implementations of
  BGP Link-State Short Path First (SPF) routing, i.e., this specification. used.
</t>
</section>
<section anchor="Acknowledgements" numbered="true" toc="default">
  <name>Acknowledgements</name>
<t>
  The
  <t>The authors would like to thank Sue Hares, Jorge Rabadan, Boris Hassanov, Dan Frost,
  Matt Anderson, Fred Baker, Lukas Krattiger, Yingzhen Qu, and Haibo Wang <contact fullname="Sue Hares"/>,
  <contact fullname="Jorge Rabadan"/>, <contact fullname="Boris Hassanov"/>,
  <contact fullname="Dan Frost"/>, <contact fullname="Matt Anderson"/>,
  <contact fullname="Fred Baker"/>, <contact fullname="Lukas Krattiger"/>,
  <contact fullname="Yingzhen Qu"/>, and <contact fullname="Haibo Wang"/> for
  their
  review reviews and comments. Thanks to Pushpasis Sarkar <contact fullname="Pushpasis Sarkar"/>
  for discussions on preventing a BGP SPF Router from being used for non-local
  traffic (i.e., transit traffic).
</t>
<t>
  The traffic).</t>
  <t>The authors extend a special thanks to Eric Rosen <contact fullname="Eric Rosen"/> for
  fruitful discussions on BGP-LS-SPF convergence as compared to IGPs.
</t>
<t> IGPs.</t>

<!--[rfced] FYI - To avoid the repetition of "The authors would like
to thank" in the Acknowledgements, we updated the text as
follows:

Original:
   The authors would like extend thanks Alvaro Retana for
   multiple AD reviews and discussions.
</t>
<t>

   The authors would to thank Ketan Talaulikar for an extensive
   shepherd review.
</t>
<t>

   The authors would like to thank Adrian Farrel, Li Zhang, and Jie Dong
   for WG last call review comments.
</t>
<t>

   The authors would to like to thank Jim Guichard for his AD review
   and discussion.
</t>
<t>

   The authors would to like to thank David Dong for his IANA review.
</t>
<t>

   The authors would to like to thank Joel Halpern for his GENART review.
</t>
<t>

   The authors would to like to thank Erik Kline, Eric Vyncke, Mahesh
   Jethanandani, and Roman Danyliw for IESG review comments.
</t>
<t>

   The authors would to like to thank John Scudder for his detailed
   IESG review and specifically for helping align the document with
   BGP documents.
</t>

Current:
   The authors would also like to thank the following people:

   *  Alvaro Retana for multiple AD reviews and discussions.

   *  Ketan Talaulikar for an extensive shepherd review.

   *  Adrian Farrel, Li Zhang, and Jie Dong for WG Last Call review
      comments.

   *  Jim Guichard for his AD review and discussion.

   *  David Dong for his IANA review.

   *  Joel Halpern for his GENART review.

   *  Erik Kline, Eric Vyncke, Mahesh Jethanandani, and Roman Danyliw
      for IESG review comments.

   *  John Scudder for his detailed IESG review and specifically for
      helping align the document with BGP documents.
-->

  <t>The authors would also like to thank the following people:</t>
<ul empty="false">
  <li><t><contact fullname="Alvaro Retana"/> for multiple AD
  reviews and discussions.</t></li>
  <li><t><contact fullname="Ketan Talaulikar"/> for an
  extensive shepherd review.</t></li>
  <li><t><contact fullname="Adrian Farrel"/>,
  <contact fullname="Li Zhang"/>, and <contact fullname="Jie Dong"/> for WG
  Last Call review comments.</t></li>
  <li><t><contact fullname="Jim Guichard"/> for his AD review and
  discussion.</t></li>
  <li><t><contact fullname="David Dong"/> for his IANA review.</t></li>
  <li><t><contact fullname="Joel Halpern"/> for his GENART review.</t></li>
  <li><t><contact fullname="Erik Kline"/>,
  <contact fullname="Eric Vyncke"/>, <contact fullname="Mahesh
  Jethanandani"/>, and <contact fullname="Roman Danyliw"/> for IESG review
  comments.</t></li>
  <li><t><contact fullname="John Scudder"/> for his detailed IESG
  review and specifically for helping align the document with BGP documents.</t></li>
</ul>
</section>
<section anchor="Contributors" numbered="true" toc="default">
<name>Contributors</name>
<t>
  In addition to the authors listed on the front page, the
<t>The following
  co-authors have people contributed substantially to the document.
</t>
<artwork align="left" name="" type="" alt=""><![CDATA[
Derek Yeung
Arrcus, Inc.
derek@arrcus.com

Gunter content of this document and should be considered
coauthors:</t>

    <contact fullname="Derek Yeung">
      <organization>Arrcus, Inc.</organization>
      <address>
        <email>derek@arrcus.com</email>
      </address>
    </contact>

    <contact fullname="Gunter Van De Velde
Nokia
gunter.van_de_velde@nokia.com

Abhay Roy
Arrcus, Inc.
abhay@arrcus.com

Venu Venugopal
Cisco Systems
venuv@cisco.com

Chaitanya Yadlapalli
AT&T
cy098d@att.com
]]></artwork> Velde">
      <organization>Nokia</organization>
      <address>
        <email>gunter.van_de_velde@nokia.com</email>
      </address>
    </contact>

    <contact fullname="Abhay Roy">
      <organization>Arrcus, Inc.</organization>
      <address>
        <email>abhay@arrcus.com</email>
      </address>
    </contact>

    <contact fullname="Venu Venugopal">
      <organization>Cisco Systems</organization>
      <address>
        <email>venuv@cisco.com</email>
      </address>
    </contact>

    <contact fullname="Chaitanya Yadlapalli">
      <organization>AT&amp;T</organization>
      <address>
        <email>cy098d@att.com</email>
      </address>
    </contact>

</section>
</section>
</middle>
<!--  *****BACK MATTER ***** -->

<back>
<references><name>References</name>
<references><name>Normative References</name>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.2119.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.2328.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2328.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.4202.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4202.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.4271.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4271.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.4760.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4760.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.5305.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5305.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.5130.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5130.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.5880.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5880.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.5925.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5925.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.6793.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6793.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.6811.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6811.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.6119.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6119.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.7606.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7606.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.8126.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.8174.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.8205.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8205.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.8405.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8405.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.8654.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8654.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.9086.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9086.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.9552.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9552.xml"/>
</references>
<references><name>Informational
<references><name>Informative References</name>
<!-- Note: Removed references to [RFC7942] and [I-D.psarkar-lsvr-bgp-spf-impl] per authors' note. -->

<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.4272.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.4456.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4272.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.4593.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4456.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.4724.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4593.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.5286.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4724.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.6952.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5286.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.7911.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6952.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.7938.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7911.xml"/>
<xi:include href="https://www.rfc-editor.org/refs/bibxml/reference.RFC.7942.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7938.xml"/>

<!-- draft-ietf-lsvr-applicability-22. IESG State: RFC Ed Queue as of 06/06/25 - C529 companion doc. -->
<xi:include href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-lsvr-applicability.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.psarkar-lsvr-bgp-spf-impl.xml"/>

<!-- [rfced] Some author comments are present in the XML. Please
confirm that no updates related to these comments are
outstanding. Note that the comments will be deleted prior to
publication.
-->

<!-- [rfced] Terminology

1) Throughout the text, the following terminology appears to be used
inconsistently. Please review these occurrences and let us know
if/how they may be made consistent.

 BGP Router vs. BGP router

 BGP SPF Router vs. BGP SPF router vs. BGP-SPF router

 BGP SPF Routing Domain vs. BGP SPF routing domain

 BGP-LS Attribute vs. BGP-LS attribute
   [Note: uppercase used in RFC 9552]

 BGP-LS Prefix Attribute vs. BGP-LS Prefix attribute

 BGP-LS-LINK NLRI vs. BGP-LS Link NLRI

 BGP-LS-SPF NLRI vs. BGP-LS-SPF Link NLRI
   [Note: are these terms different or the same?]

 BGP-LS-SPF Node NLRI vs. BGP-LS-SPF NLRI
   [Note: are these terms different or the same?]

 Decision Process vs. decision process
   [Note: uppercase used in RFC 4271]

 Remote Identifier vs. Remote Link Identifier
   [Note: are these terms different or the same?]

 Remote Node NLRI vs. Remote-Node NLRI

 UPDATE message vs. Update message vs. update message
   [Note: should this be "UPDATE message" per RFC 7606?]

2) FYI - We updated the following terms to reflect the forms on the right
for consistency. Please let us know of any objections.

 AS Number (TLV 512) -> Autonomous System (TLV 512) (per RFC 9552)
 back-off algorithm -> Back-Off algorithm (per RFC 8405)
 Error Handling -> error handling
 BGP update -> BGP Update
 BGP-LS Link Attributes -> BGP-LS Attributes (1 instance)
 BGP-LS-SPF LINK NLRI -> BGP-LS-SPF Link NLRI
 EoR Marker -> EoR marker (per RFC 4724)
 IGP metric attribute TLV (TLV 1095) -> IGP Metric (TLV 1095) (per RFC 9552)
 local and remote node descriptors ->  Local and Remote Node Descriptors
 local node descriptor -> Local Node Descriptor
 local/remote link identifiers -> Local/Remote Link Identifiers  XX
 NLRI Link -> Link NLRI
 Node identifiers -> Node Identifiers
 phase 1 -> Phase 1
 Route Reflector -> route reflector (per RFC 4456)
 SAFI BGP-LS-SPF BGP Update -> BGP-LS-SPF SAFI BGP Update
 set 1 -> Step 1
 Ships-in-the-Night -> ships-in-the-night (per other RFCs)
 SPF back-off -> SPF Back-Off (per RFC 8405)
 Treat-as-withdraw -> treat-as-withdraw (per RFC 7606)
 Unequal Cost Multi-Path -> Unequal-Cost Multipath
 Unicast -> unicast

3) In this document, we see one occurence of "BGP-LS-SPF Attribute TLV",
and it is not used in any other RFCs. Is this form correct or should it
perhaps be "BGP-LS-SPF attribute" or other?

Original:
   The BGP-LS-SPF Attribute TLV of the BGP-LS-SPF Link NLRI is defined
   to indicate the status of the link with respect to the BGP SPF
   calculation.

4) In this document, we see one occurence of "BGP-LS Node attribute".
Should this be "BGP-LS attribute" or other for consistency?

Original:
   If the BGP-LS Node attribute includes an SPF Status TLV
   (refer to Section 5.2.1.1) indicating the node is
   unreachable, the Node NLRI is ignored and the next lowest
   cost Node NLRI is selected from the CAN-LIST.

5) Should "local/remote link identifiers" perhaps be "Link
Local/Remote Identifiers" for consistency?

Original:
   For a link to be used in SPF computation for a given address family,
   i.e., IPv4 or IPv6, both routers connecting the link MUST have
   matching addresses (i.e., router interface addresses must be on the
   same subnet for numbered interfaces and the local/remote link
   identifiers (Section 6.3) must match for unnumbered interfaces).

Perhaps:
   For a link to be used in SPF computation for a given address family,
   i.e., IPv4 or IPv6, both routers connecting the link MUST have
   matching addresses (i.e., router interface addresses must be on the
   same subnet for numbered interfaces, and the Link Local/Remote
   Identifiers (Section 6.3) must match for unnumbered interfaces).

6) We note inconsistencies with "next hop". How may we update this term
for consistency?

 Next-Hop vs. Next Hop vs. next-hop vs. next hop

 Some instances in the document:
  BGP Next-Hop
  Current-Node's next-hops
  Local-RIB Next-Hop
  Local-RIB route's next-hops
  MP_REACH_NLRI Next-Hop
  The Next Hop in the MP_REACH_NLRI attribute
  (i.e., next hops)
  the next-hop for...

Perhaps:
  BGP Next-Hop (per RFC 9552)
  Local-RIB Next-Hop
  MP_REACH_NLRI Next-Hop

  When used in general: lowercase open form and hyphenated
    when preceding a noun (e.g., "The next-hop list is set
    to the internal loopback next hop").
-->

<!-- [rfced] Abbreviations

1) FYI - We have added expansions for the following abbreviations
per Section 3.6 of RFC 7322 ("RFC Style Guide"). Please review each
expansion in the document carefully to ensure correctness.

 Autonomous System (AS)
 Bidirectional Forwarding Detection (BFD)
 Network Layer Reachability Information (NLRI)
 Unequal-Cost Multipath (UCMP)

2) We note "LSDB" and "LSNDB". Are these different databases or
should they be updated for consistency?

 Link-State Database (LSDB) (per RFC 9552)
 Link-State NLRI Database (LSNDB)
-->

<!-- [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.

Note that our script did not flag any words in particular, but this should
still be reviewed as a best practice.
-->

</references>
</references>
</back>
</rfc>