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rfc:rfc7752

Internet Engineering Task Force (IETF) H. Gredler, Ed. Request for Comments: 7752 Individual Contributor Category: Standards Track J. Medved ISSN: 2070-1721 S. Previdi

                                                   Cisco Systems, Inc.
                                                             A. Farrel
                                                Juniper Networks, Inc.
                                                                S. Ray
                                                            March 2016
North-Bound Distribution of Link-State and Traffic Engineering (TE)
                       Information Using BGP

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).

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7752.

Gredler, et al. Standards Track [Page 1] RFC 7752 Link-State Info Distribution Using BGP March 2016

Copyright Notice

 Copyright (c) 2016 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
 (http://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 ....................................6
 3. Carrying Link-State Information in BGP ..........................7
    3.1. TLV Format .................................................8
    3.2. The Link-State NLRI ........................................8
         3.2.1. Node Descriptors ...................................12
         3.2.2. Link Descriptors ...................................16
         3.2.3. Prefix Descriptors .................................18
    3.3. The BGP-LS Attribute ......................................19
         3.3.1. Node Attribute TLVs ................................20
         3.3.2. Link Attribute TLVs ................................23
         3.3.3. Prefix Attribute TLVs ..............................28
    3.4. BGP Next-Hop Information ..................................31
    3.5. Inter-AS Links ............................................32
    3.6. Router-ID Anchoring Example: ISO Pseudonode ...............32
    3.7. Router-ID Anchoring Example: OSPF Pseudonode ..............33
    3.8. Router-ID Anchoring Example: OSPFv2 to IS-IS Migration ....34
 4. Link to Path Aggregation .......................................34
    4.1. Example: No Link Aggregation ..............................35
    4.2. Example: ASBR to ASBR Path Aggregation ....................35
    4.3. Example: Multi-AS Path Aggregation ........................36
 5. IANA Considerations ............................................36
    5.1. Guidance for Designated Experts ...........................37
 6. Manageability Considerations ...................................38
    6.1. Operational Considerations ................................38
         6.1.1. Operations .........................................38
         6.1.2. Installation and Initial Setup .....................38
         6.1.3. Migration Path .....................................38

Gredler, et al. Standards Track [Page 2] RFC 7752 Link-State Info Distribution Using BGP March 2016

         6.1.4. Requirements on Other Protocols and
                Functional Components ..............................38
         6.1.5. Impact on Network Operation ........................38
         6.1.6. Verifying Correct Operation ........................39
    6.2. Management Considerations .................................39
         6.2.1. Management Information .............................39
         6.2.2. Fault Management ...................................39
         6.2.3. Configuration Management ...........................40
         6.2.4. Accounting Management ..............................40
         6.2.5. Performance Management .............................40
         6.2.6. Security Management ................................41
 7. TLV/Sub-TLV Code Points Summary ................................41
 8. Security Considerations ........................................42
 9. References .....................................................43
    9.1. Normative References ......................................43
    9.2. Informative References ....................................45
 Acknowledgements ..................................................47
 Contributors ......................................................47
 Authors' Addresses ................................................48

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
 (NLRI) encoding format.  The mechanism is applicable to physical and
 virtual links.  The mechanism described is subject to policy control.

Gredler, et al. Standards Track [Page 3] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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.
 The collection of link-state and TE information and its distribution
 to consumers is shown in the following figure.
                         +-----------+
                         | Consumer  |
                         +-----------+
                               ^
                               |
                         +-----------+
                         |    BGP    |               +-----------+
                         |  Speaker  |               | Consumer  |
                         +-----------+               +-----------+
                           ^   ^   ^                       ^
                           |   |   |                       |
           +---------------+   |   +-------------------+   |
           |                   |                       |   |
     +-----------+       +-----------+             +-----------+
     |    BGP    |       |    BGP    |             |    BGP    |
     |  Speaker  |       |  Speaker  |    . . .    |  Speaker  |
     +-----------+       +-----------+             +-----------+
           ^                   ^                         ^
           |                   |                         |
          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
 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

Gredler, et al. Standards Track [Page 4] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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

1.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

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]
 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

Gredler, et al. Standards Track [Page 5] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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.

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].

Gredler, et al. Standards Track [Page 6] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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. 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.

Gredler, et al. Standards Track [Page 7] RFC 7752 Link-State Info Distribution Using BGP March 2016

3.1. TLV Format

 Information in the new Link-State NLRIs and attributes is encoded in
 Type/Length/Value triplets.  The TLV format is shown in Figure 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                        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.  Unrecognized types MUST be
 preserved and propagated.  In order to compare NLRIs with unknown
 TLVs, all TLVs MUST be ordered in ascending order by TLV Type.  If
 there are more TLVs of the same type, then the TLVs MUST be ordered
 in ascending order of the TLV value within the TLVs with the same
 type by treating the entire Value field as an opaque hexadecimal
 string and comparing leftmost octets first, regardless of the length
 of the string.  All TLVs that are not specified as mandatory are
 considered optional.

3.2. The Link-State NLRI

 The MP_REACH_NLRI and MP_UNREACH_NLRI attributes are BGP's containers
 for carrying opaque information.  Each Link-State NLRI describes
 either a node, a link, or a prefix.
 All non-VPN link, node, and prefix information SHALL be encoded using
 AFI 16388 / SAFI 71.  VPN link, node, and prefix information SHALL be
 encoded using AFI 16388 / SAFI 72.
 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.

Gredler, et al. Standards Track [Page 8] RFC 7752 Link-State Info Distribution Using BGP March 2016

 The format of the Link-State NLRI is shown in the following figures.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            NLRI Type          |     Total NLRI Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                  Link-State NLRI (variable)                 //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Figure 5: Link-State AFI 16388 / SAFI 71 NLRI Format
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            NLRI Type          |     Total NLRI Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                       Route Distinguisher                     +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                  Link-State NLRI (variable)                 //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Figure 6: Link-State VPN AFI 16388 / SAFI 72 NLRI Format
 The Total NLRI Length field contains the cumulative length, in
 octets, of the rest of the NLRI, not including the NLRI Type field or
 itself.  For VPN applications, it also includes the length of the
 Route Distinguisher.
                 +------+---------------------------+
                 | Type | NLRI Type                 |
                 +------+---------------------------+
                 |  1   | Node NLRI                 |
                 |  2   | Link NLRI                 |
                 |  3   | IPv4 Topology Prefix NLRI |
                 |  4   | IPv6 Topology Prefix NLRI |
                 +------+---------------------------+
                          Table 1: NLRI Types

Gredler, et al. Standards Track [Page 9] RFC 7752 Link-State Info Distribution Using BGP March 2016

 Route Distinguishers are defined and discussed in [RFC4364].
 The Node NLRI (NLRI Type = 1) is shown in the following figure.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |  Protocol-ID  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Identifier                          |
   |                            (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

Gredler, et al. Standards Track [Page 10] RFC 7752 Link-State Info Distribution Using BGP March 2016

 The IPv4 and IPv6 Prefix NLRIs (NLRI Type = 3 and Type = 4) use the
 same format, as shown in the following figure.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |  Protocol-ID  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Identifier                          |
   |                            (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                           |
          +-------------+----------------------------------+
                     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 4,
 the Protocol-ID 'Static configuration' SHOULD be used.
 Both OSPF and IS-IS MAY run multiple routing protocol instances over
 the same link.  See [RFC6822] and [RFC6549].  These instances define
 independent "routing universes".  The 64-bit Identifier field is used
 to identify the routing universe where the NLRI belongs.  The NLRIs
 representing link-state objects (nodes, links, or prefixes) from the
 same routing universe MUST have the same 'Identifier' value.  NLRIs

Gredler, et al. Standards Track [Page 11] RFC 7752 Link-State Info Distribution Using BGP March 2016

 with different 'Identifier' values MUST be considered to be from
 different routing universes.  Table 3 lists the 'Identifier' values
 that are defined as well-known in this document.
           +------------+----------------------------------+
           | Identifier | Routing Universe                 |
           +------------+----------------------------------+
           |     0      | Default Layer 3 Routing topology |
           +------------+----------------------------------+
               Table 3: Well-Known Instance Identifiers
 If a given protocol does not support multiple routing universes, then
 it SHOULD set the Identifier field according to Table 3.  However, an
 implementation MAY make the 'Identifier' configurable for a given
 protocol.
 Each Node Descriptor and Link Descriptor consists of one or more
 TLVs, as described in the following sections.

3.2.1. 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 link attribute described in Section 3.3.2.
 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
 and BGP-LS Identifier (see Section 3.2.1.4) to disambiguate the
 Router-IDs, as described in Section 3.2.1.1.

3.2.1.1. 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).

Gredler, et al. Standards Track [Page 12] RFC 7752 Link-State Info Distribution Using BGP March 2016

 (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.
 In Section 3.2.1.4, a set of sub-TLVs is described, which allows
 specification of a flexible key for any given node/link information
 such that global uniqueness of the NLRI is ensured.

3.2.1.2. 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 length of this
 TLV is variable.  The value contains one or more Node Descriptor
 Sub-TLVs defined in Section 3.2.1.4.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //              Node Descriptor Sub-TLVs (variable)            //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 10: Local Node Descriptors TLV Format

3.2.1.3. 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 length of this TLV is variable.  The value
 contains one or more Node Descriptor Sub-TLVs defined in
 Section 3.2.1.4.

Gredler, et al. Standards Track [Page 13] RFC 7752 Link-State Info Distribution Using BGP March 2016

    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

3.2.1.4. 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 |        4 |
         |        514         | OSPF Area-ID      |        4 |
         |        515         | IGP Router-ID     | Variable |
         +--------------------+-------------------+----------+
                   Table 4: Node Descriptor Sub-TLVs
 The sub-TLV values in Node Descriptor TLVs are defined as follows:
 Autonomous System:  Opaque value (32-bit AS Number)
 BGP-LS Identifier:  Opaque value (32-bit ID).  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 NLRI belongs.
    The Area Identifier allows different NLRIs of the same router to
    be discriminated.
 IGP Router-ID:  Opaque value.  This is a mandatory TLV.  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

Gredler, et al. Standards Track [Page 14] RFC 7752 Link-State Info Distribution Using BGP March 2016

    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.
    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
    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.

3.2.1.5. 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 7.2 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 9 bits MUST be set to 0.  Bits R are reserved
 and SHOULD be set to 0 when originated and ignored on receipt.
 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.

Gredler, et al. Standards Track [Page 15] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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 need to be generated
 where each NLRI contains an unique MT-ID.  In the BGP-LS attribute of
 a Node NLRI, one MT-ID TLV containing the array of MT-IDs of all
 topologies where the node is reachable is allowed.

3.2.2. Link Descriptors

 The Link Descriptor field is a set of Type/Length/Value (TLV)
 triplets.  The format of each TLV is shown in Section 3.1.  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.
 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],
 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.

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 The following TLVs are valid 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 3.2.1.5  |
 |           | Identifier          |              |                  |
 +-----------+---------------------+--------------+------------------+
                     Table 5: 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 are included in the Link
    Descriptor but not the link local/remote Identifier TLV.  The link
    local/remote identifiers MAY be included in the link attribute.
    If interface and neighbor addresses are not present and the link
    local/remote identifiers are present, then the link local/remote
    Identifier TLV is included in the Link Descriptor.
    The Multi-Topology Identifier TLV is included in Link Descriptor
    if that information is present.

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3.2.3. 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 valid as Prefix
 Descriptors in the IPv4/IPv6 Prefix NLRI:
 +-------------+---------------------+----------+--------------------+
 |   TLV Code  | Description         |  Length  | Reference          |
 |    Point    |                     |          | (RFC/Section)      |
 +-------------+---------------------+----------+--------------------+
 |     263     | Multi-Topology      | variable | Section 3.2.1.5    |
 |             | Identifier          |          |                    |
 |     264     | OSPF Route Type     |    1     | Section 3.2.3.1    |
 |     265     | IP Reachability     | variable | Section 3.2.3.2    |
 |             | Information         |          |                    |
 +-------------+---------------------+----------+--------------------+
                    Table 6: Prefix Descriptor TLVs

3.2.3.1. OSPF Route Type

 The OSPF Route Type TLV is an optional TLV that MAY be present in
 Prefix NLRIs.  It is used to identify the OSPF route type of the
 prefix.  It is used when an OSPF prefix is 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
 where the Type and Length fields of the TLV are defined in Table 6.
 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)

Gredler, et al. Standards Track [Page 18] RFC 7752 Link-State Info Distribution Using BGP March 2016

 o  External 2 (0x4)
 o  NSSA 1 (0x5)
 o  NSSA 2 (0x6)

3.2.3.2. IP Reachability Information

 The IP Reachability Information TLV is a mandatory TLV that 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 6.  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.

3.3. 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.

Gredler, et al. Standards Track [Page 19] RFC 7752 Link-State Info Distribution Using BGP March 2016

3.3.1. Node Attribute TLVs

 Node attribute TLVs are the TLVs that may be encoded in the BGP-LS
 attribute with a Node NLRI.  The following Node Attribute TLVs are
 defined:
 +-------------+----------------------+----------+-------------------+
 |   TLV Code  | Description          |   Length | Reference         |
 |    Point    |                      |          | (RFC/Section)     |
 +-------------+----------------------+----------+-------------------+
 |     263     | Multi-Topology       | variable | Section 3.2.1.5   |
 |             | Identifier           |          |                   |
 |     1024    | Node Flag Bits       |        1 | Section 3.3.1.1   |
 |     1025    | Opaque Node          | variable | Section 3.3.1.5   |
 |             | Attribute            |          |                   |
 |     1026    | Node Name            | variable | Section 3.3.1.3   |
 |     1027    | IS-IS Area           | variable | Section 3.3.1.2   |
 |             | Identifier           |          |                   |
 |     1028    | IPv4 Router-ID of    |        4 | [RFC5305]/4.3     |
 |             | Local Node           |          |                   |
 |     1029    | IPv6 Router-ID of    |       16 | [RFC6119]/4.1     |
 |             | Local Node           |          |                   |
 +-------------+----------------------+----------+-------------------+
                     Table 7: Node Attribute TLVs

3.3.1.1. Node Flag Bits TLV

 The Node Flag Bits TLV carries a bit mask describing node attributes.
 The value is a variable-length bit array of flags, where each bit
 represents a node capability.
    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

Gredler, et al. Standards Track [Page 20] RFC 7752 Link-State Info Distribution Using BGP March 2016

 The bits are defined as follows:
      +-----------------+-------------------------+------------+
      |       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 8: Node Flag Bits Definitions

3.3.1.2. 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

3.3.1.3. 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.

Gredler, et al. Standards Track [Page 21] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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.
 Although [RFC5301] describes an IS-IS-specific extension, usage of
 the Node Name TLV is possible for all protocols.  How a router
 derives and injects node names, e.g., OSPF nodes, is outside of the
 scope of this document.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                     Node Name (variable)                    //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      Figure 17: Node Name Format

3.3.1.4. 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.

3.3.1.5. 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].

Gredler, et al. Standards Track [Page 22] RFC 7752 Link-State Info Distribution Using BGP March 2016

    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

3.3.2. 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 3.1.  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.

Gredler, et al. Standards Track [Page 23] RFC 7752 Link-State Info Distribution Using BGP March 2016

 The following Link Attribute TLVs are valid in the BGP-LS attribute
 with a Link NLRI:
 +-----------+---------------------+--------------+------------------+
 |  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 3.3.2.3  |
 |    1093   | Link Protection     |    22/20     | [RFC5307]/1.2    |
 |           | Type                |              |                  |
 |    1094   | MPLS Protocol Mask  |     ---      | Section 3.3.2.2  |
 |    1095   | IGP Metric          |     ---      | Section 3.3.2.4  |
 |    1096   | Shared Risk Link    |     ---      | Section 3.3.2.5  |
 |           | Group               |              |                  |
 |    1097   | Opaque Link         |     ---      | Section 3.3.2.6  |
 |           | Attribute           |              |                  |
 |    1098   | Link Name           |     ---      | Section 3.3.2.7  |
 +-----------+---------------------+--------------+------------------+
                     Table 9: Link Attribute TLVs

3.3.2.1. 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.

Gredler, et al. Standards Track [Page 24] RFC 7752 Link-State Info Distribution Using BGP March 2016

3.3.2.2. 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
 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 10: MPLS Protocol Mask TLV Codes

Gredler, et al. Standards Track [Page 25] RFC 7752 Link-State Info Distribution Using BGP March 2016

3.3.2.3. 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

3.3.2.4. 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 (the two most
 significant bits are ignored).  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

3.3.2.5. 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).

Gredler, et al. Standards Track [Page 26] RFC 7752 Link-State Info Distribution Using BGP March 2016

    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.

3.3.2.6. 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

3.3.2.7. 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

Gredler, et al. Standards Track [Page 27] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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.
    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

3.3.3. 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 MUST be
 reflected into the BGP-LS attribute with a prefix NLRI.  This section
 describes the different attributes related to the IPv4/IPv6 prefixes.
 Prefix Attribute TLVs SHOULD be used when advertising NLRI types 3
 and 4 only.  The following Prefix Attribute TLVs are defined:
 +---------------+----------------------+----------+-----------------+
 |    TLV Code   | Description          |   Length | Reference       |
 |     Point     |                      |          |                 |
 +---------------+----------------------+----------+-----------------+
 |      1152     | IGP Flags            |        1 | Section 3.3.3.1 |
 |      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 3.3.3.6 |
 |               | Attribute            |          |                 |
 +---------------+----------------------+----------+-----------------+
                    Table 11: Prefix Attribute TLVs

Gredler, et al. Standards Track [Page 28] RFC 7752 Link-State Info Distribution Using BGP March 2016

3.3.3.1. 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:
    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 12: IGP Flag Bits Definitions

3.3.3.2. 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.

Gredler, et al. Standards Track [Page 29] RFC 7752 Link-State Info Distribution Using BGP March 2016

3.3.3.3. 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.

3.3.3.4. 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.

3.3.3.5. 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.

Gredler, et al. Standards Track [Page 30] RFC 7752 Link-State Info Distribution Using BGP March 2016

    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.

3.3.3.6. 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 11.  Length is variable.

3.4. 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

Gredler, et al. Standards Track [Page 31] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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 originators, 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.

3.5. 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

3.6. 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 31.  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
 contains the ISO-ID of Pseudonode1, 1920.0000.2001.02.  The IGP

Gredler, et al. Standards Track [Page 32] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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 31: IS-IS Pseudonodes

3.7. 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 32.
 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

Gredler, et al. Standards Track [Page 33] RFC 7752 Link-State Info Distribution Using BGP March 2016

 o  Remote Node Descriptor
       TLV #515: IGP Router-ID: 33.33.33.34
       TLV #514: OSPF Area-ID: ID:0.0.0.0
   +-----------------+    +-----------------+    +-----------------+
   |      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 32: OSPF Pseudonodes

3.8. 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.

4. 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

Gredler, et al. Standards Track [Page 34] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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.

4.1. Example: No Link Aggregation

 Consider Figure 33.  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 33: No Link Aggregation

4.2. 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 34.  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.

Gredler, et al. Standards Track [Page 35] RFC 7752 Link-State Info Distribution Using BGP March 2016

        AS1   :   AS2
              :
         R1-------R3
          |   :   |
          |   :   |
          |   :   |
         R2-------R4
              :
              :
       Figure 34: ASBR Link Aggregation

4.3. 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 35.  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 35: Multi-AS Aggregation

5. IANA Considerations

 IANA has assigned address family number 16388 (BGP-LS) in the
 "Address Family Numbers" registry with this document 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:

Gredler, et al. Standards Track [Page 36] RFC 7752 Link-State Info Distribution Using BGP March 2016

 o  "BGP-LS NLRI-Types" registry
    Value 0 is reserved.  The maximum value is 65535.  The registry
    has been populated with the values shown in Table 1.  Allocations
    within the registry require documentation of the proposed use of
    the allocated value (Specification Required) and approval by the
    Designated Expert assigned by the IESG (see [RFC5226]).
 o  "BGP-LS Protocol-IDs" registry
    Value 0 is reserved.  The maximum value is 255.  The registry has
    been populated with the values shown in Table 2.  Allocations
    within the registry require documentation of the proposed use of
    the allocated value (Specification Required) and approval by the
    Designated Expert assigned by the IESG (see [RFC5226]).
 o  "BGP-LS Well-Known Instance-IDs" registry
    The registry has been populated with the values shown in Table 3.
    New allocations from the range 1-31 use the IANA allocation policy
    "Specification Required" and require approval by the Designated
    Expert assigned by the IESG (see [RFC5226]).  Values in the range
    32 to 2^64-1 are for "Private Use" and are not recorded by IANA.
 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 registry has been populated with the values shown in
    Table 13.  Allocations within the registry require documentation
    of the proposed use of the allocated value (Specification
    Required) and approval by the Designated Expert assigned by the
    IESG (see [RFC5226]).

5.1. 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 [RFC5226] and to
 verify that the document is permanently and publicly available.  The
 DE is also expected to check the clarity of purpose and use of the
 requested code points.  Last, the DE must verify that any
 specification produced in the IETF that requests one of these code
 points has been made available for review by the IDR working group
 and that any specification produced outside the IETF does not
 conflict with work that is active or already published within the
 IETF.

Gredler, et al. Standards Track [Page 37] RFC 7752 Link-State Info Distribution Using BGP March 2016

6. Manageability Considerations

 This section is structured as recommended in [RFC5706].

6.1. Operational Considerations

6.1.1. 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 corresponding forwarding state impact.  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.  Furthermore, it is anticipated that
 distribution of this NLRI will be handled by dedicated route
 reflectors providing a level of isolation and fault containment
 between different NLRI types.

6.1.2. Installation and Initial Setup

 Configuration parameters defined in Section 6.2.3 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.

6.1.3. 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 6.2.3), so the extension
 can be gradually rolled out in the network.

6.1.4. 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.

6.1.5. 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.

Gredler, et al. Standards Track [Page 38] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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.

6.1.6. 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.

6.2. Management Considerations

6.2.1. 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.

6.2.2. Fault Management

 If an implementation of BGP-LS detects a malformed attribute, then it
 MUST use the 'Attribute Discard' action as per [RFC7606], Section 2.
 An implementation of BGP-LS MUST perform the following syntactic
 checks for determining if a message is malformed.
 o  Does the sum of all TLVs found in the BGP-LS attribute correspond
    to the BGP-LS path attribute length?
 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 Node, Link or Prefix
    Descriptor NLRI attribute correspond to the Total NLRI Length
    field of the Node, Link, or Prefix Descriptors?
 o  Does any fixed-length TLV correspond to the TLV Length field in
    this document?

Gredler, et al. Standards Track [Page 39] RFC 7752 Link-State Info Distribution Using BGP March 2016

6.2.3. 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 a pair of
 ASN and BGP-LS identifiers (Section 3.2.1.4) per flooding set in
 which the node participates.

6.2.4. Accounting Management

 Not Applicable.

6.2.5. 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
 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.

Gredler, et al. Standards Track [Page 40] RFC 7752 Link-State Info Distribution Using BGP March 2016

6.2.6. Security Management

 An operator SHOULD define an import policy to limit inbound updates
 as follows:
 o  Drop all updates from consumer peers.
 An implementation MUST have the means to limit inbound updates.

7. 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 3.2.1.2  |
 |           | Descriptors         |              |                  |
 |    257    | Remote Node         |     ---      | Section 3.2.1.3  |
 |           | 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 3.2.1.5  |
 |    264    | OSPF Route Type     |     ---      | Section 3.2.3    |
 |    265    | IP Reachability     |     ---      | Section 3.2.3    |
 |           | Information         |              |                  |
 |    512    | Autonomous System   |     ---      | Section 3.2.1.4  |
 |    513    | BGP-LS Identifier   |     ---      | Section 3.2.1.4  |
 |    514    | OSPF Area-ID        |     ---      | Section 3.2.1.4  |
 |    515    | IGP Router-ID       |     ---      | Section 3.2.1.4  |
 |    1024   | Node Flag Bits      |     ---      | Section 3.3.1.1  |
 |    1025   | Opaque Node         |     ---      | Section 3.3.1.5  |
 |           | Attribute           |              |                  |
 |    1026   | Node Name           |   variable   | Section 3.3.1.3  |
 |    1027   | IS-IS Area          |   variable   | Section 3.3.1.2  |
 |           | Identifier          |              |                  |
 |    1028   | IPv4 Router-ID of   |   134/---    | [RFC5305]/4.3    |
 |           | Local Node          |              |                  |

Gredler, et al. Standards Track [Page 41] RFC 7752 Link-State Info Distribution Using BGP March 2016

 |    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 3.3.2.3  |
 |    1093   | Link Protection     |    22/20     | [RFC5307]/1.2    |
 |           | Type                |              |                  |
 |    1094   | MPLS Protocol Mask  |     ---      | Section 3.3.2.2  |
 |    1095   | IGP Metric          |     ---      | Section 3.3.2.4  |
 |    1096   | Shared Risk Link    |     ---      | Section 3.3.2.5  |
 |           | Group               |              |                  |
 |    1097   | Opaque Link         |     ---      | Section 3.3.2.6  |
 |           | Attribute           |              |                  |
 |    1098   | Link Name           |     ---      | Section 3.3.2.7  |
 |    1152   | IGP Flags           |     ---      | Section 3.3.3.1  |
 |    1153   | IGP Route Tag       |     ---      | [RFC5130]        |
 |    1154   | IGP Extended Route  |     ---      | [RFC5130]        |
 |           | Tag                 |              |                  |
 |    1155   | Prefix Metric       |     ---      | [RFC5305]        |
 |    1156   | OSPF Forwarding     |     ---      | [RFC2328]        |
 |           | Address             |              |                  |
 |    1157   | Opaque Prefix       |     ---      | Section 3.3.3.6  |
 |           | Attribute           |              |                  |
 +-----------+---------------------+--------------+------------------+
          Table 13: Summary Table of TLV/Sub-TLV Code Points

8. Security Considerations

 Procedures and protocol extensions defined in this document do not
 affect the BGP security model.  See the Security Considerations
 section of [RFC4271] for a discussion of BGP security.  Also refer to
 [RFC4272] and [RFC6952] for analysis of security issues for BGP.
 In the context of the BGP peerings associated with this document, a
 BGP speaker MUST NOT accept updates from a consumer peer.  That is, a
 participating BGP speaker should be aware of the nature of its
 relationships for link-state relationships and should protect itself

Gredler, et al. Standards Track [Page 42] RFC 7752 Link-State Info Distribution Using BGP March 2016

 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 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.

9. References

9.1. 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,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
            DOI 10.17487/RFC2328, April 1998,
            <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc3209>.

Gredler, et al. Standards Track [Page 43] RFC 7752 Link-State Info Distribution Using BGP March 2016

 [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,
            <http://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,
            <http://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,
            <http://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,
            <http://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,
            <http://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, <http://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,
            <http://www.rfc-editor.org/info/rfc5120>.
 [RFC5130]  Previdi, S., Shand, M., Ed., and C. Martin, "A Policy
            Control Mechanism in IS-IS Using Administrative Tags",
            RFC 5130, DOI 10.17487/RFC5130, February 2008,
            <http://www.rfc-editor.org/info/rfc5130>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC5301]  McPherson, D. and N. Shen, "Dynamic Hostname Exchange
            Mechanism for IS-IS", RFC 5301, DOI 10.17487/RFC5301,
            October 2008, <http://www.rfc-editor.org/info/rfc5301>.

Gredler, et al. Standards Track [Page 44] RFC 7752 Link-State Info Distribution Using BGP March 2016

 [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
            Engineering", RFC 5305, DOI 10.17487/RFC5305, October
            2008, <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc5340>.
 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, DOI 10.17487/RFC5890, August 2010,
            <http://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, <http://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, <http://www.rfc-editor.org/info/rfc6549>.
 [RFC6822]  Previdi, S., Ed., Ginsberg, L., Shand, M., Roy, A., and D.
            Ward, "IS-IS Multi-Instance", RFC 6822,
            DOI 10.17487/RFC6822, December 2012,
            <http://www.rfc-editor.org/info/rfc6822>.
 [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,
            <http://www.rfc-editor.org/info/rfc7606>.

9.2. Informative References

 [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,
            <http://www.rfc-editor.org/info/rfc1918>.
 [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
            RFC 4272, DOI 10.17487/RFC4272, January 2006,
            <http://www.rfc-editor.org/info/rfc4272>.

Gredler, et al. Standards Track [Page 45] RFC 7752 Link-State Info Distribution Using BGP March 2016

 [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
            Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
            2006, <http://www.rfc-editor.org/info/rfc4364>.
 [RFC4655]  Farrel, A., Vasseur, JP., and J. Ash, "A Path Computation
            Element (PCE)-Based Architecture", RFC 4655,
            DOI 10.17487/RFC4655, August 2006,
            <http://www.rfc-editor.org/info/rfc4655>.
 [RFC5073]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "IGP Routing
            Protocol Extensions for Discovery of Traffic Engineering
            Node Capabilities", RFC 5073, DOI 10.17487/RFC5073,
            December 2007, <http://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,
            <http://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, <http://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, <http://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,
            <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc6952>.

Gredler, et al. Standards Track [Page 46] RFC 7752 Link-State Info Distribution Using BGP March 2016

 [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,
            <http://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, <http://www.rfc-editor.org/info/rfc7770>.

Acknowledgements

 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.

Contributors

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

Gredler, et al. Standards Track [Page 47] RFC 7752 Link-State Info Distribution Using BGP March 2016

Authors' Addresses

 Hannes Gredler (editor)
 Individual Contributor
 Email: hannes@gredler.at
 Jan Medved
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA  95134
 United States
 Email: jmedved@cisco.com
 Stefano Previdi
 Cisco Systems, Inc.
 Via Del Serafico, 200
 Rome  00142
 Italy
 Email: sprevidi@cisco.com
 Adrian Farrel
 Juniper Networks, Inc.
 Email: adrian@olddog.co.uk
 Saikat Ray
 Email: raysaikat@gmail.com

Gredler, et al. Standards Track [Page 48]

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