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BGP Link-State Shortest Path First (SPF) Routing
draft-ietf-lsvr-bgp-spf-31

Document Type Active Internet-Draft (lsvr WG)
Authors Keyur Patel , Acee Lindem , Shawn Zandi , Wim Henderickx
Last updated 2024-06-17
Replaces draft-keyupate-lsvr-bgp-spf
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Send notices to Gunter Van de Velde <gunter.van_de_velde@nokia.com>, aretana.ietf@gmail.com, victor@jvknet.com, ketant.ietf@gmail.com
draft-ietf-lsvr-bgp-spf-31
Link State Vector Routing (LSVR) Working Group                  K. Patel
Internet-Draft                                              Arrcus, Inc.
Intended status: Standards Track                               A. Lindem
Expires: 19 December 2024                           LabN Consulting, LLC
                                                                S. Zandi
                                                                LinkedIn
                                                           W. Henderickx
                                                                   Nokia
                                                            17 June 2024

            BGP Link-State Shortest Path First (SPF) Routing
                       draft-ietf-lsvr-bgp-spf-31

Abstract

   Many Massively Scaled Data Centers (MSDCs) have converged on
   simplified layer 3 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 use BGP Link-State 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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 19 December 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  BGP Shortest Path First (SPF) Motivation  . . . . . . . .   4
     1.3.  Document Overview . . . . . . . . . . . . . . . . . . . .   6
     1.4.  Requirements Language . . . . . . . . . . . . . . . . . .   6
   2.  Base BGP Protocol Relationship  . . . . . . . . . . . . . . .   6
   3.  BGP Link-State (BGP-LS) Relationship  . . . . . . . . . . . .   7
   4.  BGP SPF Peering Models  . . . . . . . . . . . . . . . . . . .   7
     4.1.  BGP Single-Hop Peering on Network Node Connections  . . .   7
     4.2.  BGP Peering Between Directly-Connected Nodes  . . . . . .   8
     4.3.  BGP Peering in Route-Reflector or Controller Topology . .   8
   5.  BGP Shortest Path Routing (SPF) Protocol Extensions . . . . .   9
     5.1.  BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . . .   9
       5.1.1.  BGP-LS-SPF NLRI TLVs  . . . . . . . . . . . . . . . .   9
       5.1.2.  BGP-LS Attribute  . . . . . . . . . . . . . . . . . .  10
     5.2.  Extensions to BGP-LS  . . . . . . . . . . . . . . . . . .  10
       5.2.1.  Node NLRI Usage . . . . . . . . . . . . . . . . . . .  11
         5.2.1.1.  BGP-LS-SPF Node NLRI Attribute SPF Status TLV . .  11
       5.2.2.  Link NLRI Usage . . . . . . . . . . . . . . . . . . .  12
         5.2.2.1.  BGP-LS Link NLRI Adddress Family Link Descriptor
                 TLV . . . . . . . . . . . . . . . . . . . . . . . .  13
         5.2.2.2.  BGP-LS-SPF Link NLRI Attribute SPF Status TLV . .  13
       5.2.3.  IPv4/IPv6 Prefix NLRI Usage . . . . . . . . . . . . .  14
         5.2.3.1.  BGP-LS-SPF Prefix NLRI Attribute SPF Status
                 TLV . . . . . . . . . . . . . . . . . . . . . . . .  15
       5.2.4.  BGP-LS Attribute Sequence-Number TLV  . . . . . . . .  15
     5.3.  NEXT_HOP Attribute Manipulation . . . . . . . . . . . . .  16
   6.  Decision Process with SPF Algorithm . . . . . . . . . . . . .  17
     6.1.  BGP SPF NLRI Selection  . . . . . . . . . . . . . . . . .  18
       6.1.1.  BGP Self-Originated NLRI  . . . . . . . . . . . . . .  19
     6.2.  Dual Stack Support  . . . . . . . . . . . . . . . . . . .  19
     6.3.  SPF Calculation based on BGP-LS-SPF NLRI  . . . . . . . .  20
     6.4.  IPv4/IPv6 Unicast Address Family Interaction  . . . . . .  24
     6.5.  NLRI Advertisement  . . . . . . . . . . . . . . . . . . .  24
       6.5.1.  Link/Prefix Failure Convergence . . . . . . . . . . .  25
       6.5.2.  Node Failure Convergence  . . . . . . . . . . . . . .  25
   7.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  26

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     7.1.  Processing of BGP-LS-SPF TLVs . . . . . . . . . . . . . .  26
     7.2.  Processing of BGP-LS-SPF NLRIs  . . . . . . . . . . . . .  27
     7.3.  Processing of BGP-LS Attribute  . . . . . . . . . . . . .  27
     7.4.  BGP-LS-SPF Link State NLRI Database Synchronization . . .  28
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     8.1.  BGP-LS-SPF Allocation in SAFI Parameters Registry . . . .  28
     8.2.  BGP-LS Address Family Link Descriptor . . . . . . . . . .  28
     8.3.  BGP-LS-SPF Assignments to BGP-LS NLRI and Attribute TLV
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  28
     8.4.  BGP-LS-SPF Node NLRI Attribute SPF Status TLV Status
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.5.  BGP-LS-SPF Link NLRI Attribute SPF Status TLV Status
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.6.  BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV Status
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  30
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
   10. Management Considerations . . . . . . . . . . . . . . . . . .  31
     10.1.  Configuration  . . . . . . . . . . . . . . . . . . . . .  31
     10.2.  Link Metric Configuration  . . . . . . . . . . . . . . .  31
     10.3.  Unnumbered Link Configuration  . . . . . . . . . . . . .  32
     10.4.  Adjacency End-of-RIB (EOR) Marker Requirement  . . . . .  32
     10.5.  backoff-config . . . . . . . . . . . . . . . . . . . . .  32
     10.6.  BGP-LS-SPF NLRI Readvertisemnt Delay . . . . . . . . . .  32
     10.7.  Operational Data . . . . . . . . . . . . . . . . . . . .  32
     10.8.  BGP-LS-SPF Address Family Session Isolation  . . . . . .  33
   11. Implementation Status . . . . . . . . . . . . . . . . . . . .  33
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  33
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  34
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     14.2.  Informational References . . . . . . . . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

1.  Introduction

   Many Massively Scaled Data Centers (MSDCs) have converged on
   simplified layer 3 routing.  Furthermore, requirements for
   operational simplicity has led many of these MSDCs to converge on BGP
   [RFC4271] as their single routing protocol for both their fabric
   routing and their Data Center Interconnect (DCI) routing [RFC7938].
   This document describes an alternative solution which leverages BGP-
   LS [I-D.ietf-idr-rfc7752bis] and the Shortest Path First algorithm
   used by Internal Gateway Protocols (IGPs).

   This document leverages both the BGP protocol [RFC4271] and the BGP-
   LS [I-D.ietf-idr-rfc7752bis] protocols.  The relationship, as well as
   the scope of changes is described respectively in Section 2 and
   Section 3.  The modifications to [RFC4271] for BGP SPF described

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   herein only apply to IPv4 and IPv6 as underlay unicast Subsequent
   Address Families Identifiers (SAFIs).  Operations for any other BGP
   SAFIs are outside the scope of this document.

   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 advertisement.  These advantages can
   reduce the overhead in MSDCs where there is a high degree of Equal
   Cost Multi-Path (ECMPs) and the topology is very stable.
   Additionally, using an SPF-based computation can support fast
   convergence and the computation of Loop-Free Alternatives (LFAs).
   The SPF LFA extensions defined in [RFC5286] can be similarly applied
   to BGP SPF calculations.  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 [RFC4456] or
   controllers.

1.1.  Terminology

   This specification reuses terms defined in section 1.1 of [RFC4271]
   including BGP speaker, NLRI, and Route.

   Additionally, this document introduces the following terms:

   BGP SPF Routing Domain:  A set of BGP routers that are under a single
      administrative domain and exchange link-state information using
      the BGP-LS-SPF SAFI and compute routes using BGP SPF as described
      herein.

   BGP-LS-SPF NLRI:  This refers to BGP-LS Network Layer Reachability
      Information (NLRI) that is being advertised in the BGP-LS-SPF SAFI
      (Section 5.1) and is being used for BGP SPF route computation.

   Dijkstra Algorithm:  An algorithm for computing the shortest path
      from a given node in a graph to every other node in the graph.

1.2.  BGP Shortest Path First (SPF) Motivation

   Given that [RFC7938] already describes how BGP could be used as the
   sole routing protocol in an MSDC, one might question the motivation
   for defining an alternate 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 overlay.  However, BGP SPF offers some
   unique advantages above and beyond standard BGP distance-vector
   routing.  With BGP SPF, the standard hop-by-hop peering model is
   relaxed.

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   A primary advantage is that all BGP SPF 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)
   [RFC5286], Shared Risk Link Groups (SRLGs) [RFC4202], and other
   routing enhancements without advertisement of additional BGP paths
   [RFC7911] or other extensions.

   With the BGP SPF decision process as defined in Section 6, 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.

   Another primary advantage is a potential reduction in NLRI
   advertisement.  With standard BGP distance-vector routing, a single
   link failure may impact 100s or 1000s prefixes and result in the
   withdrawal or re-advertisement of the attendant NLRI.  With BGP SPF,
   only the BGP SPF 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.

   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 only the latest version of the
   NLRI from the originator is required (sed Section 4).  Given that
   verification of the adjacencies is done outside of BGP, each BGP SPF
   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.

   Given BGP-LS NLRI is already defined [I-D.ietf-idr-rfc7752bis], this
   functionality can be reused for BGP-LS-SPF NLRI.

   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 NLRIs.
   In many MSDC fabrics, the IPv4 and IPv6 topologies are congruent
   (refer to Section 5.2.2 and Section 5.2.3).  Although 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.

   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.

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1.3.  Document Overview

   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
   [I-D.ietf-idr-rfc7752bis] (Section 3).  The BGP peering models, as
   well as their respective trade-offs are then discussed in Section 4.
   The remaining sections, which make up the bulk of the document,
   define the protocol enhancements necessary to support BGP SPF
   including BGP-LS Extensions (Section 5), replacement of the base BGP
   decision process with the SPF computation (Section 6), and BGP SPF
   error handling (Section 7).

1.4.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Base BGP Protocol Relationship

   With the exception of the decision process, the BGP SPF extensions
   leverage the BGP protocol [RFC4271] without change.  This includes
   the BGP protocol Finite State Machine, BGP messages and their
   encodings, processing of BGP messages, BGP attributes and path
   attributes, BGP NLRI encodings, and any error handling defined in
   [RFC4271], [RFC4760], and [RFC7606].

   Due to the changes to 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 be
   advertised.  If received, all path attributes MUST be accepted,
   validated, and propagated consistent with the BGP protocol [RFC4271],
   even if not needed by BGP SPF.

   Section 9 of [RFC4271] 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 according to policy.

   The BGP SPF extension fundamentally changes the decision process, as
   described herein.  Specifically:

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   1.  BGP advertisements are readvertised to neighbors immediately
       without waiting or dependence on the route computation as
       specified in phase 3 of the base BGP decision process.  Multiple
       peering models are supported as specified in Section 4.

   2.  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.

   3.  Phase 2 of the base BGP protocol decision process is replaced
       with the Shortest Path First (SPF) algorithm, also known as the
       Dijkstra algorithm.

3.  BGP Link-State (BGP-LS) Relationship

   [I-D.ietf-idr-rfc7752bis] 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 advertised using the BGP-LS AFI.  The BGP-
   LS extensions defined in [I-D.ietf-idr-rfc7752bis] make use of the
   decision process defined in [RFC4271].  Rather than reusing the BGP-
   LS SAFI, the BGP-LS-SPF SAFI (Section 5.1) is introduced to insure
   backward compatibility for the BGP-LS SAFI usage.

   The BGP SPF extensions reuse the format of the Link-State NLRI, the
   BGP-LS Attribute, and the TLVs defined in [I-D.ietf-idr-rfc7752bis].
   The usage of is described in Section 5.2.  The usage of other BGP-LS
   TLVs or extensions is not precluded and is, in fact, expected.
   However, the details are beyond the scope of this document and may be
   specified in future documents.

4.  BGP SPF Peering Models

   Depending on the topology, scaling, capabilities of the BGP SPF
   speakers, and redundancy requirements, various peering models are
   supported.  The only requirement is that all BGP SPF speakers in the
   BGP SPF routing domain adhere to this specification.

4.1.  BGP Single-Hop Peering on Network Node Connections

   The simplest peering model is the one where EBGP single-hop sessions
   are established over direct point-to-point links interconnecting the
   nodes in the BGP SPF routing domain.  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, then the link is considered up from a BGP SPF
   perspective and the corresponding BGP-LS-SPF Link NLRI is advertised.

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   An End-of-RIB (EoR) Marker [RFC4724] for the BGP-LS-SPF SAFI MAY be
   expected prior to advertising the BGP-LS Link NLRI for to peer.

   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.

   If the session goes down, the corresponding Link NLRI are withdrawn.
   Topologically, this would be equivalent to the peering model in
   [RFC7938] where there is a BGP session on every link in the data
   center switch fabric.  The content of the Link NLRI is described in
   Section 5.2.2.

4.2.  BGP Peering Between Directly-Connected Nodes

   In this model, BGP SPF speakers peer with all directly-connected
   nodes but the sessions may be between loopback addresses (i.e., two-
   hop sessions) and the direct connection discovery and liveliness
   detection for the interconnecting links are independent of the BGP
   protocol.  The BFD protocol [RFC5880] is RECOMMENDED for liveliness
   detection.  Usage of other liveliness 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
   SPF 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 [RFC4760], the link is operational as determined
   using liveliness detection mechanisms, and, optionally, the EoR
   Marker has been received as described in the Section 4.1.  This is
   much like the previous peering model only peering is between loopback
   addresses and the interconnecting links can be unnumbered.  However,
   since there are BGP sessions between every 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 over the single-hop peering model Section 4.1.

   An End-of-RIB (EoR) Marker [RFC4724] for the BGP-LS-SPF SAFI MAY also
   be expected prior to advertising the BGP-LS Link NLRI for the link(s)
   to this peer.

4.3.  BGP Peering in Route-Reflector or Controller Topology

   In this model, BGP SPF speakers peer solely with one or more Route
   Reflectors [RFC4456] or controllers.  As in the previous model,
   direct connection discovery and liveliness 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 as per the chosen liveness detection mechanism.

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   This peering model, known as sparse peering, allows for fewer BGP
   sessions and, consequently, fewer instances of the same NLRI received
   from multiple peers.  Normally, the route-reflectors or controller
   BGP sessions would be on 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.  This is discussed in greater detail in
   [I-D.ietf-lsvr-applicability].

   The controller may use constraints to determine when to advertise
   BGP-LS-SPF NLRI for BGP-LS peers.  For example, a controller may
   defer advertisement until the EoR marker has been received from both
   BGP peers and both have received each other's NLRI.  These
   constraints are outside the scope of this document and, since they
   are internal to the controller, need not be standardized.

5.  BGP Shortest Path Routing (SPF) Protocol Extensions

5.1.  BGP-LS Shortest Path Routing (SPF) SAFI

   This document introduces the BGP-LS-SPF SAFI with a value of 80.  The
   SPF-based decision process (Section 6) applies only to the BGP-LS-SPF
   SAFI and MUST NOT be used with other combinations of the BGP-LS AFI
   (16388).  In order for two BGP SPF speakers to exchange BGP-LS-SPF
   NLRI, they MUST exchange the Multiprotocol Extensions Capability
   [RFC4760] to ensure that they are both capable of properly processing
   such 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.

5.1.1.  BGP-LS-SPF NLRI TLVs

   All the TLVs defined for BGP-LS [I-D.ietf-idr-rfc7752bis] are
   applicable and can be used with the BGP-LS-SPF SAFI to describe
   links, nodes, and prefixes comprising BGP-SPF LSDB information.

   The NLRI and comprising TLVs MUST be encoded as specified in section
   5.1 [I-D.ietf-idr-rfc7752bis].  TLVs specified as mandatory in
   [I-D.ietf-idr-rfc7752bis] are considered mandatory for the BGP-LS-SPF
   SAFI as well.  If a mandatory TLV is not present, the NLRI MUST NOT
   be used in the BGP SPF route calculation.  All the other TLVs are
   considered as optional TLVs.  Documents specifying usage of optional
   TLV for BGP SPF MUST address backward compatibility.

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5.1.2.  BGP-LS Attribute

   The BGP-LS attribute of the BGP-LS-SPF SAFI uses exactly same format
   of the BGP-LS AFI [I-D.ietf-idr-rfc7752bis].  In other words, all the
   TLVs used in the BGP-LS attribute of the BGP-LS AFI are applicable
   and 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.

   All the TLVs defined for the BGP-LS Attribute
   [I-D.ietf-idr-rfc7752bis] are applicable and can be used with the
   BGP-LS-SPF SAFI to carry link, node, and prefix properties and
   attributes.

   The BGP-LS attribute may potentially be quite large depending on the
   amount of link-state information associated with a single Link- State
   NLRI.  The BGP specification [RFC4271] mandates a maximum BGP message
   size of 4096 octets.  It is RECOMMENDED that an implementation
   support [RFC8654] in order to accommodate a greater amount of
   information within the BGP-LS Attribute.  BGP SPF speakers MUST
   ensure that they limit the TLVs included in the BGP-LS Attribute to
   ensure that a BGP update message for a single Link-State NLRI does
   not cross the maximum limit for a BGP message.  The determination of
   the types of TLVs to be included by the BGP SPF speaker originating
   the attribute is outside the scope of this document.  When a BGP SPF
   speaker finds that it is exceeding the maximum BGP message size due
   to addition or update of some other BGP Attribute (e.g., AS_PATH), it
   MUST consider the BGP-LS Attribute to be malformed and the attribute
   discard handling of [RFC7606] applies.

5.2.  Extensions to BGP-LS

   [I-D.ietf-idr-rfc7752bis] 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.

   The protocol identifier specified in the Protocol-ID field
   [I-D.ietf-idr-rfc7752bis] represents the origin of the advertised
   NLRI.  For Node NLRI and Link NLRI, the specified Protocol-ID MUST be
   the direct protocol (4).  Node or Link NLRI with a Protocol-ID other

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   than the direct protocol is considered malformed.  For Prefix NLRI,
   the specified Protocol-ID MUST be the origin of the prefix.  The
   local and remote node descriptors for all NLRI MUST include the BGP
   Router-ID (TLV 516) [RFC9086] and the AS Number (TLV 512)
   [I-D.ietf-idr-rfc7752bis].  The BGP Confederation Member (TLV 517)
   [RFC9086] is not applicable.

5.2.1.  Node NLRI Usage

   The Node NLRI MUST be advertised unconditionally by all routers in
   the BGP SPF routing domain.

5.2.1.1.  BGP-LS-SPF Node NLRI Attribute SPF Status TLV

   A BGP-LS Attribute 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 Section 6.5.2) or to indicate the node is not to be used
   for transit (i.e., non-local) traffic (refer to Section 6.3).  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.

    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 Status Values: 0 - Reserved
                      1 - Node unreachable with respect to BGP SPF
                      2 - Node does not support transit with respect
                          to BGP SPF
                      3-254 - Undefined
                      255 - Reserved

   The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
   Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
   (1184).

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   If a BGP SPF speaker received the Node NLRI but the SPF Status TLV is
   not received, then any previously received information is considered
   as implicitly withdrawn and the update is propagated to other BGP SPF
   speakers.  A BGP SPF speaker receiving a BGP Update containing a SPF
   Status TLV in the BGP-LS attribute [I-D.ietf-idr-rfc7752bis] with a
   value that is unknown SHOULD be advertised to other BGP SPF speakers.
   However, a BGP SPF speaker MUST NOT use the Status TLV in its SPF
   computation.  An implementation MAY log this condition for further
   analysis.

5.2.2.  Link NLRI Usage

   The criteria for advertisement of Link NLRI are discussed in
   Section 4.

   Link NLRI is advertised with unique local and remote node descriptors
   dependent on the IP addressing.  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.  For links supporting having both IPv4 and IPv6
   addresses, both sets of descriptors MAY be included in the same Link
   NLRI.

   For unnumbered links, the Link Local/Remote Identifiers (TLV 258) are
   used.  The Link Remote Identifier isn't normally exchanged in 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 be advertised.  When 0 is advertised and there
   parallel unnumbered links between a pair of BGP SPF speakers, there
   may be transient intervals where the BGP SPF speakers don't agree on
   which of the parallel unnumbered links are operational.  For this
   reason, it is RECOMMENDED that the Link Remote Identifiers be known
   (e.g., discovered using alternate mechanisms or configured) in the
   presence of parallel unnumbered links.

   The link descriptors are described in table 4 of
   [I-D.ietf-idr-rfc7752bis].  Additionally, an address family link
   descriptor is defined to determine whether an unnumbered link can be
   used in the IPv4 SPF, the IPv6, or both (refer to section
   Section 5.2.2.1).

   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., interface addresses must match the neighbor
   addresses).

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   The IGP metric attribute TLV (TLV 1095) MUST be advertised.  If a BGP
   SPF speaker receives a Link NLRI without an IGP metric attribute TLV,
   then it MUST consider the received NLRI as a malformed (refer to
   Section 7).  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 metric would be to give each
   interface a metric of 1 making it effectively a hop count.

   The usage of other link attribute TLVs is beyond the scope of this
   document.

5.2.2.1.  BGP-LS Link NLRI Adddress Family Link Descriptor TLV

   For unnumberred 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 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.

      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 (266)                  |      Length (1 Octet)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Address Family|
     +-+-+-+-+-+-+-+-+

     Addres Family Values: 0 - Reserved
                           1 - IPv4 SPF Computation
                           2 - IPv6 SPF Computation
                           3-254 - Undefined
                           255 - Reserved

5.2.2.2.  BGP-LS-SPF Link NLRI Attribute SPF Status TLV

   This 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 Section 6.5.1.  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
   Section 6.3.  A single TLV type is shared by the Node, Link, and

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   Prefix NLRI.  The TLV type is 1184.

      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 Status Values: 0 - Reserved
                        1 - Link Unreachable with respect to BGP SPF
                        2-254 - Undefined
                        255 - Reserved

   The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
   Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
   (1184).  This implies that a BGP Update will include a single NLRI
   unless they all have the same status.

   If a BGP SPF speaker received the Link NLRI but the SPF Status TLV is
   not received, then any previously received information is considered
   as implicitly withdrawn and the update is propagated to other BGP SPF
   speakers.  A BGP SPF speaker receiving a BGP Update containing an SPF
   Status TLV in the BGP-LS attribute [I-D.ietf-idr-rfc7752bis] with a
   value that is undefined SHOULD be advertised to other BGP SPF
   speakers.  However, a BGP SPF speaker MUST NOT use the Status TLV in
   its SPF computation.  An implementation MAY log this information for
   further analysis.

5.2.3.  IPv4/IPv6 Prefix NLRI Usage

   IPv4/IPv6 Prefix NLRI is advertised with a Local Node Descriptor and
   the prefix and length.  The Prefix Descriptors field includes the IP
   Reachability Information TLV (TLV 265) as described in
   [I-D.ietf-idr-rfc7752bis].  The Prefix Metric TLV (TLV 1155) MUST be
   advertised to be considered for route calculation.  The IGP Route Tag
   TLV (TLV 1153) MAY be advertised.  The usage of other BGP-LS
   attribute TLVs is beyond the scope of this document.

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5.2.3.1.  BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV

   A BGP-LS Attribute TLV to 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 as discussed in Section 6.5.1.  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.

        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 Status Values: 0 - Reserved
                          1 - Prefix Unreachable with respect to SPF
                          2-254 - Undefined
                          255 - Reserved

   The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
   Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
   (1184).  This implies that a BGP Update cannot contain multiple NLRI.

   If a BGP SPF speaker received the Prefix NLRI but the SPF Status TLV
   is not received, then any previously received information is
   considered as implicitly withdrawn and the update is propagated to
   other BGP SPF speakers.  A BGP SPF speaker receiving a BGP Update
   containing an SPF Status TLV in the BGP-LS attribute
   [I-D.ietf-idr-rfc7752bis] with a value that is undefined SHOULD be
   advertised to other BGP SPF speakers.  However, a BGP SPF speaker
   MUST NOT use the Status TLV in its SPF computation.  An
   implementation MAY log this information for further analysis.

5.2.4.  BGP-LS Attribute Sequence-Number TLV

   A BGP-LS Attribute 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 TLV contains an 8-octet sequence number.  The usage of the
   Sequence Number TLV is described in Section 6.1.

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      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)            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sequence Number: The 64-bit strictly-increasing sequence number MUST
   be incremented for every self-originated version of a BGP-LS-SPF
   NLRI.  BGP SPF speakers implementing this specification MUST use
   available mechanisms to preserve the sequence number's strictly
   increasing property for the deployed life of the BGP SPF 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.

   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
   SPF speaker completely loses its sequence number state (e.g., the BGP
   SPF speaker hardware is replaced or experiences a cold-start), the
   BGP NLRI selection rules (see Section 6.1) insure convergence, albeit
   not immediately.

   If the Sequence-Number TLV is not received, then the corresponding
   NLRI is considered as malformed and MUST be handled as 'Treat-as-
   withdraw'.  An implementation MAY log an error for further analysis.

5.3.  NEXT_HOP Attribute Manipulation

   The rules for setting the next hop information for the BGP-LS-SPF
   SAFI follow the specification in section 5.5 of
   [I-D.ietf-idr-rfc7752bis].  All BGP peers that support SPF extensions
   will locally compute the Local-RIB Next-Hop as a result of the SPF
   process.

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6.  Decision Process with SPF Algorithm

   The Decision Process described in [RFC4271] 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 SPF 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, 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.

   The SPF-based Decision process replaces the BGP Decision process
   described in [RFC4271].  Since Link-State NLRI always contains the
   local node descriptor as described in Section 5.2, each NLRI is
   uniquely originated by a single BGP SPF 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 SPF speakers
   would be indicative of a configuration error or a masquerading attack
   (refer to Section 9).  These selected Node NLRI and their Link/Prefix
   NLRI 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.

   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 Section 6.1.  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 be advertised
   immediately to other peers and prior to any SPF calculation.  Note
   that the BGP MinRouteAdvertisementIntervalTimer and
   MinASOriginationIntervalTimer [RFC4271] timers are not applicable to
   the BGP-LS-SPF SAFI.  The scheduling of the SPF calculation, as
   described in Section 6.3, is an implementation issue.  Scheduling MAY
   be dampened consistent with the SPF back-off algorithm specified in
   [RFC8405].

   The Phase 3 decision function of the Decision Process [RFC4271] is
   also simplified since under normal SPF operation, a BGP SPF speaker
   MUST 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 are unchanged NLRIs or stale NLRIs, i.e., NLRI received
   with a less recent (numerically smaller) sequence number.

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6.1.  BGP SPF NLRI Selection

   The rules for all BGP-LS-SPF NLRIs selection for phase 1 of the BGP
   decision process, section 9.1.1 [RFC4271], no longer apply.

   1.  NLRI originated by 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
       stale NLRI is updated even if a BGP-LS router loses its sequence
       number state due to a 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.

   2.  The NLRI with the most recent Sequence Number TLV, i.e., highest
       sequence number is selected.

   3.  The NLRI received from the BGP SPF speaker with the numerically
       larger BGP Identifier is preferred.

   When a BGP SPF speaker completely loses its sequence number state,
   i.e., due to a cold start, or in the unlikely possibility that 64-bit
   sequence number wraps, the BGP routing domain will still converge.
   This is due to the fact that BGP SPF speakers adjacent to the router
   always accept self-originated NLRI from the associated speaker as
   more recent (rule # 1).  When a BGP SPF speaker reestablishes a
   connection with its peers, any existing sessions are taken down and
   stale NLRI are replaced.  The adjacent BGP SPF speakers update their
   NLRI advertisements and advertise to their neighbors until the BGP
   routing domain has converged.

   The modified SPF Decision Process performs an SPF calculation rooted
   at the local BGP SPF speaker using the metrics from the Link
   Attribute IGP Metric TLV (1095) and the Prefix Attribute Prefix
   Metric TLV (1155) [I-D.ietf-idr-rfc7752bis].  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 [RFC4271] including ORIGIN, MULTI_EXIT_DISC, and
   LOCAL_PREF attributes are ignored by the SPF algorithm.  The NEXT_HOP
   attribute is discussed in Section 5.3.  The AS_PATH and AS4_PATH
   [RFC6793] attributes are preserved and used for loop detection
   [RFC4271].  They are ignored during the SPF computation for BGP-LS-
   SPF NLRIs.

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6.1.1.  BGP Self-Originated NLRI

   Node, Link, or Prefix NLRI with Node Descriptors matching the local
   BGP SPF speaker are considered self-originated.  When self-originated
   NLRI is received and it doesn't match the local node's NLRI content
   (including sequence number), special processing is required.

   *  If 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 and the NLRI is readvertised to
      all peers.

   *  If 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 and the NLRI is readvertised to all
      peers.

   *  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 withdrawn.

   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
   SPF speaker (refer to Section 9).

6.2.  Dual Stack Support

   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 matter.  Normally, IPv4 next-hops are calculated
   for IPv4 prefixes and IPv6 next-hops are calculated for IPv6
   prefixes.

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6.3.  SPF Calculation based on BGP-LS-SPF NLRI

   This section details the BGP-LS-SPF local routing information base
   (RIB) calculation.  The router uses BGP-LS-SPF Node, Link, and Prefix
   NLRI 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 yield the same set of routes.  The algorithm below
   supports Equal Cost Multi-Path (ECMP) routes.  Weighted Unequal Cost
   Multi-Path routes are out of scope.

   The following abstract data structures are defined in order to
   specify the algorithm.

   *  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.

   *  Global Routing Information Base (GLOBAL-RIB) - This is the Routing
      Information Base (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".

   *  Link State NLRI Database (LSNDB) - Database of BGP-LS-SPF NLRI
      that facilitates access to all Node, Link, and Prefix NLRI.

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

   The algorithm is comprised of the steps below:

   1.  The current Local-RIB is invalidated, and the CAN-LIST is
       initialized to 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 6.  These routes are referred to
       as stale routes.

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   2.  The computing router's Node NLRI is updated in the Local-RIB with
       a cost of 0 and the Node NLRI is also added to the CAN-LIST.  The
       next-hop list is set to the internal loopback next-hop.

   3.  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 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.  The Node corresponding
       to this NLRI is referred to as the Current-Node.  If the CAN-LIST
       list is empty, the SPF calculation has completed and the
       algorithm proceeds to step 6.

   4.  All the Prefix NLRI with the same Local Node Descriptors as the
       Current-Node are considered for installation.  The next-hop(s)
       for these Prefix NLRI 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) added to the cost to reach the Current-Node.  The
       following is done for each Prefix NLRI (referred to as the
       Current-Prefix):

       *  If the BGP-LS Prefix attribute includes an SPF Status TLV
          indicating the prefix is unreachable, the Current-Prefix is
          considered unreachable and the next Prefix NLRI is examined in
          Step 4.

       *  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.

       *  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) is
          included in the Current-Prefix's NLRI Attribute, the tag(s)
          are installed in the current Local-RIB route's tag(s).

       *  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 the Local-RIB route's next-hops and the metric being
          updated and any route tags removed.  If the IGP Route Tag TLV
          (1153) is included in the Current-Prefix's NLRI Attribute, the
          tag(s) are installed in the current Local-RIB route's tag(s).

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       *  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 Local-RIB route's
          next-hops.  The algorithm below supports Equal Cost Multi-Path
          (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.  Nonetheless, step 4 (below)
          includes a set of recommendations in case such a limit is
          encountered.  Weighted Unequal Cost Multi-Path routes are out
          of scope as well.

   5.  All the Link NLRI with the same Node Identifiers as the Current-
       Node are considered for installation.  Each link is examined and
       is referred to in the following text as the Current-Link.  The
       cost of the Current-Link is the advertised IGP Metric 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 inherited from the
       Current-Node.  The following is done for each link:

       a.  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 and the next link for the Current-Node is
           examined in Step 5.

       b.  If the Current-Node NLRI attributes includes the SPF Status
           TLV (refer to Section 5.2.1.1) and the status indicates that
           the Node doesn't support transit, the next link for the
           Current-Node is processed in Step 5.

       c.  The Current-Link's Remote Node NLRI is accessed (i.e., the
           Node NLRI with the same Node identifiers as the Current-
           Link's Remote Node Descriptors).  If it exists, it is
           referred to as the Remote-Node and the algorithm proceeds as
           follows:

           *  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.

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           *  All the Link NLRI corresponding 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.

              -  For IPv4/IPv6 numbered Link Descriptors to match during
                 the IPv4 SPF computation, the Current-Link's IP4/IPv6
                 interface address link descriptor MUST match the
                 Remote-Link IPv4/IPv6 neighbor address link descriptor
                 and the Current-Link's IPv4/IPv6 neighbor address MUST
                 match the Remode-Link's IPv4/IPv6 interface address.

              -  For unnumbered links to match during the IPv4 or IPv6
                 SPF computation, Current-Link and Remote-Link's Address
                 Family link descriptor must match address family of the
                 IPv4 or IPv6 SPF computation, the Current--Link's Local
                 Identifier MUST match the Remote-Link's Remote
                 Identifier, and the Current-Link's Remote Identifier
                 MUST the Remote-Link's Local Identifier.  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
                 [I-D.ietf-idr-rfc7752bis]).

              If these conditions are satisfied for one of the Remote-
              Node's links, the bi-directional connectivity check
              succeeds and the Remote-Node may be processed further.
              The Remote-Node's Link NLRI providing bi-directional
              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.

           *  If the Remote-Link NLRI attribute includes an SPF Status
              TLV indicating the link is down, the Remote-Link NLRI is
              considered down and the next link for the Current-Node is
              examined in Step 5.

           *  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
              Current-Node added the Current-Link's IGP Metric TLV
              (1095).  The next-hop(s) for the Remote-Node are inherited
              from the Current-Link.

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           *  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 inherited from the Current-Link.

           *  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.

           *  If the Remote-Node NLRI is already on the CAN-LIST with a
              lower cost, it need not be reinserted on the CAN-LIST.

       d.  Return to step 3 to process the next lowest cost Node NLRI on
           the CAN-LIST.

   6.  The Local-RIB is examined and changes (adds, deletes,
       modifications) are installed into the GLOBAL-RIB.  For each route
       in the Local-RIB:

       *  If the route was added during the current BGP SPF computation,
          install the route into the GLOBAL-RIB.

       *  If the route modified during the current BGP SPF computation
          (e.g., metric, tags, or next-hops), update the route in the
          GLOBAL-RIB.

       *  If the route was not installed during the current BGP SPF
          computation, remove the route from the GLOBAL-RIB.

6.4.  IPv4/IPv6 Unicast Address Family Interaction

   While the BGP-LS-SPF address family and the BGP unicast address
   families may install routes into the same device routing tables, they
   operate independently (i.e., "Ships-in-the-Night" mode).  There is no
   implicit route redistribution between the BGP-LS-SPF address family
   and the BGP unicast address families.

   It is RECOMMENDED 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.

6.5.  NLRI Advertisement

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6.5.1.  Link/Prefix Failure Convergence

   A local failure prevents a link from being used in the SPF
   calculation due to the IGP bi-directional connectivity requirement.
   Consequently, local link failures SHOULD always be given priority
   over updates in order to ensure the highest priority propagation and
   optimal convergence.

   With a BGP advertisement, 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 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
   Section 5.2.2.2) indicating the link is down with respect to BGP SPF.
   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 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 NLRI
   MUST 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.

   Similarly, when a prefix becomes unreachable, a more recent version
   of the BGP-LS-SPF Prefix NLRI SHOULD be advertised with the SPF
   Status TLV (refer to Section 5.2.3.1) indicating the prefix is
   unreachable in the BGP-LS Prefix 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
   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.

6.5.2.  Node Failure Convergence

   By default [RFC4271], all the NLRI advertised by a node are withdrawn
   when a session failure is detected.  If fast failure detection such
   as BFD is utilized, and the node is on the fastest converging path,
   the most recent versions of BGP-LS-SPF NLRI may be withdrawn.  This
   results in an older version of the NLRI received on a different path
   being used until the new versions arrive and, potentially,
   unnecessary route flaps.  For the BGP-LS-SPF SAFI, NLRI received from
   the failing node SHOULD NOT be implicitly withdrawn immediately to

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   prevent such unnecessary route flaps.  The configurable
   NLRIImplicitWithdrawalDelay timer controls the interval that NLRI
   from the failed node is retained prior to implicit withdrawal after a
   BGP SPF speaker has transitioned out of Established state.  This does
   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 Section 6.5.1) and the bi-
   directional connectivity check fails during the BGP SPF calculation
   (refer to Section 6.3).  The suggested default value for the
   NLRIImplicitWithdrawalDelay timer is 2 seconds.

7.  Error Handling

   This section describes the Error Handling actions, as described in
   [RFC7606], that are specific to SAFI BGP-LS-SPF BGP Update message
   processing.

7.1.  Processing of BGP-LS-SPF TLVs

   When a BGP SPF speaker receives a BGP Update containing a malformed
   Node NLRI SPF Status TLV in the BGP-LS Attribute
   [I-D.ietf-idr-rfc7752bis], the corresponding Node NLRI is considered
   as malformed and MUST be handled as 'Treat-as-withdraw'.  An
   implementation SHOULD log an error (subject to rate-limiting) for
   further analysis.

   When a BGP SPF speaker receives a BGP Update containing a malformed
   Link NLRI SPF Status TLV in the BGP-LS Attribute
   [I-D.ietf-idr-rfc7752bis], the corresponding Link NLRI is considered
   as malformed and MUST be handled as 'Treat-as-withdraw'.  An
   implementation SHOULD log an error (subject to rate-limiting) for
   further analysis.

   When a BGP SPF speaker receives a BGP Update containing a malformed
   Prefix NLRI SPF Status TLV in the BGP-LS Attribute
   [I-D.ietf-idr-rfc7752bis], the corresponding Prefix NLRI is
   considered as malformed and MUST be handled as 'Treat-as-withdraw'.
   An implementation SHOULD log an error (subject to rate-limiting) for
   further analysis.

   When a BGP SPF speaker receives a BGP Update containing any malformed
   BGP-LS Attribute TE and IGP Metric TLV, the corresponding NLRI is
   considered as malformed and MUST be handled as 'Treat-as-withdraw'
   [RFC7606].  An implementation SHOULD log an error (subject to rate-
   limiting) for further analysis.

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   The BGP-LS Attribute consists of Node attribute TLVs, Link attribute
   TLVs, and the Prefix attribute TLVs.  Node attribute TLVs and their
   error handling rules are either defined in [I-D.ietf-idr-rfc7752bis]
   or derived from [RFC5305] and [RFC6119].  If a BGP SPF speaker
   receives a BGP-LS Attribute which is considered malformed based on
   these error handling rules, then it MUST consider the received NLRI
   as malformed and the receiving BGP SPF speaker MUST handle such
   malformed NLRI as 'Treat-as-withdraw' [RFC7606].

   Node Descriptor TLVs and their error handling rules are either
   defined in section 5.2.1 of [I-D.ietf-idr-rfc7752bis].  Node
   Attribute TLVs and their error handling rules are either defined in
   [I-D.ietf-idr-rfc7752bis] or derived from [RFC5305] and [RFC6119].

   Link Descriptor TLVs and their error handling rules are either
   defined in section 5.2.2 of [I-D.ietf-idr-rfc7752bis].  Link
   Attribute TLVs and their error handling rules are either defined in
   [I-D.ietf-idr-rfc7752bis] or derived from [RFC5305] and [RFC6119].

   Prefix Descriptor TLVs and their error handling rules are either
   defined in section 5.2.3 of [I-D.ietf-idr-rfc7752bis].  Prefix
   Attribute TLVs and their error handling rules are either defined in
   [I-D.ietf-idr-rfc7752bis] or derived from [RFC5130] and [RFC2328].

   If a BGP SPF 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 consider the received NLRI as malformed and the receiving BGP
   SPF speaker MUST handle such malformed NLRI as 'Treat-as-withdraw'
   [RFC7606].

   When a BGP SPF speaker receives a BGP Update that does not contain
   any BGP-LS Attribute, then a BGP SPF speaker MUST consider the
   corresponding NLRI as malformed and MUST handle it as 'Treat-as-
   withdraw' [RFC7606].  An implementation SHOULD log an error (subject
   to rate-limiting) for further analysis.

7.2.  Processing of BGP-LS-SPF NLRIs

   A BGP-LS-SPF Speaker MUST perform the syntactic validation checks of
   the BGP-LS-SPF NLRI listed in Section 8.2.2 of
   [I-D.ietf-idr-rfc7752bis] to determine if it is malformed.

7.3.  Processing of BGP-LS Attribute

   A BGP-LS-SPF Speaker MUST perform the syntactic validation checks of
   the BGP-LS Attribute listed in Section 8.2.2 of
   [I-D.ietf-idr-rfc7752bis] to determine if it is malformed.

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   An implementation SHOULD log an error for further analysis for
   problems detected during syntax validation.

7.4.  BGP-LS-SPF Link State NLRI Database Synchronization

   While uncommon, there may be situations where the LSNDBs of two BGP-
   LS-SPF speakers lose synchronization.  In these situations, the BGP
   session MUST be reset.  The mechanisms to detect loss of
   synchronization are beyond the scope of this document.

8.  IANA Considerations

8.1.  BGP-LS-SPF Allocation in SAFI Parameters Registry

   IANA has assigned value 80 for BGP-LS-SPF from the First Come First
   Served range in the "Subsequent Address Family Identifiers (SAFI)
   Parameters" registry.  IANA is requested to update the registration
   to reference only to this document.

8.2.  BGP-LS Address Family Link Descriptor

   IANA is requested to assign the value TBD (266 suggested) to the BGP-
   LS BGP-LS TLVs Registry for the BGP-LS Address Family Link Descriptor
   TLV (refer to section Section 5.2.2.1.

8.3.  BGP-LS-SPF Assignments to BGP-LS NLRI and Attribute TLV Registry

   IANA has assigned four TLVs for BGP-LS-SPF NLRI in the "BGP-LS NLRI
   and Attribute TLV" registry.  These TLV types include the SPF Status
   TLV and Sequence Number TLV.

       +==========+=============+=================================+
       | TLV Code | Description | Reference                       |
       | Point    |             |                                 |
       +==========+=============+=================================+
       | 1184     | SPF Status  | Section 5.2.1.1, RFCXXXX ([this |
       |          |             | document]), Section 5.2.2.2 and |
       |          |             | Section 5.2.3.1                 |
       +----------+-------------+---------------------------------+
       | 1181     | Sequence    | RFCXXXX ([this document]),      |
       |          | Number      | Section 5.2.4                   |
       +----------+-------------+---------------------------------+

                       Table 1: NLRI Attribute TLVs

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8.4.  BGP-LS-SPF Node NLRI Attribute SPF Status TLV Status Registry

   IANA is requested to create the "BGP-LS-SPF Node NLRI Attribute SPF
   Status TLV Status" Registry for status values in a new BGP SPF group.
   Initial values for this registry are provided below.  Future
   assignments are to be made using the IETF Review registration policy
   [RFC8126].

           +========+==========================================+
           | Values | Description                              |
           +========+==========================================+
           | 0      | Reserved                                 |
           +--------+------------------------------------------+
           | 1      | Node unreachable with respect to BGP SPF |
           +--------+------------------------------------------+
           | 2      | Node does not support transit traffic    |
           |        | with respect to BGP SPF                  |
           +--------+------------------------------------------+
           | 3-254  | Unassigned                               |
           +--------+------------------------------------------+
           | 255    | Reserved                                 |
           +--------+------------------------------------------+

                Table 2: BGP-LS-SPF Node NLRI Attribute SPF
                   Status TLV Status Registry Assignments

8.5.  BGP-LS-SPF Link NLRI Attribute SPF Status TLV Status Registry

   IANA is requested to create the "BGP-LS-SPF Link NLRI Attribute SPF
   Status TLV Status" Registry for status values in a new BGP SPF group.
   Initial values for this registry are provided below.  Future
   assignments are to be made using the IETF Review registration policy
   [RFC8126].

           +=======+==========================================+
           | Value | Description                              |
           +=======+==========================================+
           | 0     | Reserved                                 |
           +-------+------------------------------------------+
           | 1     | Link unreachable with respect to BGP SPF |
           +-------+------------------------------------------+
           | 3-254 | Unassigned                               |
           +-------+------------------------------------------+
           | 255   | Reserved                                 |
           +-------+------------------------------------------+

               Table 3: BGP-LS-SPF Link NLRI Attribute SPF
                  Status TLV Status Registry Assignments

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8.6.  BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV Status Registry

   IANA is requested to create the "BGP-LS-SPF Prefix NLRI Attribute SPF
   Status TLV Status" Registry for status values in a new BGP SPF group.
   Initial values for this registry are provided below.  Future
   assignments are to be made using the IETF Review registration policy
   [RFC8126].

          +=======+============================================+
          | Value | Description                                |
          +=======+============================================+
          | 0     | Reserved                                   |
          +-------+--------------------------------------------+
          | 1     | Prefix unreachable with respect to BGP SPF |
          +-------+--------------------------------------------+
          | 3-254 | Unassigned                                 |
          +-------+--------------------------------------------+
          | 255   | Reserved                                   |
          +-------+--------------------------------------------+

              Table 4: BGP-LS-SPF Prefix NLRI Attribute SPF
                  Status TLV Status Registry Assignments

9.  Security Considerations

   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 [RFC4271].  The Security Considerations discussed in
   [RFC4271] apply to the BGP SPF functionality as well.  The analysis
   of the security issues for BGP mentioned in [RFC4272] and [RFC6952]
   also applies to this document.  The analysis of Generic Threats to
   Routing Protocols done in [RFC4593] is also worth noting.

   As the modifications 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 [RFC6811] and
   [RFC8205] are out of scope as they are meant to apply for inter-
   domain BGP where multiple BGP Routing Domains are typically involved.
   The BGP-LS-SPF SAFI NLRI described in this document are typically
   advertised between EBGP or IBGP speakers under a single
   administrative domain.

   The BGP SPF protocol and the BGP-LS-SPF SAFI inherit the encoding
   from BGP-LS [I-D.ietf-idr-rfc7752bis], and consequently, inherit the
   security considerations for BGP-LS associated with encoding.
   Additionally, given that the BGP SPF protocol is used to install IPv4
   and IPv6 Unicast routes, the BGP SPF protocol is vulnerable to
   attacks to the routing control plane that aren't applicable to BGP-

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   LS.  One notable Denial-of-Service 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
   [RFC7606].

   In order to mitigate the risk of peering with BGP speakers
   masquerading as legitimate authorized BGP speakers, it is recommended
   that the TCP Authentication Option (TCP-AO) [RFC5925] be used to
   authenticate BGP sessions.  If an authorized BGP peer is compromised,
   that BGP peer could advertise modified Node, Link, or Prefix NLRI
   which result 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 be logged so that the operator can take
   corrective action.  This exposure is similar to other BGP AFI/SAFIs.

10.  Management Considerations

   This section includes unique management considerations for the BGP-
   LS-SPF address family.

10.1.  Configuration

   All routers in BGP SPF Routing Domain are under a single
   administrative domain allowing for consistent configuration.

10.2.  Link Metric Configuration

   For loopback prefixes, it is RECOMMMENDED 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.

   Algorithms such as setting the metric inversely to the link speed as
   supported in some IGP implementations MAY be supported.  However, the
   details of how the metric is computed are beyond the scope of this
   document.

   Within a BGP SPF Routing Domain, the IGP metrics for all advertised
   links SHOULD 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 be done for all routers and the same
   reference bandwidth SHOULD be used to derive the inversely
   proportional metric.  Failure to do so will result in incorrect
   routing based on link metric.

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10.3.  Unnumbered Link Configuration

   When parallel unnumbered links between BGP-SPF routers are included
   in the BGP SPF routing domain and the Remote Link Identifiers aren't
   readily discovered, it is RECOMMENDED that these the Remote Link
   Identifiers be configured so that precise NLRI Link matching can be
   done.

10.4.  Adjacency End-of-RIB (EOR) Marker Requirement

   Depending on the peering model, topology, and convergence
   requirements, an End-of-RIB (EoR) Marker [RFC4724] for the BGP-LS-SPF
   SAFI MAY be required from the peer prior to advertising a BGP-LS Link
   NLRI for the peer.  If configuration is supported, this SHOULD be
   configurable at the BGP SPF instance level and SHOULD be configured
   consistently throughout the BGP SPF routing domain.

10.5.  backoff-config

   In addition to configuration of the BGP-LS-SPF address family,
   implementations SHOULD support the "Shortest Path First (SPF) Back-
   Off Delay Algorithm for Link-State IGPs" [RFC8405].  If supported,
   configuration of the INITIAL_SPF_DELAY, SHORT_SPF_DELAY,
   LONG_SPF_DELAY, TIME_TO_LEARN, and HOLDDOWN_INTERVAL MUST be
   supported [RFC8405].  Section 6 of [RFC8405] recommends consistent
   configuration of these values throughout the IGP routing domain and
   this also applies to the BGP SPF Routing Domain.

10.6.  BGP-LS-SPF NLRI Readvertisemnt Delay

   The configuration parameter 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.

10.7.  Operational Data

   In order to troubleshoot SPF issues, implementations SHOULD 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, SPF end time, and SPF type if
   different types of SPF are supported.  Since the size of the log is
   finite, implementations SHOULD also maintain counters for the total
   number of SPF computations and the total number of SPF triggering
   events.  Additionally, to troubleshoot SPF scheduling and back-off
   [RFC8405], the current SPF back-off state, remaining time-to-learn,
   remaining hold-down interval, last trigger event time, last SPF time,
   and next SPF time should be available.

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10.8.  BGP-LS-SPF Address Family Session Isolation

   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.

11.  Implementation Status

   Note RFC Editor: Please remove this section and the associated
   references prior to publication.

   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 [RFC7942].
   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
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   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".

   The BGP-LS-SPF implementation status is documented in
   [I-D.psarkar-lsvr-bgp-spf-impl].

12.  Acknowledgements

   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 for their review and comments.  Thanks to
   Pushpasis Sarkar for discussions on preventing a BGP SPF Router from
   being used for non-local traffic (i.e., transit traffic).

   The authors extend special thanks to Eric Rosen for fruitful
   discussions on BGP-LS-SPF convergence as compared to IGPs.

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13.  Contributors

   In addition to the authors listed on the front page, the following
   co-authors have contributed to the document.

   Derek Yeung
   Arrcus, Inc.
   derek@arrcus.com

   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

14.  References

14.1.  Normative References

   [I-D.ietf-idr-rfc7752bis]
              Talaulikar, K., "Distribution of Link-State and Traffic
              Engineering Information Using BGP", Work in Progress,
              Internet-Draft, draft-ietf-idr-rfc7752bis-17, 25 August
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              idr-rfc7752bis-17>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

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   [RFC4202]  Kompella, K., Ed. and Y. Rekhter, Ed., "Routing Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4202, DOI 10.17487/RFC4202, October 2005,
              <https://www.rfc-editor.org/info/rfc4202>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC5130]  Previdi, S., Shand, M., Ed., and C. Martin, "A Policy
              Control Mechanism in IS-IS Using Administrative Tags",
              RFC 5130, DOI 10.17487/RFC5130, February 2008,
              <https://www.rfc-editor.org/info/rfc5130>.

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC6119]  Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
              Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
              February 2011, <https://www.rfc-editor.org/info/rfc6119>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <https://www.rfc-editor.org/info/rfc6793>.

   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811,
              DOI 10.17487/RFC6811, January 2013,
              <https://www.rfc-editor.org/info/rfc6811>.

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   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/info/rfc7606>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <https://www.rfc-editor.org/info/rfc8205>.

   [RFC8405]  Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
              Francois, P., and C. Bowers, "Shortest Path First (SPF)
              Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
              DOI 10.17487/RFC8405, June 2018,
              <https://www.rfc-editor.org/info/rfc8405>.

   [RFC8654]  Bush, R., Patel, K., and D. Ward, "Extended Message
              Support for BGP", RFC 8654, DOI 10.17487/RFC8654, October
              2019, <https://www.rfc-editor.org/info/rfc8654>.

   [RFC9086]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
              Ray, S., and J. Dong, "Border Gateway Protocol - Link
              State (BGP-LS) Extensions for Segment Routing BGP Egress
              Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
              2021, <https://www.rfc-editor.org/info/rfc9086>.

14.2.  Informational References

   [I-D.ietf-lsvr-applicability]
              Patel, K., Lindem, A., Zandi, S., and G. Dawra, "Usage and
              Applicability of Link State Vector Routing in Data
              Centers", Work in Progress, Internet-Draft, draft-ietf-
              lsvr-applicability-11, 12 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lsvr-
              applicability-11>.

   [I-D.psarkar-lsvr-bgp-spf-impl]
              Sarkar, P., Patel, K., Pallagatti, S., and
              sajibasil@gmail.com, "BGP Shortest Path Routing Extension
              Implementation Report", Work in Progress, Internet-Draft,

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              draft-psarkar-lsvr-bgp-spf-impl-01, 6 June 2023,
              <https://datatracker.ietf.org/doc/html/draft-psarkar-lsvr-
              bgp-spf-impl-01>.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <https://www.rfc-editor.org/info/rfc4456>.

   [RFC4593]  Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", RFC 4593, DOI 10.17487/RFC4593,
              October 2006, <https://www.rfc-editor.org/info/rfc4593>.

   [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
              Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
              DOI 10.17487/RFC4724, January 2007,
              <https://www.rfc-editor.org/info/rfc4724>.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <https://www.rfc-editor.org/info/rfc5286>.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
              <https://www.rfc-editor.org/info/rfc6952>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <https://www.rfc-editor.org/info/rfc7911>.

   [RFC7938]  Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
              BGP for Routing in Large-Scale Data Centers", RFC 7938,
              DOI 10.17487/RFC7938, August 2016,
              <https://www.rfc-editor.org/info/rfc7938>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

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Authors' Addresses

   Keyur Patel
   Arrcus, Inc.
   Email: keyur@arrcus.com

   Acee Lindem
   LabN Consulting, LLC
   301 Midenhall Way
   Cary, NC 27513
   United States of America
   Email: acee.ietf@gmail.com

   Shawn Zandi
   LinkedIn
   222 2nd Street
   San Francisco, CA 94105
   United States of America
   Email: szandi@linkedin.com

   Wim Henderickx
   Nokia
   copernicuslaan 50
   2018 Antwerp
   Belgium
   Email: wim.henderickx@nokia.com

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