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Versions: 00 01 02 03

Internet Engineering Task Force                           R. Sharma, Ed.
Internet-Draft                                               A. Banerjee
Intended status: Standards Track                             R. Sivaramu
Expires: November 7, 2016                                  Cisco Systems
                                                             May 6, 2016


           Multi-site EVPN based VXLAN using Border Gateways
                    draft-sharma-multi-site-evpn-00

Abstract

   This document describes the procedures for interconnecting two or
   more BGP based Ethernet VPN (EVPN) sites in a scalable fashion over
   an IP-only network.  The motivation is to support extension of EVPN
   sites without having to rely on typical Data Center Interconnect
   (DCI) technologies like MPLS/VPLS for the interconnection.  The
   requirements for such a deployment are very similar to the ones
   specified in RFC 7209 -- "Requirements for Ethernet VPN (EVPN)".

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
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   This Internet-Draft will expire on November 7, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   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



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   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Multi-Site EVPN Overview  . . . . . . . . . . . . . . . . . .   4
     3.1.  MS-EVPN Interconnect Requirements . . . . . . . . . . . .   4
     3.2.  MS-EVPN Interconnect concept and framework  . . . . . . .   5
   4.  Multi-site EVPN Interconnect Procedures . . . . . . . . . . .   8
     4.1.  Border Auto-Discovery Route . . . . . . . . . . . . . . .   8
     4.2.  Border Gateway Provisioning . . . . . . . . . . . . . . .  10
       4.2.1.  Border Gateway Designated Forwarder Election  . . . .  11
       4.2.2.  All-active Border Gateway . . . . . . . . . . . . . .  11
       4.2.3.  Multi-path Border Gateway . . . . . . . . . . . . . .  12
     4.3.  EVPN route processing at Border Gateway . . . . . . . . .  12
     4.4.  Multi-Destination tree between Border Gateways  . . . . .  14
     4.5.  Inter-site Unicast traffic  . . . . . . . . . . . . . . .  14
     4.6.  Inter-site Multi-destination traffic  . . . . . . . . . .  15
     4.7.  Host Mobility . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Convergence . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Fabric to Border Gateway Failure  . . . . . . . . . . . .  15
     5.2.  Border Gateway to Border Gateway Failures . . . . . . . .  15
   6.  Interoperability  . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Isolation of Fault Domains  . . . . . . . . . . . . . . . . .  16
   8.  Loop detection and Prevention . . . . . . . . . . . . . . . .  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Additional Stuff . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   BGP based Ethernet VPNs (EVPNs) are being used to support various VPN
   topologies with the motivation and requirements being discussed in
   detail in RFC7209 [RFC7209].  EVPN has been used to provide a Network
   Virtualization Overly (NVO) solution with a variety of tunnel
   encapsulation options over IP as described in [DCI-EVPN-OVERLAY].
   EVPN used for the Data center interconnect (DCI) at the WAN Edge is
   discussed in [DCI-EVPN-OVERLAY].  The EVPN DCI procedures are defined
   for IP and MPLS hand-off at the site boundaries.



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   In the current EVPN deployments, there is a need to segment the EVPN
   domains within a Data Center (DC) primarily due to the service
   architecture and the scaling requirements around it.  The number of
   routes, tunnel end-points, and next-hops needed in the DC are larger
   than some of the hardware elements that are being deployed.  Network
   operators would like to ensure that they have means to have smaller
   sites within the data center, if they so desire, without having to
   have traditional DCI technologies to inter-connect them.  In essence,
   they want smaller multi-site EVPN domains with an IP backbone.

   Network operators today are using the Virtual Network Identifier
   (VNI) to designate a service.  However, they would like to have this
   service available to a smaller set of nodes within the DC for
   administrative reasons; in essence they want to break up the EVPN
   domain to multiple smaller sites.  An advantage of having a smaller
   footprint for these EVPN sites, implies that the various fault
   isolation domains are now more constrained.  It is also feasible to
   have features that can re-use the VNI space across these sites if
   desired.  The above mentioned motivations for having smaller multi-
   site EVPN domains are over and above the ones that are already
   detailed in RFC7209 [RFC7209].

   In this document we focus primarily on the VXLAN encapsulation for
   EVPN deployments.  We assume that the underlay provides simple IP
   connectivity.  We go into the details of the IP/VXLAN hand-off
   mechanisms, to interconnect these smaller sites, within the data
   center itself.  We describe this deployment model as a scalable
   multi-site EVPN (MS-EVPN) deployment.  The procedures described here
   go into substantial detail regarding interconnecting L2 and L3,
   unicast and multicast domains across multiple EVPN sites.

1.1.  Requirements Language

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

2.  Terminology

   o  Border Gateway (BG): This is the node that interacts with nodes
      within a site and with nodes that are external to the site.  For
      example, in a leaf-spine data center fabric, it can be a leaf, a
      Spine, or a separate device acting as gateway to interconnect the
      sites.

   o  All-Active Border Gateway: A Virtual set of shared Border Gateways
      (or Next-hops) acting as Multiple entry-exit points for a site.




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   o  Single-Active Border Gateway: A Virtual set of unique border
      Gateways (or Next-hops) acting as a Multiple entry-exit points for
      a site.

   o  A-D: Auto-discovery.

3.  Multi-Site EVPN Overview

   In this section we describe the motivation, requirements, and
   framework of the multi-site EVPN enhancements.

3.1.  MS-EVPN Interconnect Requirements

   In this section we discuss the requirements and motivation for
   interconnecting different EVPN sites within a data center.  In
   general any interconnect technology has the following requirements:

   a.  Scalability: Multi-Site EVPN (MS-EVPN) should be able to
       interconnect multiple sites in a scalable fashion.  In other
       words, interconnecting such sites should not lead to one giant
       fabric with full mesh of end-to-end VXLAN tunnels across leafs in
       different sites.  This leads to scale issues with respect to
       managing large number of tunnel end-points and a large number of
       tunnel next-hops.  Also a huge flat fabric rules out option of
       ingress replication (IR) trees as number of replications becomes
       practically unachievable due to the internal bandwidth needed in
       hardware.

   b.  Multi-Destination traffic over unicast-only cloud: MS-EVPN
       mechanisms should be able to provide an efficient forwarding
       mechanism for multi-destination frames even if the underlay
       inter-site network is not capable of forwarding multicast frames.
       This requirement is meant to ensure that for the solution to work
       there are no additional constraints being requested of the IP
       network.  This allows for use of existing network elements as-is.

   c.  Maintain Site-specific Administrative control: The MS-EVPN
       technology should be able to interconnect fabrics from different
       Administrative domains.  It is possible that different sites have
       different VLAN-VNI mappings, use different underlay routing
       protocols, and/or have different PIM-SM group ranges etc.  It is
       expected that the technology should not impose any additional
       constraints on the various administrative domains.

   d.  Isolate fault domains: MS-EVPN technology hand-off should have
       capability to isolate traffic cross site boundaries and prevent
       defects to percolate from one site to another.  As an example, a




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       broadcast storm in a site should not lead to meltdown of all
       other sites.

   e.  Loop detection and prevention: In the scenarios where flood
       domains are stretched across fabrics, interconnecting sites are
       very vulnerable to loops and flood storms.  There is a need to
       provide comprehensive loop detection and prevention capabilities.

   f.  Plug-and-play and extensibility: Addition of new sites or
       increasing capacity of existing sites should be achievable in a
       completely plug-and-play fashion.  This essentially means that
       all control plane and forwarding states (L2 or L3 interconnect)
       should be built in downstream allocation mode.  MS-EVPN should
       not pose any maximum requirements on the scale and capacity, it
       should be easily extendable on those metrics.

3.2.  MS-EVPN Interconnect concept and framework

   EVPN with an IP-only interconnect is conceptualized as multiple site-
   local EVPN control planes and IP forwarding domains interconnected
   via a single common EVPN control and IP forwarding domain.  Every
   EVPN node is identified with a unique site-scope identifier.  A site-
   local EVPN domain consists of EVPN nodes with the same site
   identifier.  Border gateways on one hand are also part of site-
   specific EVPN domain and on other hand part of a common EVPN domain
   to interconnect with Border Gateways from other sites.  Although a
   border gateway has only a single explicit site-id (that of the site
   it is a member of), it can be considered to also have a second
   implicit site-id, that of the interconnect-domain which has
   membership of all the BG's from all sites that are being
   interconnected.  This implicit site-id membership is derived by the
   presence of the Border A-D route announced by that border gateway
   node.

   These border gateways discover each other through EVPN Border A-D
   routes and act as both control and forwarding plane gateway across
   sites.  This will facilitate site-specific nodes to visualize all
   other sites to be reachable only via its Border Gateways.

   We describe the MS-EVPN deployment model using the topology below.
   In the topology there are 3 sites, Site A, Site B, and Site C that
   are inter-connected using IP.  This entire topology is deemed to be
   part of the same Data Center.  In most deployments these sites can be
   thought of as pods, which may span a rack, a row, or multiple rows in
   the data center, depending on the size of domain desired for scale
   and fault and/or administrative isolation domains.





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   ____________________________
   | ooo Encapsulation tunnel |
   | X X X  Leaf-spine fabric |
   |__________________________|


     Site A (EVPN site A)               Site B (EVPN site B)
    ___________________________      ____________________________
   |      X X X X X X X X     |      |      X X X X X X X X     |
   |         X X X X          |      |         X X X X          |
   |        o       o         |      |        o       o         |
   |BG-1 Site A    BG-2 Site A|      |BG-1 Site B    BG-2 Site B|
    ___________________________      ____________________________
           o           o                o               o
            o           o              o               o
             o           o            o               o
              o           o          o               o
          _______________________________________________
          |                                             |
          |                                             |
          |        Inter-site common EVPN site          |
          |                                             |
          |                                             |
          _______________________________________________
                        o                   o
                         o                 o
                          o               o
                           o             o
                      ___________________________
                      | BG-1 Site C    BG-2 Site C|
                      |         X X X X           |
                      |      X X X X X X X X      |
                      _____________________________
                       Site C (EVPN site C)

                                 Figure 1

   In this topology, site-local nodes are connected to each other by
   iBGP EVPN peering and Border Gateways are connected by eBGP Muti-hop
   EVPN peering via inter-site cloud.  We explicitly spell this out to
   ensure that we can re-use BGP semantics of route announcement between
   and across the sites.  There are other BGP mechanisms to instantiate
   this and they are not discussed in this document.  This implies that
   each domain has its own AS number associated with it.  In the
   topology, only 2 border gateway per site are shown; this is more for
   ease of illustration and explanation.  The technology poses no such
   limitation.  As mentioned earlier, site-specific EVPN domain will
   consists of only site-local nodes in the sites.  A Border Gateway is



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   logically partitioned into site specific EVPN domain towards the site
   and into common EVPN domain towards other sites.  This facilitates
   them to acts as control and forwarding plane gateway for forwarding
   traffic across sites.

   EVPN nodes with in a site will discover each other via regular EVPN
   procedures and build site-local bidirectional VXLAN tunnels and
   multi-destination trees from leaves to Border Gateways.  Border
   Gateways will discover each other by Border A-D routes (defined in
   Section 4.1) and build inter-site bi-directional VXLAN tunnels and
   Multi-destination trees between them.  We thus build an end-to-end
   bidirectional forwarding path across all sites by stitching (and not
   by stretching end-to-end) site-local VXLAN tunnels with inter-site
   VXLAN tunnels.

   In essence, a MS-EVPN fabric is proposed to be built in complete
   downstream and modular fashion.

   o  Site-local Bridging domains are interconnected ONLY via Border
      Gateways with Bridging domains from other sites.  Such
      interconnect do not assume uniform mappings of mac-vrf VNI-VLAN
      across sites and stitches such bridging domains in complete
      downstream fashion using EVPN route advertisements.

   o  Site-local Routing domains are interconnected ONLY via Border
      Gateways with Routing domains from other sites.  Such interconnect
      do not assume uniform mappings of IP VRF-VNI across sites and
      stitches such routing domains in complete downstream fashion using
      EVPN route advertisements.

   o  Site-local Flood domains are interconnected ONLY via Border
      Gateways with flood domains from other sites.  Such interconnect
      do not assume uniform mappings of mac-vrf VNI across sites (or
      mechanisms to build flood domains with in site) and stitches such
      flood domains in complete downstream fashion using EVPN route
      advertisements.  It however do not exclude possibility of building
      an end-to-end flood domain, if desired for other reasons.

   The above architecture satisfies the constraints laid out in
   Section 3.1.  For example, the size of a domain may be made dependent
   on the route and next-hop scale that can be supported by the
   deployment of the network nodes.  There are no constraints on the
   network that connects the nodes within the domain or across the
   domains.  In the event multicast capability is available and enabled,
   the nodes can use those resources.  In the event the underlay is
   connected using unicast semantics, creation of ingress replication
   lists ensure that multi-destination frames reach their destinations.
   The domains may have their own deployment constraints, and the



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   overlay does not need any form of stretching.  It is within the
   control of the administrator with respect to containing fault
   isolation domains.  The automated discovery of the border nodes needs
   no further configurations for existing deployed domains.

4.  Multi-site EVPN Interconnect Procedures

   In this section we describe the new functionalities in the Border
   Gateway nodes for interconnecting EVPN sites within the DC.

4.1.  Border Auto-Discovery Route

   These routes are generated by Border Gateways and imported by leafs
   and Border Gateways.  These routes serve following purpose:

   o  Discover Border Gateways from same site.  This will help in
      finding designated forwarder for inter-site Multi-destination
      traffic.  Once designated forwarder election is complete, inter-
      site Multi-destination traffic will be forwarded by DF winner.

   o  Discover Border Gateways from other sites.  This will help in
      deciding which VXLAN tunnels should be terminated for inter-site
      traffic.  Along with the Type 3 routes, this may help in optimal
      traffic flow within the common core for multi-destination frames.

   A Border A-D route type specific EVPN NLRI is defined as follows.  It
   is proposed to be a new route type in EVPN NLRI defined in RFC7432
   [RFC7432].























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              +--------------------------------------------+
              | Site identifier (2 octet)                  |
              +--------------------------------------------+
              | Sequence number (2 octet)                  |
              +--------------------------------------------+
              | IP Address Length (1 octet)                |
              +--------------------------------------------+
              | Gateway IP (4 or 16 octets)                |
              +--------------------------------------------+
              | VNI Label (3 octets)                       |
              +--------------------------------------------+
              | Multi-destination flow Priority (1 octet)  |
              +--------------------------------------------+
              | Multi-destination forwarder (1 octet)      |
              +--------------------------------------------+



                                 Figure 2

   o  Site Identifier: This is used to distinguish A-D routes received
      from border gateways in same site or in different sites.  Border
      gateways discover each other by processing these A-D routes from
      different sites.  These site identifier can be explicitly
      configured or the BGP Autonomous system (AS) number can be
      automatically carried as the site-identifier.

   o  Sequence number: Monotonically increasing sequence number added by
      Border Gateway while sending A-D route.  In case there are
      multiple Border A-D routes, the one with the highest sequence
      number is honored while processing.

   o  IP Address Length: Number of bytes in the Gateway IP field, 4
      bytes for IPv4 address or 16 bytes for IPv6 address.

   o  Gateway IP: This is the unique IP address of the Border gateways.
      This Gateway IP will be used to build Multi-destination trees.

   o  VNI Label: This is the MAC-VRF VNI or the IP-VRF VNI.

   o  Multi-destination flow Priority : This field is optional and is 0
      if not used.  This field can be used to assist in forwarder
      election for multi-destination traffic by assigning higher
      priority among border gateways of same site.  This forwarder
      election is per MAC-VRF or IP-VRF VNI.






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   o  Multi-destination forwarder : This field is set to TRUE once DF
      election is complete for Multi-destination traffic and announcing
      Border Gateway is the DF winner.

   These A-D routes are advertised with mac-VRF and IP-VRF RTs depending
   on whether the VNI carried is a mac-VRF VNI or an IP VRF VNI.

   After a Border Gateway is provisioned, Border A-D routes will be
   announced after some delay interval from all border gateways.  This
   will provide sufficient time to learn Border A-D routes from other
   Border Gateways.

   Border Gateways between same site will run a Designated forwarder
   election per MAC-VRF VNI for multi-destination traffic across the
   site.  Border A-D routes coming from different site will not trigger
   DF election and will only be cached to terminate VXLAN tunnels from
   such border gateways.

   Multi-destination flow priority will be assigned (based on optional
   policies) to prefer a border gateway for DF election per MAC or IP
   VRF VNI for multi-destination traffic and will be used in DF election
   to prefer higher priority border gateway as forwarder.

   As has been defined in the specifications, Type 2, Type 3, and Type 5
   routes carry downstream VNI labels.  These A-D routes will help to
   pre-build VXLAN tunnels in the common EVPN domain for L2, L3, and
   Multi-Destination traffic.  Also these A-D routes will help in
   correlating next-hop of EVPN routes and will facilitate in rewriting
   next-hop attributes before re-advertising these routes from other
   sites to a given site.  This provides flexibility to keep different
   VNI-VLAN mapping in different sites and still able to interconnect L3
   and L2 domains.

   All control plane and data plane states are interconnected in a
   complete downstream fashion.  For example, BGP import rules for a
   Type 3 route should be able to extend a flood domain for a VNI and
   flood traffic destined to advertised EVPN node should carry the VNI
   which is announced in Type 3 route.  Similarly Type 2, Type 5 control
   and forwarding states should be interconnected in a complete
   downstream fashion.

4.2.  Border Gateway Provisioning

   Border Gateway nodes manage both the control-plane communications and
   the data forwarding plane for any inter-site traffic.  Border Gateway
   functionality in an EVPN site SHOULD be enabled on more than one node
   in the network for redundancy and high-availability purposes.  Any
   external Type-2/Type-5 routes that are received by the BGs of a site



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   are advertised to all the intra-site nodes by all the BGs.  For
   internal Type-2/Type-5 routes received by the BG's from the intra-
   site nodes, all the BGs of a site would advertise them to the remote
   BG's, so any L2/L3 known unicast traffic to internal destinations
   could be sent to any one of the local BG's by remote sources.  For
   known L2 and L3 unicast traffic, all of the individual border gateway
   nodes will behave either as single logical forwarding node or a set
   of active forwarding nodes.  This can be perceived by intra-site
   nodes as multiple entry/exit points for inter-site traffic.  For
   unknown unicast/multi-destination traffic, there must be a designated
   forwarder election mechanism to determine which node would perform
   the primary forwarding role at any given point in time, to ensure
   there is no duplication of traffic for any given flow (See
   Section 4.2.1).

4.2.1.  Border Gateway Designated Forwarder Election

   In the presence of more than one Border Gateway nodes in a site,
   forwarding of multi-destination L2 or L3 traffic both into the site
   and out of the site needs to be carried out by a single node.  This
   DF election could be done independently by each candidate border
   gateway, by subjecting an ordered "candidate list" of all the BG's
   present in the same site (identified by reception of the Border A-D
   routes per-VNI with the same site-id as itself) to a hash-function on
   a per-VNI basis.  All the candidate border gateways of the same site
   are required to use a uniform hash-function to yield the same result.
   Failure events which lead to a BG losing all of its connectivity to
   the IP interconnect backbone should trigger the BG to withdraw its
   Border A-D route(s), to indicate to other BG's of the site that it is
   no longer a candidate BG.  Also there is a possibility of configuring
   policies to prefer a Border gateway over others and pick as DF
   winner.

   There are two modes proposed for Border gateway provisioning.

4.2.2.  All-active Border Gateway

   In this mode all border gateways share same gateway IP and rewrite
   EVPN next-hop attributes with a shared logical next-hop entity.
   However these Gateways will maintain unique gateway IP to facilitate
   building IR trees from site-local nodes to forward Multi-Destination
   traffic.  EVPN Type 2, Type 5 routes will be advertised to the nodes
   in the site from all border gateways and Border gateway will run DF
   election per VNI for Multi destination traffic.  Type 3 routes will
   be advertised by all Border gateways but only DF will forward inter-
   site traffic.





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   This mode is useful when there is no preference between different
   border-gateways to forward traffic from different VNIs.  Standard
   data plane hashing of VXLAN header will load balance traffic among
   Border Gateways.

   Additionally, it is recommended that border gateway be enabled in the
   All-active mode wherein the BG functionality is available to the rest
   of the network as a single logical entity (as in Anycast) for inter-
   site communication.  In the absence of capability for All-active, the
   BG could be enabled as individual gateways (Single-Active BG) wherein
   a single node will perform the active BG role for a given flow at a
   given time.

4.2.3.  Multi-path Border Gateway

   In this mode, Border gateways will rewrite EVPN Next-hop attributes
   with unique next-hop entities.  This provides flexibility to apply
   usual policies and pick per-VRF, per-VNI or per-flow primary/backup
   border Gateways.  Hence, an intra-site node will see each BG as a
   next-hop for any external L2 or L3 unicast destination, and would
   perform an ECMP path selection to load-balance traffic sent to
   external destinations.  In case an intra-site node is not capable of
   performing ECMP hash based path-selection (possibly some L2
   forwarding implementations), the node is expected to choose one of
   the BG's as its designated forwarder.  EVPN Type 2, Type 5 routes
   will be advertised to the nodes in the site from all border gateways
   and Border gateway will run DF election per VNI for Multi destination
   traffic.  Type 3 routes will be advertised by all Border gateways but
   only DF will forward inter-site traffic.

4.3.  EVPN route processing at Border Gateway

   Border gateways will build EVPN peering on processing A-D routes from
   other Border gateways.  Route targets MAY be auto-generated based on
   some site-specific identifier.  If BGP AS number is used as site-
   specific identifier, import and export route targets can be auto-
   generated as explained in RFC7432 [RFC7432].  This will facilitate
   site-local nodes to import routes from other nodes in same site and
   from its Border Gateways.  Also this will prevent routes exchange
   between nodes from different sites.  However, in this auto-generated
   scheme, import mechanism on Border Gateway should be relaxed to allow
   unconditional import of Border A-D routes from other border gateways.
   Also the routes which are imported at Border Gateway and re-
   advertised should implement a mechanism to avoid looping of updates
   should they come back at Border Gateways.

   Type 2/Type 5 EVPN routes will be rewritten with Border Gateway IP,
   Border Gateway system mac as next-hop and re-advertised.  Only EVPN



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   routes received from discovered Border gateways with different site
   identifiers will be rewritten and re-advertised.  This will avoid
   rewriting every EVPN update if border gateways are also acting as
   Route reflector (RR) for site-local EVPN peering.  Also this will
   help in interoperating MS-EVPN fabric with sites which do not have
   Border Gateway functionality.

   There are few mechanisms suggested below for re-advertising these
   inter-site routes to a site and provide connectivity of inter-site
   hosts and subnets.

   o  All routes everywhere : In this mode all inter-site EVPN Type2/
      Type5 routes are downloaded on site-local leafs from Border
      Gateways.  In other words, every leaf in the MS-EVPN fabric will
      have routes from every intra-site and inter-site leafs.  This
      mechanism is best-fit for the scenarios where inter-site traffic
      is as volumonous as intra-site flow traffic.  Also this mechanism
      preserves usual glean processing, silent host discovery and
      unknown traffic handling at the leafs.

   o  Default routing to Border Gateways : In this mode, all received
      inter-site EVPN Type 2/Type 5 routes will be installed only at
      Border Gateways and will not be advertised in the site.  Border
      Gateways will inject Type 5 default routes to site-local nodes and
      avoid re-advertising Type 2 from other sites.  This mode provides
      scaling advantage by not downloading all inter-site routes to
      every leaf in MS-EVPN fabric.  This mechanism MAY require glean
      processing and unknown traffic handling to be tailored to provide
      efficient traffic forwarding.

   o  Site-scope flow registry and discovery : This mechanism provides
      scaling advantage by downloading inter-site routes on-demand.  It
      provides scaling advantages of default routing with out need to
      tailor glean processing and unknown traffic handling at the leafs.
      Leafs will create on-demand flow registry on their border Gateways
      and based on this flow registry border gateways will advertise
      Type 2 routes in a site.  In other words, assuming that we have a
      trigger to send the EVPN routes that are needed by the site for
      conversational learning from the Border Gateways, we can optimize
      on the control plane state that is needed at the various leaf
      nodes.  Hardware programming can be further optimized based on
      actual conversations needed by the leaf, as opposed to to the ones
      needed by the site.  We will describe a mechanism in the appendix
      with respect to ARP processing at the Border Gateway.

   Type 3 routes will be imported and processed on border gateways from
   other border gateways but MUST NOT be advertised again.  In both
   modes (All-active and Multipath), Type 3 routes will be generated and



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   advertised by all Border Gateways with unique gateway IP.  This will
   facilitate building fast converging flood domain connectivity inter-
   site and intra-site and on same time avoiding duplicate traffic by
   electing DF winner to forward multi-destination inter-site traffic.

4.4.  Multi-Destination tree between Border Gateways

   The procedures described here recommends building an Ingress
   Replication (IR) tree between Border Gateways.  This will facilitate
   every site to independently build site-specific Multi destination
   trees.  Multi-destination end-to-end trees between leafs could be PIM
   (site 1) + IR (between border Gateways) + PIM(site 2) or IR-IR-IR or
   PIM-IR-IR.  However this does not rule out using IR-PIM-IR or end-to-
   end PIM to build multi-destination trees end-to-end.

   Border Gateways will generate Type 3 routes with unique gateway IP
   and advertise to Border Gateways of other sites.  These Type 3 routes
   will help in building IR trees between border gateways.  However only
   DF winner per VNI will forward multi-destination traffic across
   sites.

   As Border Gateways are part of both site-specific and inter-site
   Multi-destination IR trees, split-horizon mechanism will be used to
   avoid loops.  Multi-destination tree with Border gateway as root to
   other sites (or Border-Gateways) will be in a separate horizon group.
   Similarity Multi-destination IR tree with Border Gateway as root to
   site-local nodes will be in another split horizon group.

   If PIM is used to build Multi-Destination trees in site-specific
   domain, all Border gateway will join such PIM trees and draw multi-
   destination traffic.  However only DF Border Gateway will forward
   traffic towards other sites.

4.5.  Inter-site Unicast traffic

   As site-local nodes will see all inter-site EVPN routes via Border
   Gateways, VXLAN tunnels will be built between leafs and site-local
   Border Gateways and Inter-site VXLAN tunnels will be built between
   Border gateways in different sites.  An end-to-end VXLAN
   bidirectional forwarding path between inter-site leafs will consist
   of VXLAN tunnel from leaf (say Site A) to its Border Gateway, another
   VXLAN tunnel from Border Gateway to Border Gateway in another site
   (say site B) and Border gateway to leaf (in site B).  Such
   arrangement of tunnels are very scalable as a full mesh of VXLAN
   tunnels across inter-site leafs is substituted by combination of
   intra-site and inter-site tunnels.





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   L2 and L3 unicast frames from site-local leafs will reach border
   gateway using VXLAN encapsulation.  At Border gateway, VXLAN header
   is stripped out and another VXLAN header is pushed to sent frames to
   destination site Border Gateway.  Destination site Border gateway
   will strip off VXLAN header and push another VXLAN header to send
   frame to the destination site leaf.

4.6.  Inter-site Multi-destination traffic

   Multi-destination traffic will be forwarded from one site to other
   site only by DF for that VNI.  As frames reach Border Gateway from
   site-local nodes, VXLAN header will be popped and another VXLAN
   header (derived from downstream Type3 EVPN routes) will be pushed to
   forward frame to destination site border gateway.  Similarly
   destination site Border Gateway will strip off VXLAN header and
   forward frame after pushing another VXLAN header towards the
   destination leaf.

   As explained in Section 4.4, split horizon mechanism will be used to
   avoid looping of inter-site multi-destination frames.

4.7.  Host Mobility

   Host movement handling will be same as defined in RFC7432 [RFC7432].
   When host moves, EVPN Type 2 routes with updated sequence number will
   be propagated to every EVPN node.  When a host moves inter-site, only
   Border gateways may see EVPN updates with both next-hop attributes
   and sequence number changes and leafs may see updates only with
   updated sequence numbers.  However in other cases both Border gateway
   and leafs may see next-hop and sequence number changes.

5.  Convergence

5.1.  Fabric to Border Gateway Failure

   If a Border Gateway is lost, Border gateway next-hop will be
   withdrawn for Type 2 routes.  Also per-VNI DF election will be
   triggered to chose new DF.  DF new winner will become forwarder of
   Multi-destination inter-site traffic.

5.2.  Border Gateway to Border Gateway Failures

   In case where inter-site cloud has link failures, direct forwarding
   path between border gateways can be lost.  In this case, traffic from
   one site can reach other site via border gateway of an intermediate
   site.  However this will be addressed like regular underlay failure
   and traffic terminations end-points will still stay same for inter-
   site traffic flows.



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6.  Interoperability

   The procedures defined here are only for Border Gateways.  Therefore
   other EVPN nodes in the network should be RFC7432 [RFC7432] compliant
   to operate in such topologies.

   As the procedures described here are applicable only after receiving
   Border A-D route, if other domains are connected which are not
   capable of such multi-site gateway model, they can work in regular
   EVPN mode.  The exact procedures will be detailed in a future version
   of the draft.

7.  Isolation of Fault Domains

   Isolation of network defects requires policies like storm control,
   security ACLs etc to be implemented at site boundaries.  Border
   gateways should be capable of inspecting inner payload of packets
   received from VXLAN tunnels and enforce configured policies to
   prevent defects percolating from one part to rest of the network.

8.  Loop detection and Prevention

   This has already been addressed in the Section 4.2.1.  We are in
   essence using the Designated Forwarder and Split Horizon procedures
   to break loops in this network.

9.  Acknowledgements

   This authors would like to thank Max Ardica, Lukas Krattiger, Anuj
   Mittal, Lilian Quan, Veera Ravinutala, for their review and comments.

10.  IANA Considerations

   TBD.

11.  Security Considerations

   TBD.

12.  References

12.1.  Normative References

   [DCI-EVPN-OVERLAY]
              A. Sajassi et. al., "A Network Virtualization Overlay
              Solution using EVPN", 2016, <https://tools.ietf.org/html/
              draft-ietf-bess-evpn-overlay-02>.




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

   [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, <http://www.rfc-editor.org/info/rfc7432>.

12.2.  Informative References

   [RFC7209]  Sajassi, A., Aggarwal, R., Uttaro, J., Bitar, N.,
              Henderickx, W., and A. Isaac, "Requirements for Ethernet
              VPN (EVPN)", RFC 7209, DOI 10.17487/RFC7209, May 2014,
              <http://www.rfc-editor.org/info/rfc7209>.

Appendix A.  Additional Stuff

   TBD.

Authors' Addresses

   Rajesh Sharma (editor)
   Cisco Systems
   170 W Tasman Drive
   San Jose, CA
   USA

   Email: rajshr@cisco.com


   Ayan Banerjee
   Cisco Systems
   170 W Tasman Drive
   San Jose, CA
   USA

   Email: ayabaner@cisco.com


   Raghava Sivaramu
   Cisco Systems
   170 W Tasman Drive
   San Jose, CA
   USA

   Email: raghavas@cisco.com



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