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BGP Transport VPNs

Document Type Replaced Internet-Draft (individual)
Authors Kaliraj Vairavakkalai , Natrajan Venkataraman , Balaji Rajagopalan
Last updated 2020-03-07
Replaced by draft-kaliraj-idr-bgp-classful-transport-planes
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Network Working Group                                   K. Vairavakkalai
Internet-Draft                                           N. Venkataraman
Intended status: Standards Track                          B. Rajagopalan
Expires: September 8, 2020                        Juniper Networks, Inc.
                                                          March 07, 2020

                           BGP Transport VPNs


   This document specifies a mechanism, referred to as "service
   mapping", to express association of overlay routes with underlay
   routes using BGP.  The document describes a framework for service
   mapping, and specifies BGP protocol procedures that enable
   dissimination of the service mapping information that may span across
   administrative domains.  It makes it possible to advertise multiple
   tunnels to the same destination.

   A new BGP transport address family is defined for this purpose that
   uses BGP-VPN [RFC4364] technology and follows MPLS-BGP [RFC8277] NLRI
   encoding.  This new address family is called "Transport-VPN".

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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

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

   This Internet-Draft will expire on September 8, 2020.

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Copyright Notice

   Copyright (c) 2020 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
   ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Transport Class . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Transport RIB . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Transport Routing Instance  . . . . . . . . . . . . . . . . .   6
   6.  Nexthop Resolution Scheme . . . . . . . . . . . . . . . . . .   6
   7.  BGP Transport-VPN Family NLRI . . . . . . . . . . . . . . . .   7
   8.  Comparison with other families using RFC-8277 encoding  . . .   7
   9.  Protocol Procedures . . . . . . . . . . . . . . . . . . . . .   8
   10. OAM considerations  . . . . . . . . . . . . . . . . . . . . .  10
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     14.2.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   To facilitate service mapping, the tunnels in a network can be
   grouped by the purpose they serve into a "Transport Class".  The
   tunnels could be created using any signaling protocol, such as LDP,
   RSVP, BGP-LU or SPRING.  The tunnels could also use native IP or
   IPv6, as long as the tunnels can carry MPLS payload.  Tunnels may
   exist between different pair of end points.  Multiple tunnels may
   exist between the same pair of end points.

   Thus, a Transport Class consists of tunnels created by many protocols
   terminating in various nodes, each satisfying the properties of the
   class.  For example, a "Gold" transport class may consist of tunnels

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   that traverse the shortest path with fast re-route protection, a
   "Silver" transport class may hold tunnels that traverse shortest
   paths without protection, a "To NbrAS Foo" transport class may hold
   tunnels that exit to neighboring AS Foo, and so on.

   The extensions specified in this document can be used to create a BGP
   transport tunnel that potentially spans domains, while preserving its
   Transport Class.  Examples of domain are Autonomous System (AS), or
   IGP area.  Within each domain, there is a second level underlay
   tunnel used by BGP to cross the domain.  The second level underlay
   tunnels could be hetrogeneous: Each domain may use a different type
   of tunnel, or use a differnet signaling protocol.  A domain boundary
   is demarcated by a rewrite of BGP nexthop to 'self' while re-
   advertising tunnel routes in BGP.  The path uses MPLS label-switching
   when crossing inter-AS links and uses the native intra-AS tunnel of
   the desired transport class when traversing within a domain.

   Overlay routes carry sufficient indication of the Transport Class
   they should be encapsulated over.  A "route resolution" procedure on
   the ingress node selects from the Transport Class an appropriate
   tunnel whose destination matches the nexthop of the overlay route.
   If the overlay route is carried in BGP, the protocol nexthop (or,
   PNH) is generally carried as an attribute of the route.  The PNH of
   the overlay route is also referred to as "service endpoint".  The
   service endpoint may exist in the same domain as the service ingress
   node or lie in a different domain, adjacent or non-adjacent.

   This document describes mechanisms to:

      Model a "Transport Class" as "Transport RIB" on a router,
      consisting of tunnel ingress routes of a certain class.

      Enable service routes to resolve over an intended Transport Class
      by using the corresponding Transport RIB for finding nexthop

      Advertise tunnel ingress routes in a Transport RIB via BGP without
      any path hiding, using BGP VPN technology and Add-path.  Such that
      overlay routes in the receiving domains can also resolve over
      tunnels of associated Transport Class.

      Provide a way for co-operating domains to reconcile between
      independently administered extended community namespaces, and
      interoperate between different transport signaling protocols in
      each domain.

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   In this document we focus mainly on MPLS LSPs as transport tunnels,
   but the mechanisms would work in similar manner for non-MPLS
   transport tunnels too, provided the tunnel can carry MPLS payload.

2.  Terminology

   LSP: Label Switched Path

   TE : Traffic Engineering

   SN : Service Node

   BN : Border Node

   TN : Transport Node, P-router

   BGP-VPN : VPNs built using RFC4364 mechanisms

   RT : Route-Target extended community

   RD : Route-Distinguisher

   PNH : Protocol-Nexthop

   Service Family : BGP address family used for advertising routes for
   "data traffic", as opposed to tunnels

   Transport Family : BGP address family used for advertising tunnels,
   which are in turn used by service routes for resolution

   Transport Tunnel : A tunnel over which a service may place traffic.
   These tunnels can be GRE, UDP, LDP, RSVP, or SR-TE

   Tunnel Domain : A domain of the network containing SN and BN, under a
   single administrative control that has a tunnel between SN and BN.
   An end-to-end tunnel spanning several adjacent tunnel domains can be
   created by "stitching" them together using labels.

   Transport Class : A group of transport tunnels offering the same type
   of service.

   Transport Class RT : A BGP-VPN Route-Target used to identify a
   specific Transport Class

   Transport RIB : At the SN and BN, a Transport Class has an associted
   Transport RIB that holds its tunnel routes.

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   Transport RTI : A Routing Instance; container of Transport RIB, and
   associated Transport Class RT and RD.

   Transport-VPN : Set of Transport RTIs importing same Transport Class
   RT.  These are inturn stitched together to span across tunnel domain
   boundaries using a mechanism similar to Inter-AS option-b to swap
   labels at BN (nexthop-self).

   Mapping Community : Community on a service route, that maps it to
   resolve over a Transport Class

3.  Transport Class

   A Transport Class is defined as a set of transport tunnels that share
   certain characteristics useful for underlay selection.

   On the wire, a transport class is represented as the Transport Class
   RT, which is a regular Route-Target extended community.

   A Transport Class is configured at SN and BN, along with attributes
   like RD and Route-Target.  Creation of a Transport Class instantiates
   the associated Transport RIB and a Transport routing instance to
   contain them all.

   The operator may configure a BN to classify a tunnel into an
   appropriate Transport Class, which causes the tunnel's ingress routes
   to be installed in the corresponding Transport RIB.  These tunnel
   routes may then be advertised into BGP.

   Alternatively, a router receiving the transport routes in BGP with
   appropriate signaling information can associate those ingress routes
   to the appropriate Transport Class.  E.g. for Transport-VPN
   family(SAFI TBD) routes, the Transport Class RT indicates the
   Transport Class.  For BGP-LU family(SAFI 4) routes, import policy
   based on Communities or inter-AS source-peer may be used to place the
   route in the desired Transport Class.

   When the ingress route is received via SRTE [SRTE], which encodes the
   Transport Class as an integer "Color" in the NLRI as
   "Color:Endpoint", the Color can be mapped to a Transport Class during
   import processing.  The Color could map to a Community, or Route-
   Target that installs the ingress route for "Endpoint" in the
   appropriate Transport RIB.  The SRTE route when advertised out to BGP
   speakers will then be advertised in Transport-VPN family with
   Transport Class RT and a new label.  The MPLS swap route thus
   installed for the new label will pop the label and deliver
   decapsulated-traffic into the path determined by SRTE route.

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4.  Transport RIB

   A Transport RIB is a routing-only RIB that is not installed in
   forwarding path.  However, the routes in this RIB are used to resolve
   reachability of overlay routes' PNH.  Transport RIB is created when
   the Transport Class it represents is configured.

   Overlay routes that want to use a specific Transport Class confine
   the scope of nexthop resolution to the set of routes contained in the
   corresponding Transport RIB.  This Transport RIB is the "Routing
   Table" referred in Section RFC4271 [1]

   Routes in a Transport RIB are exported out in 'Transport-VPN' address

5.  Transport Routing Instance

   A BGP VPN routing instance that is a container for the Transport
   RIBs.  It imports, and exports routes in this RIB with Transport
   Class RT.  Tunnel destination addresses in this routing instance's
   context come from the "provider namespace".  This is different from
   user VRFs for e.g., which contain prefixes in "customer namespace"

   The Transport Routing instance uses the RD and RT configured for the
   Transport Class.

6.  Nexthop Resolution Scheme

   An implementation may provide an option for the service route to
   resolve over less preferred Transport Classes, should the resolution
   over preferred, or "primary" Transport Class fail.

   To accomplish this, the set of service routes may be associated with
   a user-configured "resolution scheme", which consists of the primary
   Transport Class, and an ordered list of fallback Transport Classes.

   A community called as "Mapping Community" is configured for a
   "resolution scheme".  A Mapping community maps to exactly one
   resolution scheme.

   When a resolution scheme comprises of a primary Transport Class
   without any fallback, the Transport Class RT associated with the
   primary Transport Class is used as the Mapping Community.

   A BGP service route is associated with a resolution scheme during
   import processing.  The import processing matches against "Mapping
   Community" on the service route and determines the resolution scheme
   that should be used when resolving the route's PNH.  If the route

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   contains more than one Mapping Communities, the first one mapping to
   a resolution scheme is chosen.

   A transport route received in BGP Transport-VPN family should use a
   resolution scheme that contains only the primary Transport Class
   without any fallbacks.  The primary Transport Class is identified by
   the Transport Class RT carried on the route.  Thus Transport Class RT
   serves as the Mapping Community for Transport-VPN routes.

7.  BGP Transport-VPN Family NLRI

   The Transport-VPN family will use the existing AFI of IPv4 or IPv6,
   and a new SAFI TBD "Transport-VPN" that will apply to both IPv4 and
   IPv6 AFIs.

   The "Transport-VPN" SAFI NLRI itself is encoded as specified in [RFC8277].

   When AFI is IPv4 the "Prefix" portion of Transport-VPN family NLRI
   consists of an 8-byte RD followed by an IPv4 prefix.  When AFI is
   IPv6 the "Prefix" consists of an 8-byte RD followed by an IPv6

   Attributes on a Transport-VPN route include the Route-Target extended
   community, which is used to leak the route into the right Transport
   RIBs on SNs and BNs in the network.

8.  Comparison with other families using RFC-8277 encoding

   SAFI 128 (Inet-VPN) is a RF8277 encoded family that carries service
   prefixes in the NLRI, where the prefixes come from the customer
   namespaces, and are contexualized into separate user virtual service
   RIBs called VRFs, using RFC4364 procedures.

   SAFI 4 (BGP-LU) is a RFC8277 encoded family that carries transport
   prefixes in the NLRI, where the prefixes come from the provider

   SAFI TBD (Transport-VPN) is a RFC8277 encoded family that carries
   transport prefixes in the NLRI, where the prefixes come from the
   provider namespace, but are contexualized into separate Transport
   RIBs, using RFC4364 procedures.

   It is worth noting that SAFI 128 has been used to carry transport
   prefixes in "L3VPN Inter-AS Carrier's carrier" scenario, where BGP-
   LU/LDP prefixes in CsC VRF are advertised in SAFI 128 to the remote-
   end baby carrier.

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   In this document a new AFI/SAFI is used instead of reusing SAFI 128
   to carry these transport routes, because it is operationally
   advantageous to segregate transport and service prefixes into
   separate address families, RIBs.  E.g.  It allows to safely enable
   "per-prefix" label allocation scheme for Transport-VPN prefixes
   without affecting SAFI 128 service prefixes which may have huge
   scale. "per prefix" label allocation scheme keeps the routing churn
   local during topology changes.  A new family also facilitates having
   a different readvertisement path of the transport family routes in a
   network than the service route readvertisement path. viz. Service
   routes are exchanged over an EBGP multihop sessions between
   Autonomous systems with nexthop unchanged; whereas Transport-VPN
   routes are readvertised over EBGP single hop sessions with "nexthop-
   self" rewrite over inter-AS links.

   The Transport-VPN family is similar in vein to BGP-LU, in that it
   carries transport prefixes.  The only difference is, it also carries
   in Route Target an indication of which Transport Class the transport
   prefix belongs to, and uses RD to disambiguate multiple instances of
   the same transport prefix in a BGP Update.

9.  Protocol Procedures

   This section summarizes the procedures followed by various nodes
   speaking Transport-VPN family

   Preparing the network for deploying Transport-VPNs

      Operator decides on the Transport Classes that exist in the
      network, and allocates a Route-Target to identify each Transport

      Operator configures Transport Classes on the SNs and BNs in the
      network with unique Route-Distinguishers and Route-Targets.

      Implementations may provide automatic generation and assignment of
      RD, RT values for a transport routing instance; they should also
      provide a way to manually override the automatic mechanism, in
      order to deal with any conflicts that may arise with existing RD,
      RT values in the network.

   Origination of Transport-VPN route:

      At the ingress node of the tunnel's egress domain, the tunneling
      protocols install routes in the Transport RIB associated with the
      Transport Class the tunnel belongs to.  The ingress node then
      advertises this tunnel route into BGP as a Transport-VPN route

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      with NLRI RD:TunnelEndpoint, attaching a Route-Target that
      identifies the Transport Class.

      Alternatively, the egress node of the tunnel i.e. the tunnel
      endpoint can originate the BGP Transport-VPN route, with NLRI
      RD:TunnelEndpoint and PNH TunnelEndpoint, which will resolve over
      the tunnel route at the ingress node.  When the tunnel is up, the
      Transport-VPN route will become usable and get re-advertised.

      Unique RD is used by the originator of a Transport-VPN route to
      disambiguate the multiple BGP advertisements for a transport end

   Ingress node receiving Transport-VPN route

      On receiving a BGP Transport-VPN route with a PNH that is not
      directly connected, e.g. an IBGP-route, the Route-Target on the
      route indicates which Transport Class this route belongs to.  The
      routes in the associated Transport RIB are used to resolve the
      received PNH.  If there does not exist a route in the Transport
      RIB for the PNH, the Transport-VPN route is considered unusable,
      and MUST not be re-advertised further.

   Border node readvertising Transport-VPN route with nexthop self:

      The BN allocates an MPLS label to advertise upstream in Transport-
      VPN NLRI.  The BN also installs an MPLS swap-route for that label
      that swaps the incoming label with a label received from the
      downstream BGP speaker, or pops the incoming label.  And then
      pushes received traffic to the transport tunnel or direct
      interface that the Transport-VPN route's PNH resolved over.

   Border node receiving Transport-VPN route on EBGP :

      If the route is received with PNH that is known to be directly
      connected, e.g.  EBGP single-hop peering address, the directly
      connected interface is checked for MPLS forwarding capability.  No
      other nexthop resolution process is performed, as the inter-AS
      link can be used for any Transport Class.

      If the inter-AS links should honor Transport Class, then the BN
      should follow procedures of an Ingress node described above, and
      perform nexthop resolution process.  The interface routes should
      be installed in the Transport RIB belonging to the associated
      Transport Class.

   Avoiding path-hiding through Route Reflectors

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      When multiple BNs exist that advertise a RDn:PEn prefix to RRs,
      the RRs may hide all but one of the BNs, unless ADDPATH [RFC7911]
      is used for the Transport-VPN family.  This is similar to L3VPN
      option-B scenarios.  Hence ADDPATH should be used for Transport-
      VPN family, to avoid path-hiding through RRs.

   Ingress node receiving service route with mapping community

      Service routes received with mapping community resolve using
      Transport RIBs determined by the resolution scheme.  If the
      resolution process does not find an usable Transport-VPN route or
      tunnel route in any of the Transport RIBs, the service route MUST
      be considered unusable for forwarding purpose.

   Coordinating between domains using different community namespaces.

      Domains not agreeing on RT, RD, Mapping-community values because
      of independently administered community namespaces may deploy
      mechanisms to map and rewrite the Route-target values on domain
      boundaries, using per ASBR import policies.  This is no different
      than any other BGP VPN family.  Mechanisms employed in inter-AS
      VPN deployments may be used with the Transport-VPN family also.

      Though RD can also be rewritten on domain boundaries, deploying
      unique RDs is strongly recommended, because it helps in trouble
      shooting by uniquely identifying originator of a route, and avoids

      Future versions of this document may define a new format of Route-
      Target extended-community to carry Transport Class, to avoid
      collision with regular Route Target namespace used by service

10.  OAM considerations


11.  IANA Considerations

   This document makes following requests of IANA.

   New BGP SAFI code for "Transport-VPN".  Value TBD.

   This will be used to create new AFI,SAFI pairs for IPv4, IPv6
   Transport-VPN families. viz:

   o  "Inet, Transport-VPN".  AFI/SAFI = "1/TBD" for carrying IPv4
      Transport-VPN prefixes.

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   o  "Inet6, Transport-VPN".  AFI/SAFI = "2/TBD" for carrying IPv6
      Transport-VPN prefixes.

   Note to RFC Editor: this section may be removed on publication as an

12.  Security Considerations

   Mechanisms described in this document carry Transport routes in a new
   BGP address family.  That minimizes possibility of these routes
   leaking outside the expected domain or mixing with service routes.

   When redistributing between SAFI 4 and SAFI TBD Transport-VPN routes,
   there is a possibility of SAFI 4 routes mixing with SAFI 1 service
   routes.  To avoid such scenarios, it is recommended that
   implementations support keeping SAFI 4 routes in a separate transport
   RIB, distinct from service RIB that contain SAFI 1 service routes.

13.  Acknowledgements

   The authors thank Jeff Haas, John Scudder, Navaneetha Krishnan, Ravi
   M R, Chandrasekar Ramachandran, Shradha Hegde, Richard Roberts,
   Krzysztof Szarkowicz, John E Drake, Srihari Sangli, Vijay Kestur,
   Santosh Kolenchery for the valuable discussions.

   The decision to not reuse SAFI 128 and create a new address-family to
   carry these transport-routes was based on suggestion made by Richard
   Roberts and Krzysztof Szarkowicz.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <>.

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   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,

   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,

   [SRTE]     Previdi, S., Ed., "Advertising Segment Routing Policies in
              BGP", 11 2019, <

14.2.  URIs


Authors' Addresses

   Kaliraj Vairavakkalai
   Juniper Networks, Inc.
   1133 Innovation Way,
   Sunnyvale, CA  94089


   Natarajan Venkataraman
   Juniper Networks, Inc.
   1133 Innovation Way,
   Sunnyvale, CA  94089


   Balaji Rajagopalan
   Juniper Networks, Inc.
   Electra, Exora Business Park~Marathahalli - Sarjapur Outer
             Ring Road,
   Bangalore, KA  560103


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