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Versions: (draft-ketant-idr-rfc7752bis) 00 01 02

Inter-Domain Routing                                  K. Talaulikar, Ed.
Internet-Draft                                             Cisco Systems
Obsoletes: 7752 (if approved)                           November 1, 2019
Intended status: Standards Track
Expires: May 4, 2020


Distribution of Link-State and Traffic Engineering Information Using BGP
                      draft-ietf-idr-rfc7752bis-02

Abstract

   In a number of environments, a component external to a network is
   called upon to perform computations based on the network topology and
   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
   new BGP Network Layer Reachability Information (NLRI) encoding
   format.  The mechanism is applicable to physical and virtual 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 a number of small changes and clarifications to
   the previous specification.

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 4, 2020.



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Copyright Notice

   Copyright (c) 2019 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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Motivation and Applicability  . . . . . . . . . . . . . . . .   5
     2.1.  MPLS-TE with PCE  . . . . . . . . . . . . . . . . . . . .   5
     2.2.  ALTO Server Network API . . . . . . . . . . . . . . . . .   7
   3.  BGP Speaker Roles for BGP-LS  . . . . . . . . . . . . . . . .   7
   4.  Carrying Link-State Information in BGP  . . . . . . . . . . .   9
     4.1.  TLV Format  . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  The Link-State NLRI . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Node Descriptors  . . . . . . . . . . . . . . . . . .  14
       4.2.2.  Link Descriptors  . . . . . . . . . . . . . . . . . .  18
       4.2.3.  Prefix Descriptors  . . . . . . . . . . . . . . . . .  21
     4.3.  The BGP-LS Attribute  . . . . . . . . . . . . . . . . . .  23
       4.3.1.  Node Attribute TLVs . . . . . . . . . . . . . . . . .  23
       4.3.2.  Link Attribute TLVs . . . . . . . . . . . . . . . . .  27
       4.3.3.  Prefix Attribute TLVs . . . . . . . . . . . . . . . .  32
     4.4.  Private Use . . . . . . . . . . . . . . . . . . . . . . .  35
     4.5.  BGP Next-Hop Information  . . . . . . . . . . . . . . . .  36
     4.6.  Inter-AS Links  . . . . . . . . . . . . . . . . . . . . .  36
     4.7.  Handling of Unreachable IGP Nodes . . . . . . . . . . . .  37
     4.8.  Router-ID Anchoring Example: ISO Pseudonode . . . . . . .  38
     4.9.  Router-ID Anchoring Example: OSPF Pseudonode  . . . . . .  39
     4.10. Router-ID Anchoring Example: OSPFv2 to IS-IS Migration  .  40
   5.  Link to Path Aggregation  . . . . . . . . . . . . . . . . . .  41
     5.1.  Example: No Link Aggregation  . . . . . . . . . . . . . .  41
     5.2.  Example: ASBR to ASBR Path Aggregation  . . . . . . . . .  41
     5.3.  Example: Multi-AS Path Aggregation  . . . . . . . . . . .  42
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42
     6.1.  Guidance for Designated Experts . . . . . . . . . . . . .  43
   7.  Manageability Considerations  . . . . . . . . . . . . . . . .  44
     7.1.  Operational Considerations  . . . . . . . . . . . . . . .  44



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       7.1.1.  Operations  . . . . . . . . . . . . . . . . . . . . .  44
       7.1.2.  Installation and Initial Setup  . . . . . . . . . . .  44
       7.1.3.  Migration Path  . . . . . . . . . . . . . . . . . . .  44
       7.1.4.  Requirements on Other Protocols and Functional
               Components  . . . . . . . . . . . . . . . . . . . . .  45
       7.1.5.  Impact on Network Operation . . . . . . . . . . . . .  45
       7.1.6.  Verifying Correct Operation . . . . . . . . . . . . .  45
     7.2.  Management Considerations . . . . . . . . . . . . . . . .  45
       7.2.1.  Management Information  . . . . . . . . . . . . . . .  45
       7.2.2.  Fault Management  . . . . . . . . . . . . . . . . . .  45
       7.2.3.  Configuration Management  . . . . . . . . . . . . . .  48
       7.2.4.  Accounting Management . . . . . . . . . . . . . . . .  48
       7.2.5.  Performance Management  . . . . . . . . . . . . . . .  48
       7.2.6.  Security Management . . . . . . . . . . . . . . . . .  49
   8.  TLV/Sub-TLV Code Points Summary . . . . . . . . . . . . . . .  49
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  50
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  51
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  52
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  52
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  52
     12.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Appendix A.  Changes from RFC 7752  . . . . . . . . . . . . . . .  57
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  59

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) in order 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
   cross the visibility of more than one TED or that require 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 in order 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 new BGP Network Layer Reachability Information




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   (NLRI) encoding format.  The mechanism is applicable to physical and
   virtual 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 metric and 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 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 the 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 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



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   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 of 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 obsoletes [RFC7752] by completely replacing that
   document.  It makes a number of 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).

   o  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.

   o  If a router wants to compute a 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 uses a technique called "loose-hop-expansion" [RFC3209]



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   and selects the exit ABR and other ABRs or AS Border Routers (ASBRs)
   using the IGP-computed shortest path topology for the remainder of
   the path.  This may lead to sub-optimal paths, makes alternate/back-
   up path computation hard, and might result in no TE path being found
   when one really 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 obviously 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 sub-optimal 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:         +---------+
                |    |     | BGP with Link-State NLRI
                |    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.







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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 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 prefix 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    +--------+     BGP with    +---------+
     +--------+   |  Protocol  |  ALTO  | Link-State NLRI |   BGP   |
     | Client |<--+------------| Server |<----------------| Speaker |
     +--------+   |            |        |                 |         |
                  |            +--------+                 +---------+
     +--------+   |
     | Client |<--+
     +--------+

         Figure 3: ALTO Server Using Network Topology Information

3.  BGP Speaker Roles for BGP-LS

   In the illustration shown 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




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   of the BGP Speakers which are then used through the rest of this
   document.

   o  BGP-LS Producer: The 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 area, then likely
      they are originating the same link-state information into BGP-LS.
      R1 may also source information from sources other than IGP, e.g.
      its local node information.  The term BGP-LS Producer refers to
      the BGP Speaker that is originating link-state information into
      BGP.

   o  BGP-LS Consumer: The 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.  The term BGP-
      LS Consumer refers to the consumer application/process and not the
      BGP Speaker.  This document only covers the BGP implementation.
      The consumer application and the design of interface between BGP
      and consumer application may be implementation specific and
      outside the scope of this document.

   o  BGP-LS Propagator: The BGP Speaker RRm propagates the BGP-LS
      information between the BGP Speaker Rn and the BGP Speaker RR1.
      The BGP implementation on RRm is doing the propagation of BGP-LS
      updates and performing BGP best path calculations.  Similarly, the
      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 best path calculations.  The term BGP-LS Propagator
      refers to the BGP Speaker that is performing BGP protocol
      processing on the link-state information.

   The above roles are not mutually exclusive.  The same BGP Speaker may
   be the producer for some link-state information and propagator for
   some other link-state information while also providing this
   information to a consumer application.  Nothing precludes a BGP
   implementation performing some of the validation and processing on
   behalf of the BGP-LS Consumer as long as it does not impact the
   semantics of its role as BGP-LS Propagator as described in this
   document.

   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.







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4.  Carrying Link-State Information in BGP

   This specification contains two parts: definition of a new BGP NLRI
   that describes links, nodes, and prefixes comprising IGP link-state
   information and definition of a new BGP path attribute (BGP-LS
   Attribute) that carries link, node, and prefix properties and
   attributes, such as the link and prefix metric or auxiliary Router-
   IDs of nodes, etc.

   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 at a BGP-LS Producer
   that originate link-state information into BGP-LS.

4.1.  TLV Format

   Information in the new 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 unrecognized or unexpected TLVs
   MUST NOT result in the NLRI or the BGP-LS Attribute being considered
   as malformed.

   In order 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 in ascending order based on the value
   field.  Comparison of the value fields is performed by treating the



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   entire field as an opaque hexadecimal string.  Standard string
   comparison rules apply.  NLRIs having TLVs which do not follow the
   above ordering rules MUST be considered as malformed by a BGP-LS
   Propagator.  This ensures that multiple copies of the same NLRI from
   multiple BGP-LS Producers and the ambiguity arising there from is
   prevented.

   All TLVs within the NLRI that are not specified as mandatory are
   considered optional.  All TLVs within the BGP-LS Attribute are
   considered optional unless specified otherwise.

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

4.2.  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 describes either a node, a link, and 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.

   In order 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 (multi-protocol 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 Link-State
   but does not support a particular Link-State NLRI type.  In order to
   allow 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 [RFC7606] for Link-
   State address-family.  An implementation MUST handle unrecognized
   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.




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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            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                     +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //                  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 |
                | 65000-65535 | Private Use               |
                +-------------+---------------------------+

                            Table 1: NLRI Types

   Route Distinguishers are defined and discussed in [RFC4364].

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



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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     |                            (64 bits)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Local Node Descriptors (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                          |
     |                            (64 bits)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Local Node Descriptors (variable)             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Remote Node Descriptors (variable)            //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                  Link Descriptors (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.

















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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+
     |  Protocol-ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Identifier                          |
     |                            (64 bits)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //              Local Node Descriptors (variable)              //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Prefix Descriptors (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                           |
            |   200-255   | Private Use                      |
            +-------------+----------------------------------+

                       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 5,
   the Protocol-ID 'Static configuration' SHOULD be used.

   A router MAY run multiple protocol instances of OSPF or ISIS where by
   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 64-bit Identifier field carries
   a BGP-LS Instance Identifier (Instance-ID) 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 MUST have the same Identifier field value.



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   NLRIs with different Identifier field values MUST be considered to be
   from different IGP routing instances.  The Identifier field value 0
   is RECOMMENDED to be used when there is only a single protocol
   instance in the network where BGP-LS is operational.

   An implementation which supports multiple IGP instances MUST support
   the configuration of unique BGP-LS Instance-IDs at the routing
   protocol instance level.  The network operator MUST assign consistent
   BGP-LS Instance-ID values on all BGP-LS Producers within a given IGP
   domain.  Unique BGP-LS Instance-ID values MUST be assigned to routing
   protocol instances operating in different IGP domains.  This allows
   the BGP-LS Consumer to build an accurate segregated multi-domain
   topology based on the Identifier field even when the topology is
   advertised via BGP-LS by multiple BGP-LS Producers in the network.

   When the above described semantics and recommendations are not
   followed, a BGP-LS Consumer may see duplicate link-state objects for
   the same node, link or prefix when there are multiple BGP-LS
   Producers deployed.  This may also result in the BGP-LS Consumers
   getting an inaccurate network-wide topology.

   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 in-
   consistent link-state objects hanging around in the BGP-LS table.

   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 listed in Table 2.  Documents extending BGP-
   LS specifications with new NLRI Types and/or protocols MUST specify
   the NLRI Descriptors for them.

4.2.1.  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].  These auxiliary Router-IDs MUST be
   included in the node attribute described in Section 4.3.1 and MAY be
   included in link attribute described in Section 4.3.2.  The
   advertisement of the TE Router-IDs 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.





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   It is desirable that the Router-ID assignments inside the Node
   Descriptor 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 RFC 1918 [RFC1918].  BGP-LS uses the Autonomous System (AS) Number
   to disambiguate the Router-IDs, as described in Section 4.2.1.1.

4.2.1.1.  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 Area-ID, Router-ID,
   Protocol-ID, Multi-Topology ID, and 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.

   The mapping of the Instance-ID to the Identifier field as described
   earlier along with a set of sub-TLVs described in Section 4.2.1.4,
   allows specification of a flexible key for any given node/link
   information such that global uniqueness of the NLRI is ensured.

4.2.1.2.  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 4.2.1.4.









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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //              Node Descriptor Sub-TLVs (variable)            //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 10: Local Node Descriptors TLV Format

4.2.1.3.  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 4.2.1.4.

      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

4.2.1.4.  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




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   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 value different.

   BGP-LS Identifier:  Opaque value (32-bit ID).  This is an optional
      TLV.  In conjunction with Autonomous System Number (ASN), uniquely
      identifies the BGP-LS domain.  The combination of ASN and BGP-LS
      ID MUST be globally unique.  All BGP-LS speakers within an IGP
      flooding-set (set of IGP nodes within which an LSP/LSA is flooded)
      MUST use the same ASN, BGP-LS ID tuple.  If an IGP domain consists
      of multiple flooding-sets, then all BGP-LS speakers within the IGP
      domain SHOULD use the same ASN, BGP-LS ID tuple.

   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 Area Identifier allows different NLRIs of the same
      router to be discriminated on a per area basis.  It is not used
      for NLRIs when carrying information that is derived from AS-scope
      LSAs as it 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.  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.

      There can be at most 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 in order to compare NLRIs, even when an implementation



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      encounters an unknown sub-TLV.  Using stable sorting, an
      implementation can do binary comparison of NLRIs and hence allow
      incremental deployment of new key sub-TLVs.

   The BGP-LS Identifier was introduced by [RFC7752] and it's use is
   being deprecated by this document.  Implementations MUST continue to
   support this sub-TLV for backward compatibility.  The default value
   of 0 is RECOMMENDED to be use when a BGP-LS Producer includes this
   sub-TLV when originating information into BGP-LS.  Implementations
   MAY provide an option to configure this value for backward
   compatibility reasons.  The use of the Instance-ID in the Identifier
   field is the RECOMMENDED way of segregation of different IGP domains
   in BGP-LS.

4.2.2.  Link Descriptors

   The Link Descriptor field is a set of Type/Length/Value (TLV)
   triplets.  The format of each TLV is shown in Section 4.1.  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.  In order to fully describe a
   single logical link, two originating routers advertise a half-link
   each, i.e., two Link NLRIs are advertised for a given point-to-point
   link.

   A BGP-LS Consumer should not consider a link between two nodes as
   being available unless it has received the two Link NLRIs
   corresponding to the half-link representation of that link from both
   the nodes.  This check is similar to the 'two way connectivity check'
   that is performed by link-state IGPs and is also required to be done
   by BGP-LS Consumers of link-state topology.

   A BGP-LS Producer MAY supress the advertisement of a Link NLRI,
   corresponding to a half link, from a link-state IGP unless it 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 may be leveraged before passing information for any
   half-link that is reported from these IGPs in to BGP-LS.  This
   ensures that only those Link State IGP adjacencies which are
   established get reported via Link NLRIs.  Such a 'two way
   connectivity check' may be also 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, defined in [RFC5305], [RFC5307],



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   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 | Description         |  IS-IS TLV   | Reference        |
   |   Point   |                     |   /Sub-TLV   | (RFC/Section)    |
   +-----------+---------------------+--------------+------------------+
   |    258    | Link Local/Remote   |     22/4     | [RFC5307] / 1.1  |
   |           | Identifiers         |              |                  |
   |    259    | IPv4 interface      |     22/6     | [RFC5305] / 3.2  |
   |           | address             |              |                  |
   |    260    | IPv4 neighbor       |     22/8     | [RFC5305] / 3.3  |
   |           | address             |              |                  |
   |    261    | IPv6 interface      |    22/12     | [RFC6119] / 4.2  |
   |           | address             |              |                  |
   |    262    | IPv6 neighbor       |    22/13     | [RFC6119] / 4.3  |
   |           | address             |              |                  |
   |    263    | Multi-Topology      |     ---      | Section 4.2.2.1  |
   |           | Identifier          |              |                  |
   +-----------+---------------------+--------------+------------------+

                       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 IP 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.  IPv6 link-local addresses
      MUST NOT be carried in the IPv6 address TLVs as descriptors of a
      link as 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
      also in the case of links having only IPv6 link-local addressing
      on them.

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



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   The TLVs/sub-TLVs corresponding to the interface addresses and/or the
   local/remote identfiers may not always be signaled in the IGPs unless
   their advertisement is enabled specifically.  In such cases, a BGP-LS
   Producer may not be able to generate valid Link NLRIs for such link
   advertisements from the IGPs.

4.2.2.1.  Multi-Topology ID

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

   Semantics of the IS-IS MT-ID are defined in Section 7.1 and 7.2 of
   RFC 5120 [RFC5120].  Semantics of the OSPF MT-ID are defined in
   Section 3.7 of RFC 4915 [RFC4915].  If the value in the MT-ID TLV is
   derived from OSPF, then the upper 5 bits of the MT-ID field MUST be
   set to 0.

   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 ID TLV Format

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

   The MT-ID TLV MAY be present in a Link Descriptor, a Prefix
   Descriptor, or the BGP-LS attribute of a Node NLRI.  In 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
   Descriptor or Prefix Descriptor, multiple NLRIs MUST be generated
   where each NLRI contains a single unique MT-ID.  When used in the
   Link or Prefix Descriptor TLV for IS-IS, the Bits R are reserved and
   MUST be set to 0 (as per Section 7.2 of RFC 5120 [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




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   allowed.  When used in the Node Attribute TLV for IS-IS, the Bits R
   are set as per Section 7.1 of RFC 5120 [RFC5120].

4.2.3.  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  | Description         |  Length  | Reference          |
   |    Point    |                     |          | (RFC/Section)      |
   +-------------+---------------------+----------+--------------------+
   |     263     | Multi-Topology      | variable | Section 4.2.2.1    |
   |             | Identifier          |          |                    |
   |     264     | OSPF Route Type     |    1     | Section 4.2.3.1    |
   |     265     | IP Reachability     | variable | Section 4.2.3.2    |
   |             | Information         |          |                    |
   +-------------+---------------------+----------+--------------------+

                      Table 5: Prefix Descriptor TLVs

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

4.2.3.1.  OSPF Route Type

   The OSPF Route Type TLV is a mandatory 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 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






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

   o  Intra-Area (0x1)

   o  Inter-Area (0x2)

   o  External 1 (0x3)

   o  External 2 (0x4)

   o  NSSA 1 (0x5)

   o  NSSA 2 (0x6)

4.2.3.2.  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.  Its purpose is
   to glue a particular BGP service NLRI by virtue of its BGP next hop
   to a given node in the LSDB.  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 the most significant
   octets of the prefix, i.e., 1 octet for prefix length 1 up to 8, 2
   octets for prefix length 9 to 16, 3 octets for prefix length 17 up to
   24, 4 octets for prefix length 25 up to 32, etc.








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4.3.  The BGP-LS Attribute

   The BGP-LS Attribute 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, described in the following section.  This attribute SHOULD
   only be included with Link-State NLRIs.  This attribute MUST be
   ignored for all other address families.

   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 BGP-LS Attribute may potentially grow large in size depending on
   the amount of link-state information associated with a single Link-
   State NLRI.  The BGP specification [RFC4271] mandates a maximum BGP
   message size of 4096 octets.  It is RECOMMENDED that an
   implementation support [RFC8654] in order to accommodate larger size
   of information within the BGP-LS Attribute.  BGP-LS Producers MUST
   ensure that they limit the TLVs included in the BGP-LS Attribute to
   ensure that a BGP update message for a single Link-State NLRI does
   not cross the maximum limit for a BGP message.  The determination of
   the types of TLVs to be included MAY be made by the BGP-LS Producer
   based on the BGP-LS Consumer applications requirement and is outside
   the scope of this document.  When a BGP-LS Propagator finds that it
   is exceeding the maximum BGP message size due to addition or update
   of some other BGP Attribute (e.g.  AS_PATH), it MUST consider the
   BGP-LS Attribute to be malformed and handle the propagation as
   described in Section 7.2.2.

4.3.1.  Node Attribute TLVs

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

















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   +-------------+----------------------+----------+-------------------+
   |   TLV Code  | Description          |   Length | Reference         |
   |    Point    |                      |          | (RFC/Section)     |
   +-------------+----------------------+----------+-------------------+
   |     263     | Multi-Topology       | variable | Section 4.2.2.1   |
   |             | Identifier           |          |                   |
   |     1024    | Node Flag Bits       |        1 | Section 4.3.1.1   |
   |     1025    | Opaque Node          | variable | Section 4.3.1.5   |
   |             | Attribute            |          |                   |
   |     1026    | Node Name            | variable | Section 4.3.1.3   |
   |     1027    | IS-IS Area           | variable | Section 4.3.1.2   |
   |             | Identifier           |          |                   |
   |     1028    | IPv4 Router-ID of    |        4 | [RFC5305] / 4.3   |
   |             | Local Node           |          |                   |
   |     1029    | IPv6 Router-ID of    |       16 | [RFC6119] / 4.1   |
   |             | Local Node           |          |                   |
   +-------------+----------------------+----------+-------------------+

                       Table 6: Node Attribute TLVs

4.3.1.1.  Node Flag Bits TLV

   The Node Flag Bits TLV carries a bit mask 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| Rsvd|
     +-+-+-+-+-+-+-+-+-+

                   Figure 15: Node Flag Bits TLV Format

   The bits are defined as follows:














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        +-----------------+-------------------------+------------+
        |       Bit       | Description             | Reference  |
        +-----------------+-------------------------+------------+
        |       'O'       | Overload Bit            | [ISO10589] |
        |       'T'       | Attached Bit            | [ISO10589] |
        |       'E'       | External Bit            | [RFC2328]  |
        |       'B'       | ABR Bit                 | [RFC2328]  |
        |       'R'       | Router Bit              | [RFC5340]  |
        |       'V'       | V6 Bit                  | [RFC5340]  |
        | Reserved (Rsvd) | Reserved for future use |            |
        +-----------------+-------------------------+------------+

                    Table 7: Node Flag Bits Definitions

4.3.1.2.  IS-IS Area Identifier TLV

   An IS-IS node can be part of one or more IS-IS areas.  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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                 Area Identifier (variable)                  //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 16: IS-IS Area Identifier TLV Format

4.3.1.3.  Node Name TLV

   The Node Name TLV is optional.  Its structure and encoding 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, it can be a subset of
   the FQDN (e.g., a hostname), or it can be any string operators want
   to use for the router.  The use of FQDN or a subset 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, that
   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.




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   [RFC5301] describes an IS-IS-specific extension and [RFC5642]
   describes an OSPF extension for advertisement of Node Name which MAY
   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

4.3.1.4.  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 in its own TLV.

4.3.1.5.  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,
   e.g., a new IGP link-state attribute being defined and the protocol-
   neutral BGP-LS extensions being published.  A router, for example,
   could use this extension in order to advertise the native protocol's
   Node Attribute TLVs, such as the OSPF Router Informational
   Capabilities TLV defined in [RFC7770] or the IGP TE Node Capability
   Descriptor TLV described in [RFC5073].

      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



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4.3.2.  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 4.1.  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, 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:




































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   +-----------+---------------------+--------------+------------------+
   |  TLV Code | Description         |  IS-IS TLV   | Reference        |
   |   Point   |                     |   /Sub-TLV   | (RFC/Section)    |
   +-----------+---------------------+--------------+------------------+
   |    1028   | IPv4 Router-ID of   |   134/---    | [RFC5305] / 4.3  |
   |           | Local Node          |              |                  |
   |    1029   | IPv6 Router-ID of   |   140/---    | [RFC6119] / 4.1  |
   |           | Local Node          |              |                  |
   |    1030   | IPv4 Router-ID of   |   134/---    | [RFC5305] / 4.3  |
   |           | Remote Node         |              |                  |
   |    1031   | IPv6 Router-ID of   |   140/---    | [RFC6119] / 4.1  |
   |           | Remote Node         |              |                  |
   |    1088   | Administrative      |     22/3     | [RFC5305] / 3.1  |
   |           | group (color)       |              |                  |
   |    1089   | Maximum link        |     22/9     | [RFC5305] / 3.4  |
   |           | bandwidth           |              |                  |
   |    1090   | Max. reservable     |    22/10     | [RFC5305] / 3.5  |
   |           | link bandwidth      |              |                  |
   |    1091   | Unreserved          |    22/11     | [RFC5305] / 3.6  |
   |           | bandwidth           |              |                  |
   |    1092   | TE Default Metric   |    22/18     | Section 4.3.2.3  |
   |    1093   | Link Protection     |    22/20     | [RFC5307] / 1.2  |
   |           | Type                |              |                  |
   |    1094   | MPLS Protocol Mask  |     ---      | Section 4.3.2.2  |
   |    1095   | IGP Metric          |     ---      | Section 4.3.2.4  |
   |    1096   | Shared Risk Link    |     ---      | Section 4.3.2.5  |
   |           | Group               |              |                  |
   |    1097   | Opaque Link         |     ---      | Section 4.3.2.6  |
   |           | Attribute           |              |                  |
   |    1098   | Link Name           |     ---      | Section 4.3.2.7  |
   +-----------+---------------------+--------------+------------------+

                       Table 8: Link Attribute TLVs

4.3.2.1.  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.

4.3.2.2.  MPLS Protocol Mask TLV

   The MPLS Protocol Mask TLV carries a bit mask describing which MPLS
   signaling protocols are enabled.  The length of this TLV is 1.  The




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   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:

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

                   Table 9: MPLS Protocol Mask TLV Codes

4.3.2.3.  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 less 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



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4.3.2.4.  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 have a length of 1 octet.  Since the
   ISIS small metrics are of 6 bit size, the two most significant bits
   MUST be set to 0 and MUST be ignored by 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

4.3.2.5.  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 IPv4 (SRLG) TLV
   (Type 138) defined in [RFC5307] and the IPv6 SRLG TLV (Type 139)
   defined in [RFC6119].  In Link-State NLRI, both IPv4 and IPv6 SRLG
   information are carried in a single TLV.



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4.3.2.6.  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,
   e.g., a new IGP link-state attribute being defined and the 'protocol-
   neutral' BGP-LS extensions being published.

      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

4.3.2.7.  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, it can be a subset of the FQDN, or it can be any string
   operators want to use for the link.  The use of FQDN or a subset 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, that
   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.











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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     Link Name (variable)                    //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 24: Link Name TLV Format

4.3.3.  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   | Description           |   Length | Reference      |
   |     Point     |                       |          |                |
   +---------------+-----------------------+----------+----------------+
   |      1152     | IGP Flags             |        1 | Section        |
   |               |                       |          | 4.3.3.1        |
   |      1153     | IGP Route Tag         |      4*n | [RFC5130]      |
   |      1154     | IGP Extended Route    |      8*n | [RFC5130]      |
   |               | Tag                   |          |                |
   |      1155     | Prefix Metric         |        4 | [RFC5305]      |
   |      1156     | OSPF Forwarding       |        4 | [RFC2328]      |
   |               | Address               |          |                |
   |      1157     | Opaque Prefix         | variable | Section        |
   |               | Attribute             |          | 4.3.3.6        |
   +---------------+-----------------------+----------+----------------+

                      Table 10: Prefix Attribute TLVs

4.3.3.1.  IGP Flags TLV

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










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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |D|N|L|P| Resvd.|
     +-+-+-+-+-+-+-+-+

                      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] |
           | Reserved | Reserved for future use.  |           |
           +----------+---------------------------+-----------+

                    Table 11: IGP Flag Bits Definitions

4.3.3.2.  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

   Length is a multiple of 4.

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








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4.3.3.3.  Extended IGP Route Tag TLV

   The Extended IGP 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: Extended IGP Route Tag TLV Format

   Length is a multiple of 8.

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

4.3.3.4.  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

   Length is 4.

4.3.3.5.  OSPF Forwarding Address TLV

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





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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Forwarding Address (variable)                //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 29: OSPF Forwarding Address TLV Format

   Length is 4 for an IPv4 forwarding address, and 16 for an IPv6
   forwarding address.

4.3.3.6.  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 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 Prefix Attribute TLV is to bridge the document lag between,
   e.g., a new IGP link-state attribute being defined and the protocol-
   neutral BGP-LS extensions being published.

   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

   Type is as specified in Table 10.  Length is variable.

4.4.  Private Use

   TLVs for Vendor Private use are supported using the code point range
   reserved as indicated in Section 6.  For such TLV use in the NLRI or
   BGP-LS Attribute, the format as described in Section 4.1 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



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   immediately following the Total NLRI Length field of the Link-State
   NLRI format as described in Section 4.2 to carry the Enterprise Code.
   The Enterprise Codes are listed at <http://www.iana.org/assignments/
   enterprise-numbers>.  This enables use vendor specific extensions
   without conflicts.

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

4.5.  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 a link-local
   IPv6 address.  The link-local IPv6 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 attribute 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 attribute 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 attribute.

4.6.  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] [RFC5316].  In other cases, an
   implementation SHOULD provide a means to inject inter-AS links into
   BGP-LS.  The exact mechanism used to provision the inter-AS links is
   outside the scope of this document and are described in
   [I-D.ietf-idr-bgpls-inter-as-topology-ext].







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4.7.  Handling of Unreachable IGP Nodes

   The origination and propagation of IGP link-state information via BGP
   needs to provide a consistent and true view of the topology of the
   IGP domain.  BGP-LS provides an abstraction of the protocol specifics
   and BGP-LS Consumers may be varied types of applications.  While the
   information propagated via BGP-LS from a link-state routing protocol
   is sourced from that protocol's LSDB, it does not serve as a true
   reflection of the originating router's LSDB since it does not include
   the LSA/LSP sequence number information.  The sequence numbers are
   not included since a single NLRI update may be put together with
   information that is coming from multiple LSAs/LSPs.

   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 shown
   are in area 1.

   A BGP-LS Consumer talks to a BGP route-reflector (RR) R0 which is
   aggregating the BGP-LS feed from the BGP-LS Producers R2 and R3.
   Here R2 and R3 provide a redundant topology feed via BGP-LS to R0.
   Normally, R0 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 best path decision process.

                         Consumer
                            ^
                            |
                            R0
                    (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 its impact on the OSPF LSDB at R2 and
   R3.

   Now, R5 will remove the link 5-6 from its Router LSA and this updated
   LSA is available at R2.  R2 also has a stale copy of R6's Router LSA



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   which still has the link 6-5 in it.  Based on this view in its LSDB,
   R2 will advertise only the half-link 6-5 that it derives from R6's
   stale Router LSA.

   At the same time, R6 has removed the link 6-5 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 5-6 in it.  Based on it's
   LSDB, R3 will advertise only the half-link 5-6 that it has derived
   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 R0.  When viewed together, it
   would not detect or realize that the area 1 is actually partitioned.
   Also if R2 continues to report Link-State NLRIs corresponding to the
   stale copy of Router LSA of R4 and R6 nodes then R0 would prefer them
   over the valid Link-State NLRIs for R4 and R6 that it is receiving
   from R3 based on its BGP decision process.  This would result in the
   BGP-LS Consumer getting stale and inaccurate topology information.
   This problems 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 MUST withdraw all link-state objects advertised by
   it in BGP when the node that originated its corresponding LSP/LSAs is
   determined to have become unreachable in the IGP and it MUST re-
   advertise those link-state objects only after that node becomes
   reachable again in the IGP domain.

4.8.  Router-ID Anchoring Example: ISO Pseudonode

   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 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



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   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

4.9.  Router-ID Anchoring Example: OSPF Pseudonode

   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 10.1.1.1 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:

   o  Local Node Descriptor

         TLV #515: IGP Router-ID: 11.11.11.11

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

   o  Remote Node Descriptor

         TLV #515: IGP Router-ID: 11.11.11.11:10.1.1.1

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

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

   o  Local Node Descriptor

         TLV #515: IGP Router-ID: 11.11.11.11:10.1.1.1

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



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   o  Remote Node Descriptor

         TLV #515: IGP Router-ID: 33.33.33.34

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

            10.1.1.1/24              10.1.1.2/24
   +-------------+    +-------------+    +-------------+
   |   Node1     |    | Pseudonode1 |    |    Node2    |
   | 11.11.11.11 |--->| 11.11.11.11 |--->| 33.33.33.34 |
   |             |    |   10.1.1.1  |    |             |
   |   Area 0    |    |   Area 0    |    |    Area 0   |
   +-------------+    +-------------+    +-------------+

                        Figure 33: OSPF Pseudonodes

   The LAN subnet 10.1.1.0/24 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.

4.10.  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 a
   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, that
   initially were OSPFv2-only routers and then IS-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
   protocol.




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5.  Link to Path Aggregation

   Distribution of all links available in the global Internet is
   certainly possible; however, it 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 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.

5.1.  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-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

5.2.  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.





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

                     Figure 35: ASBR Link Aggregation

5.3.  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

6.  IANA Considerations

   IANA has assigned address family number 16388 (BGP-LS) in the
   "Address Family Numbers" registry with [RFC7752] as a reference.

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

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

   IANA has created a new "Border Gateway Protocol - Link State (BGP-LS)
   Parameters" registry at <http://www.iana.org/assignments/bgp-ls-
   parameters>.  All of the following registries are BGP-LS specific and
   are accessible under this registry:



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   o  "BGP-LS NLRI-Types" registry

      Value 0 is reserved.  The maximum value is 65535.  The range
      65000-65535 is for Private Use. The registry has been populated
      with the values shown in Table 1.  Allocations within the registry
      under the "Expert Review" policy require documentation of the
      proposed use of the allocated value and approval by the Designated
      Expert assigned by the IESG (see [RFC8126]).

   o  "BGP-LS Protocol-IDs" registry

      Value 0 is reserved.  The maximum value is 255.  The range 200-255
      is for Private Use. The registry has been populated with the
      values shown in Table 2.  Allocations within the registry under
      the "Expert Review" policy require documentation of the proposed
      use of the allocated value and approval by the Designated Expert
      assigned by the IESG (see [RFC8126]).

   o  "BGP-LS Well-Known Instance-IDs" registry

      This registry was setup via [RFC7752] and is no longer required.
      It may be retained as deprecated.

   o  "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and
      Attribute TLVs" registry

      Values 0-255 are reserved.  Values 256-65535 will be used for code
      points.  The range 65000-65535 is for Private Use. The registry
      has been populated with the values shown in Table 12.  Allocations
      within the registry under the "Expert Review" policy require
      documentation of the proposed use of the allocated value and
      approval by the Designated Expert assigned by the IESG (see
      [RFC8126]).

6.1.  Guidance for Designated Experts

   In all cases of review by the Designated Expert (DE) described here,
   the DE is expected to ascertain the existence of suitable
   documentation (a specification) as described in [RFC8126].  The DE is
   also expected to check the clarity of purpose and use of the
   requested code points.  Additionally, the DE must verify that any
   request for one of these code points has been made available for
   review and comment within the IETF: the DE will post the request to
   the IDR Working Group mailing list (or a successor mailing list
   designated by the IESG).  If the request comes from within the IETF,
   it should be documented in an Internet-Draft.  Lastly, the DE must
   ensure that any other request for a code point does not conflict with
   work that is active or already published within the IETF.



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   The IANA also requests (per [RFC8126]) that all registries with
   "Expert review" allocation policies have a "Change Controller"
   assigned.  For these four registries, the assigned "Change
   Controllers" are the chairs of the IDR working group or a successor
   as designated by the IESG.

7.  Manageability Considerations

   This section is structured as recommended in [RFC5706].

7.1.  Operational Considerations

7.1.1.  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.  It is
   expected that the distribution of this NLRI SHOULD be handled by
   dedicated route reflectors in most deployments providing a level of
   isolation and fault containment between different NLRI types.  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.

7.1.2.  Installation and Initial Setup

   Configuration parameters defined in Section 7.2.3 SHOULD be
   initialized to the following default values:

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

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

7.1.3.  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 7.2.3), so the extension
   can be gradually rolled out in the network.






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7.1.4.  Requirements on Other Protocols and Functional Components

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

7.1.5.  Impact on Network Operation

   Frequency of Link-State NLRI updates could interfere with regular BGP
   prefix distribution.  A network operator MAY use a dedicated Route-
   Reflector infrastructure to distribute Link-State NLRIs.

   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.

7.1.6.  Verifying Correct Operation

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

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

7.2.  Management Considerations

7.2.1.  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.

7.2.2.  Fault Management

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

   A Link-State NLRI MUST NOT be considered as malformed or invalid
   based on the inclusion/exclusion of TLVs or contents of the TLV
   fields (i.e. semantic errors), as described in Section 4.1 and
   Section 4.2.

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




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   o  Does the sum of all TLVs found in the BGP MP_REACH_NLRI attribute
      correspond to the BGP MP_REACH_NLRI length?

   o  Does the sum of all TLVs found in the BGP MP_UNREACH_NLRI
      attribute correspond to the BGP MP_UNREACH_NLRI length?

   o  Does the sum of all TLVs found in a Link-State NLRI correspond to
      the Total NLRI Length field of all its Descriptors?

   o  Is the length of the TLVs and, when the TLV is recognized then,
      its sub-TLVs in the NLRI valid?

   o  Has the syntactic correctness of the NLRI fields been verified as
      per [RFC7606]?

   o  Has the rule regarding ordering of TLVs been followed as described
      in Section 4.1?

   When the error determined allows for the router to skip the malformed
   NLRI(s) and continue processing of the rest of the update message
   (e.g. when the TLV ordering rule is violated), then it MUST handle
   such malformed NLRIs as 'Treat-as-withdraw'.  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 'session reset' when
   the session is only being used for BGP-LS or when it 'AFI/SAFI
   disable' action is not possible.

   A BGP-LS Attribute MUST NOT be considered as malformed or invalid
   based on the inclusion/exclusion of TLVs or contents of the TLV
   fields (i.e. semantic errors), as described in Section 4.1 and
   Section 4.3.

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

   o  Does the sum of all TLVs found in the BGP-LS Attribute correspond
      to the BGP-LS Attribute length?

   o  Has the syntactic correctness of the Attributes (including BGP-LS
      Attribute) been verified as per [RFC7606]?

   o  Is the length of each TLV and, when the TLV is recognized then,
      its sub-TLVs in the BGP-LS Attribute valid?





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   When the error determined allows for the router to skip the malformed
   BGP-LS Attribute and continue processing of the rest of the 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 then 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 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/withdraw.
   This also makes it possible for a network operator to trace back to
   the BGP-LS Propagator which actually detected a fault with the BGP-LS
   Attribute.

   An implementation SHOULD log an error for any errors found during
   syntax validation for further analysis.

   A BGP-LS Propagator 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):

   o  is a mandatory TLV present or not?

   o  is the length of a fixed length TLV correct or the length of a
      variable length TLV a valid/permissible?

   o  are the values of TLV fields valid or permissible?

   o  are the inclusion and use of TLVs/sub-TLVs with specific Link-
      State NLRI types valid?

   Each TLV MAY indicate the valid and permissible values and their
   semantics that can to be used only by a BGP-LS Consumer for its



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   semantic validation.  However, the handling of any errors may be
   specific to the particular application and outside the scope of this
   document.  A BGP-LS Consumer should ignore unrecognized and
   unexpected TLV types in both the NLRI and BGP-LS Attribute portions
   and not consider their presence as an error.

7.2.3.  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 SHOULD allow the operator to configure a 64-bit
   Instance-ID.

   An implementation SHOULD allow the operator to configure ASN and BGP-
   LS identifiers (refer Section 4.2.1.4).

   An implementation SHOULD allow the operator to configure the maximum
   size of the BGP-LS Attribute that may be used on a BGP-LS Producer.

7.2.4.  Accounting Management

   Not Applicable.

7.2.5.  Performance Management

   An implementation SHOULD provide the following statistics:

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

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

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

   o  Total number of locally originated Link-State NLRIs



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

7.2.6.  Security Management

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

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

   An implementation MUST have the means to limit inbound updates.

8.  TLV/Sub-TLV Code Points Summary

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

   +-----------+---------------------+--------------+------------------+
   |  TLV Code | Description         |  IS-IS TLV/  | Reference        |
   |   Point   |                     |   Sub-TLV    | (RFC/Section)    |
   +-----------+---------------------+--------------+------------------+
   |    256    | Local Node          |     ---      | Section 4.2.1.2  |
   |           | Descriptors         |              |                  |
   |    257    | Remote Node         |     ---      | Section 4.2.1.3  |
   |           | Descriptors         |              |                  |
   |    258    | Link Local/Remote   |     22/4     | [RFC5307] / 1.1  |
   |           | Identifiers         |              |                  |
   |    259    | IPv4 interface      |     22/6     | [RFC5305] / 3.2  |
   |           | address             |              |                  |
   |    260    | IPv4 neighbor       |     22/8     | [RFC5305] / 3.3  |
   |           | address             |              |                  |
   |    261    | IPv6 interface      |    22/12     | [RFC6119] / 4.2  |
   |           | address             |              |                  |
   |    262    | IPv6 neighbor       |    22/13     | [RFC6119] / 4.3  |
   |           | address             |              |                  |
   |    263    | Multi-Topology ID   |     ---      | Section 4.2.2.1  |
   |    264    | OSPF Route Type     |     ---      | Section 4.2.3    |
   |    265    | IP Reachability     |     ---      | Section 4.2.3    |
   |           | Information         |              |                  |
   |    512    | Autonomous System   |     ---      | Section 4.2.1.4  |
   |    513    | BGP-LS Identifier   |     ---      | Section 4.2.1.4  |
   |           | (deprecated)        |              |                  |
   |    514    | OSPF Area-ID        |     ---      | Section 4.2.1.4  |
   |    515    | IGP Router-ID       |     ---      | Section 4.2.1.4  |
   |    1024   | Node Flag Bits      |     ---      | Section 4.3.1.1  |
   |    1025   | Opaque Node         |     ---      | Section 4.3.1.5  |



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   |           | Attribute           |              |                  |
   |    1026   | Node Name           |   variable   | Section 4.3.1.3  |
   |    1027   | IS-IS Area          |   variable   | Section 4.3.1.2  |
   |           | Identifier          |              |                  |
   |    1028   | IPv4 Router-ID of   |   134/---    | [RFC5305] / 4.3  |
   |           | Local Node          |              |                  |
   |    1029   | IPv6 Router-ID of   |   140/---    | [RFC6119] / 4.1  |
   |           | Local Node          |              |                  |
   |    1030   | IPv4 Router-ID of   |   134/---    | [RFC5305] / 4.3  |
   |           | Remote Node         |              |                  |
   |    1031   | IPv6 Router-ID of   |   140/---    | [RFC6119] / 4.1  |
   |           | Remote Node         |              |                  |
   |    1088   | Administrative      |     22/3     | [RFC5305] / 3.1  |
   |           | group (color)       |              |                  |
   |    1089   | Maximum link        |     22/9     | [RFC5305] / 3.4  |
   |           | bandwidth           |              |                  |
   |    1090   | Max. reservable     |    22/10     | [RFC5305] / 3.5  |
   |           | link bandwidth      |              |                  |
   |    1091   | Unreserved          |    22/11     | [RFC5305] / 3.6  |
   |           | bandwidth           |              |                  |
   |    1092   | TE Default Metric   |    22/18     | Section 4.3.2.3  |
   |    1093   | Link Protection     |    22/20     | [RFC5307] / 1.2  |
   |           | Type                |              |                  |
   |    1094   | MPLS Protocol Mask  |     ---      | Section 4.3.2.2  |
   |    1095   | IGP Metric          |     ---      | Section 4.3.2.4  |
   |    1096   | Shared Risk Link    |     ---      | Section 4.3.2.5  |
   |           | Group               |              |                  |
   |    1097   | Opaque Link         |     ---      | Section 4.3.2.6  |
   |           | Attribute           |              |                  |
   |    1098   | Link Name           |     ---      | Section 4.3.2.7  |
   |    1152   | IGP Flags           |     ---      | Section 4.3.3.1  |
   |    1153   | IGP Route Tag       |     ---      | [RFC5130]        |
   |    1154   | IGP Extended Route  |     ---      | [RFC5130]        |
   |           | Tag                 |              |                  |
   |    1155   | Prefix Metric       |     ---      | [RFC5305]        |
   |    1156   | OSPF Forwarding     |     ---      | [RFC2328]        |
   |           | Address             |              |                  |
   |    1157   | Opaque Prefix       |     ---      | Section 4.3.3.6  |
   |           | Attribute           |              |                  |
   +-----------+---------------------+--------------+------------------+

            Table 12: Summary Table of TLV/Sub-TLV Code Points

9.  Security Considerations

   Procedures and protocol extensions defined in this document do not
   affect the BGP security model.  See the Security Considerations




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   section of [RFC4271] for a discussion of BGP security.  Also refer to
   [RFC4272] and [RFC6952] for analysis of security issues for BGP.

   In the context of the BGP peerings associated with this document, a
   BGP speaker MUST NOT accept updates from a peer that is only
   providing information to a BGP-LS Consumer.  That is, a participating
   BGP speaker should be aware of the nature of its relationships for
   link-state relationships and should protect itself from peers sending
   updates that either represent erroneous information feedback loops or
   are false input.  Such protection can be achieved by manual
   configuration of consumer peers at the BGP speaker.

   An operator SHOULD employ a mechanism to protect a BGP speaker
   against DDoS attacks from BGP-LS Consumers.  The principal attack a
   consumer may apply is to attempt to start multiple sessions either
   sequentially or simultaneously.  Protection can be applied by
   imposing rate limits.

   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 consumers are configured to
   receive such information.

10.  Contributors

   The following persons contributed significant text to RFC7752 and
   this document.  They should be considered as co-authors.

   Hannes Gredler
   Rtbrick
   Email: hannes@rtbrick.com

   Jan Medved
   Cisco Systems Inc.
   USA
   Email: jmedved@cisco.com

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

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



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   Saikat Ray
   Individual
   USA
   Email: raysaikat@gmail.com

11.  Acknowledgements

   This document update to the BGP-LS specification [RFC7752] is a
   result of feedback and inputs 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 LAN subnet for OSPF.

   We would like to thank Balaji Rajagopalan, Srihari Sangli, Shraddha
   Hegde, Andrew Stone, Jeff Tantsura, Acee Lindem, Jie Dong, Aijun Wang
   and Nandan Saha for their review and feedback on this document.

   We would like to thank Robert Varga for the significant contribution
   he gave 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.

12.  References

12.1.  Normative References

   [ISO10589]
              International Organization for Standardization,
              "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, 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>.





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   [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>.

   [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>.



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   [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>.

   [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>.

   [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>.







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   [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>.

   [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>.

   [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>.

   [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>.

12.2.  Informative References

   [I-D.ietf-idr-bgpls-inter-as-topology-ext]
              Wang, A., Chen, H., Talaulikar, K., Zhuang, S., and S. Ma,
              "BGP-LS Extension for Inter-AS Topology Retrieval", draft-
              ietf-idr-bgpls-inter-as-topology-ext-07 (work in
              progress), September 2019.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              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., 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>.



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   [RFC5073]  Vasseur, J., Ed. and J. Le Roux, Ed., "IGP Routing
              Protocol Extensions for Discovery of Traffic Engineering
              Node Capabilities", RFC 5073, DOI 10.17487/RFC5073,
              December 2007, <https://www.rfc-editor.org/info/rfc5073>.

   [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>.

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316,
              December 2008, <https://www.rfc-editor.org/info/rfc5316>.

   [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>.

   [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>.

   [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>.




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

   2.   In Section 4.1, clarification about the TLV handling aspects
        that are applicable to both the NLRI and BGP-LS Attribute parts
        and those that are applicable only for the NLRI portion.  An
        implementation may have missed the part about handling of
        unrecognized TLV and so, based on [RFC7606] guidelines, might
        discard the unknown NLRI types.  This aspect is now
        unambiguously clarified in Section 4.2.  Also, the TLVs in the
        BGP-LS Attribute that are not ordered are not to be considered
        as malformed.

   3.   Clarification of mandatory and optional TLVs in both NLRI and
        BGP-LS Attribute portions all through the document.

   4.   Handling of the growth of the BGP-LS Attribute is covered in
        Section 4.3.

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

   6.   Clarification on the use of Identifier field in the Link-State
        NLRI in Section 4.2 is provided.  It was defined ambiguously to
        refer to only mutli-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.

   7.   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 implementators
        on its usage for identification of IGP domains as against the
        use of the Identifier doing the same functionality as the
        Instance-ID when running multiple instances of IGP routing
        protocols.

   8.   Clarification that the Area-ID TLV is mandatory in the Node
        Descriptor for origination of information from OSPF except for




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        when sourcing information from AS-scope LSAs where this TLV is
        not applicable.

   9.   Moved 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
        describe the differences in the applicability of the R flags
        when MT-ID TLV is used as link descriptor TLV and Prefix
        Attribute TLV.  MT-ID TLV use is now elevated to SHOULD when it
        is enabled in the underlying IGP.

   10.  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.

   11.  Update the usage of OSPF Route Type TLV to mandate its use for
        OSPF prefixes in Section 4.2.3.1 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.

   12.  Clarification on the length of the Node Flag Bits TLV to be one
        octet.

   13.  Updated the Node Name TLV in Section 4.3.1.3 with the OSPF
        specification.

   14.  Clarification on the size of the IS-IS Narrow Metric
        advertisement via the IGP Metric TLV and the handling of the
        unused bits.

   15.  Clarified the advertisement of the prefix corresponding to the
        LAN segment in an OSPF network in Section 4.9.

   16.  Introduced Private Use TLV code point space and specified their
        encoding in Section 4.4.

   17.  Introduced Section 4.7 where issues related to consistency of
        reporting IGP link-state along with their solutions are covered.

   18.  Handling of large size of BGP-LS Attribute with growth in BGP-LS
        information is explained in Section 4.3 along with mitigation of
        errors arising out of it.

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



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Internet-Draft   Link-State Info Distribution Using BGP    November 2019


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

   21.  Change to the management of BGP-LS IANA registries from
        "Specification Required" to "Expert Review" along with updated
        guidelines for Designated Experts.

Author's Address

   Ketan Talaulikar (editor)
   Cisco Systems
   India

   Email: ketant@cisco.com





































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