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Versions: 00 01 02

Inter-Domain Routing                                       K. Talaulikar
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                                K. Swamy
Expires: March 7, 2019                                     Cisco Systems
                                                                S. Zandi
                                                                G. Dawra
                                                                LinkedIn
                                                              M. Durrani
                                                                 Equinix
                                                       September 3, 2018


             BGP Link-State Extensions for BGP-only Fabric
               draft-ketant-idr-bgp-ls-bgp-only-fabric-01

Abstract

   BGP is used as the only routing protocol in some networks today.  In
   such networks, it is useful to get a detailed view of the nodes and
   underlying links in the topology along with their attributes similar
   to one available when using link state routing protocols.  Such a
   view of a BGP-only fabric enables use cases like traffic engineering
   and forwarding of services along paths other than the BGP best path
   selection.

   This document defines extensions to the BGP Link-state address-family
   (BGP-LS) and the procedures for advertisement of the topology in a
   BGP-only network.  It also describes a specific use-case for traffic
   engineering based on Segment Routing.

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].

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 7, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Topology Collection Mechanism . . . . . . . . . . . . . . . .   3
   3.  Advertising BGP-only Network Topology . . . . . . . . . . . .   5
     3.1.  Node Advertisements . . . . . . . . . . . . . . . . . . .   5
     3.2.  Link Advertisements . . . . . . . . . . . . . . . . . . .   6
     3.3.  Prefix Advertisements . . . . . . . . . . . . . . . . . .   8
     3.4.  TE Policy Advertisements  . . . . . . . . . . . . . . . .   9
   4.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Advertisement of Router's Node Attributes . . . . . . . .  10
     4.2.  Advertisement of Router's Local Links Attributes  . . . .  11
     4.3.  Advertisement of Router's Prefix Attributes . . . . . . .  13
     4.4.  Advertisement of Router's TE Policy Attributes  . . . . .  14
   5.  Usage of BGP Topology . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Topology View for Monitoring  . . . . . . . . . . . . . .  15
     5.2.  SR-TE in BGP Networks . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   7.  Manageability Considerations  . . . . . . . . . . . . . . . .  17
     7.1.  Operational Considerations  . . . . . . . . . . . . . . .  17
       7.1.1.  Operations  . . . . . . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20




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1.  Introduction

   Network operators are going for a BGP-only routing protocol for
   certain networks like Massively Scaled Data Centers (MSDCs).  BGP
   MSDC [RFC7938] describes the requirement, design and operational
   aspects for use of BGP as the only routing protocol in MSDCs.  The
   underlying link and topology information between BGP routers is
   hidden or abstracted in this design from the underlay routing for
   improving scalability and stability in a large scale network.  On the
   flip side, there is no detailed topology view similar to one
   available in form of the Traffic Engineering (TE) Database (TED) when
   running link state routing protocols like OSPF [RFC2328] with
   extensions specified in OSPF-TE [RFC3630].

   BGP-LS [RFC7752] enables advertisement of a link state topology via
   BGP that can be consumed by a controller or in general any software
   component to get a complete topology view of the network.  BGP-LS
   extensions for advertisement of a BGP topology for the Egress Peer
   Engineering (EPE) use-case SR Central EPE
   [I-D.ietf-spring-segment-routing-central-epe] are specified in BGP-LS
   EPE [I-D.ietf-idr-bgpls-segment-routing-epe].  This document
   leverages the BGP-LS TLVs defined for BGP-LS EPE and other BGP-LS
   documents and specifies the procedures for advertising the underlying
   topology in a more generic BGP-only fabric use-case.

   This document specifies the operations and procedures when using the
   design involving EBGP single-hop sessions over direct point-to-point
   links connecting the network nodes (refer BGP MSDC [RFC7938] for
   details).  Certain modifications and other considerations are
   required when using a different design using IBGP or EGBP multihop
   and these would be specified in a future version of this document.
   While a data-center design is used as a reference, the procedures for
   topology advertisement may also apply to other networks with BGP-only
   fabric or to BGP-only portions of a larger network topology.

2.  Topology Collection Mechanism

   BGP-LS [RFC7752] has been defined to allow BGP to convey topology
   information in the form of Link-State objects - node, link and
   prefix.  The properties of each of these objects are encoded as BGP-
   LS attributes.  Applications need a topological view and visibility
   even for networks where BGP is the only routing protocol.  In such
   networks, each BGP router advertises its local information which
   includes its node, links and prefix attributes via BGP-LS.

   Figure 1 describes a typical deployment scenario.  Every BGP router
   in the network is enabled for BGP-LS and forms BGP-LS sessions with
   one or more centralized BGP speakers over which it conveys its local



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   topology information.  Each BGP router MAY receive the topology
   information from all other BGP routers via the centralized BGP
   speakers.  This way, any BGP router (as also the centralized BGP
   speakers) MAY obtain aggregated Link-State information for the entire
   BGP network.  An external component (e.g. a controller) can obtain
   this information from the centralized BGP speakers or directly by
   doing BGP-LS peering to the BGP routers.  An internal software
   component on any of the BGP routers (e.g.  TE module) can also
   receive the entire BGP network topology information from its local
   BGP process.

                +------------+
                | Controller |
                +------------+
                      ^
                      |
                      v
             +-------------------+
             |    BGP Speaker    |       +------------+
             |  (Centralized)    |       | Controller |
             +-------------------+       +------------+
                   ^   ^   ^                   ^
                   |   |   |                   |
       +-----------+   |   +---------------+   |
       |               |                   |   |
       v               v                   v   v
  +-----------+    +-----------+         +-----------+     +-----------+
  |    BGP    |    |    BGP    |         |    BGP    | <-->| Local     |
  |  Router   |    |  Router   |  . . .  |  Router   |     | Consumer  |
  +-----------+    +-----------+         +-----------+     +-----------+
       ^                ^                    ^
       |                |                    |
   Local Info       Local Info            Local Info
  (node & links)  (node & links)         (node & links)

                   Figure 1: Link State info collection

   The design described above relies on the base BGP IPv4 or IPv6
   routing underlay or any other mechanism for reachability for the BGP-
   LS session establishment with the centralized BGP speakers.  Another
   alternate design would be to enable BGP-LS as well on the hop by hop
   EBGP sessions in the underlay.  This approach results in the topology
   information being flooded via BGP-LS hop-by-hop along the BGP routers
   in the network.  Other peering designs for BGP-LS sessions may also
   be possible and they are not precluded by this document and may be
   specified in a future version of this document.





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3.  Advertising BGP-only Network Topology

   This sections specifies the BGP-LS TLVs and sub-TLVs and their use
   for advertising the topology of a BGP-only network in the form of
   BGP-LS Node, Link and Prefix NLRIs.

   BGP-LS [RFC7752] defines the BGP-LS NLRI that can be a Node NLRI, a
   Link NLRI or a Prefix NLRI.  BGP-LS EPE
   [I-D.ietf-idr-bgpls-segment-routing-epe] specifies the BGP Protocol
   ID to be used for signaling EPE information and the same is used for
   advertising of BGP topology as well.

   BGP-LS TE [I-D.ietf-idr-te-lsp-distribution] defines the BGP-LS NLRI
   that can be used to advertise the RSVP-TE or Segment Routing (SR)
   policies instantiated on a BGP Router head-end along with their
   properties and state.

   The corresponding BGP-LS attribute is a Node Attribute, a Link
   Attribute or a Prefix Attribute.  BGP-LS [RFC7752] defines the TLVs
   that map link-state information to BGP-LS NLRI and the BGP-LS
   attribute.

3.1.  Node Advertisements

   BGP-LS [RFC7752] defines Node NLRI Type as follows and also defines
   the Node Descriptor TLVs:

     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)            //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   BGP-LS EPE [I-D.ietf-idr-bgpls-segment-routing-epe] introduces
   additional Node Descriptor TLVs for BGP protocol for EPE use-case and
   the same are used by this document.

   The following Node Descriptors TLVs MUST appear in the Node NLRI as
   Local Node Descriptors:

   o  BGP Router-ID, which contains the BGP Identifier of the
      originating BGP router



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   o  Autonomous System Number, which contains the advertising router
      ASN.

   The following Node Attribute TLVs are defined in respective documents
   to signal the router properties and capabilities ( Section 4.1
   defines the procedures for their advertisements):

   +--------+--------------+-------------------------------------------+
   | TLV    | Description  |                        Reference Document |
   | Code   |              |                                           |
   | Point  |              |                                           |
   +--------+--------------+-------------------------------------------+
   | 1026   | Node Name    |                                 [RFC7752] |
   | 1161   | SID/Label    | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
   | 1034   | SRGB &       | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
   |        | Capabilities |                                           |
   | 1036   | SR Local     | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
   |        | Block        |                                           |
   | 266    | Node MSD     | [I-D.ietf-idr-bgp-ls-segment-routing-msd] |
   +--------+--------------+-------------------------------------------+

                       Table 1: Node Attribute TLVs

3.2.  Link Advertisements

   BGP-LS [RFC7752] defines Link NLRI Type as follows and also defines
   the Node and Link Descriptor TLVs used:

     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)                //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The following Node Descriptors TLVs MUST appear in the Link NLRI as
   Local Node Descriptors:





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   o  BGP Router-ID, which contains the BGP Identifier of the
      originating BGP router

   o  Autonomous System Number, which contains the advertising router
      ASN.

   The following Node Descriptors TLVs MUST appear in the Link NLRI as
   Remote Node Descriptors:

   o  BGP Router-ID, which contains the BGP Identifier of the peer BGP
      router

   o  Autonomous System Number, which contains the peer ASN.

   The following Link Descriptors TLVs MUST appear in the Link NLRI as
   Link Descriptors:

   o  Link Local/Remote Identifiers containing the 4-octet Link Local
      Identifier followed by the 4-octet value 0 indicating the Link
      Remote Identifier is unknown

   In addition, the following Link Descriptors TLVs SHOULD appear in the
   Link NLRI as Link Descriptors based on the address family used for
   setting up the BGP Peering:

   o  IPv4 Interface Address contains the address of the local interface
      through which the BGP session is established using IPv4 address.

   o  IPv6 Interface Address contains the address of the local interface
      through which the BGP session is established using IPv6 address.

   o  IPv4 Neighbor Address contains the IPv4 address of the peer
      interface used by the BGP session establishment using IPv4
      address.

   o  IPv6 Neighbor Address contains the IPv6 address of the peer
      interface used by the BGP session establishment using IPv6
      address.

   The following Node Attribute TLVs are defined in respective documents
   which are used to signal the router's local links' properties and
   capabilities ( Section 4.2 defines the procedures for their
   advertisements) :








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   +------+----------------+-------------------------------------------+
   | TLV  | Description    | Reference Document                        |
   | Code |                |                                           |
   | Poin |                |                                           |
   |  t   |                |                                           |
   +------+----------------+-------------------------------------------+
   | 1088 | Administrative | [RFC7752]                                 |
   |      | group (color)  |                                           |
   | 1089 | Maximum link   | [RFC7752]                                 |
   |      | bandwidth      |                                           |
   | 1092 | TE Default     | [RFC7752]                                 |
   |      | Metric         |                                           |
   | 1096 | SRLG           | [RFC7752]                                 |
   | 1098 | Link Name      | [RFC7752]                                 |
   | 267  | Link MSD       | [I-D.ietf-idr-bgp-ls-segment-routing-msd] |
   | 1172 | L2 Bundle      | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
   |      | Member         |                                           |
   | 1104 | Unidirectional | [I-D.ietf-idr-te-pm-bgp]                  |
   |      | link delay     |                                           |
   | 1105 | Min/Max        | [I-D.ietf-idr-te-pm-bgp]                  |
   |      | Unidirectional |                                           |
   |      | link delay     |                                           |
   | 1106 | Min/Max        | [I-D.ietf-idr-te-pm-bgp]                  |
   |      | Unidirectional |                                           |
   |      | link delay     |                                           |
   | 1107 | Unidirectional | [I-D.ietf-idr-te-pm-bgp]                  |
   |      | packet loss    |                                           |
   | 1101 | EPE Peer Node  | [I-D.ietf-idr-bgpls-segment-routing-epe]  |
   |      | SID            |                                           |
   | 1102 | EPE Peer Adj   | [I-D.ietf-idr-bgpls-segment-routing-epe]  |
   |      | SID            |                                           |
   | 1103 | EPE Peer Set   | [I-D.ietf-idr-bgpls-segment-routing-epe]  |
   |      | SID            |                                           |
   +------+----------------+-------------------------------------------+

                       Table 2: Link Attribute TLVs

3.3.  Prefix Advertisements

   BGP-LS [RFC7752] defines Prefix NLRI Type as follows and also defines
   the Node and Prefix Descriptor TLVs used:










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


   The following Node Descriptors TLVs MUST appear in the Node NLRI as
   Local Node Descriptors:

   o  BGP Router-ID, which contains the BGP Identifier of the
      originating BGP router

   o  Autonomous System Number, which contains the advertising router
      ASN.

   The Prefix Descriptor MUST contain the IP Reachability information
   TLV to identify the prefix.

   The following Prefix Attribute TLVs are defined in respective
   documents which are used to signal the router's own prefix properties
   and capabilities ( Section 4.3 defines the procedures for their
   advertisements):

   +---------+-------------+-------------------------------------------+
   | TLV     | Description |                        Reference Document |
   | Code    |             |                                           |
   | Point   |             |                                           |
   +---------+-------------+-------------------------------------------+
   | 1158    | Prefix SID  | [I-D.ietf-idr-bgp-ls-segment-routing-ext] |
   +---------+-------------+-------------------------------------------+

                      Table 3: Prefix Attribute TLVs

3.4.  TE Policy Advertisements

   BGP-LS TE [I-D.ietf-idr-te-lsp-distribution] defines TE Policy NLRI
   Type as follows and also defines the Headend Node and TE Policy
   Descriptor TLVs used:





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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+
    |  Protocol-ID  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Identifier                             |
    |                        (64 bits)                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                Headend (Node Descriptors)                   //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                TE Policy Descriptors (variable)             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The Node Descriptors TLVs are the same as specified in Section 3.1.
   The semantics for the TE Policy Descriptor TLVs and the its Attribute
   TLVs are used as specified in BGP-LS TE
   [I-D.ietf-idr-te-lsp-distribution].

4.  Procedures

   In a network where BGP is the only routing protocol, the BGP-LS
   session is used to advertise the necessary information about the
   local node properties, its local links' properties and where
   necessary the prefix's owned by the node.  TE Policies, that are
   instantiated on the local node (i.e. when it is the head-end for the
   policy), along with their properties are also advertised via the BGP-
   LS session.  This information, once collected across all BGP routers
   in the network, provides a complete topology view of the network.
   Many of these attributes are not part of the base BGP protocol
   operations and are either configured or provided by other components
   on the router.  BGP-LS performs the role of collecting this
   information and propagating it across the BGP network.

   The following sections describe the procedures for the propagation of
   the BGP-LS NLRIs on a BGP router into the BGP-LS session.  These
   procedures for propagation of BGP topology information via BGP-LS
   SHOULD be applied only in deployments and use-cases where necessary
   and SHOULD NOT be applied in every BGP deployment when BGP-LS is
   enabled.  Implementations MAY provide a configuration option to
   enable these procedures in required deployments.

4.1.  Advertisement of Router's Node Attributes

   Advertisement of the Node NLRI via BGP-LS by each BGP router in a
   BGP-only network enables the discovery of all the router nodes in the
   topology.  The Node NLRI MUST be generated by a BGP router only for




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   itself and even when there are no attributes to be advertised along
   with it.

   The Node attributes defined currently related to SEGMENT ROUTING
   [I-D.ietf-spring-segment-routing] have been described in Table 1 and
   are to be advertised when SR is enabled.  This includes:

   o  The Segment Routing Global Block (SRGB) provisioned on the router
      which is used by BGP for BGP SR [I-D.ietf-idr-bgp-prefix-sid] and
      other SR control plane protocols on the router MUST be advertised.
      The value for Flags field in the TLV is not defined for BGP
      protocol and MUST be set to 0 by the originator and ignored by
      receivers.

   o  The Segment Routing Local Block (SRLB) provisioned on the router
      which may be used by BGP for BGP-LS EPE
      [I-D.ietf-idr-bgpls-segment-routing-epe] for BGP Peering SIDs
      SHOULD be advertised.  The value for Flags field in the TLV is not
      defined for BGP protocol and MUST be set to 0 by the originator
      and ignored by receivers.

   o  The Node level MSD provides the Node's capabilities for SR SID
      operations and SHOULD be advertised.

   The Node Name Attribute SHOULD be advertised when available.

   This document introduces some of the TE concepts into BGP-only
   networks.  However, the advertisement and need for provisioning of a
   TE Router-ID is not required.  The BGP Router-ID along with the ASN
   provides similar capability for uniquely identifying a BGP router in
   the network.

   Other Node Attributes applicable to a BGP Router may also be included
   and this document does not describe the exhaustive list.

4.2.  Advertisement of Router's Local Links Attributes

   Each BGP router in a BGP-only network also advertises its local links
   using the Link NLRIs thru BGP-LS.  The Link NLRI for a given link
   between two BGP routers is advertised as uni-directional logical
   "half-link" and its link descriptors allow the correlation between
   the two NLRIs "half-links" originated by the peering routers to
   describe the bi-directional logical link and its attributes on both
   routers.

   A Link NLRI MUST be generated by a BGP router for each of its local
   link over which it is establishing a BGP session to its neighbors.
   The Link NLRI MUST be generated even when there are no link



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   attributes to be advertised for it.  The Link NLRI represents a
   peering adjacency between BGP routers and its association with the
   underlying Layer 3 link.  When the underlying Layer 3 link or the BGP
   session on top of it goes down, the Link NLRI MUST be withdrawn by
   the BGP router.  Advertisement of the Link NLRIs via BGP-LS by each
   BGP router in a BGP-only network enables the discovery of all the
   active links in the topology.

   The monitoring of links, detecting of their failures and notification
   to BGP may be performed using mechanisms like BFD.  When failures are
   detected and the BGP session over it goes down, then the
   corresponding Link NLRI is also withdrawn.  This enables faster
   detection of failures and verification of the underlying links.

   The discovery of all the links in the BGP-only network relies on the
   design that uses EBGP sessions over each interconnecting link using
   the link IP addresses (refer BGP MSDC [RFC7938]).  When doing EBGP
   multi-hop sessions between directly connected BGP routers, the
   underlying link information would need to learn by some discovery
   protocol or provisioning entity.  The mechanisms to learn the
   underlying link information for BGP-LS advertisements are outside the
   scope of this document.  However, to provide a true link topology
   picture, the advertisement of underlying links is RECOMMENDED for
   most use-cases instead of a single EBGP peering representation of a
   link between the routers.

   TE attributes for links have been traditionally associated with Link
   State Routing protocols.  However, with the ability to discover the
   link topology via BGP-LS as specified in this document, the TE
   attributes and their principles can also be applied to a network
   running BGP alone.  The TE attributes for a link have been described
   in Table 2 and are to be advertised when TE use-cases are enabled.
   This includes:

   o  The maximum bandwidth of a link is its protocol independent
      attribute and SHOULD be advertised.

   o  TE concepts of Administrative Groups (also known as affinities)
      and Shared Risk Link Groups (SRLGs) MAY be provisioned locally on
      links and then MUST be advertised.

   o  The BGP base protocol does not operate with link metrics, however,
      a TE metric concept can be introduced in a BGP only network as
      well for TE use-cases.  Implementations MAY provide the ability to
      provision TE metric value for a link for BGP use including a
      different default value for it.  The TE metric attribute SHOULD be
      advertised for each link when configured and its default value is




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      taken as 1.  When not advertised for a link, implementations who
      intend to use the TE metric MUST assume the value to be 1.

   o  The delay and loss TE metrics for links are measured via MPLS
      PerfMon [RFC6374] and their measurement mechanism over a link are
      independent of the routing protocol.  The same mechanism MAY be
      enabled in BGP-only networks and their values advertised via BGP-
      LS.

   The Link attributes defined currently related to the Segment Routing
   feature BGP-LS EPE [I-D.ietf-idr-bgpls-segment-routing-epe] have been
   described in Table 2 and are to be advertised when SR use-cases are
   enabled.  This includes:

   o  The BGP Peering SIDs provide a functionality similar to Adjacency-
      SID (refer SEGMENT ROUTING [I-D.ietf-spring-segment-routing] ) in
      BGP-only networks.  Implementations SHOULD allocate the BGP Peer-
      Adjacency SID for all its links and the BGP Peer-Node SID for all
      its peer routers.  Implementations MAY allocate the BGP Peer-Set
      SID based on local configuration.

   o  The Link level MSD provides the per link capabilities for SR SID
      operations and SHOULD be advertised when the router links have
      differing capabilities.

   The use of Layer 3 bundle links which comprise of multiple layer 2
   member links are often used in BGP networks.  When BGP session is
   configured over such a layer 3 link, the link attributes of the
   underlying layer 2 links MAY be advertised individually using the L2
   Bundle Member TLV.  The applicable attributes for the L2 links are
   described in BGP-LS SR [I-D.ietf-idr-bgp-ls-segment-routing-ext] .

   The Link Name Attribute MAY be advertised when available.

   Other Link Attributes applicable to a BGP Router may also be included
   and this document does not describe the exhaustive list.

4.3.  Advertisement of Router's Prefix Attributes

   Advertisement of the Prefix NLRI via BGP-LS is required only in
   specific use-cases.  Since the base BGP protocol along with its
   extensions already signals Prefix reachability via different NLRIs,
   there is no necessity to always duplicate the information via BGP-LS
   session.  However, for specific use-cases related to SR Traffic
   Engineering (SR-TE), it is required for each router to advertise it's
   Prefix SID(s) (refer SEGMENT ROUTING
   [I-D.ietf-spring-segment-routing]) that can be used to direct traffic
   via specific BGP routers.  Advertising such BGP Prefix SID for every



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   BGP router provides this key attribute via BGP-LS and avoids the
   requirement for the consumer of the topology information (e.g. a
   controller or local TE process) to tap into other BGP NLRI
   information.

   Advertisement of the Prefix NLRI via BGP-LS MUST be done only for its
   locally configured prefixes (e.g. its loopback interface address) and
   when BGP is advertising the BGP Prefix SID (BGP SR
   [I-D.ietf-idr-bgp-prefix-sid]) for it.

   The Prefix attributes defined currently related to SEGMENT ROUTING
   [I-D.ietf-spring-segment-routing] have been described in Table 3 and
   are to be advertised when SR is enabled.  This includes:

   o  The BGP Prefix SID provisioned on the router for the prefix MUST
      be advertised.  The SID MUST be advertised as the index to be
      consistent with the Label-Index TLV of BGP Prefix SID attribute.
      The algorithm is not defined for BGP and MUST be set to 0 by the
      originator and ignored by the receiver.  The flags are defined as
      the most significant 8 bits of the 16 bit field defined for Label-
      Index TLV in BGP SR [I-D.ietf-idr-bgp-prefix-sid].

   Other Prefix Attributes applicable may also be included and this
   document does not describe the exhaustive list.

4.4.  Advertisement of Router's TE Policy Attributes

   TE Policies that are setup using RSVP-TE or SR-TE mechanisms MAY be
   instantiated on a BGP router.  One use-case that results in such SR
   Policy instantiation on a BGP router is described later in this
   document in Section 5.2.  Advertising such TE Policies instantiated
   for every BGP router as head-end via BGP-LS provides the consumer of
   the topology information (e.g. a controller or local TE process) a
   policy view of the BGP fabric as well.

   The procedures for advertisement of the TE Policy NLRI via BGP-LS
   MUST be done only for its locally instantiated TE Policies and as
   specified in BGP-LS TE [I-D.ietf-idr-te-lsp-distribution]).
   Implementation MAY provide configuration options to control the
   specific set of TE Policies that are to be advertised from the local
   node.

5.  Usage of BGP Topology

   This section describes some of the use-cases for the building of the
   BGP topology information as specified in this document and leveraging
   it for enabling new functionality.




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5.1.  Topology View for Monitoring

   The BGP-LS advertisement of the BGP topology as specified in this
   document provides a live topology view of the BGP network for an
   application or controller that is monitoring the network.  The
   topology view is from the BGP protocol perspective and includes the
   underlying links as well that aids in network monitoring as well as
   diagnostics use-cases.  BGP-LS is the de-facto protocol for
   northbound propagation of network topology related information for
   most IGP networks and extending this capability for BGP-only networks
   allows existing controllers and applications to consume the
   information with some incremental BGP protocol awareness.

5.2.  SR-TE in BGP Networks

   The SR-TE use-case for BGP builds on top of the BGP SR
   [I-D.ietf-idr-bgp-prefix-sid] functionality and also described in BGP
   SR MSDC [I-D.ietf-spring-segment-routing-msdc].The BGP SR Prefix SID
   signaled provides the basic connectivity between all BGP routers
   using their loopback addresses.  This provides the basic best-effort
   paths in the network using the base BGP decision process that is
   unchanged.  BGP and other overlay routes and services recurse on top
   of these loopback addresses of the egress nodes and the forwarding is
   done via the BGP SR Prefix SID labels in the underlay.  While this
   version of the document focuses on the examples with MPLS dataplane
   instantiation for SR, the same is applicable for the IPv6 dataplane
   instantiation (SRv6) as well.

   SR-TE for BGP provides underlay paths through the network for the
   overlay routes and services with specific SLA requirements and use-
   cases like path disjointness, low latency paths, inclusion or
   exclusion and other TE considerations.

   SR-TE [I-D.ietf-spring-segment-routing-policy] specifies the SR-TE
   architecture and the SR Policy construct.  The BGP SR-TE
   [I-D.ietf-idr-segment-routing-te-policy] describes the extensions to
   BGP for signaling of SR Policies from a controller to the SR-TE
   headend BGP router.  BGP-LS has been extended to allow signaling of
   the SR Policies from SR-TE head-end to controllers via BGP-LS SR-TE
   [I-D.ietf-idr-te-lsp-distribution] which allows the controllers to
   learn the state of SR Policies instantiated on routers in the
   network.  This document completes the missing piece that is related
   to getting the BGP topology information from all the routers to a
   controller as well the local SRTE process on each router for their
   path computation requirements.

   The signaling of SR Polices from controller to SR-TE headend and
   reporting of the state back to the controller can also be done using



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   PCEP (PCEP SR [I-D.ietf-pce-segment-routing], PCE Initiated
   [RFC8281], PCEP Stateful [RFC8231]).  However, the BGP topology
   learning via BGP-LS which is specified in this document is also
   required for the deployments that uses PCEP in the BGP-only network.

   The topology collected via BGP-LS in a BGP-only fabric in a Segment
   Routing deployment comprise of:

   o  The properties of every BGP router node and the Prefix SID to
      reach that node.

   o  The properties of all the links between the BGP routers and the
      Peer-Adjacency-SIDs (and other EPE SIDs) corresponding to them
      that allow directing traffic over specific links and/or to
      specific neighbors.

   o  The properties and state of the SR Policies instantiatied on each
      of the BGP routers along with their end points, their properties
      and most importantly the Binding SID to steer traffic into the SR
      Policies.

   This topology information allows a computation node to build SR
   Policies for services over the BGP fabric for a given traffic
   engineering objective at any given node.

   The topology of the BGP fabric is distributed to a centralized
   controller or application for use-cases that need a centralized
   computation of SR Policy which can then be signaled to the SR-TE
   head-end node via PCEP or BGP-SRTE.  The topology is also available
   at any node in the BGP fabric to be used by its local SR-TE process
   to perform path computation for its own SR Policies for use-cases
   that are addressed by local computation.

   A high level summary of the key topology information advertised via
   BGP-LS by BGP routers can be used for TE computations as follows

   o  The BGP SR Prefix SIDs and the BGP EPE Peering Adjacency SIDs
      provide the equivalent of the IGP Prefix and Adjacency SIDs and
      can be used to direct traffic to a specific BGP router and over a
      specific BGP peer session or link respectively.  Traffic for the
      BGP SR Prefix SIDs follow the path computed by the BGP decision
      process.

   o  The TE administrative group (also known as affinities) and SRLG
      attributes can be configured over links to enable computation of
      paths with inclusion and exclusion of specific links or paths that
      are mutually disjoint.




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   o  The enabling of link delay and loss measurements and their
      advertisements can help monitoring the link quality and carve out
      paths based on latency and other SLA requirements.

   o  The signaling of the Node and Link MSD allows controllers to
      instantiate SR Policies based on the capability of the routers.

   This section attempts to highlight and describe at a high level some
   of the possible SR-TE solutions and use-cases in a BGP-only network.
   Further details on these use-cases can be found in SR-TE
   [I-D.ietf-spring-segment-routing-policy].

6.  IANA Considerations

   None

7.  Manageability Considerations

   This section is structured as recommended in [RFC5706].

7.1.  Operational Considerations

7.1.1.  Operations

   Existing BGP and BGP-LS operational procedures apply.  No additional
   operation procedures are defined in this document.

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.

9.  Acknowledgements

10.  References

10.1.  Normative References

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-08
              (work in progress), May 2018.






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   [I-D.ietf-idr-bgp-ls-segment-routing-msd]
              Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
              "Signaling MSD (Maximum SID Depth) using Border Gateway
              Protocol Link-State", draft-ietf-idr-bgp-ls-segment-
              routing-msd-02 (work in progress), August 2018.

   [I-D.ietf-idr-bgp-prefix-sid]
              Previdi, S., Filsfils, C., Lindem, A., Sreekantiah, A.,
              and H. Gredler, "Segment Routing Prefix SID extensions for
              BGP", draft-ietf-idr-bgp-prefix-sid-27 (work in progress),
              June 2018.

   [I-D.ietf-idr-bgpls-segment-routing-epe]
              Previdi, S., Filsfils, C., Patel, K., Ray, S., and J.
              Dong, "BGP-LS extensions for Segment Routing BGP Egress
              Peer Engineering", draft-ietf-idr-bgpls-segment-routing-
              epe-15 (work in progress), March 2018.

   [I-D.ietf-idr-te-lsp-distribution]
              Previdi, S., Talaulikar, K., Dong, J., Chen, M., Gredler,
              H., and J. Tantsura, "Distribution of Traffic Engineering
              (TE) Policies and State using BGP-LS", draft-ietf-idr-te-
              lsp-distribution-09 (work in progress), June 2018.

   [I-D.ietf-idr-te-pm-bgp]
              Ginsberg, L., Previdi, S., Wu, Q., Tantsura, J., and C.
              Filsfils, "BGP-LS Advertisement of IGP Traffic Engineering
              Performance Metric Extensions", draft-ietf-idr-te-pm-
              bgp-10 (work in progress), March 2018.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

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

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






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

10.2.  Informative References

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Jain, D., Mattes, P., Rosen,
              E., and S. Lin, "Advertising Segment Routing Policies in
              BGP", draft-ietf-idr-segment-routing-te-policy-04 (work in
              progress), July 2018.

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-12 (work in progress), June
              2018.

   [I-D.ietf-spring-segment-routing-central-epe]
              Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
              Afanasiev, "Segment Routing Centralized BGP Egress Peer
              Engineering", draft-ietf-spring-segment-routing-central-
              epe-10 (work in progress), December 2017.

   [I-D.ietf-spring-segment-routing-msdc]
              Filsfils, C., Previdi, S., Dawra, G., Aries, E., and P.
              Lapukhov, "BGP-Prefix Segment in large-scale data
              centers", draft-ietf-spring-segment-routing-msdc-09 (work
              in progress), May 2018.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
              bogdanov@google.com, b., and P. Mattes, "Segment Routing
              Policy Architecture", draft-ietf-spring-segment-routing-
              policy-01 (work in progress), June 2018.

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

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <https://www.rfc-editor.org/info/rfc3630>.





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

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

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374,
              DOI 10.17487/RFC6374, September 2011,
              <https://www.rfc-editor.org/info/rfc6374>.

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

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

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

Authors' Addresses

   Ketan Talaulikar
   Cisco Systems
   Pune  411057
   India

   Email: ketant@cisco.com






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   Clarence Filsfils
   Cisco Systems
   Brussels
   Belgium

   Email: cfilsfil@cisco.com


   Krishna Swamy
   Cisco Systems
   San Jose
   USA

   Email: kriswamy@cisco.com


   Shawn Zandi
   LinkedIn
   USA

   Email: szandi@linkedin.com


   Gaurav Dawra
   LinkedIn
   USA

   Email: gdawra.ietf@gmail.com


   Muhammad Durrani
   Equinix
   USA

   Email: mdurrani@equinix.com
















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