BESS Workgroup                                           J. Rabadan, Ed.
Internet-Draft                                              S. Sathappan
Intended status: Standards Track                               V. Prabhu
Expires: 8 September 2022                                          Nokia
                                                                  W. Lin
                                                                 Juniper
                                                            P. Brissette
                                                           Cisco Systems
                                                            7 March 2022


      Ethernet VPN Virtual Private Wire Services Gateway Solution
                   draft-sr-bess-evpn-vpws-gateway-00

Abstract

   Ethernet VPN Virtual Private Wire Services (EVPN VPWS) need to be
   deployed in high scale multi-domain networks, where each domain can
   use a different transport technology, such as MPLS, VXLAN or Segment
   Routing with MPLS or IPv6 Segment Identifiers (SIDs).  While the
   transport interworking solutions on border routers spare the border
   routers from having to process service routes, they do not always
   meet the multi-homing, redundancy, and operational requirements, or
   provide the isolation that each domain requires.  This document
   analyzes the scenarios in which an interconnect solution for EVPN
   VPWS using EVPN Domain Gateways is needed, and adds the required
   extensions to support it.

Status of This Memo

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

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

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

   This Internet-Draft will expire on 8 September 2022.







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Copyright Notice

   Copyright (c) 2022 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  EVPN Interconnect Options . . . . . . . . . . . . . . . .   3
     1.3.  When is the Service Interworking Solution Required for EVPN
           VPWS  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.4.  Service Gateway Extensions for EVPN VPWS  . . . . . . . .   9
   2.  Conventions used in this document . . . . . . . . . . . . . .  10
   3.  Service Interworking procedures for EVPN VPWS . . . . . . . .  10
     3.1.  Redistribution of EVPN Routes Across Domains  . . . . . .  10
     3.2.  EVPN Domain Anycast Gateways for redundancy . . . . . . .  12
     3.3.  EVPN Multi-Homing for Domain Gateway Redundancy (I-ES)  .  14
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Ethernet VPN Virtual Private Wire Services (EVPN VPWS) [RFC8214] need
   to be deployed in high scale multi-domain networks, where each domain
   can use a different transport technology, such as MPLS, VXLAN or
   Segment Routing with MPLS or IPv6 Segment Identifiers (SIDs).  While
   the transport interworking solutions on border routers spare the
   border routers from having to process service routes, they do not
   always meet the multi-homing, redundancy, and operational
   requirements, or provide the isolation that each domain requires.
   This document analyzes the scenarios in which an interconnect
   solution for EVPN VPWS using EVPN Domain Gateways is needed, and adds



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   the required extensions to support it.

1.1.  Terminology

   This section summarizes the terminology that is used throughout the
   rest of the document.

   *  BR: Border Router, router that provides connectivity between
      domains, typically an Area Border Router (ABR) or Autonomous
      System Border Router (ASBR).

   *  I-PE: Ingress Provider Edge router

   *  E-PE: Egress Provider Edge router

   *  ES and ESI: Ethernet Segment and Ethernet Segment Identifier, as
      defined in [I-D.ietf-bess-rfc7432bis].

   *  I-ES and I-ESI: Interconnect Ethernet Segment and Interconnect
      Ethernet Segment Identifier.  An I-ES is defined for multihoming
      to the domains to which a Service Gateway is attached [RFC9014].

   *  NVO: Network Virtualization Overlay.

   *  EVPN Domain and EVPN Domain Gateway: two PEs are in the same EVPN
      Domain if they are attached to the same service and the packets
      between them do not require a data path lookup of the inner frame
      in any intermediate router.  An EVPN Domain is typically a group
      of PE, P and Border Routers that belong to the same IGP instance
      or BGP domain.  EVPN services are instantiated on the PEs and
      Border Routers, which are referred to as EVPN Domain Gateways in
      this document.  An EVPN Domain Gateway connects two or more EVPN
      Domains and is configured with multiple Domain identifiers (EVPN
      Domain-ID) in the VPWS that connects those EVPN Domains.  Each
      EVPN Domain-ID representing an EVPN Domain.  Another definition of
      EVPN Domain Gateway is a Border Router that implements the Service
      Interworking procedures described in this document.

   *  Domain: in this document Domain and EVPN Domain are used
      interchangeably.

1.2.  EVPN Interconnect Options

   This section describes the EVPN [I-D.ietf-bess-rfc7432bis] high level
   interconnect options and discusses their applicability to EVPN VPWS.

   1.  Service Interworking solution:




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                A-D per EVI   A-D per EVI      A-D per EVI
                RD2 tag2 L22  RD3 tag3 SID33   RD4 tag4 vni44
              <------------+ <--------------+ <-------------+
              +------------+ +--------------+ +-------------+
              |            | |              | |             |
            I-PE           BR-1             BR-2           E-PE
         +-------+       +-------+       +-------+       +-------+
         |+-----+|       |+-----+|       |+-----+|       |+-----+|
    CE1--||VPWS1||       ||VPWS1||       ||VPWS1||       ||VPWS1||-->CE2
         |+-----+|       |+-----+|       |+-----+|       |+-----+|
         +-------+       +-------+       +-------+       +-------+
              |  SR-MPLS   | |     SRv6     | |   VXLAN      |
              +------------+ +--------------+ +--------------+
              <------------> <--------------> <-------------->
                 Domain-1        Domain-2         Domain-3

              Figure 1: Service Interworking Interconnect

       [RFC9014] section 4 describes an end-to-end EVPN interconnect
       solution using EVPN Domain Gateways, or simply Gateways.  The
       Gateways provide connectivity across EVPN Domains, where those
       Domains can use MPLS tunnels, overlay tunnels (e.g., VXLAN) or
       Segment Routing tunnels.  Procedures are extrapolated to SRv6
       domains too.  The Gateways provide independence in terms of the
       Route Targets and Route Distinguishers used in each Domain, or
       the type of multicast tree used for BUM traffic in each domain,
       while keeping the key EVPN properties end-to-end, such as MAC
       mobility, MAC protection or ARP suppression.  The Gateways also
       provide all-active and single-active multi-homing redundancy by
       extending the concept of the multi-homing Ethernet Segment for
       interconnect domains (I-ES).  In this document, we refer to this
       solution as the Service Interworking option, and the Border
       Routers play the role of EVPN Domain Gateways.  Since [RFC9014]
       section 4 only describes the solution for EVPN multi-point
       services, this document extends the procedures to support EVPN
       VPWS services with the required extensions.  Figure 1 illustrates
       the Service Interworking solution across domains of different
       transport encapsulations when applied to EVPN VPWS services.

   2.  Inter-domain Option-B solution:











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                          NHSelf           NHSelf      A-D per EVI
                        L22<-L33         L33<-L44      RD4 tag4 L44
              <------------+ <--------------+ <-------------+
              +------------+ +--------------+ +-------------+
              |            | |              | |             |
            I-PE           BR-1             BR-2           E-PE
         +-------+       +-------+       +-------+       +-------+
         |+-----+|       |       |       |       |       |+-----+|
    CE1--||VPWS1||       |       |       |       |       ||VPWS1||-->CE2
         |+-----+|       |       |       |       |       |+-----+|
         +-------+       +-------+       +-------+       +-------+
              |  SR-MPLS   | |   SR-MPLS    | |  SR-MPLS     |
              +------------+ +--------------+ +--------------+
              <------------> <--------------> <-------------->
                 Domain-1        Domain-2         Domain-3

                     Figure 2: Inter-domain Option-B

       [RFC8365] section 10 provides an alternative interconnect
       solution for EVPN services by using Border Routers that re-write
       the EVPN BGP next-hops and program a swap operation of the VNIs
       or MPLS labels (depending on whether the encapsulation is NVO-
       based or MPLS-based).  This solution does not require the
       instantiation of Services on the Border Routers that perform a
       lookup on the inner destination MAC (as in the case of
       [RFC9014]), however the solution is limited to the interconnect
       of domains of the same encapsulation.  In addition, the solution
       does not support per-ES mass withdraw of the EVPN MAC/IP
       Advertisement routes, as described in [RFC8365].  In this
       document we refer to this solution as Inter-domain Option-B.
       Figure 2 illustrates this model when applied to EVPN VPWS, where
       the three domains are all now of the same encapsulation, and
       there is no service instantiation on the Border Routers.

   3.  Transport Interworking solution:
















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                                                       A-D per EVI
                                                       RD4 tag4 L44
              <---------------------------------------------+
              +------------+ +--------------+ +-------------+
              |            | |              | |             |
            I-PE           BR-1             BR-2           E-PE
         +-------+       +-------+       +-------+       +-------+
         |+-----+|       |Transp |       |Transp |       |+-----+|
    CE1--||VPWS1||       |IW     |       |IW     |       ||VPWS1||-->CE2
         |+-----+|       |       |       |       |       |+-----+|
         +-------+       +-------+       +-------+       +-------+
              |  SR-MPLS   | |   SRv6       | |  SR-MPLS     |
              +------------+ +--------------+ +--------------+
              <------------> <--------------> <-------------->
                 Domain-1        Domain-2         Domain-3

                 Figure 3: Transport Interworking option

       Other proposals are currently being investigated, in the context
       of SRv6 to MPLS interworking, e.g.,
       [I-D.agrawal-spring-srv6-mpls-interworking].  In these solutions,
       the Border Routers do not change the EVPN BGP next-hops, or
       process EVPN routes for that matter.  The Border Routers provide
       stitching between MPLS and SRv6 tunnels.  In this case, the
       solution allows the interconnect of domains of different
       encapsulation, as long as the ingress and egress PEs support the
       same encapsulation.  A variation of this solution is the Inter-
       domain Option-C solution, where a BGP LU (Label Unicast) tunnel
       provides the stitching across the domains, as long as all the
       domains use the same encapsulation.  In this document, we refer
       to this solution as Transport Interworking option.  Figure 3
       illustrates this model when applied to EVPN VPWS, where I-PE and
       E-PE are attached to domains of the same encapsulation.
       Intermediate domains, e.g., Domain-2, can be of encapsulations
       different from the ones used at the ingress and egress Domains.
       The EVPN route is not processed or changed by the Border Routers.

1.3.  When is the Service Interworking Solution Required for EVPN VPWS

   The three interconnect solutions described in Section 1.2 are valid,
   however, this section describes the requirements that make the
   Service Interworking solution needed.  Those requirements are:

   a.  Per-domain EVPN Multi-Homing

       The Service Interworking solution allows the use of different
       Ethernet Segment Identifiers (ESI) per domain, as well as the
       implementation of the aliasing and backup procedures on a per-



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       domain basis.  The use of different ESIs per domain may help
       guarantee the uniqueness of the ESI when different domains
       independently managed and operated are interconnected.  The
       implementation of independent aliasing and backup procedures per
       domain, spares the need for propagation of the EVPN A-D per ES
       routes by the Border Routers (which are EVPN Domain Gateways in
       the Service Interworking solution).  These A-D per ES routes are
       consumed within the domain, which results in a significant
       reduction of the number of routes that the ingress PEs need to
       process.  Another consequence of the processing of A-D per ES
       routes per domain, is a faster convergence in case of ES PE or
       link failure, since A-D per ES routes are no longer propagated by
       all the Border Routers along the domains, but processed by the
       Border Routers of the originating domain.  Per-domain EVPN Multi-
       Homing procedures are not possible in the Inter-domain Option-B
       or Transport Interworking solutions.

   b.  Per-ES Mass Withdrawal

       In order to benefit from the per-ES mass withdrawal property of
       EVPN Multi-Homing, the received BGP next-hops of the selected
       EVPN A-D per EVI and A-D per ES routes need to match on a PE.
       This cannot be guaranteed in an Inter-domain Option-B solution,
       as described in [RFC8365] section 10.2.2., however it is always
       the case in the Service Interworking or Transport Interworking
       solution.

   c.  Per-domain Route Distinguishers (RDs) and Route Targets (RTs)

       In case of merge of domains coming from different administrative
       entities, the uniqueness of RDs and RTs across domains for the
       same service is not guaranteed.  Hence the re-write of RD/RTs at
       the Border Routers may be required.  If that is the case, the
       Service Interworking solution provides the support for re-writing
       RD/RTs.  The Inter-domain Option-B may allow re-writing RD/RTs,
       however, it is not considered a common practice.  The Transport
       Interworking solution does not support the translation of RD/RTs.

   d.  Ethernet Tag IDs per domain

       Similar to per-domain RDs and RTs, re-writing of Ethernet Tag IDs
       used in the A-D per EVI routes may be needed in case of
       interconnect of domains that belong to different administrative
       entities.  This can be only supported by a Service Interworking
       solution.

   e.  Control Word, Flow Label and MTU (Maximum Transfer Unit)
       signaling per domain



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       As described in [I-D.ietf-bess-rfc7432bis], the use of Control
       Word and Flow Label, as well as the MTU are signaled in the EVPN
       Layer 2 Attributes extended community along with the A-D per EVI
       routes.  The signaling and use of Control Word is recommended in
       those domains where P routers can get confused when hashing based
       on the tunneled EVPN packet payload, but the Control Word may not
       be needed in some domains.  Similarly, the Flow Label introduces
       an additional level of entropy in EVPN encapsulated packets, that
       may be needed in some domains but adding unnecessary extra
       overhead in other domains.  Different MTUs may be supported in
       different domains, due to the domains running on different
       physical media.  A Service Interworking model allows the
       signaling and use of Control Word, Flow Label, and Layer-2 MTU on
       a per domain basis.  This is not the case in the other two models
       analyzed in this document.

   f.  Heterogeneous Encapsulations

       Interconnecting domains that use different encapsulations (e.g.,
       VXLAN, SRv6, MPLS, SR-MPLS, etc.) is a common requirement.  This
       becomes important in case the domains have different platform
       features, or migrations to new encapsulations or transport types
       are needed.  In the Service Interworking model the EVPN routes
       are generated and consumed at every Border Router (which is an
       EVPN Domain Gateway), hence the encapsulation indicated along
       with the route can be advertised independently at each Border
       Router.  That is not the case in the models 2 and 3 in
       Section 1.2.  The Inter-domain Option-B model requires the same
       encapsulation in each of the domains the Border Router connects,
       whereas the Transport Interworking model requires that at least
       the ingress and egress domains have the same encapsulation.

   g.  Per-domain EVPN Service OAM

       [RFC9062] defines the Service OAM requirements for EVPN services.
       When applied to the Interconnect solutions, the three solutions
       in Section 1.2 allow for the use of MEPs and MIPs on the ingress
       and egress PEs, but only the Service Interworking solution
       supports MEPs and MIPs on the Border Routers.  In other words,
       per-domain EVPN Service OAM is only supported in the Service
       Interworking option.

   The above requirements and their support across the Interconnect
   solutions are summarized in Table 1.







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   +======================+==============+==============+==============+
   | Requirement          | Service      | Inter-domain | Transport    |
   |                      | Interworking | Option-B     | Interworking |
   +======================+==============+==============+==============+
   | Per-domain           | Yes          | No           | No           |
   | EVPN Multi-          |              |              |              |
   | Homing               |              |              |              |
   +----------------------+--------------+--------------+--------------+
   | Per-ES Mass          | Yes          | No           | Yes          |
   | Withdrawal           |              |              |              |
   +----------------------+--------------+--------------+--------------+
   | Per-domain RD/       | Yes          | Yes*         | No           |
   | RTs                  |              |              |              |
   +----------------------+--------------+--------------+--------------+
   | Ethernet Tag         | Yes          | No           | No           |
   | IDs per domain       |              |              |              |
   +----------------------+--------------+--------------+--------------+
   | Control Word,        | Yes          | No           | No           |
   | Flow Label and       |              |              |              |
   | MTU signaling        |              |              |              |
   | per domain           |              |              |              |
   +----------------------+--------------+--------------+--------------+
   | Heterogeneous        | Yes          | No           | Yes**        |
   | encapsulations       |              |              |              |
   +----------------------+--------------+--------------+--------------+
   | Per-domain           | Yes          | No           | No           |
   | EVPN Service         |              |              |              |
   | OAM                  |              |              |              |
   +----------------------+--------------+--------------+--------------+

             Table 1: EVPN VPWS Interconnect Options Comparison

   * Although possible, it is unusual to re-write RD/RTs in the Inter-
   domain Option-B solution

   ** Supported only when the ingress and egress domains are of the same
   encapsulation

1.4.  Service Gateway Extensions for EVPN VPWS

   The rest of the document specifies the extensions required for the
   EVPN Domain Gateways to implement the Service Interworking solution
   to deploy end-to-end EVPN VPWS services.  In a nutshell, the AD per
   EVI routes advertised by I-PE and E-PE are redistributed across
   domains, while ES and A-D per ES routes advertised by these PEs are
   not redistributed by the EVPN Domain Gateways.  In addition, this
   document defines how Gateway redundancy works using either an Anycast
   Gateway solution, or by extending the I-ES concept already defined



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   for multi-point EVPN services in [RFC9014].

2.  Conventions used in this document

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

3.  Service Interworking procedures for EVPN VPWS

   This section describes the EVPN VPWS extensions on the EVPN Domain
   Gateways (or simply Gateways) to support the Service Interworking
   model.  An EVPN Domain Gateway in this context is a Border Router
   that connects EVPN Domains and implements the Service Interworking
   model of Section 1.2.  Section 3.1 specifies the Gateway rules to
   redistribute EVPN routes.  When redundant Gateways attached to two or
   more EVPN Domains are deployed, there are two redundancy mechanisms
   that can be used.  Section 3.2 describes a redundancy method that we
   refer to as "Anycast" and is based on the redundant Gateways behaving
   as a single system for the remote PEs.  Section 3.3 describes the
   redundancy based on I-ES, as an extension of the I-ES procedures
   specified in [RFC9014], only for EVPN VPWS services.  The Anycast
   redundancy does not require the use of I-ES and supports single-
   active multi-homing connectivity, but it will not support all-active,
   aliasing, backup, or mass withdraw features that are supported along
   with the use of I-ES and EVPN Multi-Homing.

3.1.  Redistribution of EVPN Routes Across Domains

   The EVPN Domain Gateways MUST establish separate BGP sessions for
   sending/receiving EVPN routes to/from each different Domain to which
   they are attached.  We refer to redistribution of an EVPN route to
   all the procedures in the Gateway that involve the reception and
   process of the source domain EVPN route, the programming of the
   forwarding path for the route, and the readvertisement of the route
   to a different domain (the next destination domain).

   The reception and processing of EVPN routes for an EVPN VPWS service
   follows [RFC8214].  If the D-PATH attribute is contained in the EVPN
   A-D per EVI route, loop detection and best path selection follows
   [I-D.sr-bess-evpn-dpath].  The Gateway imports the valid best EVPN
   A-D per EVI route required for an Ethernet Tag ID based on the import
   Route Target.  If a non-zero ESI is included in the route, the
   [RFC8214] procedures for aliasing, backup, and mass withdraw are
   followed on the Gateway.




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   If an A-D per EVI route for a service is successfully imported and
   processed, forwarding state is programmed in the data path using the
   MPLS label, VNI or SRv6 SID that was received in the EVPN A-D per EVI
   route.  In addition, depending on the encapsulation of the route's
   next destination domain, the router allocates a new MPLS label, VNI
   or SRv6 SID and programs a data path switching operation between the
   identifiers of the source and next destination domains.  Immediately
   after, the Gateway re-advertises the route to the BGP speaker in the
   next domain.  We refer to the source domain as the domain from which
   the Gateway receives the route, and the next domain as the EVPN
   Domain in which the Gateway redistributes the route.  The following
   considerations apply to the redistributed EVPN A-D per EVI routes:

   a.  The redistributed A-D per EVI route MUST carry a different RD
       than the source A-D per EVI route did.  This ensures that, in
       case of redundant Gateways, there is full path visibility in the
       next domain where the route is advertised.

   b.  The redistributed route MAY carry the same set of Route Targets
       as the source route did, if the source and next destination
       domains use different encapsulations, however translation or re-
       write of Route Targets SHOULD be supported in this case.  In case
       the source and next destination domains use the same
       encapsulation, the Gateway MUST use either different import Route
       Targets in the two domains, or use different Ethernet Tag IDs to
       create forwarding state in the two domains.  This ensures the
       Gateway does not loop packets back to the source domain and the
       redistributed routes are not leaked back to the source domain.

   c.  The ESI of the redistributed route MUST be set to zero or the
       value of the I-ESI defined in the Gateway (if any).

   d.  The Ethernet Tag ID of the redistributed route MAY have the same
       value as the source route.  Translation of the Ethernet Tag IDs
       SHOULD be supported though.

   e.  The EVPN Layer 2 Attributes extended community is regenerated for
       the redistributed route.  The value of the P and B flags are set
       based on the Gateway's I-ES and MUST NOT be propagated from the
       source route.  The Control Word, Flow Label flags, as well as the
       MTU, MAY be set to different values from the source A-D route.

   f.  The encapsulation specific attributes of the redistributed route
       are regenerated based on the encapsulation of the next domain.
       That includes the encoding of the A-D per EVI route NLRI as
       specified in [RFC8214] or [RFC8365], or the addition of the SRv6
       Services TLV as in [I-D.ietf-bess-srv6-services].




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   g.  The redistributed route should carry the Communities, Extended
       Communities and Large Communities of the source route, except for
       Route Targets (which are reoriginated), EVPN Extended Communities
       and BGP Encapsulation Extended Communities [RFC9012].  EVPN
       Extended Communities and BGP Encapsulation Extended Communities
       MUST NOT be propagated across domains.

   h.  The redistributed A-D per EVI route MUST update the D-PATH
       attribute of the received route, or add the D-PATH attribute if
       the received route did not contain a D-PATH
       [I-D.sr-bess-evpn-dpath].

   EVPN VPWS services also make use of multi-homing routes, that is,
   EVPN A-D per ES routes and Ethernet Segment routes.  These multi-
   homing routes are processed in the Gateway as in [RFC8214].  The A-D
   per ES and Ethernet Segment routes are only processed in the context
   of the domain they are received, and they MUST NOT be redistributed
   to any other domain.  A-D per ES and Ethernet Segment routes may be
   originated at the Gateway though, if the Gateway is attached to an
   I-ES, as described in Section 3.3.

3.2.  EVPN Domain Anycast Gateways for redundancy

   The Anycast Service Gateway redundancy is specified as follows:

   a.  All the Anycast Gateways attached to the same two domains MUST
       redistribute the EVPN A-D per EVI routes between domains as per
       Section 3.1 with the following considerations:

       *  No I-ES is used on the Gateways, therefore the ESI value MUST
          be set to zero when redistributing EVPN A-D per EVI routes.

       *  All the redundant Gateways may set the same (or different)
          Ethernet Tag ID in the redistributed A-D per EVI route.

   b.  All Anycast Gateways MUST process the received D-PATH attribute
       and update the D-PATH (with the source domain-id) when
       redistributing the A-D per EVI route to the next domain.  The
       D-PATH attribute will avoid control plane loops.

   As an illustration of this redundancy method, suppose all four
   Service Gateways in Figure 4 are configured as Anycast Service
   Gateways, and local and remote Ethernet Tag IDs are configured as 1,
   2 and 3 on all routers in the domains 1, 2 and 3 respectively.







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               A-D per EVI    A-D per EVI
              RD11 tag1 L111  RD21 tag2 SID21
             <------------+ <--------------+
             +------------+ +--------------+ +-------------+
             |  Domain-1  | |   Domain-2   | |  Domain-3   |
             |            BR-11            BR-21       A-D per EVI
             |          +-------+       +-------+      RD4 tag3 vni33
             |          |+-----+|       |+-----+|     <---------+
           I-PE    +--> ||VPWS1||-----> ||VPWS1||--+      E-PE
        +-------+  |    |+-----+|       |+-----+|  |    +-------+
        |+-----+|--+    +-------+       +-------+  |    |+-----+|
   CE1--||VPWS1||         | |              | |     +--> ||VPWS1||-->CE2
        |+-----+|         BR-12            BR-22        |+-----+|
        +-------+       +-------+       +-------+       +-------+
             |          |+-----+|       |+-----+|          |
             |          ||VPWS1||       ||VPWS1||          |
             |          |+-----+|       |+-----+|          |
             |          +-------+       +-------+          |
             |  SR-MPLS   | |     SRv6     | |   VXLAN     |
             +------------+ +--------------+ +-------------+
               A-D per EVI    A-D per EVI
              RD12 tag1 L121  RD22 tag2 SID22
             <------------+ <--------------+

                        Figure 4: Anycast Redundancy


   In the example in Figure 4 E-PE advertises an EVPN A-D per EVI route
   for Ethernet Tag ID 3.  Both BR-21 and BR-22 import the route and
   redistribute it with Ethernet Tag ID 2 and new RD and encapsulation
   into domain-2.  When redistributing, both BR-21 and BR-22 update (if
   it existed before) or insert a D-PATH attribute with the domain-id of
   domain-3.  That prevents BR-21 and BR-22 from redistributing back
   into domain-3 each other's route [I-D.sr-bess-evpn-dpath].  BR-11 and
   BR-12 import the routes after best path selection and perform the
   same process and redistribution into domain-1.  I-PE will receive two
   routes for Ethernet Tag ID 1, from BR-11 and BR-12, and will perform
   best path selection for Ethernet Tag ID 1.  Based on the best path
   selection carried out by I-PE and the BRs along the way, all flows
   from CE1 to CE2 will follow, e.g., I-PE, BR-11, BR-21 and E-PE.  In
   case of failure on any of the BRs in the data path, the routers will
   select the alternate route for the Ethernet Tag ID.  The same control
   plane exchange and traffic flow happen in the reverse direction,
   where I-PE becomes the egress PE and E-PE the ingress PE.

   As illustrated in Figure 4, this model does not support per-flow load
   balancing (all-active multi-homing) to all the BR nodes along the way
   from CE to CE.



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3.3.  EVPN Multi-Homing for Domain Gateway Redundancy (I-ES)

   EVPN Multi-Homing procedures can be used on the EVPN Domain Gateways.
   For that, an I-ES and its assigned I-ESI will be configured on the
   Gateways for multihoming.  The I-ES concept is introduced in
   [RFC9014], and it is used in this document for EVPN VPWS services.
   This I-ES represents a domain to the next domain, in both directions.
   Therefore two or more Gateways attached to the same two domains will
   use the same I-ESI when advertising routes to the two domains.

   The Gateways attached to the same I-ES:

   a.  Advertise EVPN Ethernet Segment routes and A-D per ES routes for
       the I-ES.  Those routes are not redistributed beyond the Domain
       into which they are originated.

   b.  Receive Ethernet Segment and A-D per ES routes from the I-ES
       peer(s), and use them for I-ES Designated Forwarding (DF)
       Election and mass withdraw respectively, as described in
       [RFC8214] and [I-D.ietf-bess-rfc7432bis].

   c.  Set the I-ESI into the EVPN A-D per EVI routes that are
       redistributed across domains.  P and B flags are set based on the
       result of the DF Election [RFC8214].

   d.  Identify loops if the received EVPN A-D per EVI routes include a
       local domain-id in the D-PATH attribute.  Also EVPN A-D per EVI
       routes that include a local ESI MUST NOT be redistributed to
       another domain, irrespective of the presence of the D-PATH
       attribute.

   Figure 5 illustrates the use of I-ES or EVPN Multi-Homing procedures
   in EVPN Domain Gateways.  In the example, BR-11 and BR-12 are
   attached to I-ES-1 (with ESI-1 as identifier), whereas BR-21 and
   BR-22 are attached to I-ES-2 (using ESI-2).
















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               A-D per EVI           A-D per EVI
              RD11 tag1 ESI-1 L111   RD21 tag2 ESI-2 SID21
             <------------+ <--------------+
             +------------+ +--------------+ +-------------+
             |  Domain-1  | |   Domain-2   | |  Domain-3   |
             |            BR-11            BR-21       A-D per EVI
             |          +-------+       +-------+      RD4 tag3 vni33
             |          |+-----+|       |+-----+|     <---------+
           I-PE    +--> ||VPWS1||-+---> ||VPWS1||--+      E-PE
        +-------+  |    |+-----+| | +-> |+-----+|  |    +-------+
        |+-----+|--+    +-------+ | |   +-------+  +--> |+-----+|
   CE1--||VPWS1||  I-ES1  | |     | |      | |    I-ES2 ||VPWS1||-->CE2
        |+-----+|--+      BR-12   | |      BR-22   +--> |+-----+|
        +-------+  |    +-------+ +-|-> +-------+  |    +-------+
             |     |    |+-----+|   |   |+-----+|  |       |
             |     +--> ||VPWS1||---+-> ||VPWS1||--+       |
             |          |+-----+|       |+-----+|          |
             |          +-------+       +-------+          |
             |  SR-MPLS   | |     SRv6     | |   VXLAN     |
             +------------+ +--------------+ +-------------+
               A-D per EVI          A-D per EVI
              RD12 tag1 ESI-1 L121  RD22 tag2 ESI-2 SID22
             <------------+ <--------------+

                        Figure 5: EVPN Multi-Homing

   E-PE advertises an A-D per EVI route for tag3, that gets
   redistributed by BR-21/BR-22 first, and BR-11/BR-12 later,
   translating the Ethernet Tag ID and encapsulation in each
   redistribution.  The BR nodes implement the EVPN Multi-Homing
   procedures for their own Ethernet Segment as in [RFC8214], and set
   the P and B flags accordingly when redistributing the A-D per EVI
   routes, to indicate the forwarding mode to the receiving nodes.  If
   I-ES-1 and I-ES-2 are defined as all-active multi-homing Ethernet
   Segments, per-flow load balancing will be performed not only by the
   I-PE to the Gateways in domain-1, but also by the Gateways at each
   domain of the EVPN VPWS service, as depicted in Figure 5.  The same
   control plane exchange and traffic flow happen in the reverse
   direction, where I-PE becomes the egress PE and E-PE the ingress PE.

   I-ES-1 and I-ES-2 are independent of each other, e.g., I-ES-1 can
   work in single-active mode, whereas I-ES-2 uses all-active mode.  If
   that is the case, BR-11 and BR-12 run Designated Forwarded (DF)
   Election and BR-11 signals P=1 and B=0 (in the EVPN Layer 2
   Attributes extended community) if it is elected as DF, whereas BR-12
   signals P=0 and B=1 if elected as Backup DF router.  I-PE then sends
   all traffic to BR-11, and BR-21/BR-22 send all traffic to BR-11 in
   the reverse direction.  Since BR-21/BR-22 work in all-active mode,



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   they both signal P=1/B=0 to both, E-PE and BR-11/BR-12.  Therefore
   traffic from BR-11/BR-12 is sprayed to both BR-21/BR-22, and so is
   traffic from E-PE.

   The Anycast Gateway and the EVPN Multi-Homing redundancy solutions
   can coexist.  The Gateways of the same redundancy group MUST
   implement the same redundancy method, but different redundancy
   Gateway groups MAY implement different methods.  In the example, BR-
   11/BR-12 constitutes a redundancy group and BR-21/BR-22 constitutes a
   different redundancy group.

4.  Security Considerations

   To be added in a future version.

5.  IANA Considerations

   None.

6.  Acknowledgments


7.  Contributors

8.  References

8.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8365]  Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
              Uttaro, J., and W. Henderickx, "A Network Virtualization
              Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
              DOI 10.17487/RFC8365, March 2018,
              <https://www.rfc-editor.org/info/rfc8365>.









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   [I-D.sr-bess-evpn-dpath]
              Rabadan, J., Sathappan, S., Gautam, M., Brissette, P., and
              W. Lin, "Domain Path (D-PATH) for Ethernet VPN (EVPN)
              Interconnect Networks", Work in Progress, Internet-Draft,
              draft-sr-bess-evpn-dpath-01, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-sr-bess-evpn-dpath-
              01.txt>.

   [I-D.ietf-bess-rfc7432bis]
              Sajassi, A., Burdet, L. A., Drake, J., and J. Rabadan,
              "BGP MPLS-Based Ethernet VPN", Work in Progress, Internet-
              Draft, draft-ietf-bess-rfc7432bis-03, 28 February 2022,
              <https://www.ietf.org/archive/id/draft-ietf-bess-
              rfc7432bis-03.txt>.

   [RFC9014]  Rabadan, J., Ed., Sathappan, S., Henderickx, W., Sajassi,
              A., and J. Drake, "Interconnect Solution for Ethernet VPN
              (EVPN) Overlay Networks", RFC 9014, DOI 10.17487/RFC9014,
              May 2021, <https://www.rfc-editor.org/info/rfc9014>.

   [RFC8214]  Boutros, S., Sajassi, A., Salam, S., Drake, J., and J.
              Rabadan, "Virtual Private Wire Service Support in Ethernet
              VPN", RFC 8214, DOI 10.17487/RFC8214, August 2017,
              <https://www.rfc-editor.org/info/rfc8214>.

   [I-D.ietf-bess-srv6-services]
              Dawra, G., Filsfils, C., Talaulikar, K., Raszuk, R.,
              Decraene, B., Zhuang, S., and J. Rabadan, "SRv6 BGP based
              Overlay Services", Work in Progress, Internet-Draft,
              draft-ietf-bess-srv6-services-12, 5 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-bess-srv6-
              services-12.txt>.

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,
              <https://www.rfc-editor.org/info/rfc9012>.

8.2.  Informative References

   [RFC9062]  Salam, S., Sajassi, A., Aldrin, S., Drake, J., and D.
              Eastlake 3rd, "Framework and Requirements for Ethernet VPN
              (EVPN) Operations, Administration, and Maintenance (OAM)",
              RFC 9062, DOI 10.17487/RFC9062, June 2021,
              <https://www.rfc-editor.org/info/rfc9062>.






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   [I-D.agrawal-spring-srv6-mpls-interworking]
              Agrawal, S., ALI, Z., Filsfils, C., Voyer, D., and Z. Li,
              "SRv6 and MPLS interworking", Work in Progress, Internet-
              Draft, draft-agrawal-spring-srv6-mpls-interworking-07, 21
              February 2022, <https://www.ietf.org/archive/id/draft-
              agrawal-spring-srv6-mpls-interworking-07.txt>.

Authors' Addresses

   J. Rabadan (editor)
   Nokia
   520 Almanor Avenue
   Sunnyvale, CA 94085
   United States of America
   Email: jorge.rabadan@nokia.com


   S. Sathappan
   Nokia
   520 Almanor Avenue
   Sunnyvale, CA 94085
   United States of America
   Email: senthil.sathappan@nokia.com


   V. Prabhu
   Nokia
   600 March Rd
   Kanata ON K2K 2T6
   Canada
   Email: vinod.prabhu@nokia.com


   W. Lin
   Juniper
   United States of America
   Email: wlin@juniper.net


   P. Brissette
   Cisco Systems
   Canada
   Email: pbrisset@cisco.com








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