path: root/doc/rfc/rfc9552.txt

Internet Engineering Task Force (IETF)                K. Talaulikar, Ed.
Request for Comments: 9552                                 Cisco Systems
Obsoletes: 7752, 9029                                      December 2023
Category: Standards Track                                               
ISSN: 2070-1721


Distribution of Link-State and Traffic Engineering Information Using BGP

Abstract

   In many environments, a component external to a network is called
   upon to perform computations based on the network topology and the
   current state of the connections within the network, including
   Traffic Engineering (TE) information.  This is information typically
   distributed by IGP routing protocols within the network.

   This document describes a mechanism by which link-state and TE
   information can be collected from networks and shared with external
   components using the BGP routing protocol.  This is achieved using a
   BGP Network Layer Reachability Information (NLRI) encoding format.
   The mechanism applies to physical and virtual (e.g., tunnel) IGP
   links.  The mechanism described is subject to policy control.

   Applications of this technique include Application-Layer Traffic
   Optimization (ALTO) servers and Path Computation Elements (PCEs).

   This document obsoletes RFC 7752 by completely replacing that
   document.  It makes some small changes and clarifications to the
   previous specification.  This document also obsoletes RFC 9029 by
   incorporating the updates that it made to RFC 7752.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9552.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Motivation and Applicability
     2.1.  MPLS-TE with PCE
     2.2.  ALTO Server Network API
   3.  BGP Speaker Roles for BGP-LS
   4.  Advertising IGP Information into BGP-LS
   5.  Carrying Link-State Information in BGP
     5.1.  TLV Format
     5.2.  The Link-State NLRI
       5.2.1.  Node Descriptors
       5.2.2.  Link Descriptors
       5.2.3.  Prefix Descriptors
     5.3.  The BGP-LS Attribute
       5.3.1.  Node Attribute TLVs
       5.3.2.  Link Attribute TLVs
       5.3.3.  Prefix Attribute TLVs
     5.4.  Private Use
     5.5.  BGP Next-Hop Information
     5.6.  Inter-AS Links
     5.7.  OSPF Virtual Links and Sham Links
     5.8.  OSPFv2 Type 4 Summary-LSA & OSPFv3 Inter-Area-Router-LSA
     5.9.  Handling of Unreachable IGP Nodes
     5.10. Router-ID Anchoring Example: ISO Pseudonode
     5.11. Router-ID Anchoring Example: OSPF Pseudonode
     5.12. Router-ID Anchoring Example: OSPFv2 to IS-IS Migration
   6.  Link to Path Aggregation
     6.1.  Example: No Link Aggregation
     6.2.  Example: ASBR to ASBR Path Aggregation
     6.3.  Example: Multi-AS Path Aggregation
   7.  IANA Considerations
     7.1.  BGP-LS Registries
       7.1.1.  BGP-LS NLRI Types Registry
       7.1.2.  BGP-LS Protocol-IDs Registry
       7.1.3.  BGP-LS Well-Known Instance-IDs Registry
       7.1.4.  BGP-LS Node Flags Registry
       7.1.5.  BGP-LS MPLS Protocol Mask Registry
       7.1.6.  BGP-LS IGP Prefix Flags Registry
       7.1.7.  BGP-LS TLVs Registry
     7.2.  Guidance for Designated Experts
   8.  Manageability Considerations
     8.1.  Operational Considerations
       8.1.1.  Operations
       8.1.2.  Installation and Initial Setup
       8.1.3.  Migration Path
       8.1.4.  Requirements for Other Protocols and Functional
               Components
       8.1.5.  Impact on Network Operation
       8.1.6.  Verifying Correct Operation
     8.2.  Management Considerations
       8.2.1.  Management Information
       8.2.2.  Fault Management
       8.2.3.  Configuration Management
       8.2.4.  Accounting Management
       8.2.5.  Performance Management
       8.2.6.  Security Management
   9.  TLV/Sub-TLV Code Points Summary
   10. Security Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Appendix A.  Changes from RFC 7752
   Acknowledgements
   Contributors
   Author's Address

1.  Introduction

   The contents of a Link-State Database (LSDB) or of an IGP's Traffic
   Engineering Database (TED) describe only the links and nodes within
   an IGP area.  Some applications, such as end-to-end Traffic
   Engineering (TE), would benefit from visibility outside one area or
   Autonomous System (AS) to make better decisions.

   The IETF has defined the Path Computation Element (PCE) [RFC4655] as
   a mechanism for achieving the computation of end-to-end TE paths that
   crosses the visibility of more than one TED or that requires CPU-
   intensive or coordinated computations.  The IETF has also defined the
   ALTO server [RFC5693] as an entity that generates an abstracted
   network topology and provides it to network-aware applications.

   Both a PCE and an ALTO server need to gather information about the
   topologies and capabilities of the network to be able to fulfill
   their function.

   This document describes a mechanism by which link-state and TE
   information can be collected from networks and shared with external
   components using the BGP routing protocol [RFC4271].  This is
   achieved using a BGP Network Layer Reachability Information (NLRI)
   encoding format.  The mechanism applies to physical and virtual
   (e.g., tunnel) links.  The mechanism described is subject to policy
   control.

   A router maintains one or more databases for storing link-state
   information about nodes and links in any given area.  Link attributes
   stored in these databases include: local/remote IP addresses, local/
   remote interface identifiers, link IGP metric, link TE metric, link
   bandwidth, reservable bandwidth, per Class-of-Service (CoS) class
   reservation state, preemption, and Shared Risk Link Groups (SRLGs).
   The router's BGP - Link State (BGP-LS) process can retrieve topology
   from these LSDBs and distribute it to a consumer, either directly or
   via a peer BGP Speaker (typically a dedicated route reflector), using
   the encoding specified in this document.

   An illustration of the collection of link-state and TE information
   and its distribution to consumers is shown in Figure 1 below.

               +-----------+
               | Consumer  |
               +-----------+
                     ^
                     |
               +-----------+             +-----------+
               |    BGP    |             |    BGP    |
               |  Speaker  |<----------->|  Speaker  |  +-----------+
               |    RR1    |             |    RRm    |  | Consumer  |
               +-----------+             +-----------+  +-----------+
                   ^   ^                       ^             ^
                   |   |                       |             |
             +-----+   +---------+             +---------+   |
             |                   |                       |   |
       +-----------+       +-----------+             +-----------+
       |    BGP    |       |    BGP    |             |    BGP    |
       |  Speaker  |       |  Speaker  |    . . .    |  Speaker  |
       |    R1     |       |     R2    |             |    Rn     |
       +-----------+       +-----------+             +-----------+
             ^                   ^                         ^
             |                   |                         |
            IGP                 IGP                       IGP

           Figure 1: Collection of Link-State and TE Information

   A BGP Speaker may apply a configurable policy to the information that
   it distributes.  Thus, it may distribute the real physical topology
   from the LSDB or the TED.  Alternatively, it may create an abstracted
   topology, where virtual, aggregated nodes are connected by virtual
   paths.  Aggregated nodes can be created, for example, out of multiple
   routers in a Point of Presence (POP).  Abstracted topology can also
   be a mix of physical and virtual nodes and physical and virtual
   links.  Furthermore, the BGP Speaker can apply policy to determine
   when information is updated to the consumer so that there is a
   reduction in information flow from the network to the consumers.
   Mechanisms through which topologies can be aggregated or virtualized
   are outside the scope of this document.

   This document focuses on the specifications related to the
   origination of IGP-derived information and their propagation via BGP-
   LS.  It also describes the advertisement into BGP-LS of information,
   either configured or derived, that is local to a node.  In general,
   the procedures in this document form part of the base BGP-LS protocol
   specification and apply to information from other sources that are
   introduced into BGP-LS.

   This document obsoletes [RFC7752] by completely replacing that
   document.  It makes some small changes and clarifications to the
   previous specification as documented in Appendix A.

1.1.  Requirements Language

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

2.  Motivation and Applicability

   This section describes use cases from which the requirements can be
   derived.

2.1.  MPLS-TE with PCE

   As described in [RFC4655], a PCE can be used to compute MPLS-TE paths
   within a "domain" (such as an IGP area) or across multiple domains
   (such as a multi-area AS or multiple ASes).

   *  Within a single area, the PCE offers enhanced computational power
      that may not be available on individual routers, sophisticated
      policy control and algorithms, and coordination of computation
      across the whole area.

   *  If a router wants to compute an MPLS-TE path across IGP areas,
      then its own TED lacks visibility of the complete topology.  That
      means that the router cannot determine the end-to-end path and
      cannot even select the right exit router (Area Border Router
      (ABR)) for an optimal path.  This is an issue for large-scale
      networks that need to segment their core networks into distinct
      areas but still want to take advantage of MPLS-TE.

   Previous solutions used per-domain path computation [RFC5152].  The
   source router could only compute the path for the first area because
   the router only has full topological visibility for the first area
   along the path but not for subsequent areas.  Per-domain path
   computation selects the exit ABR and other ABRs or AS Border Routers
   (ASBRs) as loose-hops [RFC3209] and using the IGP-computed shortest
   path topology for the remainder of the path.  This may lead to
   suboptimal paths, makes alternate/back-up path computation hard, and
   might result in no TE path being found when one does exist.

   The PCE presents a computation server that may have visibility into
   more than one IGP area or AS or may cooperate with other PCEs to
   perform distributed path computation.  The PCE needs access to the
   TED for the area(s) it serves, but [RFC4655] does not describe how
   this is achieved.  Many implementations make the PCE a passive
   participant in the IGP so that it can learn the latest state of the
   network, but this may be suboptimal when the network is subject to a
   high degree of churn or when the PCE is responsible for multiple
   areas.

   The following figure shows how a PCE can get its TED information
   using the mechanism described in this document.

                +----------+                           +---------+
                |  -----   |                           |   BGP   |
                | | TED |<-+-------------------------->| Speaker |
                |  -----   |   TED synchronization     |         |
                |    |     |        mechanism          +---------+
                |    |     |
                |    v     |
                |  -----   |
                | | PCE |  |
                |  -----   |
                +----------+
                     ^
                     | Request/
                     | Response
                     v
       Service  +----------+   Signaling  +----------+
       Request  | Head-End |   Protocol   | Adjacent |
       -------->|  Node    |<------------>|   Node   |
                +----------+              +----------+

     Figure 2: External PCE Node Using a TED Synchronization Mechanism

   The mechanism in this document allows the necessary TED information
   to be collected from the IGP within the network, filtered according
   to configurable policy, and distributed to the PCE as necessary.

2.2.  ALTO Server Network API

   An ALTO server [RFC5693] is an entity that generates an abstracted
   network topology and provides it to network-aware applications over a
   web-service-based API.  Example applications are peer-to-peer (P2P)
   clients or trackers, or Content Distribution Networks (CDNs).  The
   abstracted network topology comes in the form of two maps: a Network
   Map that specifies the allocation of prefixes to Partition
   Identifiers (PIDs) and a Cost Map that specifies the cost between
   PIDs listed in the Network Map. For more details, see [RFC7285].

   ALTO abstract network topologies can be auto-generated from the
   physical topology of the underlying network.  The generation would
   typically be based on policies and rules set by the operator.  Both
   prefix and TE data are required: prefix data is required to generate
   ALTO Network Maps and TE (topology) data is required to generate ALTO
   Cost Maps.  Prefix data is carried and originated in BGP, and TE data
   is originated and carried in an IGP.  The mechanism defined in this
   document provides a single interface through which an ALTO server can
   retrieve all the necessary prefixes and network topology data from
   the underlying network.  Note that an ALTO server can use other
   mechanisms to get network data, for example, peering with multiple
   IGP and BGP Speakers.

   The following figure shows how an ALTO server can get network
   topology information from the underlying network using the mechanism
   described in this document.

     +--------+
     | Client |<--+
     +--------+   |
                  |    ALTO    +--------+     Topology    +---------+
     +--------+   |  Protocol  |  ALTO  | Sync Mechanism  |   BGP   |
     | Client |<--+------------| Server |<----------------| Speaker |
     +--------+   |            |        |                 |         |
                  |            +--------+                 +---------+
     +--------+   |
     | Client |<--+
     +--------+

          Figure 3: ALTO Server Using Network Topology Information

3.  BGP Speaker Roles for BGP-LS

   In Figure 1, the BGP Speakers can be seen playing different roles in
   the distribution of information using BGP-LS.  This section
   introduces terms that explain the different roles of the BGP Speakers
   that are then used throughout the rest of this document.

   BGP-LS Producer:  The term BGP-LS Producer refers to a BGP Speaker
      that is originating link-state information into BGP.  BGP Speakers
      R1, R2, ... Rn originate link-state information from their
      underlying link-state IGP protocols into BGP-LS.  If R1 and R2 are
      in the same IGP flooding domain, then they would ordinarily
      originate the same link-state information into BGP-LS.  R1 may
      also originate information from sources other than IGP, e.g., its
      local node information.

   BGP-LS Consumer:  The term BGP-LS Consumer refers to a consumer
      application/process and not a BGP Speaker.  BGP Speakers RR1 and
      Rn are handing off the BGP-LS information that they have collected
      to a consumer application.  The BGP protocol implementation and
      the consumer application may be on the same or different nodes.
      This document only covers the BGP implementation.  The consumer
      application and the design of the interface between BGP and the
      consumer application may be implementation specific and are
      outside the scope of this document.  The communication of
      information MUST be unidirectional (i.e., from a BGP Speaker to
      the BGP-LS Consumer application), and a BGP-LS Consumer MUST NOT
      be able to send information to a BGP Speaker for origination into
      BGP-LS.

   BGP-LS Propagator:  The term BGP-LS Propagator refers to a BGP
      Speaker that is performing BGP protocol processing on the link-
      state information.  BGP Speaker RRm propagates the BGP-LS
      information between BGP Speaker Rn and BGP Speaker RR1.  The BGP
      implementation on RRm is propagating BGP-LS information.  It
      performs handling of BGP-LS UPDATE messages and performs the BGP
      Decision Process as part of deciding what information is to be
      propagated.  Similarly, BGP Speaker RR1 is receiving BGP-LS
      information from R1, R2, and RRm and propagating the information
      to the BGP-LS Consumer after performing BGP Decision Process.

   The above roles are not mutually exclusive.  The same BGP Speaker may
   be the BGP-LS Producer for some link-state information and BGP-LS
   Propagator for some other link-state information while also providing
   this information to a BGP-LS Consumer.

   The rest of this document refers to the role when describing
   procedures that are specific to that role.  When the role is not
   specified, then the said procedure applies to all BGP Speakers.

4.  Advertising IGP Information into BGP-LS

   The origination and propagation of IGP link-state information via BGP
   needs to provide a consistent and accurate view of the topology of
   the IGP domain.  BGP-LS provides an abstraction of the IGP specifics,
   and BGP-LS Consumers may be varied types of applications.

   The link-state information advertised in BGP-LS from the IGPs is
   derived from the IGP LSDB built using the OSPF Link-State
   Advertisements (LSAs) or the IS-IS Link-State Packets (LSPs).
   However, it does not serve as a verbatim reflection of the
   originating router's LSDB.  It does not include the LSA/LSP sequence
   number information since a single link-state object may be put
   together with information that is coming from multiple LSAs/LSPs.
   Also, not all of the information carried in LSAs/LSPs may be required
   or suitable for advertisement via BGP-LS (e.g., ASBR reachability in
   OSPF, OSPF virtual links, link-local-scoped information, etc.).  The
   LSAs/LSPs that are purged or aged out are not included in the BGP-LS
   advertisement even though they may be present in the LSDB (e.g., for
   the IGP flooding purposes).  The information from the LSAs/LSPs that
   is invalid or malformed or that which needs to be ignored per the
   respective IGP protocol specifications are also not included in the
   BGP-LS advertisement.

   The details of the interface between IGPs and BGP for the
   advertisement of link-state information are outside the scope of this
   document.  In some cases, the information derived from IGP processing
   (e.g., combination of link-state object from across multiple LSAs/
   LSPs, leveraging reachability and two-way connectivity checks, etc.)
   is required for the advertisement of link-state information into BGP-
   LS.

5.  Carrying Link-State Information in BGP

   The link-state information is carried in BGP UPDATE messages as: (1)
   BGP NLRI information carried within MP_REACH_NLRI and MP_UNREACH_NLRI
   attributes that describes link, node, or prefix objects and (2) a BGP
   path attribute (BGP-LS Attribute) that carries properties of the
   link, node, or prefix objects such as the link and prefix metric,
   auxiliary Router-IDs of nodes, etc.

   It is desirable to keep the dependencies on the protocol source of
   this attribute to a minimum and represent any content in an IGP-
   neutral way, such that applications that want to learn about a link-
   state topology do not need to know about any OSPF or IS-IS protocol
   specifics.

   This section mainly describes the procedures for a BGP-LS Producer to
   originate link-state information into BGP-LS.

5.1.  TLV Format

   Information in the Link-State NLRIs and the BGP-LS Attribute is
   encoded in Type/Length/Value triplets.  The TLV format is shown in
   Figure 4 and applies to both the NLRI and the BGP-LS Attribute
   encodings.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                        Value (variable)                     //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 4: TLV Format

   The Length field defines the length of the value portion in octets
   (thus, a TLV with no value portion would have a length of zero).  The
   TLV is not padded to 4-octet alignment.  Unknown and unsupported
   types MUST be preserved and propagated within both the NLRI and the
   BGP-LS Attribute.  The presence of unknown or unexpected TLVs MUST
   NOT result in the NLRI or the BGP-LS Attribute being considered
   malformed.  An example of an unexpected TLV is when a TLV is received
   along with an update for a link-state object other than the one that
   the TLV is specified as associated with.

   To compare NLRIs with unknown TLVs, all TLVs within the NLRI MUST be
   ordered in ascending order by TLV Type.  If there are multiple TLVs
   of the same type within a single NLRI, then the TLVs sharing the same
   type MUST be first in ascending order based on the Length field
   followed by ascending order based on the Value field.  Comparison of
   the Value fields is performed by treating the entire field as opaque
   binary data and ordered lexicographically (i.e., treating each byte
   of binary data as a symbol to compare, with the symbols ordered by
   their numerical value).  NLRIs having TLVs that do not follow the
   above ordering rules MUST be considered as malformed by a BGP-LS
   Propagator.  This insistence on canonical ordering ensures that
   multiple variant copies of the same NLRI from multiple BGP-LS
   Producers and the ambiguity arising therefrom is prevented.

   For both the NLRI and BGP-LS Attribute parts, all TLVs are considered
   as optional except where explicitly specified as mandatory or
   required in specific conditions.

   The TLVs within the BGP-LS Attribute SHOULD be ordered in ascending
   order by TLV type.  The BGP-LS Attribute with unordered TLVs MUST NOT
   be considered malformed.

   The origination of the same link-state information by multiple BGP-LS
   Producers may result in differences and inconsistencies due to the
   inclusion or exclusion of optional TLVs.  Different optional TLVs in
   the NLRI results in multiple NLRIs being generated for the same link-
   state object.  Different optional TLVs in the BGP-LS Attribute may
   result in the propagation of partial information.  To address these
   inconsistencies, the BGP-LS Consumer will need to recognize and merge
   the duplicate information or deal with missing information.  The
   deployment of BGP-LS Producers that consistently originate the same
   set of optional TLVs is recommended to mitigate such situations.

5.2.  The Link-State NLRI

   The MP_REACH_NLRI and MP_UNREACH_NLRI attributes are BGP's containers
   for carrying opaque information.  This specification defines three
   Link-State NLRI types that describe either a node, a link, or a
   prefix.

   All non-VPN link, node, and prefix information SHALL be encoded using
   AFI 16388 / SAFI 71.  VPN link, node, and prefix information SHALL be
   encoded using AFI 16388 / SAFI 72.

   For two BGP Speakers to exchange Link-State NLRI, they MUST use BGP
   Capabilities Advertisement to ensure that they are both capable of
   properly processing such NLRI.  This is done as specified in
   [RFC4760] by using capability code 1 (multiprotocol BGP), with AFI
   16388 / SAFI 71 for BGP-LS and AFI 16388 / SAFI 72 for BGP-LS-VPN.

   New Link-State NLRI types may be introduced in the future.  Since
   supported NLRI type values within the address family are not
   expressed in the Multiprotocol BGP (MP-BGP) capability [RFC4760], it
   is possible that a BGP Speaker has advertised support for BGP-LS but
   does not support a particular Link-State NLRI type.  To allow the
   introduction of new Link-State NLRI types seamlessly in the future
   without the need for upgrading all BGP Speakers in the propagation
   path (e.g., a route reflector), this document deviates from the
   default handling behavior specified by Section 5.4 (paragraph 2) of
   [RFC7606] for Link-State address family.  An implementation MUST
   handle unknown Link-State NLRI types as opaque objects and MUST
   preserve and propagate them.

   The format of the Link-State NLRI is shown in the following figures.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            NLRI Type          |     Total NLRI Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //                  Link-State NLRI (variable)                 //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 5: Link-State AFI 16388 / SAFI 71 NLRI Format

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            NLRI Type          |     Total NLRI Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                Route Distinguisher (8 octets)                 +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //                  Link-State NLRI (variable)                 //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 6: Link-State VPN AFI 16388 / SAFI 72 NLRI Format

   The Total NLRI Length field contains the cumulative length, in
   octets, of the rest of the NLRI, not including the NLRI Type field or
   itself.  For VPN applications, it also includes the length of the
   Route Distinguisher.

                   +======+===========================+
                   | Type | NLRI Type                 |
                   +======+===========================+
                   |  1   | Node NLRI                 |
                   +------+---------------------------+
                   |  2   | Link NLRI                 |
                   +------+---------------------------+
                   |  3   | IPv4 Topology Prefix NLRI |
                   +------+---------------------------+
                   |  4   | IPv6 Topology Prefix NLRI |
                   +------+---------------------------+

                           Table 1: NLRI Types

   Route Distinguishers are defined and discussed in [RFC4364].

   The Node NLRI (NLRI Type = 1) is shown in the following figure.

      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
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     +                           (8 octets)                          +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //             Local Node Descriptors TLV (variable)           //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 7: The Node NLRI Format

   The Link NLRI (NLRI Type = 2) is shown in the following figure.

      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
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     +                           (8 octets)                          +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //            Local Node Descriptors TLV (variable)            //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //            Remote Node Descriptors TLV (variable)           //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Link Descriptors TLVs (variable)              //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 8: The Link NLRI Format

   The IPv4 and IPv6 Prefix NLRIs (NLRI Type = 3 and Type = 4) use the
   same format as shown in the following figure.

      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
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     +                           (8 octets)                          +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //            Local Node Descriptors TLV (variable)            //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //             Prefix Descriptors TLVs (variable)              //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 9: The IPv4/IPv6 Topology Prefix NLRI Format

   The Protocol-ID field can contain one of the following values:

            +=============+==================================+
            | Protocol-ID | NLRI information source protocol |
            +=============+==================================+
            |      1      | IS-IS Level 1                    |
            +-------------+----------------------------------+
            |      2      | IS-IS Level 2                    |
            +-------------+----------------------------------+
            |      3      | OSPFv2                           |
            +-------------+----------------------------------+
            |      4      | Direct                           |
            +-------------+----------------------------------+
            |      5      | Static configuration             |
            +-------------+----------------------------------+
            |      6      | OSPFv3                           |
            +-------------+----------------------------------+

                      Table 2: Protocol Identifiers

   The 'Direct' and 'Static configuration' protocol types SHOULD be used
   when BGP-LS is sourcing local information.  For all information
   derived from other protocols, the corresponding Protocol-ID MUST be
   used.  If BGP-LS has direct access to interface information and wants
   to advertise a local link, then the Protocol-ID 'Direct' SHOULD be
   used.  For modeling virtual links, such as described in Section 6,
   the Protocol-ID 'Static configuration' SHOULD be used.

   A router may run multiple protocol instances of OSPF or IS-IS whereby
   it becomes a border router between multiple IGP domains.  Both OSPF
   and IS-IS may also run multiple routing protocol instances over the
   same link.  See [RFC8202] and [RFC6549].  These instances define
   independent IGP routing domains.  The Identifier field carries an
   8-octet BGP-LS Instance Identifier (Instance-ID) number that is used
   to identify the IGP routing domain where the NLRI belongs.  The NLRIs
   representing link-state objects (nodes, links, or prefixes) from the
   same IGP routing instance should have the same BGP-LS Instance-ID.
   NLRIs with different BGP-LS Instance-IDs are considered to be from
   different IGP routing instances.

   To support multiple IGP instances, an implementation needs to support
   the configuration of unique BGP-LS Instance-IDs at the routing
   protocol instance level.  The BGP-LS Instance-ID 0 is RECOMMENDED to
   be used when there is only a single protocol instance in the network
   where BGP-LS is operational.  The network operator MUST assign the
   same BGP-LS Instance-IDs on all BGP-LS Producers within a given IGP
   domain.  Unique BGP-LS Instance-IDs MUST be assigned to routing
   protocol instances operating in different IGP domains.  This can
   allow the BGP-LS Consumer to build an accurate segregated multi-
   domain topology based on the BGP-LS Instance-ID.

   When the above-described semantics and recommendations are not
   followed, a BGP-LS Consumer may see more than one link-state object
   for the same node, link, or prefix (each with a different BGP-LS
   Instance-ID) when there are multiple BGP-LS Producers deployed.  This
   may also result in the BGP-LS Consumers getting an inaccurate
   network-wide topology.

   Each Node Descriptor, Link Descriptor, and Prefix Descriptor consists
   of one or more TLVs, as described in the following sections.  These
   Descriptor TLVs are applicable for the Node, Link, and Prefix NLRI
   Types for the protocols that are listed in Table 2.  Documents
   extending BGP-LS specifications with new NLRI Types and/or protocols
   MUST specify the NLRI descriptors for them.

   When adding, removing, or modifying a TLV/sub-TLV from a Link-State
   NLRI, the BGP-LS Producer MUST withdraw the old NLRI by including it
   in the MP_UNREACH_NLRI.  Not doing so can result in duplicate and
   inconsistent link-state objects hanging around in the BGP-LS table.

5.2.1.  Node Descriptors

   Each link is anchored by a pair of Router-IDs that are used by the
   underlying IGP, namely a 48-bit ISO System-ID for IS-IS and a 32-bit
   Router-ID for OSPFv2 and OSPFv3.  An IGP may use one or more
   additional auxiliary Router-IDs, mainly for Traffic Engineering
   purposes.  For example, IS-IS may have one or more IPv4 and IPv6 TE
   Router-IDs [RFC5305] [RFC6119].  When configured, these auxiliary TE
   Router-IDs (TLV 1028/1029) MUST be included in the node attribute
   described in Section 5.3.1 and MAY be included in the link attribute
   described in Section 5.3.2.  The advertisement of the TE Router-IDs
   can help a BGP-LS Consumer to correlate multiple link-state objects
   (e.g., in different IGP instances or areas/levels) to the same node
   in the network.

   It is desirable that the Router-ID assignments inside the Node
   Descriptors are globally unique.  However, there may be Router-ID
   spaces (e.g., ISO) where no global registry exists, or worse, Router-
   IDs have been allocated following the private-IP allocation described
   in [RFC1918].  BGP-LS uses the Autonomous System Number to
   disambiguate the Router-IDs, as described in Section 5.2.1.1.

5.2.1.1.  Globally Unique Node/Link/Prefix Identifiers

   One problem that needs to be addressed is the ability to identify an
   IGP node globally (by "globally", we mean within the BGP-LS database
   collected by all BGP-LS Speakers that talk to each other).  This can
   be expressed through the following two requirements:

   (A)   The same node MUST NOT be represented by two keys (otherwise,
         one node will look like two nodes).

   (B)   Two different nodes MUST NOT be represented by the same key
         (otherwise, two nodes will look like one node).

   We define an "IGP domain" to be the set of nodes (hence, by
   extension, links and prefixes) within which each node has a unique
   IGP representation by using the combination of OSPF Area-ID, Router-
   ID, Protocol-ID, Multi-Topology Identifier (MT-ID), and BGP-LS
   Instance-ID.  The problem is that BGP may receive node/link/prefix
   information from multiple independent "IGP domains", and we need to
   distinguish between them.  Moreover, we can't assume there is always
   one and only one IGP domain per AS.  During IGP transitions, it may
   happen that two redundant IGPs are in place.

   Furthermore, in deployments where BGP-LS is used to advertise
   topology from multiple ASes, the Autonomous System Number (ASN) is
   used to distinguish topology information reported from different
   ASes.

   The BGP-LS Instance-ID carried in the Identifier field, as described
   earlier along with a set of sub-TLVs described in Section 5.2.1.4,
   allows specification of a flexible key for any given node/link
   information such that the global uniqueness of the NLRI is ensured.
   Since the BGP-LS Instance-ID is operator assigned, its allocation
   scheme can ensure that each IGP domain is uniquely identified even
   across a multi-AS network.

5.2.1.2.  Local Node Descriptors

   The Local Node Descriptors TLV contains Node Descriptors for the node
   anchoring the local end of the link.  This is a mandatory TLV in all
   three types of NLRIs (node, link, and prefix).  The Type is 256.  The
   length of this TLV is variable.  The value contains one or more Node
   Descriptor sub-TLVs defined in Section 5.2.1.4.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //              Node Descriptor Sub-TLVs (variable)            //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 10: Local Node Descriptors TLV Format

5.2.1.3.  Remote Node Descriptors

   The Remote Node Descriptors TLV contains Node Descriptors for the
   node anchoring the remote end of the link.  This is a mandatory TLV
   for Link NLRIs.  The Type is 257.  The length of this TLV is
   variable.  The value contains one or more Node Descriptor sub-TLVs
   defined in Section 5.2.1.4.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //              Node Descriptor Sub-TLVs (variable)            //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 11: Remote Node Descriptors TLV Format

5.2.1.4.  Node Descriptor Sub-TLVs

   The Node Descriptor sub-TLV type code points and lengths are listed
   in the following table:

    +====================+================================+==========+
    | Sub-TLV Code Point | Description                    |   Length |
    +====================+================================+==========+
    |        512         | Autonomous System              |        4 |
    +--------------------+--------------------------------+----------+
    |        513         | BGP-LS Identifier (deprecated) |        4 |
    +--------------------+--------------------------------+----------+
    |        514         | OSPF Area-ID                   |        4 |
    +--------------------+--------------------------------+----------+
    |        515         | IGP Router-ID                  | Variable |
    +--------------------+--------------------------------+----------+

                    Table 3: Node Descriptor Sub-TLVs

   The sub-TLV values in Node Descriptor TLVs are defined as follows:

   Autonomous System:  Opaque value (32-bit AS Number).  This is an
      optional TLV.  The value SHOULD be set to the AS Number associated
      with the BGP process originating the link-state information.  An
      implementation MAY provide a configuration option on the BGP-LS
      Producer to use a different value, e.g., to avoid collisions when
      using private AS Numbers.

   BGP-LS Identifier:  Opaque value (32-bit ID).  This is an optional
      TLV that has been deprecated by this document (refer to Appendix A
      for more details).  It MAY be advertised for compatibility with
      [RFC7752] implementations.  See the final paragraph of this
      section for further considerations and a recommended default
      value.

   OSPF Area-ID:  Used to identify the 32-bit area to which the
      information advertised in the NLRI belongs.  This is a mandatory
      TLV when originating information from OSPF that is derived from
      area-scope LSAs.  The OSPF Area Identifier allows different NLRIs
      of the same router to be differentiated on a per-area basis.  It
      is not used for NLRIs when carrying information that is derived
      from AS-scope LSAs as that information is not associated with a
      specific area.

   IGP Router-ID:  Opaque value.  This is a mandatory TLV when
      originating information from IS-IS, OSPF, 'Direct', or 'Static
      configuration'.  For an IS-IS non-pseudonode, this contains a
      6-octet ISO Node-ID (ISO System-ID).  For an IS-IS pseudonode
      corresponding to a LAN, this contains the 6-octet ISO Node-ID of
      the Designated Intermediate System (DIS) followed by a 1-octet,
      nonzero PSN identifier (7 octets in total).  For an OSPFv2 or
      OSPFv3 non-pseudonode, this contains the 4-octet Router-ID.  For
      an OSPFv2 pseudonode representing a LAN, this contains the 4-octet
      Router-ID of the Designated Router (DR) followed by the 4-octet
      IPv4 address of the DR's interface to the LAN (8 octets in total).
      Similarly, for an OSPFv3 pseudonode, this contains the 4-octet
      Router-ID of the DR followed by the 4-octet interface identifier
      of the DR's interface to the LAN (8 octets in total).  The TLV
      size in combination with the protocol identifier enables the
      decoder to determine the type of the node.  For 'Direct' or
      'Static configuration', the value SHOULD be taken from an IPv4 or
      IPv6 address (e.g., loopback interface) configured on the node.
      When the node is running an IGP protocol, an implementation MAY
      choose to use the IGP Router-ID for 'Direct' or 'Static
      configuration'.

   At most, there MUST be one instance of each sub-TLV type present in
   any Node Descriptor.  The sub-TLVs within a Node Descriptor MUST be
   arranged in ascending order by sub-TLV type.  This needs to be done
   to compare NLRIs, even when an implementation encounters an unknown
   sub-TLV.  Using stable sorting, an implementation can do a binary
   comparison of NLRIs and hence allow incremental deployment of new key
   sub-TLVs.

   The BGP-LS Identifier was introduced by [RFC7752], and its use is
   being deprecated by this document.  Implementations SHOULD support
   the advertisement of this sub-TLV for backward compatibility in
   deployments where there are BGP-LS Producer implementations that
   conform to [RFC7752] to ensure consistency of NLRI encoding for link-
   state objects.  The default value of 0 is RECOMMENDED to be used when
   a BGP-LS Producer includes this sub-TLV when originating information
   into BGP-LS.  Implementations SHOULD provide an option to configure
   this value for backward compatibility reasons.  As a reminder, the
   use of the BGP-LS Instance-ID that is carried in the Identifier field
   is the way of segregation of link-state objects of different IGP
   domains in BGP-LS.

5.2.2.  Link Descriptors

   The Link Descriptor field is a set of Type/Length/Value (TLV)
   triplets.  The format of each TLV is shown in Section 5.1.  The Link
   Descriptor TLVs uniquely identify a link among multiple parallel
   links between a pair of anchor routers.  A link described by the Link
   Descriptor TLVs actually is a "half-link", a unidirectional
   representation of a logical link.  To fully describe a single logical
   link, two anchor routers advertise a half-link each, i.e., two Link
   NLRIs are advertised for a given point-to-point link.

   A link between two nodes is not considered as complete (or available)
   unless it is described by the two Link NLRIs corresponding to the
   half-link representation from the pair of anchor nodes.  This check
   is similar to the 'two-way connectivity check' that is performed by
   link-state IGPs.

   An implementation MAY suppress the advertisement of a Link NLRI,
   corresponding to a half-link, from a link-state IGP unless the IGP
   has verified that the link is being reported in the IS-IS LSP or OSPF
   Router LSA by both the nodes connected by that link.  This 'two-way
   connectivity check' is performed by link-state IGPs during their
   computation and can be leveraged before passing information for any
   half-link that is reported from these IGPs into BGP-LS.  This ensures
   that only those link-state IGP adjacencies that are established get
   reported via Link NLRIs.  Such a 'two-way connectivity check' could
   also be required in certain cases (e.g., with OSPF) to obtain the
   proper link identifiers of the remote node.

   The format and semantics of the Value fields in most Link Descriptor
   TLVs correspond to the format and semantics of Value fields in IS-IS
   Extended IS Reachability sub-TLVs, which are defined in [RFC5305],
   [RFC5307], and [RFC6119].  Although the encodings for Link Descriptor
   TLVs were originally defined for IS-IS, the TLVs can carry data
   sourced by either IS-IS or OSPF.

   The following TLVs are defined as Link Descriptors in the Link NLRI:

     +================+===================+============+=============+
     | TLV Code Point | Description       | IS-IS TLV/ | Reference   |
     |                |                   |  Sub-TLV   |             |
     +================+===================+============+=============+
     |      258       | Link Local/Remote |    22/4    | [RFC5307],  |
     |                | Identifiers       |            | Section 1.1 |
     +----------------+-------------------+------------+-------------+
     |      259       | IPv4 interface    |    22/6    | [RFC5305],  |
     |                | address           |            | Section 3.2 |
     +----------------+-------------------+------------+-------------+
     |      260       | IPv4 neighbor     |    22/8    | [RFC5305],  |
     |                | address           |            | Section 3.3 |
     +----------------+-------------------+------------+-------------+
     |      261       | IPv6 interface    |   22/12    | [RFC6119],  |
     |                | address           |            | Section 4.2 |
     +----------------+-------------------+------------+-------------+
     |      262       | IPv6 neighbor     |   22/13    | [RFC6119],  |
     |                | address           |            | Section 4.3 |
     +----------------+-------------------+------------+-------------+
     |      263       | Multi-Topology    |    ---     | Section     |
     |                | Identifier        |            | 5.2.2.1     |
     +----------------+-------------------+------------+-------------+

                       Table 4: Link Descriptor TLVs

   The information about a link present in the LSA/LSP originated by the
   local node of the link determines the set of TLVs in the Link
   Descriptor of the link.

      If interface and neighbor addresses, either IPv4 or IPv6, are
      present, then the interface/neighbor address TLVs MUST be
      included, and the Link Local/Remote Identifiers TLV MUST NOT be
      included in the Link Descriptor.  The Link Local/Remote
      Identifiers TLV MAY be included in the link attribute when
      available.  IPv4/IPv6 link-local addresses MUST NOT be carried in
      the IPv4/IPv6 interface/neighbor address TLVs (259/260/261/262) as
      descriptors of a link since they are not considered unique.

      If interface and neighbor addresses are not present and the link
      local/remote identifiers are present, then the Link Local/Remote
      Identifiers TLV MUST be included in the Link Descriptor.  The Link
      Local/Remote identifiers MUST be included in the Link Descriptor
      and in the case of links having only IPv6 link-local addressing on
      them.

      The Multi-Topology Identifier TLV MUST be included as a Link
      Descriptor if the underlying IGP link object is associated with a
      non-default topology.

   The TLVs/sub-TLVs corresponding to the interface addresses and/or the
   local/remote identifiers may not always be signaled in the IGPs
   unless their advertisement is enabled specifically.  In such cases,
   it is valid to advertise a BGP-LS Link NLRI without any of these
   identifiers.

5.2.2.1.  Multi-Topology Identifier

   The Multi-Topology Identifier (MT-ID) TLV carries one or more IS-IS
   or OSPF Multi-Topology Identifiers for a link, node, or prefix.

   The semantics of the IS-IS MT-ID are defined in Sections 7.1 and 7.2
   of [RFC5120].  The semantics of the OSPF MT-ID are defined in
   Section 3.7 of [RFC4915].  If the value in the MT-ID TLV is derived
   from OSPF, then the upper R bits of the MT-ID field MUST be set to 0
   and only the values from 0 to 127 are valid for the MT-ID.

   The format of the MT-ID TLV is shown in the following figure.

      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             |          Length=2*n           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |R R R R|  Multi-Topology ID 1  |             ....             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //             ....             |R R R R|  Multi-Topology ID n  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 12: Multi-Topology Identifier TLV Format

   The Type is 263, the length is 2*n, and n is the number of MT-IDs
   carried in the TLV.

   The MT-ID TLV MAY be included as a Link Descriptor, as a Prefix
   Descriptor, or in the BGP-LS Attribute of a Node NLRI.  When included
   as a Link or Prefix Descriptor, only a single MT-ID TLV containing
   the MT-ID of the topology where the link or the prefix is reachable
   is allowed.  In case one wants to advertise multiple topologies for a
   given Link or Prefix Descriptor, multiple NLRIs MUST be generated
   where each NLRI contains a single unique MT-ID.  When used as a Link
   or Prefix Descriptor for IS-IS, the Bits R are reserved and MUST be
   set to 0 (as per Section 7.2 of [RFC5120]) when originated and
   ignored on receipt.

   In the BGP-LS Attribute of a Node NLRI, one MT-ID TLV containing the
   array of MT-IDs of all topologies where the node is reachable is
   allowed.  When used in the Node Attribute TLV for IS-IS, the Bits R
   are set as per Section 7.1 of [RFC5120].

5.2.3.  Prefix Descriptors

   The Prefix Descriptor field is a set of Type/Length/Value (TLV)
   triplets.  Prefix Descriptor TLVs uniquely identify an IPv4 or IPv6
   prefix originated by a node.  The following TLVs are defined as
   Prefix Descriptors in the IPv4/IPv6 Prefix NLRI:

   +================+===========================+==========+===========+
   | TLV Code Point | Description               |  Length  | Reference |
   +================+===========================+==========+===========+
   |      263       | Multi-Topology            | variable | Section   |
   |                | Identifier                |          | 5.2.2.1   |
   +----------------+---------------------------+----------+-----------+
   |      264       | OSPF Route Type           |    1     | Section   |
   |                |                           |          | 5.2.3.1   |
   +----------------+---------------------------+----------+-----------+
   |      265       | IP Reachability           | variable | Section   |
   |                | Information               |          | 5.2.3.2   |
   +----------------+---------------------------+----------+-----------+

                      Table 5: Prefix Descriptor TLVs

   The Multi-Topology Identifier TLV MUST be included in the Prefix
   Descriptor if the underlying IGP prefix object is associated with a
   non-default topology.

5.2.3.1.  OSPF Route Type

   The OSPF Route Type TLV is an optional TLV corresponding to Prefix
   NLRIs originated from OSPF.  It is used to identify the OSPF route
   type of the prefix.  An OSPF prefix MAY be advertised in the OSPF
   domain with multiple route types.  The Route Type TLV allows the
   discrimination of these advertisements.  The OSPF Route Type TLV MUST
   be included in the advertisement when the type is either being
   signaled explicitly in the underlying LSA or can be determined via
   another LSA for the same prefix when it is not signaled explicitly
   (e.g., in the case of OSPFv2 Extended Prefix Opaque LSA [RFC7684]).
   The route type advertised in the OSPFv2 Extended Prefix TLV
   (Section 2.1 of [RFC7684]) does not make a distinction between Type 1
   and 2 for AS external and Not-So-Stubby Area (NSSA) external routes.
   In this case, the route type to be used in the BGP-LS advertisement
   can be determined by checking the OSPFv2 External or NSSA External
   LSA for the prefix.  A similar check for the base OSPFv2 LSAs can be
   done to determine the route type to be used when the route type value
   0 is carried in the OSPFv2 Extended Prefix TLV.

   The format of the OSPF Route Type TLV is shown in the following
   figure.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Route Type   |
     +-+-+-+-+-+-+-+-+

                   Figure 13: OSPF Route Type TLV Format

   The Type and Length fields of the TLV are defined in Table 5.  The
   Route Type field follows the route types defined in the OSPF protocol
   and can be one of the following:

   *  Intra-Area (0x1)

   *  Inter-Area (0x2)

   *  External 1 (0x3)

   *  External 2 (0x4)

   *  NSSA 1 (0x5)

   *  NSSA 2 (0x6)

5.2.3.2.  IP Reachability Information

   The IP Reachability Information TLV is a mandatory TLV for IPv4 &
   IPv6 Prefix NLRI types.  The TLV contains one IP address prefix (IPv4
   or IPv6) originally advertised in the IGP topology.  A router SHOULD
   advertise an IP Prefix NLRI for each of its BGP next hops.  The
   format of the IP Reachability Information TLV is shown in the
   following figure:

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Prefix Length | IP Prefix (variable)                         //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 14: IP Reachability Information TLV Format

   The Type and Length fields of the TLV are defined in Table 5.  The
   following two fields determine the reachability information of the
   address family.  The Prefix Length field contains the length of the
   prefix in bits.  The IP Prefix field contains an IP address prefix
   followed by the minimum number of trailing bits needed to make the
   end of the field fall on an octet boundary.  Any trailing bits MUST
   be set to 0.  Thus, the IP Prefix field contains the most significant
   octets of the prefix, i.e., 1 octet for prefix length 1 up to 8, 2
   octets for prefix length 9 up to 16, 3 octets for prefix length 17 up
   to 24, 4 octets for prefix length 25 up to 32, etc.

5.3.  The BGP-LS Attribute

   The BGP-LS Attribute (assigned value 29 by IANA) is an optional, non-
   transitive BGP Attribute that is used to carry link, node, and prefix
   parameters and attributes.  It is defined as a set of Type/Length/
   Value (TLV) triplets, as described in the following section.  This
   attribute SHOULD only be included with Link-State NLRIs.  The use of
   this attribute for other address families is outside the scope of
   this document.

   The Node Attribute TLVs, Link Attribute TLVs, and Prefix Attribute
   TLVs are sets of TLVs that may be encoded in the BGP-LS Attribute
   associated with a Node NLRI, Link NLRI, and Prefix NLRI respectively.

   The size of the BGP-LS Attribute may potentially grow 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
   implementations support the extended message size for BGP [RFC8654]
   to accommodate a larger size of information within the BGP-LS
   Attribute.  BGP-LS Producers MUST ensure that the TLVs included in
   the BGP-LS Attribute does not result in a BGP UPDATE message for a
   single Link-State NLRI that crosses the maximum limit for a BGP
   message.

   An implementation MAY adopt mechanisms to avoid this problem that may
   be based on the BGP-LS Consumer applications' requirement; these
   mechanisms are beyond the scope of this specification.  However, if
   an implementation chooses to mitigate the problem by excluding some
   TLVs from the BGP-LS Attribute, this exclusion SHOULD be done
   consistently by all BGP-LS Producers within a given BGP-LS domain.
   In the event of inconsistent exclusion of TLVs from the BGP-LS
   Attribute, the result would be a differing set of attributes of the
   link-state object being propagated to BGP-LS Consumers based on the
   BGP Decision Process at BGP-LS Propagators.

   When a BGP-LS Propagator finds that it is exceeding the maximum BGP
   message size due to the addition or update of some other BGP
   Attribute (e.g., AS_PATH), it MUST consider the BGP-LS Attribute to
   be malformed, apply the 'Attribute Discard' error-handling approach
   [RFC7606], and handle the propagation as described in Section 8.2.2.
   When a BGP-LS Propagator needs to perform 'Attribute Discard' for
   reducing the BGP UPDATE message size as specified in Section 4 of
   [RFC8654], it MUST first discard the BGP-LS Attribute to enable the
   detection and diagnosis of this error condition as discussed in
   Section 8.2.2.  This brings the deployment consideration that the
   consistent propagation of BGP-LS information with a BGP UPDATE
   message size larger than 4096 octets can only happen along a set of
   BGP Speakers that all support the contents of [RFC8654].

5.3.1.  Node Attribute TLVs

   The following Node Attribute TLVs are defined for the BGP-LS
   Attribute associated with a Node NLRI:

       +================+================+==========+=============+
       | TLV Code Point | Description    |   Length | Reference   |
       +================+================+==========+=============+
       |      263       | Multi-Topology | variable | Section     |
       |                | Identifier     |          | 5.2.2.1     |
       +----------------+----------------+----------+-------------+
       |      1024      | Node Flag Bits |        1 | Section     |
       |                |                |          | 5.3.1.1     |
       +----------------+----------------+----------+-------------+
       |      1025      | Opaque Node    | variable | Section     |
       |                | Attribute      |          | 5.3.1.5     |
       +----------------+----------------+----------+-------------+
       |      1026      | Node Name      | variable | Section     |
       |                |                |          | 5.3.1.3     |
       +----------------+----------------+----------+-------------+
       |      1027      | IS-IS Area     | variable | Section     |
       |                | Identifier     |          | 5.3.1.2     |
       +----------------+----------------+----------+-------------+
       |      1028      | IPv4 Router-ID |        4 | [RFC5305],  |
       |                | of Local Node  |          | Section 4.3 |
       +----------------+----------------+----------+-------------+
       |      1029      | IPv6 Router-ID |       16 | [RFC6119],  |
       |                | of Local Node  |          | Section 4.1 |
       +----------------+----------------+----------+-------------+

                       Table 6: Node Attribute TLVs

5.3.1.1.  Node Flag Bits TLV

   The Node Flag Bits TLV carries a bitmask describing node attributes.
   The value is a 1-octet-length bit array of flags, where each bit
   represents a node-operational state or attribute.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |O|T|E|B|R|V|   |
     +-+-+-+-+-+-+-+-+

                    Figure 15: Node Flag Bits TLV Format

   The bits are defined as follows:

                    +=====+==============+============+
                    | Bit | Description  | Reference  |
                    +=====+==============+============+
                    | 'O' | Overload Bit | [ISO10589] |
                    +-----+--------------+------------+
                    | 'A' | Attached Bit | [ISO10589] |
                    +-----+--------------+------------+
                    | 'E' | External Bit | [RFC2328]  |
                    +-----+--------------+------------+
                    | 'B' | ABR Bit      | [RFC2328]  |
                    +-----+--------------+------------+
                    | 'R' | Router Bit   | [RFC5340]  |
                    +-----+--------------+------------+
                    | 'V' | V6 Bit       | [RFC5340]  |
                    +-----+--------------+------------+

                    Table 7: Node Flag Bits Definitions

   The bits that are not defined MUST be set to 0 by the originator and
   MUST be ignored by the receiver.

5.3.1.2.  IS-IS Area Identifier TLV

   An IS-IS node can be part of only a single IS-IS area.  However, a
   node can have multiple synonymous area addresses.  Each of these area
   addresses is carried in the IS-IS Area Identifier TLV.  If multiple
   area addresses are present, multiple TLVs are used to encode them.
   The IS-IS Area Identifier TLV may be present in the BGP-LS Attribute
   only when advertised in the Link-State Node NLRI.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               IS-IS Area Identifier (variable)              //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 16: IS-IS Area Identifier TLV Format

5.3.1.3.  Node Name TLV

   The Node Name TLV is optional.  The encoding semantics for the node
   name has been borrowed from [RFC5301].  The Value field identifies
   the symbolic name of the router node.  This symbolic name can be the
   Fully Qualified Domain Name (FQDN) for the router, a substring of the
   FQDN (e.g., a hostname), or any string that an operator wants to use
   for the router.  The use of the FQDN or a substring of it is strongly
   RECOMMENDED.  The maximum length of the Node Name TLV is 255 octets.

   The Value field is encoded in 7-bit ASCII.  If a user interface for
   configuring or displaying this field permits Unicode characters, then
   the user interface is responsible for applying the ToASCII and/or
   ToUnicode algorithm as described in [RFC5890] to achieve the correct
   format for transmission or display.

   [RFC5301] describes an IS-IS-specific extension, and [RFC5642]
   describes an OSPF extension for the advertisement of the node name,
   which may be encoded in the Node Name TLV.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     Node Name (variable)                    //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 17: Node Name Format

5.3.1.4.  Local IPv4/IPv6 Router-ID TLVs

   The local IPv4/IPv6 Router-ID TLVs are used to describe auxiliary
   Router-IDs that the IGP might be using, e.g., for TE and migration
   purposes such as correlating a Node-ID between different protocols.
   If there is more than one auxiliary Router-ID of a given type, then
   each one is encoded as a separate TLV.

5.3.1.5.  Opaque Node Attribute TLV

   The Opaque Node Attribute TLV is an envelope that transparently
   carries optional Node Attribute TLVs advertised by a router.  An
   originating router shall use this TLV for encoding information
   specific to the protocol advertised in the NLRI header Protocol-ID
   field or new protocol extensions to the protocol as advertised in the
   NLRI header Protocol-ID field for which there is no protocol-neutral
   representation in the BGP Link-State NLRI.  The primary use of the
   Opaque Node Attribute TLV is to bridge the document lag between a new
   IGP link-state attribute and its protocol-neutral BGP-LS extension
   being defined.  Once the protocol-neutral BGP-LS extensions are
   defined, the BGP-LS implementations may still need to advertise the
   information both within the Opaque Attribute TLV and the new TLV
   definition for incremental deployment and transition.

   In the case of OSPF, this TLV MUST NOT be used to advertise TLVs
   other than those in the OSPF Router Information (RI) LSA [RFC7770].

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Opaque Node Attributes (variable)             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 18: Opaque Node Attribute Format

   The Type is as specified in Table 6.  The length is variable.

5.3.2.  Link Attribute TLVs

   Link Attribute TLVs are TLVs that may be encoded in the BGP-LS
   Attribute with a Link NLRI.  Each 'Link Attribute' is a Type/Length/
   Value (TLV) triplet formatted as defined in Section 5.1.  The format
   and semantics of the Value fields in some Link Attribute TLVs
   correspond to the format and semantics of the Value fields in IS-IS
   Extended IS Reachability sub-TLVs, which are defined in [RFC5305] and
   [RFC5307].  Other Link Attribute TLVs are defined in this document.
   Although the encodings for Link Attribute TLVs were originally
   defined for IS-IS, the TLVs can carry data sourced by either IS-IS or
   OSPF.

   The following Link Attribute TLVs are defined for the BGP-LS
   Attribute associated with a Link NLRI:

      +================+=================+============+=============+
      | TLV Code Point | Description     | IS-IS TLV/ | Reference   |
      |                |                 |  Sub-TLV   |             |
      +================+=================+============+=============+
      |      1028      | IPv4 Router-ID  |  134/---   | [RFC5305],  |
      |                | of Local Node   |            | Section 4.3 |
      +----------------+-----------------+------------+-------------+
      |      1029      | IPv6 Router-ID  |  140/---   | [RFC6119],  |
      |                | of Local Node   |            | Section 4.1 |
      +----------------+-----------------+------------+-------------+
      |      1030      | IPv4 Router-ID  |  134/---   | [RFC5305],  |
      |                | of Remote Node  |            | Section 4.3 |
      +----------------+-----------------+------------+-------------+
      |      1031      | IPv6 Router-ID  |  140/---   | [RFC6119],  |
      |                | of Remote Node  |            | Section 4.1 |
      +----------------+-----------------+------------+-------------+
      |      1088      | Administrative  |    22/3    | [RFC5305],  |
      |                | group (color)   |            | Section 3.1 |
      +----------------+-----------------+------------+-------------+
      |      1089      | Maximum link    |    22/9    | [RFC5305],  |
      |                | bandwidth       |            | Section 3.4 |
      +----------------+-----------------+------------+-------------+
      |      1090      | Max. reservable |   22/10    | [RFC5305],  |
      |                | link bandwidth  |            | Section 3.5 |
      +----------------+-----------------+------------+-------------+
      |      1091      | Unreserved      |   22/11    | [RFC5305],  |
      |                | bandwidth       |            | Section 3.6 |
      +----------------+-----------------+------------+-------------+
      |      1092      | TE Default      |   22/18    | Section     |
      |                | Metric          |            | 5.3.2.3     |
      +----------------+-----------------+------------+-------------+
      |      1093      | Link Protection |   22/20    | [RFC5307],  |
      |                | Type            |            | Section 1.2 |
      +----------------+-----------------+------------+-------------+
      |      1094      | MPLS Protocol   |    ---     | Section     |
      |                | Mask            |            | 5.3.2.2     |
      +----------------+-----------------+------------+-------------+
      |      1095      | IGP Metric      |    ---     | Section     |
      |                |                 |            | 5.3.2.4     |
      +----------------+-----------------+------------+-------------+
      |      1096      | Shared Risk     |    ---     | Section     |
      |                | Link Group      |            | 5.3.2.5     |
      +----------------+-----------------+------------+-------------+
      |      1097      | Opaque Link     |    ---     | Section     |
      |                | Attribute       |            | 5.3.2.6     |
      +----------------+-----------------+------------+-------------+
      |      1098      | Link Name       |    ---     | Section     |
      |                |                 |            | 5.3.2.7     |
      +----------------+-----------------+------------+-------------+

                        Table 8: Link Attribute TLVs

5.3.2.1.  IPv4/IPv6 Router-ID TLVs

   The local/remote IPv4/IPv6 Router-ID TLVs are used to describe
   auxiliary Router-IDs that the IGP might be using, e.g., for TE
   purposes.  All auxiliary Router-IDs of both the local and the remote
   node MUST be included in the link attribute of each Link NLRI.  If
   there is more than one auxiliary Router-ID of a given type, then
   multiple TLVs are used to encode them.

5.3.2.2.  MPLS Protocol Mask TLV

   The MPLS Protocol Mask TLV carries a bitmask describing which MPLS
   signaling protocols are enabled.  The length of this TLV is 1.  The
   value is a bit array of 8 flags, where each bit represents an MPLS
   Protocol capability.

   Generation of the MPLS Protocol Mask TLV is only valid for and SHOULD
   only be used with originators that have local link insight, for
   example, the Protocol-IDs 'Static configuration' or 'Direct' as per
   Table 2.  The MPLS Protocol Mask TLV MUST NOT be included in NLRIs
   with the other Protocol-IDs listed in Table 2.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|R|  Reserved |
     +-+-+-+-+-+-+-+-+

                     Figure 19: MPLS Protocol Mask TLV

   The following bits are defined, and the reserved bits MUST be set to
   zero and SHOULD be ignored on receipt:

     +=====+=============================================+===========+
     | Bit | Description                                 | Reference |
     +=====+=============================================+===========+
     | 'L' | Label Distribution Protocol (LDP)           | [RFC5036] |
     +-----+---------------------------------------------+-----------+
     | 'R' | Extension to RSVP for LSP Tunnels (RSVP-TE) | [RFC3209] |
     +-----+---------------------------------------------+-----------+

                   Table 9: MPLS Protocol Mask TLV Codes

   The bits that are not defined MUST be set to 0 by the originator and
   MUST be ignored by the receiver.

5.3.2.3.  TE Default Metric TLV

   The TE Default Metric TLV carries the Traffic Engineering metric for
   this link.  The length of this TLV is fixed at 4 octets.  If a source
   protocol uses a metric width of fewer than 32 bits, then the high-
   order bits of this field MUST be padded with zero.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    TE Default Link Metric                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 20: TE Default Metric TLV Format

5.3.2.4.  IGP Metric TLV

   The IGP Metric TLV carries the metric for this link.  The length of
   this TLV is variable, depending on the metric width of the underlying
   protocol.  IS-IS small metrics are 6 bits in size but are encoded in
   a 1-octet field; therefore, the two most significant bits of the
   field MUST be set to 0 by the originator and MUST be ignored by the
   receiver.  OSPF link metrics have a length of 2 octets.  IS-IS wide
   metrics have a length of 3 octets.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //      IGP Link Metric (variable length)      //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 21: IGP Metric TLV Format

5.3.2.5.  Shared Risk Link Group TLV

   The Shared Risk Link Group (SRLG) TLV carries the Shared Risk Link
   Group information (see Section 2.3 ("Shared Risk Link Group
   Information") of [RFC4202]).  It contains a data structure consisting
   of a (variable) list of SRLG values, where each element in the list
   has 4 octets, as shown in Figure 22.  The length of this TLV is 4 *
   (number of SRLG values).

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Shared Risk Link Group Value                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                         ............                        //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Shared Risk Link Group Value                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 22: Shared Risk Link Group TLV Format

   The SRLG TLV for OSPF-TE is defined in [RFC4203].  In IS-IS, the SRLG
   information is carried in two different TLVs: the GMPLS-SRLG TLV (for
   IPv4) (Type 138) defined in [RFC5307] and the IPv6 SRLG TLV (Type
   139) defined in [RFC6119].  Both IPv4 and IPv6 SRLG information is
   carried in a single TLV.

5.3.2.6.  Opaque Link Attribute TLV

   The Opaque Link Attribute TLV is an envelope that transparently
   carries optional Link Attribute TLVs advertised by a router.  An
   originating router shall use this TLV for encoding information
   specific to the protocol advertised in the NLRI header Protocol-ID
   field or new protocol extensions to the protocol as advertised in the
   NLRI header Protocol-ID field for which there is no protocol-neutral
   representation in the BGP Link-State NLRI.  The primary use of the
   Opaque Link Attribute TLV is to bridge the document lag between a new
   IGP link-state attribute and its 'protocol-neutral' BGP-LS extension
   being defined.  Once the protocol-neutral BGP-LS extensions are
   defined, the BGP-LS implementations may still need to advertise the
   information both within the Opaque Attribute TLV and the new TLV
   definition for incremental deployment and transition.

   In the case of OSPFv2, this TLV MUST NOT be used to advertise
   information carried using TLVs other than those in the OSPFv2
   Extended Link Opaque LSA [RFC7684].  In the case of OSPFv3, this TLV
   MUST NOT be used to advertise TLVs other than those in the OSPFv3 E-
   Router-LSA or E-Link-LSA [RFC8362].

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Opaque Link Attributes (variable)            //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 23: Opaque Link Attribute TLV Format

5.3.2.7.  Link Name TLV

   The Link Name TLV is optional.  The Value field identifies the
   symbolic name of the router link.  This symbolic name can be the FQDN
   for the link, a substring of the FQDN, or any string that an operator
   wants to use for the link.  The use of the FQDN or a substring of it
   is strongly RECOMMENDED.  The maximum length of the Link Name TLV is
   255 octets.

   The Value field is encoded in 7-bit ASCII.  If a user interface for
   configuring or displaying this field permits Unicode characters, then
   the user interface is responsible for applying the ToASCII and/or
   ToUnicode algorithm as described in [RFC5890] to achieve the correct
   format for transmission or display.

   How a router derives and injects link names is outside of the scope
   of this document.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     Link Name (variable)                    //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 24: Link Name TLV Format

5.3.3.  Prefix Attribute TLVs

   Prefixes are learned from the IGP topology (IS-IS or OSPF) with a set
   of IGP attributes (such as metric, route tags, etc.) that are
   advertised in the BGP-LS Attribute with Prefix NLRI types 3 and 4.

   The following Prefix Attribute TLVs are defined for the BGP-LS
   Attribute associated with a Prefix NLRI:

     +================+=================+==========+=================+
     | TLV Code Point | Description     |   Length | Reference       |
     +================+=================+==========+=================+
     |      1152      | IGP Flags       |        1 | Section 5.3.3.1 |
     +----------------+-----------------+----------+-----------------+
     |      1153      | IGP Route Tag   |      4*n | [RFC5130]       |
     +----------------+-----------------+----------+-----------------+
     |      1154      | IGP Extended    |      8*n | [RFC5130]       |
     |                | Route Tag       |          |                 |
     +----------------+-----------------+----------+-----------------+
     |      1155      | Prefix Metric   |        4 | [RFC5305]       |
     +----------------+-----------------+----------+-----------------+
     |      1156      | OSPF Forwarding |        4 | [RFC2328]       |
     |                | Address         |          |                 |
     +----------------+-----------------+----------+-----------------+
     |      1157      | Opaque Prefix   | variable | Section 5.3.3.6 |
     |                | Attribute       |          |                 |
     +----------------+-----------------+----------+-----------------+

                      Table 10: Prefix Attribute TLVs

5.3.3.1.  IGP Flags TLV

   The IGP Flags TLV contains one octet of IS-IS and OSPF flags and bits
   originally assigned to the prefix.  The IGP Flags TLV is encoded as
   follows:

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |D|N|L|P|       |
     +-+-+-+-+-+-+-+-+

                       Figure 25: IGP Flag TLV Format

   The Value field contains bits defined according to the table below:

              +=====+===========================+===========+
              | Bit | Description               | Reference |
              +=====+===========================+===========+
              | 'D' | IS-IS Up/Down Bit         | [RFC5305] |
              +-----+---------------------------+-----------+
              | 'N' | OSPF "no unicast" Bit     | [RFC5340] |
              +-----+---------------------------+-----------+
              | 'L' | OSPF "local address" Bit  | [RFC5340] |
              +-----+---------------------------+-----------+
              | 'P' | OSPF "propagate NSSA" Bit | [RFC5340] |
              +-----+---------------------------+-----------+

                    Table 11: IGP Flag Bits Definitions

   The bits that are not defined MUST be set to 0 by the originator and
   MUST be ignored by the receiver.

5.3.3.2.  IGP Route Tag TLV

   The IGP Route Tag TLV carries original IGP Tags (IS-IS [RFC5130] or
   OSPF) of the prefix and is encoded as follows:

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                    Route Tags (one or more)                 //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 26: IGP Route Tag TLV Format

   The length is a multiple of 4.

   The Value field contains one or more Route Tags as learned in the IGP
   topology.

5.3.3.3.  IGP Extended Route Tag TLV

   The IGP Extended Route Tag TLV carries IS-IS Extended Route Tags of
   the prefix [RFC5130] and is encoded as follows:

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Extended Route Tag (one or more)             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 27: IGP Extended Route Tag TLV Format

   The length is a multiple of 8.

   The Extended Route Tag field contains one or more Extended Route Tags
   as learned in the IGP topology.

5.3.3.4.  Prefix Metric TLV

   The Prefix Metric TLV is an optional attribute and may only appear
   once.  If present, it carries the metric of the prefix as known in
   the IGP topology, as described in Section 4 of [RFC5305] (and
   therefore represents the reachability cost to the prefix).  If not
   present, it means that the prefix is advertised without any
   reachability.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Metric                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 28: Prefix Metric TLV Format

   The length is 4.

5.3.3.5.  OSPF Forwarding Address TLV

   The OSPF Forwarding Address TLV [RFC2328] [RFC5340] carries the OSPF
   forwarding address as known in the original OSPF advertisement.  The
   forwarding address can be either IPv4 or IPv6.

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Forwarding Address (variable)                //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 29: OSPF Forwarding Address TLV Format

   The length is 4 for an IPv4 forwarding address and 16 for an IPv6
   forwarding address.

5.3.3.6.  Opaque Prefix Attribute TLV

   The Opaque Prefix Attribute TLV is an envelope that transparently
   carries optional Prefix Attribute TLVs advertised by a router.  An
   originating router shall use this TLV for encoding information
   specific to the protocol advertised in the NLRI header Protocol-ID
   field or it shall use new protocol extensions for the protocol as
   advertised in the NLRI header Protocol-ID field for which there is no
   protocol-neutral representation in the BGP Link-State NLRI.  The
   primary use of the Opaque Prefix Attribute TLV is to bridge the
   document lag between a new IGP link-state attribute and its protocol-
   neutral BGP-LS extension being defined.  Once the protocol-neutral
   BGP-LS extensions are defined, the BGP-LS implementations may still
   need to advertise the information both within the Opaque Attribute
   TLV and the new TLV definition for incremental deployment and
   transition.

   In the case of OSPFv2, this TLV MUST NOT be used to advertise
   information carried using TLVs other than those in the OSPFv2
   Extended Prefix Opaque LSA [RFC7684].  In the case of OSPFv3, this
   TLV MUST NOT be used to advertise TLVs other than those in the OSPFv3
   E-Inter-Area-Prefix-LSA, E-Intra-Area-Prefix-LSA, E-AS-External-LSA,
   and E-NSSA-LSA [RFC8362].

   The format of the TLV is as follows:

      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             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //              Opaque Prefix Attributes  (variable)           //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 30: Opaque Prefix Attribute TLV Format

   The Type is as specified in Table 10.  The length is variable.

5.4.  Private Use

   TLVs for Vendor Private Use are supported using the code point range
   reserved as indicated in Section 7.  For such TLV use in the NLRI or
   BGP-LS Attribute, the format described in Section 5.1 is to be used
   and a 4-octet field MUST be included as the first field in the value
   to carry the Enterprise Code.  For a private use NLRI type, a 4-octet
   field MUST be included as the first field in the NLRI immediately
   following the Total NLRI Length field of the Link-State NLRI format
   as described in Section 5.2 to carry the Enterprise Code [ENTNUM].
   This enables the use of vendor-specific extensions without conflicts.

   Multiple instances of private-use TLVs MAY appear in the BGP-LS
   Attribute.

5.5.  BGP Next-Hop Information

   BGP link-state information for both IPv4 and IPv6 networks can be
   carried over either an IPv4 BGP session or an IPv6 BGP session.  If
   an IPv4 BGP session is used, then the next hop in the MP_REACH_NLRI
   SHOULD be an IPv4 address.  Similarly, if an IPv6 BGP session is
   used, then the next hop in the MP_REACH_NLRI SHOULD be an IPv6
   address.  Usually, the next hop will be set to the local endpoint
   address of the BGP session.  The next-hop address MUST be encoded as
   described in [RFC4760].  The Length field of the next-hop address
   will specify the next-hop address family.  If the next-hop length is
   4, then the next hop is an IPv4 address; if the next-hop length is
   16, then it is a global IPv6 address; and if the next-hop length is
   32, then there is one global IPv6 address followed by an IPv6 link-
   local address.  The IPv6 link-local address should be used as
   described in [RFC2545].  For VPN Subsequent Address Family Identifier
   (SAFI), as per custom, an 8-byte Route Distinguisher set to all zero
   is prepended to the next hop.

   The BGP Next-Hop is used by each BGP-LS Speaker to validate the NLRI
   it receives.  In case identical NLRIs are sourced by multiple BGP-LS
   Producers, the BGP Next-Hop is used to tiebreak as per the standard
   BGP path decision process.  This specification doesn't mandate any
   rule regarding the rewrite of the BGP Next-Hop.

5.6.  Inter-AS Links

   The main source of TE information is the IGP, which is not active on
   inter-AS links.  In some cases, the IGP may have information of
   inter-AS links [RFC5392] [RFC9346].  In other cases, an
   implementation SHOULD provide a means to inject inter-AS links into
   BGP-LS.  The exact mechanism used to advertise the inter-AS links is
   outside the scope of this document.

5.7.  OSPF Virtual Links and Sham Links

   In an OSPF [RFC2328] [RFC5340] network, OSPF virtual links serve to
   connect physically separate components of the backbone to establish/
   maintain continuity of the backbone area.  While OSPF virtual links
   are modeled as point-to-point, unnumbered links in the OSPF topology,
   their characteristics and purpose are different from other types of
   links in the OSPF topology.  They are advertised using a distinct
   "virtual link" type in OSPF LSAs.  The mechanism for the
   advertisement of OSPF virtual links via BGP-LS is outside the scope
   of this document.

   In an OSPF network, sham links [RFC4577] [RFC6565] are used to
   provide intra-area connectivity between VPN Routing and Forwarding
   (VRF) instances on Provider Edge (PE) routers over the VPN provider's
   network.  These links are advertised in OSPF as point-to-point,
   unnumbered links and represent connectivity over a service provider
   network using encapsulation mechanisms like MPLS.  As such, the
   mechanism for the advertisement of OSPF sham links follows the same
   procedures as other point-to-point, unnumbered links as described
   previously in this document.

5.8.  OSPFv2 Type 4 Summary-LSA & OSPFv3 Inter-Area-Router-LSA

   OSPFv2 [RFC2328] defines the type 4 summary-LSA and OSPFv3 [RFC5340]
   defines the inter-area-router-LSA for an Area Border Router (ABR) to
   advertise reachability to an AS Border Router (ASBR) that is external
   to the area yet internal to the AS.  The nature of information
   advertised by OSPF using this type of LSA does not map to either a
   node, a link, or a prefix as discussed in this document.  Therefore,
   the mechanism for the advertisement of the information carried by
   these LSAs is outside the scope of this document.

5.9.  Handling of Unreachable IGP Nodes

   Consider an OSPF network as shown in Figure 31, where R2 and R3 are
   the BGP-LS Producers and also the OSPF Area Border Routers (ABRs).
   The link between R2 and R3 is in area 0, while the other links are in
   area 1 as indicated by the a0 and a1 references respectively against
   the links.

   A BGP-LS Consumer talks to BGP route reflector RR0, which is a BGP-LS
   Propagator that is aggregating the BGP-LS feed from BGP-LS Producers
   R2 and R3.  Here, R2 and R3 provide a redundant topology feed via
   BGP-LS to RR0.  Normally, RR0 would receive two identical copies of
   all the Link-State NLRIs from both R2 and R3 and it would pick one of
   them (say R2) based on the standard BGP Decision Process.

                     BGP-LS Consumer
                            ^
                            |
                           RR0
                    (BGP Route Reflector)
                         /      \
                        /        \
                 a1    /   a0     \    a1
            R1 ------ R2 -------- R3 ------ R4
        a1  |                               |  a1
            |                               |
            R5 ---------------------------- R6
                           a1

          Figure 31: Incorrect Reporting Due to BGP Path Selection

   Consider a scenario where the link between R5 and R6 is lost (thereby
   partitioning the area 1), and consider its impact on the OSPF LSDB at
   R2 and R3.

   Now, R5 will remove the link R5-R6 from its Router LSA, and this
   updated LSA is available at R2.  R2 also has a stale copy of R6's
   Router LSA that still has the link R6-R5 in it.  Based on this view
   in its LSDB, R2 will advertise only the half-link R6-R5 that it
   derives from R6's stale Router LSA.

   At the same time, R6 has removed the link R6-R5 from its Router LSA,
   and this updated LSA is available at R3.  Similarly, R3 also has a
   stale copy of R5's Router LSA having the link R5-R6 in it.  Based on
   its LSDB, R3 will advertise only the half-link R5-R6 that it derives
   from R5's stale Router LSA.

   Now, the BGP-LS Consumer receives both the Link NLRIs corresponding
   to the half-links from R2 and R3 via RR0.  When viewed together, it
   would not detect or realize that area 1 is partitioned.  Also, if R2
   continues to report Node and Prefix NLRIs corresponding to the stale
   copy of R4's and R6's Router LSAs, then RR0 could prefer them over
   the valid Node and Prefix NLRIs for R4 and R6 that it is receiving
   from R3 depending on RR0's BGP Decision Process.  This would result
   in the BGP-LS Consumer getting stale and inaccurate topology
   information.  This problem scenario is avoided if R2 were to not
   advertise the link-state information corresponding to R4 and R6 and
   if R3 were to not advertise similarly for R1 and R5.

   A BGP-LS Producer SHOULD withdraw all link-state objects advertised
   by it in BGP when the node that originated its corresponding LSPs/
   LSAs is determined to have become unreachable in the IGP.  An
   implementation MAY continue to advertise link-state objects
   corresponding to unreachable nodes in a deployment use case where the
   BGP-LS Consumer is interested in receiving a topology feed
   corresponding to a complete IGP LSDB view.  In such deployments, it
   is expected that the problem described above is mitigated by the BGP-
   LS Consumer via appropriate handling of such a topology feed in
   addition to the use of either a direct BGP peering with the BGP-LS
   Producer nodes or mechanisms such as those described in [RFC7911]
   when using RRs.  Details of these mechanisms are outside the scope of
   this document.

   If the BGP-LS Producer does withdraw link-state objects associated
   with an IGP node based on the failure of reachability check for that
   node, then it MUST re-advertise those link-state objects after that
   node becomes reachable again in the IGP domain.

5.10.  Router-ID Anchoring Example: ISO Pseudonode

   The encoding of a broadcast LAN in IS-IS provides a good example of
   how Router-IDs are encoded.  Consider Figure 32.  This represents a
   broadcast LAN between a pair of routers.  The "real" (non-pseudonode)
   routers have both an IPv4 Router-ID and an IS-IS Node-ID.  The
   pseudonode does not have an IPv4 Router-ID.  Node1 is the DIS for the
   LAN.  Two unidirectional links, (Node1, Pseudonode1) and
   (Pseudonode1, Node2), are being generated.

   The Link NLRI of (Node1, Pseudonode1) is encoded as follows.  The IGP
   Router-ID TLV of the local Node Descriptor is 6 octets long and
   contains the ISO-ID of Node1, 1920.0000.2001.  The IGP Router-ID TLV
   of the remote Node Descriptor is 7 octets long and contains the ISO-
   ID of Pseudonode1, 1920.0000.2001.02.  The BGP-LS Attribute of this
   link contains one local IPv4 Router-ID TLV (TLV type 1028) containing
   192.0.2.1, the IPv4 Router-ID of Node1.

   The Link NLRI of (Pseudonode1, Node2) is encoded as follows.  The IGP
   Router-ID TLV of the local Node Descriptor is 7 octets long and
   contains the ISO-ID of Pseudonode1, 1920.0000.2001.02.  The IGP
   Router-ID TLV of the remote Node Descriptor is 6 octets long and
   contains the ISO-ID of Node2, 1920.0000.2002.  The BGP-LS Attribute
   of this link contains one remote IPv4 Router-ID TLV (TLV type 1030)
   containing 192.0.2.2, the IPv4 Router-ID of Node2.

     +-----------------+    +-----------------+    +-----------------+
     |      Node1      |    |   Pseudonode1   |    |      Node2      |
     |1920.0000.2001.00|--->|1920.0000.2001.02|--->|1920.0000.2002.00|
     |     192.0.2.1   |    |                 |    |     192.0.2.2   |
     +-----------------+    +-----------------+    +-----------------+

                        Figure 32: IS-IS Pseudonodes

5.11.  Router-ID Anchoring Example: OSPF Pseudonode

   The encoding of a broadcast LAN in OSPF provides a good example of
   how Router-IDs and local Interface IPs are encoded.  Consider
   Figure 33.  This represents a broadcast LAN between a pair of
   routers.  The "real" (non-pseudonode) routers have both an IPv4
   Router-ID and an Area Identifier.  The pseudonode does have an IPv4
   Router-ID, an IPv4 Interface Address (for disambiguation), and an
   OSPF Area.  Node1 is the DR for the LAN; hence, its local IP address
   198.51.100.1 is used as both the Router-ID and Interface IP for the
   pseudonode keys.  Two unidirectional links, (Node1, Pseudonode1) and
   (Pseudonode1, Node2), are being generated.

   The Link NLRI of (Node1, Pseudonode1) is encoded as follows:

   *  Local Node Descriptor

      TLV #515:  IGP Router-ID: 192.0.2.1

      TLV #514:  OSPF Area-ID: ID:0.0.0.0

   *  Remote Node Descriptor

      TLV #515:  IGP Router-ID: 192.0.2.1:198.51.100.1

      TLV #514:  OSPF Area-ID: ID:0.0.0.0

   The Link NLRI of (Pseudonode1, Node2) is encoded as follows:

   *  Local Node Descriptor

      TLV #515:  IGP Router-ID: 192.0.2.1:198.51.100.1

      TLV #514:  OSPF Area-ID: ID:0.0.0.0

   *  Remote Node Descriptor

      TLV #515:  IGP Router-ID: 192.0.2.2

      TLV #514:  OSPF Area-ID: ID:0.0.0.0

        198.51.100.1/24             198.51.100.2/24
   +-------------+    +-------------+    +-------------+
   |   Node1     |    | Pseudonode1 |    |    Node2    |
   |  192.0.2.1  |--->|  192.0.2.1  |--->|  192.0.2.2  |
   |             |    |198.51.100.1 |    |             |
   |   Area 0    |    |   Area 0    |    |    Area 0   |
   +-------------+    +-------------+    +-------------+

                        Figure 33: OSPF Pseudonodes

   The LAN subnet 198.51.100.0/24 is not included in the Router LSA of
   Node1 or Node2.  The Network LSA for this LAN advertised by the DR
   Node1 contains the subnet mask for the LAN along with the DR address.
   A Prefix NLRI corresponding to the LAN subnet is advertised with the
   Pseudonode1 used as the local node using the DR address and the
   subnet mask from the Network LSA.

5.12.  Router-ID Anchoring Example: OSPFv2 to IS-IS Migration

   Graceful migration from one IGP to another requires coordinated
   operation of both protocols during the migration period.  Such
   coordination requires identifying a given physical link in both IGPs.
   The IPv4 Router-ID provides that "glue", which is present in the Node
   Descriptors of the OSPF Link NLRI and in the link attribute of the
   IS-IS Link NLRI.

   Consider a point-to-point link between two routers, A and B, which
   initially were OSPFv2-only routers and then had IS-IS enabled on
   them.  Node A has IPv4 Router-ID and ISO-ID; node B has IPv4 Router-
   ID, IPv6 Router-ID, and ISO-ID.  Each protocol generates one Link
   NLRI for the link (A, B), both of which are carried by BGP-LS.  The
   OSPFv2 Link NLRI for the link is encoded with the IPv4 Router-ID of
   nodes A and B in the local and remote Node Descriptors, respectively.
   The IS-IS Link NLRI for the link is encoded with the ISO-ID of nodes
   A and B in the local and remote Node Descriptors, respectively.  In
   addition, the BGP-LS Attribute of the IS-IS Link NLRI contains the
   TLV type 1028 containing the IPv4 Router-ID of node A, TLV type 1030
   containing the IPv4 Router-ID of node B, and TLV type 1031 containing
   the IPv6 Router-ID of node B.  In this case, by using IPv4 Router-ID,
   the link (A, B) can be identified in both the IS-IS and OSPF
   protocols.

6.  Link to Path Aggregation

   Distribution of all links available on the global Internet is
   certainly possible; however, it is not desirable from a scaling and
   privacy point of view.  Therefore, an implementation may support a
   link to path aggregation.  Rather than advertising all specific links
   of a domain, an ASBR may advertise an "aggregate link" between a non-
   adjacent pair of nodes.  The "aggregate link" represents the
   aggregated set of link properties between a pair of non-adjacent
   nodes.  The actual methods to compute the path properties (of
   bandwidth, metric, etc.) are outside the scope of this document.  The
   decision of whether to advertise all specific links or aggregated
   links is an operator's policy choice.  To highlight the varying
   levels of exposure, the following deployment examples are discussed.

6.1.  Example: No Link Aggregation

   Consider Figure 34.  Both AS1 and AS2 operators want to protect their
   inter-AS {R1, R3}, {R2, R4} links using RSVP - Fast Reroute (RSVP-
   FRR) LSPs.  If R1 wants to compute its link-protection LSP to R3, it
   needs to "see" an alternate path to R3.  Therefore, the AS2 operator
   exposes its topology.  All BGP-TE-enabled routers in AS1 "see" the
   full topology of AS2 and therefore can compute a backup path.  Note
   that the computing router decides if the direct link between {R3, R4}
   or the {R4, R5, R3} path is used.

          AS1   :   AS2
                :
           R1-------R3
            |   :   | \
            |   :   |  R5
            |   :   | /
           R2-------R4
                :
                :

                       Figure 34: No Link Aggregation

6.2.  Example: ASBR to ASBR Path Aggregation

   The brief difference between the "no-link aggregation" example and
   this example is that no specific link gets exposed.  Consider
   Figure 35.  The only link that gets advertised by AS2 is an
   "aggregate" link between R3 and R4.  This is enough to tell AS1 that
   there is a backup path.  However, the actual links being used are
   hidden from the topology.

          AS1   :   AS2
                :
           R1-------R3
            |   :   |
            |   :   |
            |   :   |
           R2-------R4
                :
                :

                      Figure 35: ASBR Link Aggregation

6.3.  Example: Multi-AS Path Aggregation

   Service providers in control of multiple ASes may even decide to not
   expose their internal inter-AS links.  Consider Figure 36.  AS3 is
   modeled as a single node that connects to the border routers of the
   aggregated domain.

          AS1   :   AS2   :   AS3
                :         :
           R1-------R3-----
            |   :         : \
            |   :         :   vR0
            |   :         : /
           R2-------R4-----
                :         :
                :         :

                      Figure 36: Multi-AS Aggregation

7.  IANA Considerations

   As this document obsoletes [RFC7752] and [RFC9029], IANA has updated
   all registration information that references those documents to
   instead reference this document.

   IANA has assigned address family number 16388 (BGP-LS) in the
   "Address Family Numbers" registry.

   IANA has assigned SAFI values 71 (BGP-LS) and 72 (BGP-LS-VPN) in the
   "SAFI Values" registry under the "Subsequent Address Family
   Identifiers (SAFI) Parameters" registry group.

   IANA has assigned value 29 (BGP-LS Attribute) in the "BGP Path
   Attributes" registry under the "Border Gateway Protocol (BGP)
   Parameters" registry group.

   IANA has created a "Border Gateway Protocol - Link-State (BGP-LS)
   Parameters" registry group at <https://www.iana.org/assignments/bgp-
   ls-parameters>.

   This section also incorporates all the changes to the allocation
   procedures for the BGP-LS IANA registry group as well as the
   guidelines for designated experts introduced by [RFC9029].

7.1.  BGP-LS Registries

   All of the registries listed in the following subsections are
   specific to BGP-LS and are accessible under this registry.

7.1.1.  BGP-LS NLRI Types Registry

   The "BGP-LS NLRI Types" registry has been set up for assignment for
   the two-octet-sized code points for BGP-LS NLRI types and populated
   with the values shown below:

          +=============+===========================+===========+
          |     Type    | NLRI Type                 | Reference |
          +=============+===========================+===========+
          |      0      | Reserved                  |  RFC 9552 |
          +-------------+---------------------------+-----------+
          |      1      | Node NLRI                 |  RFC 9552 |
          +-------------+---------------------------+-----------+
          |      2      | Link NLRI                 |  RFC 9552 |
          +-------------+---------------------------+-----------+
          |      3      | IPv4 Topology Prefix NLRI |  RFC 9552 |
          +-------------+---------------------------+-----------+
          |      4      | IPv6 Topology Prefix NLRI |  RFC 9552 |
          +-------------+---------------------------+-----------+
          | 65000-65535 | Private Use               |  RFC 9552 |
          +-------------+---------------------------+-----------+

                        Table 12: BGP-LS NLRI Types

   A range is reserved for Private Use [RFC8126].  All other allocations
   within the registry are to be made using the "Expert Review" policy
   [RFC8126], which requires documentation of the proposed use of the
   allocated value and approval by the designated expert assigned by the
   IESG.

7.1.2.  BGP-LS Protocol-IDs Registry

   The "BGP-LS Protocol-IDs" registry has been set up for assignment for
   the one-octet-sized code points for BGP-LS Protocol-IDs and populated
   with the values shown below:

      +=============+==================================+===========+
      | Protocol-ID | NLRI information source protocol | Reference |
      +=============+==================================+===========+
      |      0      | Reserved                         |  RFC 9552 |
      +-------------+----------------------------------+-----------+
      |      1      | IS-IS Level 1                    |  RFC 9552 |
      +-------------+----------------------------------+-----------+
      |      2      | IS-IS Level 2                    |  RFC 9552 |
      +-------------+----------------------------------+-----------+
      |      3      | OSPFv2                           |  RFC 9552 |
      +-------------+----------------------------------+-----------+
      |      4      | Direct                           |  RFC 9552 |
      +-------------+----------------------------------+-----------+
      |      5      | Static configuration             |  RFC 9552 |
      +-------------+----------------------------------+-----------+
      |      6      | OSPFv3                           |  RFC 9552 |
      +-------------+----------------------------------+-----------+
      |   200-255   | Private Use                      |  RFC 9552 |
      +-------------+----------------------------------+-----------+

                      Table 13: BGP-LS Protocol-IDs

   A range is reserved for Private Use [RFC8126].  All other allocations
   within the registry are to be made using the "Expert Review" policy
   [RFC8126], which requires documentation of the proposed use of the
   allocated value and approval by the designated expert assigned by the
   IESG.

7.1.3.  BGP-LS Well-Known Instance-IDs Registry

   The "BGP-LS Well-Known Instance-IDs" registry that was set up via
   [RFC7752] is no longer required.  IANA has marked this registry
   obsolete and changed its registration procedure to "registry closed".

7.1.4.  BGP-LS Node Flags Registry

   The "BGP-LS Node Flags" registry has been created for the one-octet-
   sized flags field of the Node Flag Bits TLV (1024) and populated with
   the initial values shown below:

                +=====+======================+===========+
                | Bit | Description          | Reference |
                +=====+======================+===========+
                |  0  | Overload Bit (O-bit) |  RFC 9552 |
                +-----+----------------------+-----------+
                |  1  | Attached Bit (A-bit) |  RFC 9552 |
                +-----+----------------------+-----------+
                |  2  | External Bit (E-bit) |  RFC 9552 |
                +-----+----------------------+-----------+
                |  3  | ABR Bit (B-bit)      |  RFC 9552 |
                +-----+----------------------+-----------+
                |  4  | Router Bit (R-bit)   |  RFC 9552 |
                +-----+----------------------+-----------+
                |  5  | V6 Bit (V-bit)       |  RFC 9552 |
                +-----+----------------------+-----------+
                | 6-7 | Unassigned           |           |
                +-----+----------------------+-----------+

                       Table 14: BGP-LS Node Flags

   Allocations within the registry are to be made using the "Expert
   Review" policy [RFC8126], which requires documentation of the
   proposed use of the allocated value and approval by the designated
   expert assigned by the IESG.

7.1.5.  BGP-LS MPLS Protocol Mask Registry

   The "BGP-LS MPLS Protocol Mask" registry has been created for the
   one-octet-sized flags field of the MPLS Protocol Mask TLV (1094) and
   populated with the initial values shown below:

      +=====+===========================================+===========+
      | Bit | Description                               | Reference |
      +=====+===========================================+===========+
      |  0  | Label Distribution Protocol (L-bit)       |  RFC 9552 |
      +-----+-------------------------------------------+-----------+
      |  1  | Extension to RSVP for LSP Tunnels (R-bit) |  RFC 9552 |
      +-----+-------------------------------------------+-----------+
      | 2-7 | Unassigned                                |           |
      +-----+-------------------------------------------+-----------+

                    Table 15: BGP-LS MPLS Protocol Mask

   Allocations within the registry are to be made using the "Expert
   Review" policy [RFC8126], which requires documentation of the
   proposed use of the allocated value and approval by the designated
   expert assigned by the IESG.

7.1.6.  BGP-LS IGP Prefix Flags Registry

   The "BGP-LS IGP Prefix Flags" registry has been created for the one-
   octet-sized flags field of the IGP Flags TLV (1152) and populated
   with the initial values shown below:

          +=====+===================================+===========+
          | Bit | Description                       | Reference |
          +=====+===================================+===========+
          |  0  | IS-IS Up/Down Bit (D-bit)         |  RFC 9552 |
          +-----+-----------------------------------+-----------+
          |  1  | OSPF "no unicast" Bit (N-bit)     |  RFC 9552 |
          +-----+-----------------------------------+-----------+
          |  2  | OSPF "local address" Bit (L-bit)  |  RFC 9552 |
          +-----+-----------------------------------+-----------+
          |  3  | OSPF "propagate NSSA" Bit (P-bit) |  RFC 9552 |
          +-----+-----------------------------------+-----------+
          | 4-7 | Unassigned                        |           |
          +-----+-----------------------------------+-----------+

                     Table 16: BGP-LS IGP Prefix Flags

   Allocations within the registry are to be made using the "Expert
   Review" policy [RFC8126], which requires documentation of the
   proposed use of the allocated value and approval by the designated
   expert assigned by the IESG.

7.1.7.  BGP-LS TLVs Registry

   The "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and
   Attribute TLVs" registry was created via [RFC7752].  Per this
   document, IANA has renamed that registry to "BGP-LS NLRI and
   Attribute TLVs" and removed the column for "IS-IS TLV/Sub-TLV".  The
   registration procedures are as follows:

            +================+================================+
            | TLV Code Point | Registration Process           |
            +================+================================+
            |     0-255      | Reserved (not to be allocated) |
            +----------------+--------------------------------+
            |   256-64999    | Expert Review                  |
            +----------------+--------------------------------+
            |  65000-65535   | Private Use                    |
            +----------------+--------------------------------+

                 Table 17: BGP-LS TLVs Registration Process

   A range is reserved for Private Use [RFC8126].  All other allocations
   except for the reserved range within the registry are to be made
   using the "Expert Review" policy [RFC8126], which requires
   documentation of the proposed use of the allocated value and approval
   by the designated expert assigned by the IESG.

   The registry was pre-populated with the values shown in Table 18, and
   the reference for each allocation has been changed to this document
   and the respective section where those TLVs are specified.

7.2.  Guidance for Designated Experts

   In all cases of review by the designated expert described here, the
   designated expert is expected to check the clarity of purpose and use
   of the requested code points.  The following points apply to the
   registries discussed in this document:

   1.  Application for a code point allocation may be made to the
       designated experts at any time and MUST be accompanied by
       technical documentation explaining the use of the code point.
       Such documentation SHOULD be presented in the form of an
       Internet-Draft but MAY arrive in any form that can be reviewed
       and exchanged among reviewers.

   2.  The designated experts SHOULD only consider requests that arise
       from Internet-Drafts that have already been accepted as working
       group documents or that are planned for progression as AD-
       Sponsored documents in the absence of a suitably chartered
       working group.

   3.  In the case of working group documents, the designated experts
       MUST check with the working group chairs that there is a
       consensus within the working group to allocate at this time.  In
       the case of AD-Sponsored documents, the designated experts MUST
       check with the AD for approval to allocate at this time.

   4.  If the document is not adopted by the IDR Working Group (or its
       successor), the designated expert MUST notify the IDR mailing
       list (or its successor) of the request and MUST provide access to
       the document.  The designated expert MUST allow two weeks for any
       response.  Any comments received MUST be considered by the
       designated expert as part of the subsequent step.

   5.  The designated experts MUST then review the assignment requests
       on their technical merit.  The designated experts MAY raise
       issues related to the allocation request with the authors and on
       the IDR (or successor) mailing list for further consideration
       before the assignments are made.

   6.  The designated expert MUST ensure that any request for a code
       point does not conflict with work that is active or already
       published within the IETF.

   7.  Once the designated experts have approved, IANA will update the
       registry by marking the allocated code points with a reference to
       the associated document.

   8.  In the event that the document is a working group document or is
       AD-Sponsored and fails to progress to publication as an RFC, the
       working group chairs or AD SHOULD contact IANA to coordinate
       about marking the code points as deprecated.  A deprecated code
       point is not marked as allocated for use and is not available for
       allocation in a future document.  The WG chairs may inform IANA
       that a deprecated code point can be completely deallocated (i.e.,
       made available for new allocations) at any time after it has been
       deprecated if there is a shortage of unallocated code points in
       the registry.

8.  Manageability Considerations

   This section is structured as recommended in [RFC5706].

8.1.  Operational Considerations

8.1.1.  Operations

   Existing BGP operational procedures apply.  No new operation
   procedures are defined in this document.  It is noted that the NLRI
   information present in this document carries purely application-level
   data that has no immediate impact on the corresponding forwarding
   state computed by BGP.  As such, any churn in reachability
   information has a different impact than regular BGP updates, which
   need to change the forwarding state for an entire router.
   Distribution of the BGP-LS NLRIs SHOULD be handled by dedicated route
   reflectors in most deployments providing a level of isolation and
   fault containment between different BGP address families.  In the
   event of dedicated route reflectors not being available, other
   alternate mechanisms like separation of BGP instances or separate BGP
   sessions (e.g., using different addresses for peering) for Link-State
   information distribution SHOULD be used.

   It is RECOMMENDED that operators deploying BGP-LS enable two or more
   BGP-LS Producers in each IGP flooding domain to achieve redundancy in
   the origination of link-state information into BGP-LS.  It is also
   RECOMMENDED that operators ensure BGP peering designs that ensure
   redundancy in the BGP update propagation paths (e.g., using at least
   a pair of route reflectors) and ensure that BGP-LS Consumers are
   receiving the topology information from at least two BGP-LS Speakers.

   In a multi-domain IGP network, the correct provisioning of the BGP-LS
   Instance-IDs on the BGP-LS Producers is required for consistent
   reporting of the multi-domain link-state topology.  Refer to
   Section 5.2 for more details.

8.1.2.  Installation and Initial Setup

   Configuration parameters defined in Section 8.2.3 SHOULD be
   initialized to the following default values:

   *  The Link-State NLRI capability is turned off for all neighbors.

   *  The maximum rate at which Link-State NLRIs will be advertised/
      withdrawn from neighbors is set to 200 updates per second.

8.1.3.  Migration Path

   The proposed extension is only activated between BGP peers after
   capability negotiation.  Moreover, the extensions can be turned on/
   off on an individual peer basis (see Section 8.2.3), so the extension
   can be gradually rolled out in the network.

8.1.4.  Requirements for Other Protocols and Functional Components

   The protocol extension defined in this document does not put new
   requirements on other protocols or functional components.

8.1.5.  Impact on Network Operation

   The frequency of Link-State NLRI updates could interfere with regular
   BGP prefix distribution.  A network operator should use a dedicated
   route reflector infrastructure to distribute Link-State NLRIs as
   discussed in Section 8.1.1.

   Distribution of Link-State NLRIs SHOULD be limited to a single admin
   domain, which can consist of multiple areas within an AS or multiple
   ASes.

8.1.6.  Verifying Correct Operation

   Existing BGP procedures apply.  In addition, an implementation SHOULD
   allow an operator to:

   *  List neighbors with whom the speaker is exchanging Link-State
      NLRIs.

8.2.  Management Considerations

8.2.1.  Management Information

   The IDR Working Group has documented and continues to document parts
   of the Management Information Base and YANG models for managing and
   monitoring BGP Speakers and the sessions between them.  It is
   currently believed that the BGP session running BGP-LS is not
   substantially different from any other BGP session and can be managed
   using the same data models.

8.2.2.  Fault Management

   This section describes the fault management actions, as described in
   [RFC7606], that are to be performed for the handling of BGP UPDATE
   messages for BGP-LS.

   A Link-State NLRI MUST NOT be considered malformed or invalid based
   on the inclusion/exclusion of TLVs or contents of the TLV fields
   (i.e., semantic errors), as described in Sections 5.1 and 5.2.

   A BGP-LS Speaker MUST perform the following syntactic validation of
   the Link-State NLRI to determine if it is malformed.

   *  The sum of all TLV lengths found in the BGP MP_REACH_NLRI
      attribute corresponds to the BGP MP_REACH_NLRI length.

   *  The sum of all TLV lengths found in the BGP MP_UNREACH_NLRI
      attribute corresponds to the BGP MP_UNREACH_NLRI length.

   *  The sum of all TLV lengths found in a Link-State NLRI corresponds
      to the Total NLRI Length field of all its descriptors.

   *  The length of the TLVs and, when the TLV is recognized then, the
      length of its sub-TLVs in the NLRI are valid.

   *  The syntactic correctness of the NLRI fields has been verified as
      per [RFC7606].

   *  The rule regarding the ordering of TLVs has been followed as
      described in Section 5.1.

   *  For NLRIs carrying either a Local or Remote Node Descriptor TLV,
      there is not more than one instance of a sub-TLV present.

   When the error that is determined allows for the router to skip the
   malformed NLRI(s) and continue the processing of the rest of the BGP
   UPDATE message (e.g., when the TLV ordering rule is violated), then
   it MUST handle such malformed NLRIs as 'NLRI discard' (i.e.,
   processing similar to what is described in Section 5.4 of [RFC7606]).
   In other cases, where the error in the NLRI encoding results in the
   inability to process the BGP UPDATE message (e.g., length-related
   encoding errors), then the router SHOULD handle such malformed NLRIs
   as 'AFI/SAFI disable' when other AFI/SAFI besides BGP-LS are being
   advertised over the same session.  Alternately, the router MUST
   perform a 'session reset' when the session is only being used for
   BGP-LS or if 'AFI/SAFI disable' action is not possible.

   A BGP-LS Attribute MUST NOT be considered malformed or invalid based
   on the inclusion/exclusion of TLVs or contents of the TLV fields
   (i.e., semantic errors), as described in Sections 5.1 and 5.3.

   A BGP-LS Speaker MUST perform the following syntactic validation of
   the BGP-LS Attribute to determine if it is malformed.

   *  The sum of all TLV lengths found in the BGP-LS Attribute
      corresponds to the BGP-LS Attribute length.

   *  The syntactic correctness of the Attributes (including the BGP-LS
      Attribute) have been verified as per [RFC7606].

   *  The length of each TLV and, when the TLV is recognized then, the
      length of its sub-TLVs in the BGP-LS Attribute are valid.

   When the error that is determined allows for the router to skip the
   malformed BGP-LS Attribute and continue the processing of the rest of
   the BGP UPDATE message (e.g., when the BGP-LS Attribute length and
   the total Path Attribute Length are correct but some TLV/sub-TLV
   length within the BGP-LS Attribute is invalid), then it MUST handle
   such malformed BGP-LS Attribute as 'Attribute Discard'.  In other
   cases, where the error in the BGP-LS Attribute encoding results in
   the inability to process the BGP UPDATE message, the handling is the
   same as described above for the malformed NLRI.

   Note that the 'Attribute Discard' action results in the loss of all
   TLVs in the BGP-LS Attribute and not the removal of a specific
   malformed TLV.  The removal of specific malformed TLVs may give a
   wrong indication to a BGP-LS Consumer of that specific information
   being deleted or not available.

   When a BGP Speaker receives an UPDATE message with Link-State NLRI(s)
   in the MP_REACH_NLRI but without the BGP-LS Attribute, it is most
   likely an indication that a BGP Speaker preceding it has performed
   the 'Attribute Discard' fault handling.  An implementation SHOULD
   preserve and propagate the Link-State NLRIs, unless denied by local
   policy, in such an UPDATE message so that the BGP-LS Consumers can
   detect the loss of link-state information for that object and not
   assume its deletion/withdrawal.  This also makes it possible for a
   network operator to trace back to the BGP-LS Propagator that detected
   the fault with the BGP-LS Attribute.

   An implementation SHOULD log a message for any errors found during
   syntax validation for further analysis.

   A BGP-LS Propagator, even when it has a coexisting BGP-LS Consumer on
   the same node, should not perform semantic validation of the Link-
   State NLRI or the BGP-LS Attribute to determine if it is malformed or
   invalid.  Some types of semantic validation that are not to be
   performed by a BGP-LS Propagator are as follows (and this is not to
   be considered as an exhaustive list):

   *  presence of a mandatory TLV

   *  the length of a fixed-length TLV is correct or the length of a
      variable length TLV is valid or permissible

   *  the values of TLV fields are valid or permissible

   *  the inclusion and use of TLVs/sub-TLVs with specific Link-State
      NLRI types is valid

   Each TLV may indicate the valid and permissible values and their
   semantics that can be used only by a BGP-LS Consumer for its semantic
   validation.  However, the handling of any errors may be specific to
   the particular application and outside the scope of this document.

8.2.3.  Configuration Management

   An implementation SHOULD allow the operator to specify neighbors to
   which Link-State NLRIs will be advertised and from which Link-State
   NLRIs will be accepted.

   An implementation SHOULD allow the operator to specify the maximum
   rate at which Link-State NLRIs will be advertised/withdrawn from
   neighbors.

   An implementation SHOULD allow the operator to specify the maximum
   number of Link-State NLRIs stored in a router's Routing Information
   Base (RIB).

   An implementation SHOULD allow the operator to create abstracted
   topologies that are advertised to neighbors and create different
   abstractions for different neighbors.

   An implementation MUST allow the operator to configure an 8-octet
   BGP-LS Instance-ID.  Refer to Section 5.2 for guidance to the
   operator for the configuration of BGP-LS Instance-ID.

   An implementation SHOULD allow the operator to configure Autonomous
   System Number (ASN) and BGP-LS identifiers (refer to
   Section 5.2.1.4).

   An implementation SHOULD allow the operator to configure a 4096-byte
   size limit for a BGP-LS UPDATE message on a BGP-LS Producer or allow
   larger values when they know that all BGP-LS Speakers support the
   extended message size [RFC8654].

8.2.4.  Accounting Management

   Not Applicable.

8.2.5.  Performance Management

   An implementation SHOULD provide the following statistics:

   *  Total number of Link-State NLRI updates sent/received

   *  Number of Link-State NLRI updates sent/received, per neighbor

   *  Number of errored received Link-State NLRI updates, per neighbor

   *  Total number of locally originated Link-State NLRIs

   These statistics should be recorded as absolute counts since the
   system or session start time.  An implementation MAY also enhance
   this information by recording peak per-second counts in each case.

8.2.6.  Security Management

   An operator MUST define an import policy to limit inbound updates as
   follows:

   *  Drop all updates from peers that are only serving BGP-LS
      Consumers.

   An implementation MUST have the means to limit inbound updates.

9.  TLV/Sub-TLV Code Points Summary

   This section contains the global table of all TLVs/sub-TLVs defined
   in this document.

     +================+=========================+===================+
     | TLV Code Point | Description             | Reference Section |
     +================+=========================+===================+
     |      256       | Local Node Descriptors  | Section 5.2.1.2   |
     +----------------+-------------------------+-------------------+
     |      257       | Remote Node Descriptors | Section 5.2.1.3   |
     +----------------+-------------------------+-------------------+
     |      258       | Link Local/Remote       | Section 5.2.2     |
     |                | Identifiers             |                   |
     +----------------+-------------------------+-------------------+
     |      259       | IPv4 interface address  | Section 5.2.2     |
     +----------------+-------------------------+-------------------+
     |      260       | IPv4 neighbor address   | Section 5.2.2     |
     +----------------+-------------------------+-------------------+
     |      261       | IPv6 interface address  | Section 5.2.2     |
     +----------------+-------------------------+-------------------+
     |      262       | IPv6 neighbor address   | Section 5.2.2     |
     +----------------+-------------------------+-------------------+
     |      263       | Multi-Topology          | Section 5.2.2.1   |
     |                | Identifier              |                   |
     +----------------+-------------------------+-------------------+
     |      264       | OSPF Route Type         | Section 5.2.3.1   |
     +----------------+-------------------------+-------------------+
     |      265       | IP Reachability         | Section 5.2.3.2   |
     |                | Information             |                   |
     +----------------+-------------------------+-------------------+
     |      512       | Autonomous System       | Section 5.2.1.4   |
     +----------------+-------------------------+-------------------+
     |      513       | BGP-LS Identifier       | Section 5.2.1.4   |
     |                | (deprecated)            |                   |
     +----------------+-------------------------+-------------------+
     |      514       | OSPF Area-ID            | Section 5.2.1.4   |
     +----------------+-------------------------+-------------------+
     |      515       | IGP Router-ID           | Section 5.2.1.4   |
     +----------------+-------------------------+-------------------+
     |      1024      | Node Flag Bits          | Section 5.3.1.1   |
     +----------------+-------------------------+-------------------+
     |      1025      | Opaque Node Attribute   | Section 5.3.1.5   |
     +----------------+-------------------------+-------------------+
     |      1026      | Node Name               | Section 5.3.1.3   |
     +----------------+-------------------------+-------------------+
     |      1027      | IS-IS Area Identifier   | Section 5.3.1.2   |
     +----------------+-------------------------+-------------------+
     |      1028      | IPv4 Router-ID of Local | Sections 5.3.1.4  |
     |                | Node                    | and 5.3.2.1       |
     +----------------+-------------------------+-------------------+
     |      1029      | IPv6 Router-ID of Local | Sections 5.3.1.4  |
     |                | Node                    | and 5.3.2.1       |
     +----------------+-------------------------+-------------------+
     |      1030      | IPv4 Router-ID of       | Section 5.3.2.1   |
     |                | Remote Node             |                   |
     +----------------+-------------------------+-------------------+
     |      1031      | IPv6 Router-ID of       | Section 5.3.2.1   |
     |                | Remote Node             |                   |
     +----------------+-------------------------+-------------------+
     |      1088      | Administrative group    | Section 5.3.2     |
     |                | (color)                 |                   |
     +----------------+-------------------------+-------------------+
     |      1089      | Maximum link bandwidth  | Section 5.3.2     |
     +----------------+-------------------------+-------------------+
     |      1090      | Max. reservable link    | Section 5.3.2     |
     |                | bandwidth               |                   |
     +----------------+-------------------------+-------------------+
     |      1091      | Unreserved bandwidth    | Section 5.3.2     |
     +----------------+-------------------------+-------------------+
     |      1092      | TE Default Metric       | Section 5.3.2.3   |
     +----------------+-------------------------+-------------------+
     |      1093      | Link Protection Type    | Section 5.3.2     |
     +----------------+-------------------------+-------------------+
     |      1094      | MPLS Protocol Mask      | Section 5.3.2.2   |
     +----------------+-------------------------+-------------------+
     |      1095      | IGP Metric              | Section 5.3.2.4   |
     +----------------+-------------------------+-------------------+
     |      1096      | Shared Risk Link Group  | Section 5.3.2.5   |
     +----------------+-------------------------+-------------------+
     |      1097      | Opaque Link Attribute   | Section 5.3.2.6   |
     +----------------+-------------------------+-------------------+
     |      1098      | Link Name               | Section 5.3.2.7   |
     +----------------+-------------------------+-------------------+
     |      1152      | IGP Flags               | Section 5.3.3.1   |
     +----------------+-------------------------+-------------------+
     |      1153      | IGP Route Tag           | Section 5.3.3.2   |
     +----------------+-------------------------+-------------------+
     |      1154      | IGP Extended Route Tag  | Section 5.3.3.3   |
     +----------------+-------------------------+-------------------+
     |      1155      | Prefix Metric           | Section 5.3.3.4   |
     +----------------+-------------------------+-------------------+
     |      1156      | OSPF Forwarding Address | Section 5.3.3.5   |
     +----------------+-------------------------+-------------------+
     |      1157      | Opaque Prefix Attribute | Section 5.3.3.6   |
     +----------------+-------------------------+-------------------+

            Table 18: Summary Table of TLV/Sub-TLV Code Points

10.  Security Considerations

   Procedures and protocol extensions defined in this document do not
   affect the BGP security model.  See the Security Considerations
   section of [RFC4271] for a discussion of BGP security.  Also, refer
   to [RFC4272] and [RFC6952] for analysis of security issues for BGP.

   The operator should ensure that a BGP-LS Speaker does not accept
   UPDATE messages from a peer that only provides information to a BGP-
   LS Consumer by using the policy configuration options discussed in
   Sections 8.2.3 and 8.2.6.  Generally, an operator is aware of the
   BGP-LS Speaker's role and link-state peerings.  Therefore, the
   operator can protect the BGP-LS Speaker from peers sending updates
   that may represent erroneous information, feedback loops, or false
   input.

   An error or tampering of the link-state information that is
   originated into BGP-LS and propagated through the network for use by
   BGP-LS Consumers applications can result in the malfunction of those
   applications.  Some examples of such risks are the origination of
   incorrect information that is not present or consistent with the IGP
   LSDB at the BGP-LS Producer, incorrect ordering of TLVs in the NLRI,
   or inconsistent origination from multiple BGP-LS Producers and
   updates to either the NLRI or BGP-LS Attribute during propagation
   (including discarding due to errors).  These are not new risks from a
   BGP protocol perspective; however, in the case of BGP-LS, impact
   reflects on the consumer applications instead of BGP routing
   functionalities.

   Additionally, it may be considered that the export of link-state and
   TE information as described in this document constitutes a risk to
   confidentiality of mission-critical or commercially sensitive
   information about the network.  BGP peerings are not automatic and
   require configuration; thus, it is the responsibility of the network
   operator to ensure that only trusted BGP Speakers are configured to
   receive such information.  Similar security considerations also arise
   on the interface between BGP Speakers and BGP-LS Consumers, but their
   discussion is outside the scope of this document.

11.  References

11.1.  Normative References

   [ENTNUM]   IANA, "Private Enterprise Numbers (PENs)",
              <https://www.iana.org/assignments/enterprise-numbers/>.

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

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

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

   [RFC2545]  Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
              Extensions for IPv6 Inter-Domain Routing", RFC 2545,
              DOI 10.17487/RFC2545, March 1999,
              <https://www.rfc-editor.org/info/rfc2545>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [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>.

   [RFC4203]  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
              Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
              <https://www.rfc-editor.org/info/rfc4203>.

   [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>.

   [RFC4577]  Rosen, E., Psenak, P., and P. Pillay-Esnault, "OSPF as the
              Provider/Customer Edge Protocol for BGP/MPLS IP Virtual
              Private Networks (VPNs)", RFC 4577, DOI 10.17487/RFC4577,
              June 2006, <https://www.rfc-editor.org/info/rfc4577>.

   [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>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/info/rfc4915>.

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <https://www.rfc-editor.org/info/rfc5036>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [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>.

   [RFC5301]  McPherson, D. and N. Shen, "Dynamic Hostname Exchange
              Mechanism for IS-IS", RFC 5301, DOI 10.17487/RFC5301,
              October 2008, <https://www.rfc-editor.org/info/rfc5301>.

   [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>.

   [RFC5307]  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <https://www.rfc-editor.org/info/rfc5307>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC5642]  Venkata, S., Harwani, S., Pignataro, C., and D. McPherson,
              "Dynamic Hostname Exchange Mechanism for OSPF", RFC 5642,
              DOI 10.17487/RFC5642, August 2009,
              <https://www.rfc-editor.org/info/rfc5642>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [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>.

   [RFC6565]  Pillay-Esnault, P., Moyer, P., Doyle, J., Ertekin, E., and
              M. Lundberg, "OSPFv3 as a Provider Edge to Customer Edge
              (PE-CE) Routing Protocol", RFC 6565, DOI 10.17487/RFC6565,
              June 2012, <https://www.rfc-editor.org/info/rfc6565>.

   [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>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.

   [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>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.

   [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>.

11.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

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

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5152]  Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
              Per-Domain Path Computation Method for Establishing Inter-
              Domain Traffic Engineering (TE) Label Switched Paths
              (LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008,
              <https://www.rfc-editor.org/info/rfc5152>.

   [RFC5392]  Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5392, DOI 10.17487/RFC5392,
              January 2009, <https://www.rfc-editor.org/info/rfc5392>.

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693,
              DOI 10.17487/RFC5693, October 2009,
              <https://www.rfc-editor.org/info/rfc5693>.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions",
              RFC 5706, DOI 10.17487/RFC5706, November 2009,
              <https://www.rfc-editor.org/info/rfc5706>.

   [RFC6549]  Lindem, A., Roy, A., and S. Mirtorabi, "OSPFv2 Multi-
              Instance Extensions", RFC 6549, DOI 10.17487/RFC6549,
              March 2012, <https://www.rfc-editor.org/info/rfc6549>.

   [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>.

   [RFC7285]  Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
              Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
              "Application-Layer Traffic Optimization (ALTO) Protocol",
              RFC 7285, DOI 10.17487/RFC7285, September 2014,
              <https://www.rfc-editor.org/info/rfc7285>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [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>.

   [RFC8202]  Ginsberg, L., Previdi, S., and W. Henderickx, "IS-IS
              Multi-Instance", RFC 8202, DOI 10.17487/RFC8202, June
              2017, <https://www.rfc-editor.org/info/rfc8202>.

   [RFC9029]  Farrel, A., "Updates to the Allocation Policy for the
              Border Gateway Protocol - Link State (BGP-LS) Parameters
              Registries", RFC 9029, DOI 10.17487/RFC9029, June 2021,
              <https://www.rfc-editor.org/info/rfc9029>.

   [RFC9346]  Chen, M., Ginsberg, L., Previdi, S., and D. Xiaodong, "IS-
              IS Extensions in Support of Inter-Autonomous System (AS)
              MPLS and GMPLS Traffic Engineering", RFC 9346,
              DOI 10.17487/RFC9346, February 2023,
              <https://www.rfc-editor.org/info/rfc9346>.

Appendix A.  Changes from RFC 7752

   This section lists the high-level changes from RFC 7752 and provides
   reference to the document sections wherein those have been
   introduced.

   1.   Updated Figure 1 in Section 1 and added Section 3 to illustrate
        the different roles of a BGP implementation in conveying link-
        state information.

   2.   Clarified aspects related to advertisement of link-state
        information from IGPs into BGP-LS in Section 4.

   3.   In Section 5.1, clarified aspects about TLV handling that apply
        to both the NLRI and BGP-LS Attribute parts as well as those
        that are applicable only for the NLRI portion.  An
        implementation may have missed the part about the handling of an
        unknown TLV and so, based on [RFC7606] guidelines, might discard
        the unknown NLRI types.  This aspect is now unambiguously
        clarified in Section 5.2.  Also, the TLVs in the BGP-LS
        Attribute that are not ordered are not to be considered
        malformed.

   4.   Clarified aspects of mandatory and optional TLVs in both NLRI
        and BGP-LS Attribute portions all through the document.

   5.   In Section 5.3, the handling of a large-sized BGP-LS Attribute
        with growth in BGP-LS information is explained along with
        mitigation of errors arising out of it.

   6.   Clarified that the document describes the NLRI descriptor TLVs
        for the protocols and NLRI types specified in this document as
        well as future BGP-LS extensions must describe the same for
        other protocols and NLRI types that they introduce.

   7.   In Section 5.2, clarified the use of the Identifier field in the
        Link-State NLRI.  It was defined ambiguously to refer to only
        multi-instance IGP on a single link while it can also be used
        for multiple IGP protocol instances on a router.  The IANA
        registry is accordingly being removed.

   8.   The BGP-LS Identifier TLV in the Node Descriptors has been
        deprecated.  Its use was not well specified by [RFC7752], and
        there has been some amount of confusion between implementors on
        its usage for identification of IGP domains as against the use
        of the Identifier field carrying the BGP-LS Instance-ID when
        running multiple instances of IGP routing protocols.  The
        original purpose of the BGP-LS Identifier was that, in
        conjunction with the ASN, it would uniquely identify the BGP-LS
        domain and that the combination of ASN and BGP-LS ID would be
        globally unique.  However, the BGP-LS Instance-ID carried in the
        Identifier field in the fixed part of the NLRI also provides a
        similar functionality.  Hence, the inclusion of the BGP-LS
        Identifier TLV is not necessary.  If advertised, all BGP-LS
        Speakers within an IGP flooding-set (set of IGP nodes within
        which an LSP/LSA is flooded) had to use the same (ASN, BGP-LS
        ID) tuple, and if an IGP domain consists of multiple flooding-
        sets, then all BGP-LS Speakers within the IGP domain had to use
        the same (ASN, BGP-LS ID) tuple.

   9.   Clarified that the Area-ID TLV is mandatory in the Node
        Descriptor for the origination of information from OSPF except
        for when sourcing information from AS-scope LSAs where this TLV
        is not applicable.  Also clarified the IS-IS area and area
        addresses.

   10.  Moved the MT-ID TLV from the Node Descriptor section to under
        the Link Descriptor section since it is not a Node Descriptor
        sub-TLV.  Fixed the ambiguity in the encoding of OSPF MT-ID in
        this TLV.  Updated the IS-IS specification reference section and
        described the differences in the applicability of the R flags
        when the MT-ID TLV is used as the Link Descriptor TLV and Prefix
        Attribute TLV.  The MT-ID TLV use is now elevated to SHOULD when
        it is enabled in the underlying IGP.

   11.  Clarified that IPv6 link-local addresses are not advertised in
        the Link Descriptor TLVs and the local/remote identifiers are to
        be used instead for links with IPv6 link-local addresses only.

   12.  Updated the usage of OSPF Route Type TLV to mandate its use for
        OSPF prefixes in Section 5.2.3.1 since this is required for
        segregation of intra-area prefixes that are used to reach a node
        (e.g., a loopback) from other types of inter-area and external
        prefixes.

   13.  Clarified the specific OSPFv2 and OSPFv3 protocol TLV space to
        be used in the Node, Link, and Prefix Opaque Attribute TLVs.

   14.  Clarified that the length of the Node Flag Bits and IGP Flags
        TLVs are to be one octet.

   15.  Updated the Node Name TLV in Section 5.3.1.3 with the OSPF
        specification.

   16.  Clarified the size of the IS-IS Narrow Metric advertisement via
        the IGP Metric TLV and the handling of the unused bits.

   17.  Clarified the advertisement of the prefix corresponding to the
        LAN segment in an OSPF network in Section 5.11.

   18.  Clarified the advertisement and support for OSPF-specific
        concepts like virtual links, sham links, and Type 4 LSAs in
        Sections 5.7 and 5.8.

   19.  Introduced the Private Use TLV code point space and specified
        their encoding in Section 5.4.

   20.  In Section 5.9, introduced where issues related to the
        consistency of reporting IGP link-state along with their
        solutions are covered.

   21.  Added a recommendation for isolation of BGP-LS sessions from
        other BGP route exchanges to avoid errors and faults in BGP-LS
        affecting the normal BGP routing.

   22.  Updated the Fault Management section with detailed rules based
        on the role of the BGP Speaker in the BGP-LS information
        propagation flow.

   23.  Changed the management of BGP-LS IANA registries from
        "Specification Required" to "Expert Review" along with updated
        guidelines for designated experts, more specifically, the
        inclusion of changes introduced via [RFC9029] that are obsoleted
        by this document.

   24.  Added BGP-LS IANA registries with "Expert Review" policy for the
        flag fields of various TLVs that was missed out.  Renamed the
        BGP-LS TLV registry and removed the "IS-IS TLV/Sub-TLV" column
        from it.

Acknowledgements

   This document update to the BGP-LS specification [RFC7752] is a
   result of feedback and input from the discussions in the IDR Working
   Group.  It also incorporates certain details and clarifications based
   on implementation and deployment experience with BGP-LS.

   Cengiz Alaettinoglu and Parag Amritkar brought forward the need to
   clarify the advertisement of a LAN subnet for OSPF.

   We would like to thank Balaji Rajagopalan, Srihari Sangli, Shraddha
   Hegde, Andrew Stone, Jeff Tantsura, Acee Lindem, Les Ginsberg, Jie
   Dong, Aijun Wang, Nandan Saha, Joel Halpern, and Gyan Mishra for
   their review and feedback on this document.  Thanks to Tom Petch for
   his review and comments on the IANA Considerations section.  We would
   also like to thank Jeffrey Haas for his detailed shepherd review and
   input for improving the document.

   The detailed AD review by Alvaro Retana and his suggestions have
   helped improve this document significantly.

   We would like to thank Robert Varga for his significant contribution
   to [RFC7752].

   We would like to thank Nischal Sheth, Alia Atlas, David Ward, Derek
   Yeung, Murtuza Lightwala, John Scudder, Kaliraj Vairavakkalai, Les
   Ginsberg, Liem Nguyen, Manish Bhardwaj, Matt Miller, Mike Shand,
   Peter Psenak, Rex Fernando, Richard Woundy, Steven Luong, Tamas
   Mondal, Waqas Alam, Vipin Kumar, Naiming Shen, Carlos Pignataro,
   Balaji Rajagopalan, Yakov Rekhter, Alvaro Retana, Barry Leiba, and
   Ben Campbell for their comments on [RFC7752].

Contributors

   The following persons contributed significant text to [RFC7752] and
   this document.  They should be considered coauthors.

   Hannes Gredler
   Rtbrick
   Email: hannes@rtbrick.com


   Jan Medved
   Cisco Systems Inc.
   United States of America
   Email: jmedved@cisco.com


   Stefano Previdi
   Huawei Technologies
   Italy
   Email: stefano@previdi.net


   Adrian Farrel
   Old Dog Consulting
   Email: adrian@olddog.co.uk


   Saikat Ray
   Individual
   United States of America
   Email: raysaikat@gmail.com


Author's Address

   Ketan Talaulikar (editor)
   Cisco Systems
   India
   Email: ketant.ietf@gmail.com