Internet Engineering Task Force (IETF)                   J. Rabadan, Ed.
Request for Comments: 9014                                  S. Sathappan
Category: Standards Track                                  W. Henderickx
ISSN: 2070-1721                                                    Nokia
                                                              A. Sajassi
                                                                   Cisco
                                                                J. Drake
                                                                 Juniper
                                                                May 2021


     Interconnect Solution for Ethernet VPN (EVPN) Overlay Networks

Abstract

   This document describes how Network Virtualization Overlays (NVOs)
   can be connected to a Wide Area Network (WAN) in order to extend the
   Layer 2 connectivity required for some tenants.  The solution
   analyzes the interaction between NVO networks running Ethernet
   Virtual Private Networks (EVPNs) and other Layer 2 VPN (L2VPN)
   technologies used in the WAN, such as Virtual Private LAN Services
   (VPLSs), VPLS extensions for Provider Backbone Bridging (PBB-VPLS),
   EVPN, or PBB-EVPN.  It also describes how the existing technical
   specifications apply to the interconnection and extends the EVPN
   procedures needed in some cases.  In particular, this document
   describes how EVPN routes are processed on Gateways (GWs) that
   interconnect EVPN-Overlay and EVPN-MPLS networks, as well as the
   Interconnect Ethernet Segment (I-ES), to provide multihoming.  This
   document also describes the use of the Unknown MAC Route (UMR) to
   avoid issues of a Media Access Control (MAC) scale on Data Center
   Network Virtualization Edge (NVE) devices.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9014.

Copyright Notice

   Copyright (c) 2021 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
   2.  Conventions and Terminology
   3.  Decoupled Interconnect Solution for EVPN-Overlay Networks
     3.1.  Interconnect Requirements
     3.2.  VLAN-Based Handoff
     3.3.  PW-Based Handoff
     3.4.  Multihoming Solution on the GWs
     3.5.  Gateway Optimizations
       3.5.1.  MAC Address Advertisement Control
       3.5.2.  ARP/ND Flooding Control
       3.5.3.  Handling Failures between GW and WAN Edge Routers
   4.  Integrated Interconnect Solution for EVPN-Overlay Networks
     4.1.  Interconnect Requirements
     4.2.  VPLS Interconnect for EVPN-Overlay Networks
       4.2.1.  Control/Data Plane Setup Procedures on the GWs
       4.2.2.  Multihoming Procedures on the GWs
     4.3.  PBB-VPLS Interconnect for EVPN-Overlay Networks
       4.3.1.  Control/Data Plane Setup Procedures on the GWs
       4.3.2.  Multihoming Procedures on the GWs
     4.4.  EVPN-MPLS Interconnect for EVPN-Overlay Networks
       4.4.1.  Control plane Setup Procedures on the GWs
       4.4.2.  Data Plane Setup Procedures on the GWs
       4.4.3.  Multihoming Procedure Extensions on the GWs
       4.4.4.  Impact on MAC Mobility Procedures
       4.4.5.  Gateway Optimizations
       4.4.6.  Benefits of the EVPN-MPLS Interconnect Solution
     4.5.  PBB-EVPN Interconnect for EVPN-Overlay Networks
       4.5.1.  Control/Data Plane Setup Procedures on the GWs
       4.5.2.  Multihoming Procedures on the GWs
       4.5.3.  Impact on MAC Mobility Procedures
       4.5.4.  Gateway Optimizations
     4.6.  EVPN-VXLAN Interconnect for EVPN-Overlay Networks
       4.6.1.  Globally Unique VNIs in the Interconnect Network
       4.6.2.  Downstream-Assigned VNIs in the Interconnect Network
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   [RFC8365] discusses the use of Ethernet Virtual Private Networks
   (EVPNs) [RFC7432] as the control plane for Network Virtualization
   Overlays (NVOs), where VXLAN [RFC7348], NVGRE [RFC7637], or MPLS over
   GRE [RFC4023] can be used as possible data plane encapsulation
   options.

   While this model provides a scalable and efficient multitenant
   solution within the Data Center, it might not be easily extended to
   the Wide Area Network (WAN) in some cases, due to the requirements
   and existing deployed technologies.  For instance, a Service Provider
   might have an already deployed Virtual Private LAN Service (VPLS)
   [RFC4761] [RFC4762], VPLS extensions for Provider Backbone Bridging
   (PBB-VPLS) [RFC7041], EVPN [RFC7432], or PBB-EVPN [RFC7623] network
   that has to be used to interconnect Data Centers and WAN VPN users.
   A Gateway (GW) function is required in these cases.  In fact,
   [RFC8365] discusses two main Data Center Interconnect (DCI) solution
   groups: "DCI using GWs" and "DCI using ASBRs".  This document
   specifies the solutions that correspond to the "DCI using GWs" group.

   It is assumed that the NVO GW and the WAN Edge functions can be
   decoupled into two separate systems or integrated into the same
   system.  The former option will be referred to as "decoupled
   interconnect solution" throughout the document, whereas the latter
   one will be referred to as "integrated interconnect solution".

   The specified procedures are local to the redundant GWs connecting a
   DC to the WAN.  The document does not preclude any combination across
   different DCs for the same tenant.  For instance, a "Decoupled"
   solution can be used in GW1 and GW2 (for DC1), and an "Integrated"
   solution can be used in GW3 and GW4 (for DC2).

   While the Gateways and WAN Provider Edges (PEs) use existing
   specifications in some cases, the document also defines extensions
   that are specific to DCI.  In particular, those extensions are:

   *  The Interconnect Ethernet Segment (I-ES), an Ethernet Segment that
      can be associated to a set of pseudowires (PWs) or other tunnels.
      The I-ES defined in this document is not associated with a set of
      Ethernet links, as per [RFC7432], but rather with a set of virtual
      tunnels (e.g., a set of PWs).  This set of virtual tunnels is
      referred to as vES [VIRTUAL-ES].

   *  The use of the Unknown MAC Route (UMR) in a DCI scenario.

   *  The processing of EVPN routes on Gateways with MAC-VRFs connecting
      EVPN-Overlay and EVPN-MPLS networks, or EVPN-Overlay and EVPN-
      Overlay networks.

2.  Conventions and Terminology

   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.

   AC:  Attachment Circuit

   ARP:  Address Resolution Protocol

   BUM:  Broadcast, Unknown Unicast and Multicast (traffic)

   CE:  Customer Equipment

   CFM:  Connectivity Fault Management

   DC:  Data Center

   DCI:  Data Center Interconnect

   DF:  Designated Forwarder

   EVI:  EVPN Instance

   EVPN:  Ethernet Virtual Private Network, as in [RFC7432]

   EVPN Tunnel binding:  refers to a tunnel to a remote PE/NVE for a
      given EVI.  Ethernet packets in these bindings are encapsulated
      with the Overlay or MPLS encapsulation and the EVPN label at the
      bottom of the stack.

   ES:  Ethernet Segment

   ESI:  Ethernet Segment Identifier

   GW:  Gateway or Data Center Gateway

   I-ES and I-ESI:  Interconnect Ethernet Segment and Interconnect
      Ethernet Segment Identifier.  An I-ES is defined on the GWs for
      multihoming to/from the WAN.

   MAC  Media Access Control

   MAC-VRF:  refers to an EVI instance in a particular node

   MP2P and LSM tunnels:  refer to multipoint-to-point and label
      switched multicast tunnels

   ND:  Neighbor Discovery

   NDF:  Non-Designated Forwarder

   NVE:  Network Virtualization Edge

   NVGRE:  Network Virtualization using Generic Routing Encapsulation

   NVO:  Network Virtualization Overlay

   OAM:  Operations, Administration, and Maintenance

   PBB:  Provider Backbone Bridging

   PE:  Provider Edge

   PW:  Pseudowire

   RD:  Route Distinguisher

   RR:  Route Reflector

   RT:  Route Target

   S/C-TAG:  refers to a combination of Service Tag and Customer Tag in
      a 802.1Q frame

   TOR:  Top-Of-Rack

   UMR:  Unknown MAC Route

   vES:  virtual Ethernet Segment

   VNI/VSID:  refers to VXLAN/NVGRE virtual identifiers

   VPLS:  Virtual Private LAN Service

   VSI:  Virtual Switch Instance or VPLS instance in a particular PE

   VXLAN:  Virtual eXtensible LAN

3.  Decoupled Interconnect Solution for EVPN-Overlay Networks

   This section describes the interconnect solution when the GW and WAN
   Edge functions are implemented in different systems.  Figure 1
   depicts the reference model described in this section.  Note that,
   although not shown in Figure 1, GWs may have local Attachment
   Circuits (ACs).

                                   +--+
                                   |CE|
                                   +--+
                                     |
                                  +----+
                             +----| PE |----+
           +---------+       |    +----+    |       +---------+
   +----+  |        +---+  +----+        +----+  +---+        |  +----+
   |NVE1|--|        |   |  |WAN |        |WAN |  |   |        |--|NVE3|
   +----+  |        |GW1|--|Edge|        |Edge|--|GW3|        |  +----+
           |        +---+  +----+        +----+  +---+        |
           |  NVO-1   |      |     WAN      |      |   NVO-2  |
           |        +---+  +----+        +----+  +---+        |
           |        |   |  |WAN |        |WAN |  |   |        |
   +----+  |        |GW2|--|Edge|        |Edge|--|GW4|        |  +----+
   |NVE2|--|        +---+  +----+        +----+  +---+        |--|NVE4|
   +----+  +---------+       |              |       +---------+  +----+
                             +--------------+

   |<-EVPN-Overlay-->|<-VLAN->|<-WAN L2VPN->|<--PW-->|<--EVPN-Overlay->|
                      handoff               handoff

                   Figure 1: Decoupled Interconnect Model

   The following section describes the interconnect requirements for
   this model.

3.1.  Interconnect Requirements

   The decoupled interconnect architecture is intended to be deployed in
   networks where the EVPN-Overlay and WAN providers are different
   entities and a clear demarcation is needed.  This solution solves the
   following requirements:

   *  A simple connectivity handoff between the EVPN-Overlay network
      provider and the WAN provider so that QoS and security enforcement
      are easily accomplished.

   *  Independence of the L2VPN technology deployed in the WAN.

   *  Multihoming between GW and WAN Edge routers, including per-service
      load balancing.  Per-flow load balancing is not a strong
      requirement, since a deterministic path per service is usually
      required for an easy QoS and security enforcement.

   *  Support of Ethernet OAM and Connectivity Fault Management (CFM)
      [IEEE.802.1AG] [Y.1731] functions between the GW and the WAN Edge
      router to detect individual AC failures.

   *  Support for the following optimizations at the GW:

      -  Flooding reduction of unknown unicast traffic sourced from the
         DC Network Virtualization Edge (NVE) devices.

      -  Control of the WAN MAC addresses advertised to the DC.

      -  Address Resolution Protocol (ARP) and Neighbor Discovery (ND)
         flooding control for the requests coming from the WAN.

3.2.  VLAN-Based Handoff

   In this option, the handoff between the GWs and the WAN Edge routers
   is based on VLANs [IEEE.802.1Q].  This is illustrated in Figure 1
   (between the GWs in NVO-1 and the WAN Edge routers).  Each MAC-VRF in
   the GW is connected to a different VSI/MAC-VRF instance in the WAN
   Edge router by using a different C-TAG VLAN ID or a different
   combination of S/C-TAG VLAN IDs that matches at both sides.

   This option provides the best possible demarcation between the DC and
   WAN providers, and it does not require control plane interaction
   between both providers.  The disadvantage of this model is the
   provisioning overhead, since the service has to be mapped to a C-TAG
   or S/C-TAG VLAN ID combination at both GW and WAN Edge routers.

   In this model, the GW acts as a regular Network Virtualization Edge
   (NVE) towards the DC.  Its control plane, data plane procedures, and
   interactions are described in [RFC8365].

   The WAN Edge router acts as a (PBB-)VPLS or (PBB-)EVPN PE with
   Attachment Circuits (ACs) to the GWs.  Its functions are described in
   [RFC4761], [RFC4762], [RFC6074], [RFC7432], and [RFC7623].

3.3.  PW-Based Handoff

   If MPLS between the GW and the WAN Edge router is an option, a PW-
   based interconnect solution can be deployed.  In this option, the
   handoff between both routers is based on FEC128-based PWs [RFC4762]
   or FEC129-based PWs (for a greater level of network automation)
   [RFC6074].  Note that this model still provides a clear demarcation
   between DC and WAN (since there is a single PW between each MAC-VRF
   and peer VSI), and security/QoS policies may be applied on a per-PW
   basis.  This model provides better scalability than a C-TAG-based
   handoff and less provisioning overhead than a combined C/S-TAG
   handoff.  The PW-based handoff interconnect is illustrated in
   Figure 1 (between the NVO-2 GWs and the WAN Edge routers).

   In this model, besides the usual MPLS procedures between GW and WAN
   Edge router [RFC3031], the GW MUST support an interworking function
   in each MAC-VRF that requires extension to the WAN:

   *  If a FEC128-based PW is used between the MAC-VRF (GW) and the VSI
      (WAN Edge), the corresponding Virtual Connection Identifier (VCID)
      MUST be provisioned on the MAC-VRF and match the VCID used in the
      peer VSI at the WAN Edge router.

   *  If BGP Auto-discovery [RFC6074] and FEC129-based PWs are used
      between the GW MAC-VRF and the WAN Edge VSI, the provisioning of
      the VPLS-ID MUST be supported on the MAC-VRF and MUST match the
      VPLS-ID used in the WAN Edge VSI.

   If a PW-based handoff is used, the GW's AC (or point of attachment to
   the EVPN instance) uses a combination of a PW label and VLAN IDs.
   PWs are treated as service interfaces, defined in [RFC7432].

3.4.  Multihoming Solution on the GWs

   EVPN single-active multihoming -- i.e., per-service load-balancing
   multihoming -- is required in this type of interconnect.

   The GWs will be provisioned with a unique ES for each WAN
   interconnect, and the handoff attachment circuits or PWs between the
   GW and the WAN Edge router will be assigned an ESI for each such ES.
   The ESI will be administratively configured on the GWs according to
   the procedures in [RFC7432].  This I-ES will be referred to as "I-ES"
   hereafter, and its identifier will be referred to as "I-ESI".
   Different ESI types are described in [RFC7432].  The use of Type 0
   for the I-ESI is RECOMMENDED in this document.

   The solution (on the GWs) MUST follow the single-active multihoming
   procedures as described in [RFC8365] for the provisioned I-ESI --
   i.e., Ethernet A-D routes per ES and per EVI will be advertised to
   the DC NVEs for the multihoming functions, and ES routes will be
   advertised so that ES discovery and Designated Forwarder (DF)
   procedures can be followed.  The MAC addresses learned (in the data
   plane) on the handoff links will be advertised with the I-ESI encoded
   in the ESI field.

3.5.  Gateway Optimizations

   The following GW features are optional and optimize the control plane
   and data plane in the DC.

3.5.1.  MAC Address Advertisement Control

   The use of EVPN in NVO networks brings a significant number of
   benefits, as described in [RFC8365].  However, if multiple DCs are
   interconnected into a single EVI, each DC will have to import all of
   the MAC addresses from each of the other DCs.

   Even if optimized BGP techniques like RT constraint [RFC4684] are
   used, the number of MAC addresses to advertise or withdraw (in case
   of failure) by the GWs of a given DC could overwhelm the NVEs within
   that DC, particularly when the NVEs reside in the hypervisors.

   The solution specified in this document uses the Unknown MAC Route
   (UMR) that is advertised into a given DC by each of the DC's GWs.
   This route is defined in [RFC7543] and is a regular EVPN MAC/IP
   Advertisement route in which the MAC Address Length is set to 48, the
   MAC address is set to 0, and the ESI field is set to the DC GW's
   I-ESI.

   An NVE within that DC that understands and processes the UMR will
   send unknown unicast frames to one of the DC's GWs, which will then
   forward that packet to the correct egress PE.  Note that, because the
   ESI is set to the DC GW's I-ESI, all-active multihoming can be
   applied to unknown unicast MAC addresses.  An NVE that does not
   understand the Unknown MAC Route will handle unknown unicast as
   described in [RFC7432].

   This document proposes that local policy determine whether MAC
   addresses and/or the UMR are advertised into a given DC.  As an
   example, when all the DC MAC addresses are learned in the control/
   management plane, it may be appropriate to advertise only the UMR.
   Advertising all the DC MAC addresses in the control/management plane
   is usually the case when the NVEs reside in hypervisors.  Refer to
   [RFC8365], Section 7.

   It is worth noting that the UMR usage in [RFC7543] and the UMR usage
   in this document are different.  In the former, a Virtual Spoke
   (V-spoke) does not necessarily learn all the MAC addresses pertaining
   to hosts in other V-spokes of the same network.  The communication
   between two V-spokes is done through the Default MAC Gateway (DMG)
   until the V-spokes learn each other's MAC addresses.  In this
   document, two leaf switches in the same DC are recommended for
   V-spokes to learn each other's MAC addresses for the same EVI.  The
   leaf-to-leaf communication is always direct and does not go through
   the GW.

3.5.2.  ARP/ND Flooding Control

   Another optimization mechanism, naturally provided by EVPN in the
   GWs, is the Proxy ARP/ND function.  The GWs should build a Proxy ARP/
   ND cache table, as per [RFC7432].  When the active GW receives an
   ARP/ND request/solicitation coming from the WAN, the GW does a Proxy
   ARP/ND table lookup and replies as long as the information is
   available in its table.

   This mechanism is especially recommended on the GWs, since it
   protects the DC network from external ARP/ND-flooding storms.

3.5.3.  Handling Failures between GW and WAN Edge Routers

   Link/PE failures are handled on the GWs as specified in [RFC7432].
   The GW detecting the failure will withdraw the EVPN routes, as per
   [RFC7432].

   Individual AC/PW failures may be detected by OAM mechanisms.  For
   instance:

   *  If the interconnect solution is based on a VLAN handoff, Ethernet-
      CFM [IEEE.802.1AG] [Y.1731] may be used to detect individual AC
      failures on both the GW and WAN Edge router.  An individual AC
      failure will trigger the withdrawal of the corresponding A-D per
      EVI route as well as the MACs learned on that AC.

   *  If the interconnect solution is based on a PW handoff, the Label
      Distribution Protocol (LDP) PW Status bits TLV [RFC6870] may be
      used to detect individual PW failures on both the GW and WAN Edge
      router.

4.  Integrated Interconnect Solution for EVPN-Overlay Networks

   When the DC and the WAN are operated by the same administrative
   entity, the Service Provider can decide to integrate the GW and WAN
   Edge PE functions in the same router for obvious reasons to save as
   relates to Capital Expenditure (CAPEX) and Operating Expenses (OPEX).
   This is illustrated in Figure 2.  Note that this model does not
   provide an explicit demarcation link between DC and WAN anymore.
   Although not shown in Figure 2, note that the GWs may have local ACs.

                             +--+
                             |CE|
                             +--+
                               |
                            +----+
                       +----| PE |----+
           +---------+ |    +----+    | +---------+
   +----+  |        +---+            +---+        |  +----+
   |NVE1|--|        |   |            |   |        |--|NVE3|
   +----+  |        |GW1|            |GW3|        |  +----+
           |        +---+            +---+        |
           |  NVO-1   |       WAN      |   NVO-2  |
           |        +---+            +---+        |
           |        |   |            |   |        |
   +----+  |        |GW2|            |GW4|        |  +----+
   |NVE2|--|        +---+            +---+        |--|NVE4|
   +----+  +---------+ |              | +---------+  +----+
                       +--------------+

   |<--EVPN-Overlay--->|<-----VPLS--->|<---EVPN-Overlay-->|
                       |<--PBB-VPLS-->|
     Interconnect  ->  |<-EVPN-MPLS-->|
      options          |<--EVPN-Ovl-->|*
                       |<--PBB-EVPN-->|

   * EVPN-Ovl stands for EVPN-Overlay (and it's an interconnect option).

                  Figure 2: Integrated Interconnect Model

4.1.  Interconnect Requirements

   The integrated interconnect solution meets the following
   requirements:

   *  Control plane and data plane interworking between the EVPN-Overlay
      network and the L2VPN technology supported in the WAN,
      irrespective of the technology choice -- i.e., (PBB-)VPLS or
      (PBB-)EVPN, as depicted in Figure 2.

   *  Multihoming, including single-active multihoming with per-service
      load balancing or all-active multihoming -- i.e., per-flow load-
      balancing -- as long as the technology deployed in the WAN
      supports it.

   *  Support for end-to-end MAC Mobility, Static MAC protection and
      other procedures (e.g., proxy-arp) described in [RFC7432] as long
      as EVPN-MPLS is the technology of choice in the WAN.

   *  Independent inclusive multicast trees in the WAN and in the DC.
      That is, the inclusive multicast tree type defined in the WAN does
      not need to be the same as in the DC.

4.2.  VPLS Interconnect for EVPN-Overlay Networks

4.2.1.  Control/Data Plane Setup Procedures on the GWs

   Regular MPLS tunnels and Targeted LDP (tLDP) / BGP sessions will be
   set up to the WAN PEs and RRs as per [RFC4761], [RFC4762], and
   [RFC6074], and overlay tunnels and EVPN will be set up as per
   [RFC8365].  Note that different route targets for the DC and the WAN
   are normally required (unless [RFC4762] is used in the WAN, in which
   case no WAN route target is needed).  A single type-1 RD per service
   may be used.

   In order to support multihoming, the GWs will be provisioned with an
   I-ESI (see Section 3.4), which will be unique for each
   interconnection.  In this case, the I-ES will represent the group of
   PWs to the WAN PEs and GWs.  All the [RFC7432] procedures are still
   followed for the I-ES -- e.g., any MAC address learned from the WAN
   will be advertised to the DC with the I-ESI in the ESI field.

   A MAC-VRF per EVI will be created in each GW.  The MAC-VRF will have
   two different types of tunnel bindings instantiated in two different
   split-horizon groups:

   *  VPLS PWs will be instantiated in the WAN split-horizon group.

   *  Overlay tunnel bindings (e.g., VXLAN, NVGRE) will be instantiated
      in the DC split-horizon group.

   Attachment circuits are also supported on the same MAC-VRF (although
   not shown in Figure 2), but they will not be part of any of the above
   split-horizon groups.

   Traffic received in a given split-horizon group will never be
   forwarded to a member of the same split-horizon group.

   As far as BUM flooding is concerned, a flooding list will be composed
   of the sublist created by the inclusive multicast routes and the
   sublist created for VPLS in the WAN.  BUM frames received from a
   local Attachment Circuit (AC) will be forwarded to the flooding list.
   BUM frames received from the DC or the WAN will be forwarded to the
   flooding list, observing the split-horizon group rule described
   above.

   Note that the GWs are not allowed to have an EVPN binding and a PW to
   the same far end within the same MAC-VRF, so that loops and packet
   duplication are avoided.  In case a GW can successfully establish
   both an EVPN binding and a PW to the same far-end PE, the EVPN
   binding will prevail, and the PW will be brought down operationally.

   The optimization procedures described in Section 3.5 can also be
   applied to this model.

4.2.2.  Multihoming Procedures on the GWs

   This model supports single-active multihoming on the GWs.  All-active
   multihoming is not supported by VPLS; therefore, it cannot be used on
   the GWs.

   In this case, for a given EVI, all the PWs in the WAN split-horizon
   group are assigned to I-ES.  All the single-active multihoming
   procedures as described by [RFC8365] will be followed for the I-ES.

   The non-DF GW for the I-ES will block the transmission and reception
   of all the PWs in the WAN split-horizon group for BUM and unicast
   traffic.

4.3.  PBB-VPLS Interconnect for EVPN-Overlay Networks

4.3.1.  Control/Data Plane Setup Procedures on the GWs

   In this case, there is no impact on the procedures described in
   [RFC7041] for the B-component.  However, the I-component instances
   become EVI instances with EVPN-Overlay bindings and potentially local
   attachment circuits.  A number of MAC-VRF instances can be
   multiplexed into the same B-component instance.  This option provides
   significant savings in terms of PWs to be maintained in the WAN.

   The I-ESI concept described in Section 4.2.1 will also be used for
   the PBB-VPLS-based interconnect.

   B-component PWs and I-component EVPN-Overlay bindings established to
   the same far end will be compared.  The following rules will be
   observed:

   *  Attempts to set up a PW between the two GWs within the B-component
      context will never be blocked.

   *  If a PW exists between two GWs for the B-component and an attempt
      is made to set up an EVPN binding on an I-component linked to that
      B-component, the EVPN binding will be kept down operationally.
      Note that the BGP EVPN routes will still be valid but not used.

   *  The EVPN binding will only be up and used as long as there is no
      PW to the same far end in the corresponding B-component.  The EVPN
      bindings in the I-components will be brought down before the PW in
      the B-component is brought up.

   The optimization procedures described in Section 3.5 can also be
   applied to this interconnect option.

4.3.2.  Multihoming Procedures on the GWs

   This model supports single-active multihoming on the GWs.  All-active
   multihoming is not supported by this scenario.

   The single-active multihoming procedures as described by [RFC8365]
   will be followed for the I-ES for each EVI instance connected to the
   B-component.  Note that in this case, for a given EVI, all the EVPN
   bindings in the I-component are assigned to the I-ES.  The non-DF GW
   for the I-ES will block the transmission and reception of all the
   I-component EVPN bindings for BUM and unicast traffic.  When learning
   MACs from the WAN, the non-DF MUST NOT advertise EVPN MAC/IP routes
   for those MACs.

4.4.  EVPN-MPLS Interconnect for EVPN-Overlay Networks

   If EVPN for MPLS tunnels (referred to as "EVPN-MPLS" hereafter) are
   supported in the WAN, an end-to-end EVPN solution can be deployed.
   The following sections describe the proposed solution as well as its
   impact on the procedures from [RFC7432].

4.4.1.  Control plane Setup Procedures on the GWs

   The GWs MUST establish separate BGP sessions for sending/receiving
   EVPN routes to/from the DC and to/from the WAN.  Normally, each GW
   will set up one BGP EVPN session to the DC RR (or two BGP EVPN
   sessions if there are redundant DC RRs) and one session to the WAN RR
   (or two sessions if there are redundant WAN RRs).

   In order to facilitate separate BGP processes for DC and WAN, EVPN
   routes sent to the WAN SHOULD carry a different Route Distinguisher
   (RD) than the EVPN routes sent to the DC.  In addition, although
   reusing the same value is possible, different route targets are
   expected to be handled for the same EVI in the WAN and the DC.  Note
   that the EVPN service routes sent to the DC RRs will normally include
   a [RFC9012] BGP encapsulation extended community with a different
   tunnel type than the one sent to the WAN RRs.

   As in the other discussed options, an I-ES and its assigned I-ESI
   will be configured on the GWs for multihoming.  This I-ES represents
   the WAN EVPN-MPLS PEs to the DC but also the DC EVPN-Overlay NVEs to
   the WAN.  Optionally, different I-ESI values are configured for
   representing the WAN and the DC.  If different EVPN-Overlay networks
   are connected to the same group of GWs, each EVPN-Overlay network
   MUST get assigned a different I-ESI.

   Received EVPN routes will never be reflected on the GWs but instead
   will be consumed and re-advertised (if needed):

   *  Ethernet A-D routes, ES routes, and Inclusive Multicast routes are
      consumed by the GWs and processed locally for the corresponding
      [RFC7432] procedures.

   *  MAC/IP advertisement routes will be received and imported, and if
      they become active in the MAC-VRF, the information will be re-
      advertised as new routes with the following fields:

      -  The RD will be the GW's RD for the MAC-VRF.

      -  The ESI will be set to the I-ESI.

      -  The Ethernet-tag value will be kept from the received NLRI the
         received NLRI.

      -  The MAC length, MAC address, IP Length, and IP address values
         will be kept from the received NLRI.

      -  The MPLS label will be a local 20-bit value (when sent to the
         WAN) or a DC-global 24-bit value (when sent to the DC for
         encapsulations using a VNI).

      -  The appropriate Route Targets (RTs) and [RFC9012] BGP
         encapsulation extended community will be used according to
         [RFC8365].

   The GWs will also generate the following local EVPN routes that will
   be sent to the DC and WAN, with their corresponding RTs and [RFC9012]
   BGP encapsulation extended community values:

   *  ES route(s) for the I-ESI(s).

   *  Ethernet A-D routes per ES and EVI for the I-ESI(s).  The A-D per-
      EVI routes sent to the WAN and the DC will have consistent
      Ethernet-Tag values.

   *  Inclusive Multicast routes with independent tunnel-type value for
      the WAN and DC.  For example, a P2MP Label Switched Path (LSP) may
      be used in the WAN, whereas ingress replication may be used in the
      DC.  The routes sent to the WAN and the DC will have a consistent
      Ethernet-Tag.

   *  MAC/IP advertisement routes for MAC addresses learned in local
      attachment circuits.  Note that these routes will not include the
      I-ESI value in the ESI field.  These routes will include a zero
      ESI or a non-zero ESI for local multihomed Ethernet Segments (ES).
      The routes sent to the WAN and the DC will have a consistent
      Ethernet-Tag.

   Assuming GW1 and GW2 are peer GWs of the same DC, each GW will
   generate two sets of the above local service routes: set-DC will be
   sent to the DC RRs and will include an A-D per EVI, Inclusive
   Multicast, and MAC/IP routes for the DC encapsulation and RT.  Set-
   WAN will be sent to the WAN RRs and will include the same routes but
   using the WAN RT and encapsulation.  GW1 and GW2 will receive each
   other's set-DC and set-WAN.  This is the expected behavior on GW1 and
   GW2 for locally generated routes:

   *  Inclusive multicast routes: When setting up the flooding lists for
      a given MAC-VRF, each GW will include its DC peer GW only in the
      EVPN-MPLS flooding list (by default) and not the EVPN-Overlay
      flooding list.  That is, GW2 will import two Inclusive Multicast
      routes from GW1 (from set-DC and set-WAN) but will only consider
      one of the two, giving the set-WAN route higher priority.  An
      administrative option MAY change this preference so that the set-
      DC route is selected first.

   *  MAC/IP advertisement routes for local attachment circuits: As
      above, the GW will select only one, giving the route from the set-
      WAN a higher priority.  As with the Inclusive multicast routes, an
      administrative option MAY change this priority.

4.4.2.  Data Plane Setup Procedures on the GWs

   The procedure explained at the end of the previous section will make
   sure there are no loops or packet duplication between the GWs of the
   same EVPN-Overlay network (for frames generated from local ACs),
   since only one EVPN binding per EVI (or per Ethernet Tag in the case
   of VLAN-aware bundle services) will be set up in the data plane
   between the two nodes.  That binding will by default be added to the
   EVPN-MPLS flooding list.

   As for the rest of the EVPN tunnel bindings, they will be added to
   one of the two flooding lists that each GW sets up for the same MAC-
   VRF:

   *  EVPN-Overlay flooding list (composed of bindings to the remote
      NVEs or multicast tunnel to the NVEs).

   *  EVPN-MPLS flooding list (composed of MP2P or LSM tunnel to the
      remote PEs).

   Each flooding list will be part of a separate split-horizon group:
   the WAN split-horizon group or the DC split-horizon group.  Traffic
   generated from a local AC can be flooded to both split-horizon
   groups.  Traffic from a binding of a split-horizon group can be
   flooded to the other split-horizon group and local ACs, but never to
   a member of its own split-horizon group.

   When either GW1 or GW2 receives a BUM frame on an MPLS tunnel,
   including an ESI label at the bottom of the stack, they will perform
   an ESI label lookup and split-horizon filtering as per [RFC7432], in
   case the ESI label identifies a local ESI (I-ESI or any other nonzero
   ESI).

4.4.3.  Multihoming Procedure Extensions on the GWs

   This model supports single-active as well as all-active multihoming.

   All the [RFC7432] multihoming procedures for the DF election on
   I-ES(s), as well as the backup-path (single-active) and aliasing
   (all-active) procedures, will be followed on the GWs.  Remote PEs in
   the EVPN-MPLS network will follow regular [RFC7432] aliasing or
   backup-path procedures for MAC/IP routes received from the GWs for
   the same I-ESI.  So will NVEs in the EVPN-Overlay network for MAC/IP
   routes received with the same I-ESI.

   As far as the forwarding plane is concerned, by default, the EVPN-
   Overlay network will have an analogous behavior to the access ACs in
   [RFC7432] multihomed Ethernet Segments.

   The forwarding behavior on the GWs is described below:

   *  Single-active multihoming; assuming a WAN split-horizon group
      (comprised of EVPN-MPLS bindings), a DC split-horizon group
      (comprised of EVPN-Overlay bindings), and local ACs on the GWs:

      -  Forwarding behavior on the non-DF: The non-DF MUST block
         ingress and egress forwarding on the EVPN-Overlay bindings
         associated to the I-ES.  The EVPN-MPLS network is considered to
         be the core network, and the EVPN-MPLS bindings to the remote
         PEs and GWs will be active.

      -  Forwarding behavior on the DF: The DF MUST NOT forward BUM or
         unicast traffic received from a given split-horizon group to a
         member of its own split-horizon group.  Forwarding to other
         split-horizon groups and local ACs is allowed (as long as the
         ACs are not part of an ES for which the node is non-DF).  As
         per [RFC7432] and for split-horizon purposes, when receiving
         BUM traffic on the EVPN-Overlay bindings associated to an I-ES,
         the DF GW SHOULD add the I-ESI label when forwarding to the
         peer GW over EVPN-MPLS.

      -  When receiving EVPN MAC/IP routes from the WAN, the non-DF MUST
         NOT reoriginate the EVPN routes and advertise them to the DC
         peers.  In the same way, EVPN MAC/IP routes received from the
         DC MUST NOT be advertised to the WAN peers.  This is consistent
         with [RFC7432] and allows the remote PE/NVEs to know who the
         primary GW is, based on the reception of the MAC/IP routes.

   *  All-active multihoming; assuming a WAN split-horizon group
      (comprised of EVPN-MPLS bindings), a DC split-horizon group
      (comprised of EVPN-Overlay bindings), and local ACs on the GWs:

      -  Forwarding behavior on the non-DF: The non-DF follows the same
         behavior as the non-DF in the single-active case, but only for
         BUM traffic.  Unicast traffic received from a split-horizon
         group MUST NOT be forwarded to a member of its own split-
         horizon group but can be forwarded normally to the other split-
         horizon groups and local ACs.  If a known unicast packet is
         identified as a "flooded" packet, the procedures for BUM
         traffic MUST be followed.

      -  Forwarding behavior on the DF: The DF follows the same behavior
         as the DF in the single-active case, but only for BUM traffic.
         Unicast traffic received from a split-horizon group MUST NOT be
         forwarded to a member of its own split-horizon group but can be
         forwarded normally to the other split-horizon group and local
         ACs.  If a known unicast packet is identified as a "flooded"
         packet, the procedures for BUM traffic MUST be followed.  As
         per [RFC7432] and for split-horizon purposes, when receiving
         BUM traffic on the EVPN-Overlay bindings associated to an I-ES,
         the DF GW MUST add the I-ESI label when forwarding to the peer
         GW over EVPN-MPLS.

      -  Contrary to the single-active multihoming case, both DF and
         non-DF reoriginate and advertise MAC/IP routes received from
         the WAN/DC peers, adding the corresponding I-ESI so that the
         remote PE/NVEs can perform regular aliasing, as per [RFC7432].

   The example in Figure 3 illustrates the forwarding of BUM traffic
   originated from an NVE on a pair of all-active multihoming GWs.

        |<--EVPN-Overlay--->|<--EVPN-MPLS-->|

                +---------+ +--------------+
         +----+ BUM       +---+             |
         |NVE1+----+----> |   +-+-----+     |
         +----+  | |   DF |GW1| |     |     |
                 | |      +-+-+ |     |    ++--+
                 | |        |   |     +--> |PE1|
                 | +--->X +-+-+ |          ++--+
                 |     NDF|   | |           |
         +----+  |        |GW2<-+           |
         |NVE2+--+        +-+-+             |
         +----+  +--------+ |  +------------+
                            v
                          +--+
                          |CE|
                          +--+

                    Figure 3: Multihoming BUM Forwarding

   GW2 is the non-DF for the I-ES and blocks the BUM forwarding.  GW1 is
   the DF and forwards the traffic to PE1 and GW2.  Packets sent to GW2
   will include the ESI label for the I-ES.  Based on the ESI label, GW2
   identifies the packets as I-ES-generated packets and will only
   forward them to local ACs (CE in the example) and not back to the
   EVPN-Overlay network.

4.4.4.  Impact on MAC Mobility Procedures

   MAC Mobility procedures described in [RFC7432] are not modified by
   this document.

   Note that an intra-DC MAC move still leaves the MAC attached to the
   same I-ES, so under the rules of [RFC7432], this is not considered a
   MAC Mobility event.  Only when the MAC moves from the WAN domain to
   the DC domain (or from one DC to another) will the MAC be learned
   from a different ES, and the MAC Mobility procedures will kick in.

   The sticky-bit indication in the MAC Mobility extended community MUST
   be propagated between domains.

4.4.5.  Gateway Optimizations

   All the Gateway optimizations described in Section 3.5 MAY be applied
   to the GWs when the interconnect is based on EVPN-MPLS.

   In particular, the use of the Unknown MAC Route, as described in
   Section 3.5.1, solves some transient packet-duplication issues in
   cases of all-active multihoming, as explained below.

   Consider the diagram in Figure 2 for EVPN-MPLS interconnect and all-
   active multihoming, and the following sequence:

   (a)  MAC Address M1 is advertised from NVE3 in EVI-1.

   (b)  GW3 and GW4 learn M1 for EVI-1 and re-advertise M1 to the WAN
        with I-ESI-2 in the ESI field.

   (c)  GW1 and GW2 learn M1 and install GW3/GW4 as next hops following
        the EVPN aliasing procedures.

   (d)  Before NVE1 learns M1, a packet arrives at NVE1 with destination
        M1.  If the Unknown MAC Route had not been advertised into the
        DC, NVE1 would have flooded the packet throughout the DC, in
        particular to both GW1 and GW2.  If the same VNI/VSID is used
        for both known unicast and BUM traffic, as is typically the
        case, there is no indication in the packet that it is a BUM
        packet, and both GW1 and GW2 would have forwarded it, creating
        packet duplication.  However, because the Unknown MAC Route had
        been advertised into the DC, NVE1 will unicast the packet to
        either GW1 or GW2.

   (e)  Since both GW1 and GW2 know M1, the GW receiving the packet will
        forward it to either GW3 or GW4.

4.4.6.  Benefits of the EVPN-MPLS Interconnect Solution

   The "DCI using ASBRs" solution described in [RFC8365] and the GW
   solution with EVPN-MPLS interconnect may be seen as similar, since
   they both retain the EVPN attributes between Data Centers and
   throughout the WAN.  However, the EVPN-MPLS interconnect solution on
   the GWs has significant benefits compared to the "DCI using ASBRs"
   solution:

   *  As in any of the described GW models, this solution supports the
      connectivity of local attachment circuits on the GWs.  This is not
      possible in a "DCI using ASBRs" solution.

   *  Different data plane encapsulations can be supported in the DC and
      the WAN, while a uniform encapsulation is needed in the "DCI using
      ASBRs" solution.

   *  Optimized multicast solution, with independent inclusive multicast
      trees in DC and WAN.

   *  MPLS label aggregation: For the case where MPLS labels are
      signaled from the NVEs for MAC/IP advertisement routes, this
      solution provides label aggregation.  A remote PE MAY receive a
      single label per GW MAC-VRF, as opposed to a label per NVE/MAC-VRF
      connected to the GW MAC-VRF.  For instance, in Figure 2, PE would
      receive only one label for all the routes advertised for a given
      MAC-VRF from GW1, as opposed to a label per NVE/MAC-VRF.

   *  The GW will not propagate MAC Mobility for the MACs moving within
      a DC.  Mobility intra-DC is solved by all the NVEs in the DC.  The
      MAC Mobility procedures on the GWs are only required in case of
      mobility across DCs.

   *  Proxy-ARP/ND function on the DC GWs can be leveraged to reduce
      ARP/ND flooding in the DC or/and the WAN.

4.5.  PBB-EVPN Interconnect for EVPN-Overlay Networks

   PBB-EVPN [RFC7623] is yet another interconnect option.  It requires
   the use of GWs where I-components and associated B-components are
   part of EVI instances.

4.5.1.  Control/Data Plane Setup Procedures on the GWs

   EVPN will run independently in both components, the I-component MAC-
   VRF and B-component MAC-VRF.  Compared to [RFC7623], the DC customer
   MACs (C-MACs) are no longer learned in the data plane on the GW but
   in the control plane through EVPN running on the I-component.  Remote
   C-MACs coming from remote PEs are still learned in the data plane.
   B-MACs in the B-component will be assigned and advertised following
   the procedures described in [RFC7623].

   An I-ES will be configured on the GWs for multihoming, but its I-ESI
   will only be used in the EVPN control plane for the I-component EVI.
   No unreserved ESIs will be used in the control plane of the
   B-component EVI, as per [RFC7623].  That is, the I-ES will be
   represented to the WAN PBB-EVPN PEs using shared or dedicated B-MACs.

   The rest of the control plane procedures will follow [RFC7432] for
   the I-component EVI and [RFC7623] for the B-component EVI.

   From the data plane perspective, the I-component and B-component EVPN
   bindings established to the same far end will be compared, and the
   I-component EVPN-Overlay binding will be kept down following the
   rules described in Section 4.3.1.

4.5.2.  Multihoming Procedures on the GWs

   This model supports single-active as well as all-active multihoming.

   The forwarding behavior of the DF and non-DF will be changed based on
   the description outlined in Section 4.4.3, substituting the WAN
   split-horizon group for the B-component, and using [RFC7623]
   procedures for the traffic sent or received on the B-component.

4.5.3.  Impact on MAC Mobility Procedures

   C-MACs learned from the B-component will be advertised in EVPN within
   the I-component EVI scope.  If the C-MAC was previously known in the
   I-component database, EVPN would advertise the C-MAC with a higher
   sequence number, as per [RFC7432].  From the perspective of Mobility
   and the related procedures described in [RFC7432], the C-MACs learned
   from the B-component are considered local.

4.5.4.  Gateway Optimizations

   All the considerations explained in Section 4.4.5 are applicable to
   the PBB-EVPN interconnect option.

4.6.  EVPN-VXLAN Interconnect for EVPN-Overlay Networks

   If EVPN for Overlay tunnels is supported in the WAN, and a GW
   function is required, an end-to-end EVPN solution can be deployed.
   While multiple Overlay tunnel combinations at the WAN and the DC are
   possible (MPLSoGRE, NVGRE, etc.), VXLAN is described here, given its
   popularity in the industry.  This section focuses on the specific
   case of EVPN for VXLAN (EVPN-VXLAN hereafter) and the impact on the
   [RFC7432] procedures.

   The procedures described in Section 4.4 apply to this section, too,
   only substituting EVPN-MPLS for EVPN-VXLAN control plane specifics
   and using [RFC8365] "Local Bias" procedures instead of Section 4.4.3.
   Since there are no ESI labels in VXLAN, GWs need to rely on "Local
   Bias" to apply split horizon on packets generated from the I-ES and
   sent to the peer GW.

   This use case assumes that NVEs need to use the VNIs or VSIDs as
   globally unique identifiers within a Data Center, and a Gateway needs
   to be employed at the edge of the Data-Center network to translate
   the VNI or VSID when crossing the network boundaries.  This GW
   function provides VNI and tunnel-IP-address translation.  The use
   case in which local downstream-assigned VNIs or VSIDs can be used
   (like MPLS labels) is described by [RFC8365].

   While VNIs are globally significant within each DC, there are two
   possibilities in the interconnect network:

   1.  Globally unique VNIs in the interconnect network.  In this case,
       the GWs and PEs in the interconnect network will agree on a
       common VNI for a given EVI.  The RT to be used in the
       interconnect network can be autoderived from the agreed-upon
       interconnect VNI.  The VNI used inside each DC MAY be the same as
       the interconnect VNI.

   2.  Downstream-assigned VNIs in the interconnect network.  In this
       case, the GWs and PEs MUST use the proper RTs to import/export
       the EVPN routes.  Note that even if the VNI is downstream
       assigned in the interconnect network, and unlike option (a), it
       only identifies the <Ethernet Tag, GW> pair and not the <Ethernet
       Tag, egress PE> pair.  The VNI used inside each DC MAY be the
       same as the interconnect VNI.  GWs SHOULD support multiple VNI
       spaces per EVI (one per interconnect network they are connected
       to).

   In both options, NVEs inside a DC only have to be aware of a single
   VNI space, and only GWs will handle the complexity of managing
   multiple VNI spaces.  In addition to VNI translation above, the GWs
   will provide translation of the tunnel source IP for the packets
   generated from the NVEs, using their own IP address.  GWs will use
   that IP address as the BGP next hop in all the EVPN updates to the
   interconnect network.

   The following sections provide more details about these two options.

4.6.1.  Globally Unique VNIs in the Interconnect Network

   Considering Figure 2, if a host H1 in NVO-1 needs to communicate with
   a host H2 in NVO-2, and assuming that different VNIs are used in each
   DC for the same EVI (e.g., VNI-10 in NVO-1 and VNI-20 in NVO-2), then
   the VNIs MUST be translated to a common interconnect VNI (e.g., VNI-
   100) on the GWs.  Each GW is provisioned with a VNI translation
   mapping so that it can translate the VNI in the control plane when
   sending BGP EVPN route updates to the interconnect network.  In other
   words, GW1 and GW2 MUST be configured to map VNI-10 to VNI-100 in the
   BGP update messages for H1's MAC route.  This mapping is also used to
   translate the VNI in the data plane in both directions: that is,
   VNI-10 to VNI-100 when the packet is received from NVO-1 and the
   reverse mapping from VNI-100 to VNI-10 when the packet is received
   from the remote NVO-2 network and needs to be forwarded to NVO-1.

   The procedures described in Section 4.4 will be followed, considering
   that the VNIs advertised/received by the GWs will be translated
   accordingly.

4.6.2.  Downstream-Assigned VNIs in the Interconnect Network

   In this case, if a host H1 in NVO-1 needs to communicate with a host
   H2 in NVO-2, and assuming that different VNIs are used in each DC for
   the same EVI, e.g., VNI-10 in NVO-1 and VNI-20 in NVO-2, then the
   VNIs MUST be translated as in Section 4.6.1.  However, in this case,
   there is no need to translate to a common interconnect VNI on the
   GWs.  Each GW can translate the VNI received in an EVPN update to a
   locally assigned VNI advertised to the interconnect network.  Each GW
   can use a different interconnect VNI; hence, this VNI does not need
   to be agreed upon on all the GWs and PEs of the interconnect network.

   The procedures described in Section 4.4 will be followed, taking into
   account the considerations above for the VNI translation.

5.  Security Considerations

   This document applies existing specifications to a number of
   interconnect models.  The security considerations included in those
   documents, such as [RFC7432], [RFC8365], [RFC7623], [RFC4761], and
   [RFC4762] apply to this document whenever those technologies are
   used.

   As discussed, [RFC8365] discusses two main DCI solution groups: "DCI
   using GWs" and "DCI using ASBRs".  This document specifies the
   solutions that correspond to the "DCI using GWs" group.  It is
   important to note that the use of GWs provides a superior level of
   security on a per-tenant basis, compared to the use of ASBRs.  This
   is due to the fact that GWs need to perform a MAC lookup on the
   frames being received from the WAN, and they apply security
   procedures, such as filtering of undesired frames, filtering of
   frames with a source MAC that matches a protected MAC in the DC, or
   application of MAC-duplication procedures defined in [RFC7432].  On
   ASBRs, though, traffic is forwarded based on a label or VNI swap, and
   there is usually no visibility of the encapsulated frames, which can
   carry malicious traffic.

   In addition, the GW optimizations specified in this document provide
   additional protection of the DC tenant systems.  For instance, the
   MAC-address advertisement control and Unknown MAC Route defined in
   Section 3.5.1 protect the DC NVEs from being overwhelmed with an
   excessive number of MAC/IP routes being learned on the GWs from the
   WAN.  The ARP/ND flooding control described in Section 3.5.2 can
   reduce/suppress broadcast storms being injected from the WAN.

   Finally, the reader should be aware of the potential security
   implications of designing a DCI with the decoupled interconnect
   solution (Section 3) or the integrated interconnect solution
   (Section 4).  In the decoupled interconnect solution, the DC is
   typically easier to protect from the WAN, since each GW has a single
   logical link to one WAN PE, whereas in the Integrated solution, the
   GW has logical links to all the WAN PEs that are attached to the
   tenant.  In either model, proper control plane and data plane
   policies should be put in place in the GWs in order to protect the DC
   from potential attacks coming from the WAN.

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  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>.

   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <https://www.rfc-editor.org/info/rfc4761>.

   [RFC4762]  Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
              LAN Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
              <https://www.rfc-editor.org/info/rfc4762>.

   [RFC6074]  Rosen, E., Davie, B., Radoaca, V., and W. Luo,
              "Provisioning, Auto-Discovery, and Signaling in Layer 2
              Virtual Private Networks (L2VPNs)", RFC 6074,
              DOI 10.17487/RFC6074, January 2011,
              <https://www.rfc-editor.org/info/rfc6074>.

   [RFC7041]  Balus, F., Ed., Sajassi, A., Ed., and N. Bitar, Ed.,
              "Extensions to the Virtual Private LAN Service (VPLS)
              Provider Edge (PE) Model for Provider Backbone Bridging",
              RFC 7041, DOI 10.17487/RFC7041, November 2013,
              <https://www.rfc-editor.org/info/rfc7041>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC7543]  Jeng, H., Jalil, L., Bonica, R., Patel, K., and L. Yong,
              "Covering Prefixes Outbound Route Filter for BGP-4",
              RFC 7543, DOI 10.17487/RFC7543, May 2015,
              <https://www.rfc-editor.org/info/rfc7543>.

   [RFC7623]  Sajassi, A., Ed., Salam, S., Bitar, N., Isaac, A., and W.
              Henderickx, "Provider Backbone Bridging Combined with
              Ethernet VPN (PBB-EVPN)", RFC 7623, DOI 10.17487/RFC7623,
              September 2015, <https://www.rfc-editor.org/info/rfc7623>.

   [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>.

   [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>.

7.2.  Informative References

   [IEEE.802.1AG]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks Virtual Bridged Local Area Networks Amendment 5:
              Connectivity Fault Management", IEEE standard 802.1ag-
              2007, January 2008.

   [IEEE.802.1Q]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks--Bridges and Bridged Networks", IEEE standard
              802.1Q-2014, DOI 10.1109/IEEESTD.2014.6991462, December
              2014, <https://doi.org/10.1109/IEEESTD.2014.6991462>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
              <https://www.rfc-editor.org/info/rfc4023>.

   [RFC4684]  Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
              R., Patel, K., and J. Guichard, "Constrained Route
              Distribution for Border Gateway Protocol/MultiProtocol
              Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
              Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
              November 2006, <https://www.rfc-editor.org/info/rfc4684>.

   [RFC6870]  Muley, P., Ed. and M. Aissaoui, Ed., "Pseudowire
              Preferential Forwarding Status Bit", RFC 6870,
              DOI 10.17487/RFC6870, February 2013,
              <https://www.rfc-editor.org/info/rfc6870>.

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
              <https://www.rfc-editor.org/info/rfc7348>.

   [RFC7637]  Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation",
              RFC 7637, DOI 10.17487/RFC7637, September 2015,
              <https://www.rfc-editor.org/info/rfc7637>.

   [VIRTUAL-ES]
              Sajassi, A., Brissette, P., Schell, R., Drake, J. E., and
              J. Rabadan, "EVPN Virtual Ethernet Segment", Work in
              Progress, Internet-Draft, draft-ietf-bess-evpn-virtual-
              eth-segment-06, 9 March 2020,
              <https://tools.ietf.org/html/draft-ietf-bess-evpn-virtual-
              eth-segment-06>.

   [Y.1731]   ITU-T, "OAM functions and mechanisms for Ethernet based
              networks", ITU-T Recommendation Y.1731, August 2019.

Acknowledgments

   The authors would like to thank Neil Hart, Vinod Prabhu, and Kiran
   Nagaraj for their valuable comments and feedback.  We would also like
   to thank Martin Vigoureux and Alvaro Retana for their detailed
   reviews and comments.

Contributors

   In addition to the authors listed on the front page, the following
   coauthors have also contributed to this document:

   Ravi Shekhar
   Juniper Networks


   Anil Lohiya
   Juniper Networks


   Wen Lin
   Juniper Networks


   Florin Balus
   Cisco


   Patrice Brissette
   Cisco


   Senad Palislamovic
   Nokia


   Dennis Cai
   Alibaba


Authors' Addresses

   Jorge Rabadan (editor)
   Nokia
   777 E. Middlefield Road
   Mountain View, CA 94043
   United States of America

   Email: jorge.rabadan@nokia.com


   Senthil Sathappan
   Nokia

   Email: senthil.sathappan@nokia.com


   Wim Henderickx
   Nokia

   Email: wim.henderickx@nokia.com


   Ali Sajassi
   Cisco

   Email: sajassi@cisco.com


   John Drake
   Juniper

   Email: jdrake@juniper.net