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Versions: (draft-sajassi-l2vpn-evpn-inter-subnet-forwarding) 00 01 02 03 04 05 06 07 08

L2VPN Workgroup                                          A. Sajassi, Ed.
INTERNET-DRAFT                                                  S. Salam
Intended Status: Standards Track                               S. Thoria
                                                                   Cisco
                                                                J. Drake
                                                                 Juniper
                                                              J. Rabadan
                                                                   Nokia

Expires: January 2, 2019                                    July 2, 2018


                Integrated Routing and Bridging in EVPN
            draft-ietf-bess-evpn-inter-subnet-forwarding-04


Abstract

   EVPN provides an extensible and flexible multi-homing VPN solution
   over an MPLS/IP network for intra-subnet connectivity among Tenant
   Systems and End Devices that can be physical or virtual. However,
   there are scenarios for which there is a need for a dynamic and
   efficient inter-subnet connectivity among these Tenant Systems and
   End Devices while maintaining the multi-homing capabilities of EVPN.
   This document describes an Integrated Routing and Bridging (IRB)
   solution based on EVPN to address such requirements.


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

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

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html



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Copyright and License Notice

   Copyright (c) 2014 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
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2 EVPN PE Model for IRB Operation  . . . . . . . . . . . . . . . .  7
   3  Symmetric and Asymmetric IRB  . . . . . . . . . . . . . . . . .  8
     3.1 IRB Interface and its MAC & IP addresses . . . . . . . . . . 11
     3.2 Symmetric IRB Procedures . . . . . . . . . . . . . . . . . . 12
       3.2.1 Control Plane - Ingress PE . . . . . . . . . . . . . . . 12
       3.2.2 Control Plane - Egress PE  . . . . . . . . . . . . . . . 13
       3.2.3 Data Plane - Ingress PE  . . . . . . . . . . . . . . . . 14
       3.2.4 Data Plane - Egress PE . . . . . . . . . . . . . . . . . 14
     3.3 Asymmetric IRB Procedures  . . . . . . . . . . . . . . . . . 15
       3.3.1 Control Plane - Ingress PE . . . . . . . . . . . . . . . 15
       3.3.2 Control Plane - Egress PE  . . . . . . . . . . . . . . . 15
       3.3.3 Data Plane - Ingress PE  . . . . . . . . . . . . . . . . 16
       3.3.4 Data Plane - Egress PE . . . . . . . . . . . . . . . . . 17
   4 BGP Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.1 Router's MAC Extended Community  . . . . . . . . . . . . . . 17
   5 Operational Models for Symmetric Inter-Subnet Forwarding . . . . 18
     5.1 IRB forwarding on NVEs for Tenant Systems  . . . . . . . . . 18
       5.1.1 Control Plane Operation  . . . . . . . . . . . . . . . . 19
       5.1.2 Data Plane Operation - Inter Subnet  . . . . . . . . . . 21
       5.1.3 TS Move Operation  . . . . . . . . . . . . . . . . . . . 22
     5.2 IRB forwarding on NVEs for Subnets behind Tenant Systems . . 23
       5.2.1 Control Plane Operation  . . . . . . . . . . . . . . . . 24
       5.2.2 Data Plane Operation . . . . . . . . . . . . . . . . . . 25
   6  Inter-Subnet DCI Scenarios  . . . . . . . . . . . . . . . . . . 26
     6.1 Switching among IP subnets in different DCs without GW . . . 27
     6.2 Switching among IP subnets in different DCs with GW  . . . . 29
   7 TS Mobility  . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     7.1 TS Mobility & Optimum Forwarding for TS Outbound Traffic . . 31



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     7.2 TS Mobility & Optimum Forwarding for TS Inbound Traffic  . . 31
       7.2.1 Mobility without Route Aggregation . . . . . . . . . . . 31
   8  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 32
   9  Security Considerations . . . . . . . . . . . . . . . . . . . . 32
   10  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 32
   11  References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     11.1  Normative References . . . . . . . . . . . . . . . . . . . 32
     11.2  Informative References . . . . . . . . . . . . . . . . . . 32
   12  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 33
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33


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.

   BD: Broadcast Domain. As per [RFC7432], an EVI consists of a single
   or multiple BDs. In case of VLAN-bundle and VLAN-based service models
   (see [RFC7432]), a BD is equivalent to an EVI. In case of VLAN-aware
   bundle service model, an EVI contains multiple BDs. Also, in this
   document, BD and subnet are equivalent terms.

   BD Route Target: refers to the Broadcast Domain assigned Route Target
   [RFC4364]. In case of VLAN-aware bundle service model, all the BD
   instances in the MAC-VRF share the same Route Target.

   BT: Bridge Table. The instantiation of a BD in a MAC-VRF, as per
   [RFC7432].

   DGW: Data Center Gateway.

   Ethernet A-D route: Ethernet Auto-Discovery (A-D) route, as per
   [RFC7432].

   Ethernet NVO tunnel: refers to Network Virtualization Overlay tunnels
   with Ethernet payload. Examples of this type of tunnels are VXLAN or
   GENEVE.

   EVI: EVPN Instance spanning the NVE/PE devices that are participating
   on that EVPN, as per [RFC7432].



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   EVPN: Ethernet Virtual Private Networks, as per [RFC7432].

   GRE: Generic Routing Encapsulation.

   GW IP: Gateway IP Address.

   IPL: IP Prefix Length.

   IP NVO tunnel: it refers to Network Virtualization Overlay tunnels
   with IP payload (no MAC header in the payload).

   IP-VRF: A VPN Routing and Forwarding table for IP routes on an
   NVE/PE. The IP routes could be populated by EVPN and IP-VPN address
   families. An IP-VRF is also an instantiation of a layer 3 VPN in an
   NVE/PE.

   IRB: Integrated Routing and Bridging interface. It connects an IP-VRF
   to a BD (or subnet).

   MAC-VRF: A Virtual Routing and Forwarding table for Media Access
   Control (MAC) addresses on an NVE/PE, as per [RFC7432]. A MAC-VRF is
   also an instantiation of an EVI in an NVE/PE.

   ML: MAC address length.

   ND: Neighbor Discovery Protocol.

   NVE: Network Virtualization Edge.

   GENEVE: Generic Network Virtualization Encapsulation, [GENEVE].

   NVO: Network Virtualization Overlays.

   RT-2: EVPN route type 2, i.e., MAC/IP advertisement route, as defined
   in [RFC7432].

   RT-5: EVPN route type 5, i.e., IP Prefix route. As defined in Section
   3 of [EVPN-PREFIX].

   SBD: Supplementary Broadcast Domain. A BD that does not have any ACs,
   only IRB interfaces, and it is used to provide connectivity among all
   the IP-VRFs of the tenant. The SBD is only required in IP-VRF- to-IP-
   VRF use-cases (see Section 4.4.).

   SN: Subnet.

   TS: Tenant System.




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   VA: Virtual Appliance.

   VNI: Virtual Network Identifier. As in [RFC8365], the term is used as
   a representation of a 24-bit NVO instance identifier, with the
   understanding that VNI will refer to a VXLAN Network Identifier in
   VXLAN, or Virtual Network Identifier in GENEVE, etc. unless it is
   stated otherwise.

   VTEP: VXLAN Termination End Point, as in [RFC7348].

   VXLAN: Virtual Extensible LAN, as in [RFC7348].

   This document also assumes familiarity with the terminology of
   [RFC7432], [RFC8365] and [RFC7365].





































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1  Introduction

   EVPN provides an extensible and flexible multi-homing VPN solution
   over an MPLS/IP network for intra-subnet connectivity among Tenant
   Systems (TS's) and End Devices that can be physical or virtual; where
   an IP subnet is represented by an EVI for a VLAN-based service or by
   an <EVI, VLAN> for a VLAN-aware bundle service. However, there are
   scenarios for which there is a need for a dynamic and efficient
   inter-subnet connectivity among these Tenant Systems and End Devices
   while maintaining the multi-homing capabilities of EVPN. This
   document describes an Integrated Routing and Bridging (IRB) solution
   based on EVPN to address such requirements.

   The inter-subnet communication is traditionally achieved at
   centralized L3 Gateway (L3GW) nodes where all the inter-subnet
   communication policies are enforced. When two Tenant Systems (TS's)
   belonging to two different subnets connected to the same PE node,
   wanted to communicate with each other, their traffic needed to be
   back hauled from the PE node all the way to the centralized gateway
   nodes where inter-subnet switching is performed and then back to the
   PE node. For today's large multi-tenant data center, this scheme is
   very inefficient and sometimes impractical.

   In order to overcome the drawback of centralized L3GW approach, IRB
   functionality is needed on the PE nodes (also referred to as EVPN
   NVEs) attached to TS's in order to avoid inefficient forwarding of
   tenant traffic (i.e., avoid back-hauling and hair-pinning). A PE with
   IRB capability, can not only locally bridged the tenant intra-subnet
   traffic but also can locally route the tenant inter-subnet traffic on
   a packet by packet basis thus meeting the requirements for both intra
   and inter-subnet forwarding and avoiding non-optimum traffic
   forwarding associate with centralized L3GW approach.

   Some TS's run non-IP protocols in conjunction with their IP traffic.
   Therefore, it is important to handle both kinds of traffic optimally
   - e.g., to bridge non-IP and intra-subnet traffic and to route inter-
   subnet IP traffic. Therefore, the solution needs to meet the
   following requirements:

   R1: The solution MUST allow for both inter-subnet and intra-subnet
   traffic belonging to the same tenant to be locally routed and bridged
   respectively. The solution MUST provide IP routing for inter-subnet
   traffic and Ethernet Bridging for intra-subnet traffic.

   R2: The solution MUST support bridging of non-IP traffic.

   R3: The solution MUST allow inter-subnet switching to be disabled on
   a per VLAN basis on PEs where the traffic needs to be back hauled to



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   another node (i.e., for performing FW or DPI functionality).


2 EVPN PE Model for IRB Operation

   Since this document discusses IRB operation in relationship to EVPN
   MAC-VRF, IP-VRF, EVI, Bridge Domain (BD), Bridge Table (BT), and IRB
   interfaces, it is important to understand the relationship among
   these components. Therefore, the following PE model is demonstrated
   below to a) describe these components and b) illustrate the
   relationship among them.



      +-------------------------------------------------------------+
      |                                                             |
      |              +------------------+                    IRB PE |
      | Attachment   | +------------------+                         |
      | Circuit(AC1) | |  +----------+    |                MPLS/NVO tnl
    ----------------------*Bridge    |    |                    +-----
      |              | |  |Table(BT1)|    |    +-----------+  / \    \
      |              | |  |          *---------*           |<--> |Eth|
      |              | |  |Eth-Tag x |    |IRB1|           |  \ /    /
      |              | |  +----------+    |    |           |   +-----
      |              | |     ...          |    |  IP-VRF1  |        |
      |              | |  +----------+    |    |  RD2/RT2  |MPLS/NVO tnl
      |              | |  |Bridge    |    |    |           |   +-----
      |              | |  |Table(BT2)|    |IRB2|           |  / \    \
      |              | |  |          *---------*           |<--> |IP |
    ----------------------*Eth-Tag y |    |    +-----------+  \ /    /
      |  AC2         | |  +----------+    |                    +-----
      |              | |    MAC-VRF1      |                         |
      |              +-+    RD1/RT1       |                         |
      |                +------------------+                         |
      |                                                             |
      |                                                             |
      +-------------------------------------------------------------+


                      Figure 1: EVPN IRB PE Model

   A tenant needing IRB services on a PE, requires an IP Virtual Routing
   and Forwarding table (IP-V RF) along with one or more MAC Virtual
   Routing and Forwarding tables (MAC-VRFs). An IP-VRF, as defined in
   [RFC4364], is the instantiation of an IPVPN in a PE. A MAC-VRF, as
   defined in [RFC7432], is the instantiation of an EVI (EVPN Instancce)
   in a PE. A MAC-VRF can consists of one or more Bridge Tables (BTs)
   where each BT corresponds to a VLAN (broadcast domain - BD). If the



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   service interface for the EVPN PE is configured in VLAN-Based mode
   (i.e., section 6.1 of [RFC7432]), then there is only a single BT per
   MAC-VRF (per EVI) - i.e., there is only one tenant VLAN per EVI.
   However, if the service interface for the EVPN PE is configured in
   VLAN-Aware Bundle mode (i.e., section 6.3 of [RFC7432]), then there
   are several BTs per MAC-VRF (per EVI) - i.e., there are several
   tenant VLANs per EVI.

   Each BT is connected to a IP-VRF via a L3 interface called IRB
   interface. Since a single tenant subnet is typically (and in this
   document) represented by a VLAN (and thus supported by a single BT),
   for a given tenant there are as many BTs as there are subnets and
   thus there are also as many IRB interfaces between the tenant IP-VRF
   and the associated BTs as shown in the PE model above.

   IP-VRF is identified by its corresponding route target and route
   distinguisher and MAC-VRF is also identified by its corresponding
   route target and route distinguisher. If operating in EVPN VLAN-Based
   mode, then a receiving PE that receives an EVPN route with MAC-VRF
   route target can identify the corresponding BT; however, if operating
   in EVPN VLAN-Aware Bundle mode, then the receiving PE needs both the
   MAC-VRF route target and VLAN ID in order to identify the
   corresponding BT.

3  Symmetric and Asymmetric IRB

   This document defines and describes two types of IRB solutions -
   namely symmetric and asymmetric IRB. In symmetric IRB as its name
   implies, the lookup operation is symmetric at both ingress and egress
   PEs - i.e., both ingress and egress PEs perform lookups on both TS's
   MAC and IP addresses - i.e., ingress PE performs lookup on
   destination TS's MAC address followed by its IP address and egress PE
   performs lookup on destination TS's IP address followed by its MAC
   address as depicted in figure 2.

















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               Ingress PE                   Egress PE
         +-------------------+        +------------------+
         |                   |        |                  |
         |    +-> IP-VFF ----|---->---|-----> IP-VRF -+  |
         |    |              |        |               |  |
         |   BT1        BT2  |        |  BT3         BT2 |
         |    |              |        |               |  |
         |    ^              |        |               v  |
         |    |              |        |               |  |
         +-------------------+        +------------------+
              ^                                       |
              |                                       |
        TS1->-+                                       +->-TS2
                        Figure 2: Symmetric IRB

   In symmetric IRB as shown in figure-2, the inter-subnet forwarding
   between two PEs is done between their associated IP-VRFs. Therefore,
   the tunnel connecting these IP-VRFs can be either IP-only tunnel (in
   case of MPLS or GENEVE encapsulation) or Ethernet NVO tunnel (in case
   of VxLAN encapsulation). If it is Ethernet NOV tunnel, the TS's IP
   packet is encapsulated in an Ethernet header consisting of ingress
   and egress PEs MAC addresses - i.e., there is no need for ingress PE
   to use the destination TS's MAC address. Therefore, in symmetric IRB,
   there is no need for the ingress PE to hold destination TS's IP and
   MAC association in its ARP table. Each PE participating in symmetric
   IRB only maintains ARP entries for locally connected hosts and
   maintain MAC-VRFs/BTs for only locally configured subnets.

   In asymmetric IRB, the lookup operation is asymmetric and the ingress
   PE performs three lookups; whereas the egress PE performs a single
   lookup - i.e., the ingress PE performs lookups on destination TS's
   MAC address, followed by its IP address, followed by its MAC address
   again; whereas, the egress PE performs just a single lookup on
   destination TS's MAC address as depicted in figure 3 below.

















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            Ingress PE                       Egress PE
         +-------------------+        +------------------+
         |                   |        |                  |
         |    +-> IP-VFF ->  |        |      IP-VRF      |
         |    |           |  |        |                  |
         |   BT1        BT2  |        |  BT3         BT2 |
         |    |           |  |        |              | | |
         |    |           +--|--->----|--------------+ | |
         |    |              |        |                v |
         +-------------------+        +----------------|-+
              ^                                        |
              |                                        |
        TS1->-+                                        +->-TS2
                        Figure 3: Asymmetric IRB

   In asymmetric IRB as shown in figure-2, the inter-subnet forwarding
   between two PEs is done between their associated MAC-VRFs/BTs.
   Therefore, the MPLS or NVO tunnel used for inter-subnet forwarding
   MUST be of type Ethernet. Since at the egress PE only MAC lookup is
   performed (e.g., no IP lookup), the TS's IP packet needs to be
   encapsulated with the destination TS's MAC address. In order for
   ingress PE to perform such encapsulation, it needs to maintain TS's
   IP and MAC address association in its ARP table. Furthermore, it
   needs to maintain destination TS's MAC address in the corresponding
   BT even though it does not have the corresponding subnet locally
   configured. In other words, each PE participating in asymmetric IRB
   MUST maintain ARP entries for remote hosts (hosts connected to other
   PEs) as well as maintaining MAC-VRFs/BTs for subnets that are not
   locally present on that PE.

   The following subsection defines the control and data planes
   procedures for symmetric and asymmetric IRB on ingress and egress
   PEs. The following figure is used in description of these procedures
   where it shows a single IP-VRF and a number of BTs on each PE for a
   given tenant. The IP-VRF of the tenant (i.e., IP-VRF1) is connected
   to each BT via its associated IRB interface. Each BT on a PE is
   associated with a unique VLAN (e.g., with a BD) where in turn is
   associated with a single MAC-VRF in case of VLAN-Based mode or a
   number of BTs can be associated with a single MAC-VRF in case of
   VLAN-Aware Bundle mode. Whether the service interface on a PE is
   VLAN-Based or VLAN-Aware Bundle mode does not impact the IRB
   operation and procedures. It only impacts the setting of Ethernet tag
   field in EVPN routes as described in [RFC7432].








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                    PE 1         +---------+
              +-------------+    |         |
      TS1-----|         MACx|    |         |        PE2
    (IP1/M1)  |(BT1)        |    |         |   +-------------+
      TS5-----|      \      |    |  MPLS/  |   |MACy  (BT3)  |-----TS3
    (IP5/M5)  |Mx/IPx \     |    |  VxLAN/ |   |     /       |  (IP3/M3)
              |    (IP-VRF1)|----|  NVGRE  |---|(IP-VRF1)    |
              |       /     |    |         |   |     \       |
      TS2-----|(BT2) /      |    |         |   |      (BT1)  |-----TS4
    (IP2/M2)  |             |    |         |   |             |   (IP4/M4)
              +-------------+    |         |   +-------------+
                                 |         |
                                 +---------+

                       Figure 4: IRB forwarding


3.1 IRB Interface and its MAC & IP addresses

   To support inter-subnet forwarding on a PE, the PE acts as an IP
   Default Gateway from the perspective of the attached Tenant Systems
   where default gateway MAC and IP addresses are configured on each IRB
   interface associated with its subnet and falls into one of the
   following two options:

   1. All the PEs for a given tenant subnet use the same anycast default
   gateway IP and MAC addresses . On each PE, this default gateway IP
   and MAC addresses correspond to the IRB interface connecting the BT
   associated with the tenant's <EVI, VLAN> to the corresponding
   tenant's IP-VRF.

   2. Each PE for a given tenant subnet uses the same anycast default
   gateway IP address but its own MAC address. These MAC addresses are
   aliased to the same anycast default gateway IP address through the
   use of the Default Gateway extended community as specified in [EVPN],
   which is carried in the EVPN MAC/IP Advertisement routes. On each PE,
   this default gateway IP address along with its associated MAC
   addresses correspond to the IRB interface connecting the BT
   associated with the tenant's <EVI, VLAN> to the corresponding
   tenant's IP-VRF.

   It is worth noting that if the applications that are running on the
   TS's are employing or relying on any form of MAC security, then
   either the first model (i.e. using anycast MAC address) should be
   used to ensure that the applications receive traffic from the same
   IRB interface MAC address that they are sending to, or if the second
   model is used, then the IRB interface MAC address MUST be the one
   used in the initial ARP reply for that TS.



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   Although both of these options are equally applicable to both
   symmetric and asymmetric IRB, the option-1 is recommended because of
   the ease of anycast MAC address provisioning on not only the IRB
   interface associated with a given subnet across all the PEs
   corresponding to that EVI but also on all IRB interfaces associated
   with all the tenant's subnets across all the PEs corresponding to all
   the EVIs for that tenant. Furthermore, it simplifies the operation as
   there is no need for Default Gateway extended community advertisement
   and its associated MAC aliasing procedure.

   When a TS sends an ARP request to the PE that is attached to, the ARP
   request is sent for the IP address of the IRB interface associated
   with the TS's subnet. For example, in figure 4, TS1 is configured
   with the anycast IPx address as its default gateway IP address and
   thus when it sends an ARP request for IPx (IP address of the IRB
   interface for BT1), the PE1 sends an ARP reply with the MACx which is
   the MAC address of that IRB interface.

   In addition to anycast addresses, IRB interfaces can be configured
   with non-anycast IP addresses for the purpose of OAM (such as
   traceroute/ping to these interfaces) for both symmetric and
   asymmetric IRB. These IP addresses need to be distributed as VPN
   routes when PEs operating in symmetric IRB mode. However, they don't
   need to be distributed if the PEs are operating in asymmetric IRB
   mode and the IRB interfaces are configured with individual MACs.


3.2 Symmetric IRB Procedures

3.2.1 Control Plane - Ingress PE

   When a PE (e.g., PE1 in figure 4 above) learns MAC and IP address of
   a TS (via an ARP request), it adds the MAC address to the
   corresponding MAC-VRF/BT of that tenant's subnet and adds the IP
   address to the IP-VRF for that tenant. Furthermore, it adds this TS's
   MAC and IP address association to its ARP table. It then builds an
   EVPN MAC/IP Advertisement route (type 2) as follow and advertises it
   to other PEs participating in that tenant's VPN.

   - The Length field of the BGP EVPN NLRI for an EVPN MAC/IP
   Advertisement route MUST be either 40 (if IPv4 address is carried) or
   52 (if IPv6 address is carried).

   - Route Distinguisher (RD), Ethernet Segment Identifier, Ethernet Tag
   ID, MAC Address Length, MAC Address, IP Address Length, IP Address,
   and MPLS Label1 fields MUST be used as defined in [RFC7432] and
   [RFC8365].




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   - The MPLS Label2 field is set to either an MPLS label or a VNI
   corresponding to the tenant's IP-VRF. In case of an MPLS label, this
   field is encoded as 3 octets, where the high-order 20 bits contain
   the label value.

   Just as in [RFC7432], the RD, Ethernet Tag ID, MAC Address Length,
   MAC Address, IP Address Length, and IP Address fields are part of
   the route key used by BGP to compare routes. The rest of the fields
   are not part of the route key.

   This route is advertised along with the following two extended
   communities:
        1) Tunnel Type Extended Community
        2) Router's MAC Extended Community

   For symmetric IRB mode, Router's MAC EC is needed to carry the PE's
   overlay MAC address which is used for IP-VRF to IP-VRF communications
   with Ethernet NVO tunnel. If MPLS or IP-only NVO tunnel is used, then
   there is no need to send Router's MAC Extended Community along with
   this route.

   This route MUST be advertised with two route targets - one
   corresponding to the MAC-VRF of the tenant's subnet and another
   corresponding to the tenant's IP-VRF.


3.2.2 Control Plane - Egress PE

   When a PE (e.g., PE2 in figure 4 above) receives this EVPN MAC/IP
   Advertisement route advertisement, it performs the following:

   - Using MAC-VRF route target, it identifies the corresponding MAC-
   VRF. If the MAC-VRF exists (e.g., it is locally configured) then it
   imports the MAC address into it. Otherwise, it does not import the
   MAC address.

   - Using IP-VRF route target, it identifies the corresponding IP-VRF
   and imports the IP address into it.

   The inclusion of MPLS label2 field in this route signals to the
   receiving PE that this route is for symmetric IRB mode and MPLS
   label2 needs to be installed in forwarding path to identify the
   corresponding IP-VRF.

   If the receiving PE receives this route with both the MAC-VRF and IP-
   VRF route targets but the MAC/IP Advertisement route does not include
   MPLS label2 field and if the receiving PE does not support asymmetric
   IRB mode, then if it has the corresponding MAC-VRF, it only imports



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   the MAC address; otherwise, if it doesn't have the corresponding MAC-
   VRF, it MUST treat the route as withdraw [RFC7606].

3.2.3 Data Plane - Ingress PE

   When an Ethernet frame is received by an ingress PE (e.g., PE1 in
   figure 4 above), the PE uses the AC ID (e.g., VLAN ID) to identify
   the associated MAC-VRF/BT and it performs a lookup on the destination
   MAC address. If the MAC address corresponds to its IRB Interface MAC
   address, the ingress PE deduces that the packet must be inter-subnet
   routed. Hence, the ingress PE performs an IP lookup in the associated
   IP-VRF table. The lookup identifies BGP next hop of egress PE along
   with the tunnel/encapsulation type and the associated MPLS/VNI
   values.

   If the tunnel type is that of MPLS or IP-only NVO tunnel, then TS's
   IP packet is sent over the tunnel without any Ethernet header.
   However, if the tunnel type is that of Eternet NVO tunnel, then an
   Ethernet header needs to be added to the TS's IP packet. The source
   MAC address of this Ethernet header is set to the ingress PE's router
   MAC address and the destination MAC address of this Ethernet header
   is set to the egress PE's router MAC address. The MPLS VPN label or
   VNI fields are set accordingly and the packet is forwarded to the
   egress PE.

   If case of NVO tunnel encapsulation, the outer source IP address is
   set to the ingress PE's BGP next-hop address and outer destination IP
   address is set to the egress PE's BGP next-hop address.



3.2.4 Data Plane - Egress PE

   When the tenant's MPLS or NVO encapsulated packet is received over an
   MPLS or NVO tunnel by the egress PE, the egress PE removes NVO tunnel
   encapsulation and uses the VPN MPLS label (for MPLS encapsulation) or
   VNI (for VxLAN encapsulation) to identify the IP-VRF in which IP
   lookup needs to be performed.

   The lookup identifies a local adjacency to the IRB interface
   associated with the egress subnet's MAC-VRF/BT.

   The egress PE gets the destination TS's MAC address for that TS's IP
   address from its ARP table, it encapsulates the packet with that
   destination MAC address and a source MAC address corresponding to
   that IRB interface and sends the packet to its destination subnet
   MAC-VRF/BT.




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   The destination MAC address lookup in the MAC-VRF/BT results in local
   adjacency (e.g., local interface) over which the Ethernet frame is
   sent on.


3.3 Asymmetric IRB Procedures

3.3.1 Control Plane - Ingress PE

   When a PE (e.g., PE1 in figure 4 above) learns MAC and IP address of
   a TS (via an ARP request), it populates its MAC-VRF/BT, IP-VRF, and
   ARP table just as in the case for symmetric IRB. It then builds an
   EVPN MAC/IP Advertisement route (type 2) as follow and advertises it
   to other PEs participating in that tenant's VPN.

   - The Length field of the BGP EVPN NLRI for an EVPN MAC/IP
   Advertisement route MUST be either 37 (if IPv4 address is carried) or
   49 (if IPv6 address is carried).

   - Route Distinguisher (RD), Ethernet Segment Identifier, Ethernet Tag
   ID, MAC Address Length, MAC Address, IP Address Length, IP Address,
   and MPLS Label1 fields MUST be used as defined in [RFC7432] and
   [RFC8365].

   - The MPLS Label2 field MUST NOT be included in this route.

   Just as in [RFC7432], the RD, Ethernet Tag ID, MAC Address Length,
   MAC Address, IP Address Length, and IP Address fields are part of
   the route key used by BGP to compare routes. The rest of the fields
   are not part of the route key.

   This route is advertised along with the following extended
   communitiy:
        1) Tunnel Type Extended Community

   For asymmetric IRB mode, Router's MAC EC is not needed because
   forwarding is performed using destination TS's MAC address which is
   carried in this route advertisement.

   This route MUST always be advertised with MAC-VRF route target. It
   MAY also be advertised with a second route target corresponding to
   the IP-VRF. If only MAC-VRF route target is used, then the receiving
   PE uses the MAC-VRF route target to identify the corresponding IP-VRF
   - i.e., many MAC-VRF route targets map to the same IP-VRF for a given
   tenant.


3.3.2 Control Plane - Egress PE



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   When a PE (e.g., PE2 in figure 4 above) receives this EVPN MAC/IP
   Advertisement route advertisement, it performs the following:

   - Using MAC-VRF route target, it identifies the corresponding MAC-VRF
   and imports the MAC address into it. For asymmetric IRB mode, it is
   assumed that all PEs participating in a tenant's VPN are configured
   with all subnets and corresponding MAC-VRFs/BTs even if there are no
   locally attached TS's for some of these subnets. The reason for this
   is because ingress PE needs to do forwarding based on destination
   TS's MAC address and does proper NVO tunnel encapsulation which are
   property of a lookup in MAC-VRF/BT. An implementation may choose to
   consolidate the lookup at the ingress PE's IP-VRF with the lookup at
   the ingress PE's destination subnet MAC-VRF. Consideration for such
   consolidation of lookups is outside the scope of this document.

   - Using MAC-VRF route target, it identifies the corresponding ARP
   table for the tenant and it adds an entry to the ARP table for the
   TS's MAC and IP address association. It should be noted that the
   tenant's ARP table at the receiving PE is identified by all the MAC-
   VRF route targets for that tenant. If IP-VRF route target is included
   with this route advertisement, then it MAY be used for the
   identification of tenant's ARP table.

   For asymmetric IRB mode, the MPLS label2 field SHOULD not be included
   in the route; however, if the receiving PE receives this route with
   the MPLS label2 field, then it SHOULD ignore it.


3.3.3 Data Plane - Ingress PE

   When an Ethernet frame is received by an ingress PE (e.g., PE1 in
   figure 4 above), the PE uses the AC ID (e.g., VLAN ID) to identify
   the associated MAC-VRF/BT and it performs a lookup on the destination
   MAC address. If the MAC address corresponds to its IRB Interface MAC
   address, the ingress PE deduces that the packet must be inter-subnet
   routed. Hence, the ingress PE performs an IP lookup in the associated
   IP-VRF table. The lookup identifies a local adjacency to the IRB
   interface associated with the egress subnet's MAC-VRF/BT.

   The ingress PE gets the destination TS's MAC address for that TS's IP
   address from its ARP table, it encapsulates the packet with that
   destination MAC address and a source MAC address corresponding to
   that IRB interface and sends the packet to its destination subnet
   MAC-VRF/BT.

   The destination MAC address lookup in the MAC-VRF/BT results in BGP
   next hop address of egress PE. The ingress PE encapsulates the packet
   using Ethernet NVO tunnel of the choice (e.g., VxLAN or GENEVE) and



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   sends the packet to the egress PE. Since the packet forwarding is
   between ingress PE's MAC-VRF/BT and egress PE's MAC-VRF/BT, the
   packet encapsulation procedures follows that of [RFC7432] for MPLS
   and [RFC8365] for VxLAN encapsulations.


3.3.4 Data Plane - Egress PE

   When a tenant's Ethernet frame is received over an NVO tunnel by the
   egress PE, the egress PE removes NVO tunnel encapsulation and uses
   the VPN MPLS label (for MPLS encapsulation) or VNI (for VxLAN
   encapsulation) to identify the MAC-VRF/BT in which MAC lookup needs
   to be performed.

   The MAC lookup results in local adjacency (e.g., local interface)
   over which the packet needs to get sent.

   Note that the forwarding behavior on the egress PE is the same as
   EVPN intra-subnet forwarding described in [RFC7432] for MPLS and
   [RFC8365] for VxLAN networks. In other words, all the packet
   processing associated with the inter-subnet forwarding semantics is
   confined to the ingress PE.

   It should also be noted that [RFC7432] provides different level of
   granularity for the EVPN label.  Besides identifying bridge domain
   table, it can be used to identify the egress interface or a
   destination MAC address on that interface. If EVPN label is used for
   egress interface or individual MAC address identification, then no
   MAC lookup is needed in the egress PE for MPLS encapsulation and the
   packet can be directly forwarded to the egress interface just based
   on EVPN label lookup.


4 BGP Encoding

   This document defines one new BGP Extended Community for EVPN.

4.1 Router's MAC Extended Community

   A new EVPN BGP Extended Community called Router's MAC is introduced
   here. This new extended community is a transitive extended community
   with the Type field of 0x06 (EVPN) and the Sub-Type of 0x03. It may
   be advertised along with BGP Encapsulation Extended Community define
   in section 4.5 of [TUNNEL-ENCAP].

   The Router's MAC Extended Community is encoded as an 8-octet value as
   follows:




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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Type=0x06     | Sub-Type=0x03 |        Router's MAC           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Router's MAC Cont'd                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 5: Router's MAC Extended Community

   This extended community is used to carry the PE's MAC address for
   symmetric IRB scenarios and it is sent with RT-2.


5 Operational Models for Symmetric Inter-Subnet Forwarding

   The following sections describe two main symmetric IRB forwarding
   scenarios (within a DC - i.e., intra-DC) along with their
   corresponding procedures. In the following scenarios, without loss of
   generality, it is assumed that a given tenant is represented by a
   single IP-VPN instance. Therefore, on a given PE, a tenant is
   represented by a single IP-VRF table and one or more MAC-VRF tables.

5.1 IRB forwarding on NVEs for Tenant Systems

   This section covers the symmetric IRB procedures for the scenario
   where each Tenant System (TS) is attached to one or more NVEs and its
   host IP and MAC addresses are learned by the attached NVEs and are
   distributed to all other NVEs that are interested in participating in
   both intra-subnet and inter-subnet communications with that TS.

   In this scenario, for a given tenant, an NVE has typically one MAC-
   VRF for each tenant's subnet (VLAN) that is configured for, assuming
   VLAN-based service which is typically the case for VxLAN and NVGRE
   encapsulation and each MAC-VRF consists of a single bridge domain. In
   case of MPLS encapsulation with VLAN-aware bundling, then each MAC-
   VRF consists of multiple bridge domains (one bridge domain per VLAN).
   The MAC-VRFs on an NVE for a given tenant are associated with an IP-
   VRF corresponding to that tenant (or IP-VPN instance) via their IRB
   interfaces.

   Each NVE MUST support QoS, Security, and OAM policies per IP-VRF
   to/from the core network. This is not to be confused with the QoS,
   Security, and OAM policies per Attachment Circuits (AC) to/from the
   Tenant Systems. How this requirement is met is an implementation
   choice and it is outside the scope of this document.

   Since VxLAN and NVGRE encapsulations require inner Ethernet header



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   (inner MAC SA/DA), and since for inter-subnet traffic, TS MAC address
   cannot be used, the ingress NVE's MAC address is used as inner MAC
   SA. The NVE's MAC address is the device MAC address and it is common
   across all MAC-VRFs and IP-VRFs. This MAC address is advertised using
   the new EVPN Router's MAC Extended Community (section 6.1).

   Figure 6 below illustrates this scenario where a given tenant (e.g.,
   an IP-VPN instance) has three subnets represented by MAC-VRF1, MAC-
   VRF2, and MAC-VRF3 across two NVEs. There are five TS's that are
   associated with these three MAC-VRFs - i.e., TS1, TS4, and TS5 are
   sitting on the same subnet (e.g., same MAC-VRF/VLAN);where, TS1 and
   TS5 are associated with MAC-VRF1 on NVE1, TS4 is associated with MAC-
   VRF1 on NVE2.  TS2 is associated with MAC-VRF2 on NVE1, and TS3 is
   associated with MAC-VRF3 on NVE2. MAC-VRF1 and MAC-VRF2 on NVE1 are
   in turn associated with IP-VRF1 on NVE1 and MAC-VRF1 and MAC-VRF3 on
   NVE2 are associated with IP-VRF1 on NVE2. When TS1, TS5, and TS4
   exchange traffic with each other, only L2 forwarding (bridging) part
   of the IRB solution is exercised because all these TS's sit on the
   same subnet. However, when TS1 wants to exchange traffic with TS2 or
   TS3 which belong to different subnets, then both bridging and routing
   parts of the IRB solution are exercised. The following subsections
   describe the control and data planes operations for this IRB scenario
   in details.



                     NVE1         +---------+
               +-------------+    |         |
       TS1-----|         MACx|    |         |        NVE2
     (IP1/M1)  |(MAC-        |    |         |   +-------------+
       TS5-----| VRF1)\      |    |  MPLS/  |   |MACy  (MAC-  |-----TS3
     (IP5/M5)  |       \     |    |  VxLAN/ |   |     / VRF3) |  (IP3/M3)
               |    (IP-VRF1)|----|  NVGRE  |---|(IP-VRF1)    |
               |       /     |    |         |   |     \       |
       TS2-----|(MAC- /      |    |         |   |      (MAC-  |-----TS4
     (IP2/M2)  | VRF2)       |    |         |   |       VRF1) |   (IP4/M4)
               +-------------+    |         |   +-------------+
                                  |         |
                                  +---------+

          Figure 6: IRB forwarding on NVEs for Tenant Systems

5.1.1 Control Plane Operation

   Each NVE advertises a MAC/IP Advertisement route (i.e., Route Type 2)
   for each of its TS's with the following field set:

   - RD and ESI per [EVPN]



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   - Ethernet Tag = 0; assuming VLAN-based service
   - MAC Address Length = 48
   - MAC Address = Mi ; where i = 1,2,3,4, or 5 in the above example
   - IP Address Length = 32 or 128
   - IP Address = IPi ; where i = 1,2,3,4, or 5 in the above example
   - Label-1 = MPLS Label or VNID corresponding to MAC-VRF
   - Label-2 = MPLS Label or VNID corresponding to IP-VRF

   Each NVE advertises an RT-2 route with two Route Targets (one
   corresponding to its MAC-VRF and the other corresponding to its IP-
   VRF. Furthermore, the RT-2 is advertised with two BGP Extended
   Communities. The first BGP Extended Community identifies the tunnel
   type per section 4.5 of [TUNNEL-ENCAP] and the second BGP Extended
   Community includes the MAC address of the NVE (e.g., MACx for NVE1 or
   MACy for NVE2) as defined in section 6.1. This second Extended
   Community (for the MAC address of NVE) is only required when Ethernet
   NVO tunnel type is used. If IP NVO tunnel type is used, then there is
   no need to send this second Extended Community. It should be noted
   that IP NVO tunnel type is only applicable to symmetric IRB
   procedures.

   Upon receiving this advertisement, the receiving NVE performs the
   following:

   - It uses Route Targets corresponding to its MAC-VRF and IP-VRF for
   identifying these tables and subsequently importing this route into
   them.

   - It imports the MAC address from MAC/IP Advertisement route into the
   MAC-VRF with BGP Next Hop address as underlay tunnel destination
   address (e.g., VTEP DA for VxLAN encapsulation) and Label-1 as VNID
   for VxLAN encapsulation or EVPN label for MPLS encapsulation.

   - If the route carries the new Router's MAC Extended Community, and
   if the receiving NVE is using Ethernet NVO tunnel, then the receiving
   NVE imports the IP address into IP-VRF with NVE's MAC address (from
   the new Router's MAC Extended Community) as inner MAC DA and BGP Next
   Hop address as underlay tunnel destination address, VTEP DA for VxLAN
   encapsulation and Label-2 as IP-VPN VNID for VxLAN encapsulation.

   - If the receiving NVE is going to use MPLS encapsulation, then the
   receiving NVE imports the IP address into IP-VRF with BGP Next Hop
   address as underlay tunnel destination address, and Label-2 as IP-VPN
   label for MPLS encapsulation.

   If the receiving NVE receives a RT-2 with only a single Route Target
   corresponding to IP-VRF and Label-1, or if it receives a RT-2 with
   only a single Route Target corresponding to MAC-VRF but with both



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   Label-1 and Label-2, or if it receives a RT-2 with MAC Address Length
   of zero, then it must not import it to either IP-VRF or MAC-VRF and
   it must log an error.



5.1.2 Data Plane Operation - Inter Subnet

   The following description of the data-plane operation describes just
   the logical functions and the actual implementation may differ. Lets
   consider data-plane operation when TS1 in subnet-1 (MAC-VRF1) on NVE1
   wants to send traffic to TS3 in subnet-3 (MAC-VRF3) on NVE2.

   - NVE1 receives a packet with MAC DA corresponding to the MAC-VRF1
   IRB interface on NVE1 (the interface between MAC-VRF1 and IP-VRF1),
   and VLAN-tag corresponding to MAC-VRF1.

   - Upon receiving the packet, the NVE1 uses VLAN-tag to identify the
   MAC-VRF1. It then looks up the MAC DA and forwards the frame to its
   IRB interface.

   -  The Ethernet header of the packet is stripped and the packet is
   fed to the IP-VRF where IP lookup is performed on the destination
   address. This lookup yields an outgoing interface and the required
   encapsulation. If the encapsulation is for Ethernet NVO tunnel, then
   it includes a MAC address to be used as inner MAC DA, an IP address
   to be used as VTEP DA, and a VPN-ID to be used as VNID. The inner MAC
   SA and VTEP SA is set to NVE's MAC and IP addresses respectively. If
   it is a MPLS encapsulation, then corresponding EVPN and LSP labels
   are added to the packet. The packet is then forwarded to the egress
   NVE.

   - On the egress NVE, if the packet arrives on Ethernet NVO tunnel
   (e.g., it is VxLAN encapsulated), then the VxLAN header is removed.
   Since the inner MAC DA is the egress NVE's MAC address, the egress
   NVE knows that it needs to perform an IP lookup. It uses VNID to
   identify the IP-VRF table. If the packet is MPLS encapsulated, then
   the EVPN label lookup identifies the IP-VRF table. Next, an IP lookup
   is performed for the destination TS (TS3) which results in access-
   facing IRB interface over which the packet is sent. Before sending
   the packet over this interface, the ARP table is consulted to get the
   destination TS's MAC address.

   - The IP packet is encapsulated with an Ethernet header with MAC SA
   set to that of IRB interface MAC address and MAC DA set to that of
   destination TS (TS3) MAC address. The packet is sent to the
   corresponding MAC-VRF3 and after a lookup of MAC DA, is forwarded to
   the destination TS (TS3) over the corresponding interface.



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   In this symmetric IRB scenario, inter-subnet traffic between NVEs
   will always use the IP-VRF VNID/MPLS label. For instance, traffic
   from TS2 to TS4 will be encapsulated by NVE1 using NVE2's IP-VRF
   VNID/MPLS label, as long as TS4's host IP is present in NVE1's IP-
   VRF.

5.1.3 TS Move Operation

   When a TS move from one NVE to other, it is important that the MAC
   mobility procedures are properly executed and the corresponding MAC-
   VRF and IP-VRF tables on all participating NVEs are updated. [EVPN]
   describes the MAC mobility procedures for L2-only services for both
   single-homed TS and multi-homed TS. This section describes the
   incremental procedures and BGP Extended Communities needed to handle
   the MAC mobility for a mixed of L2 and L3 connectivity (aka IRB). In
   order to place the emphasis on the differences between L2-only versus
   L2-and-L3 use cases, the incremental procedure is described for
   single-homed TS with the expectation that the reader can easily
   extrapolate multi-homed TS based on the procedures described in
   section 15 of [EVPN].

   Lets consider TS1 in figure-6 above where it moves from NVE1 to NVE2.
   In such move, NVE2 discovers IP1/MAC1 of TS1 and realizes that it is
   a MAC move and it advertises a MAC/IP route per section 5.1.1 above
   with MAC Mobility Extended Community.

   Since NVE2 learns TS1's MAC/IP addresses locally, it updates its MAC-
   VRF1 and IP-VRF1 for TS1 with its local interface.

   If the local learning at NVE1 is performed using control or
   management planes, then these interactions serve as the trigger for
   NVE1 to withdraw the MAC and IP addresses associated with TS1.
   However, if the local learning at NVE1 is performed using data-plane
   learning, then the reception of the MAC/IP Advertisement route (for
   TS1) from NVE2 with MAC Mobility extended community serve as the
   trigger for NVE1 to withdraw the MAC and IP addresses associated with
   TS1.

   All other remote NVE devices upon receiving the MAC/IP advertisement
   route for TS1 from NVE2 with MAC Mobility extended community compare
   the sequence number in this advertisement with the one previously
   received. If the new sequence number is greater than the old one,
   then they update the MAC/IP addresses of TS1 in their corresponding
   MAC-VRFs and IP-VRFs to point to NVE2. Furthermore, upon receiving
   the MAC/IP withdraw for TS1 from NVE1, these remote PEs perform the
   cleanups for their BGP tables.





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5.2 IRB forwarding on NVEs for Subnets behind Tenant Systems

   This section covers the symmetric IRB procedures for the scenario
   where some Tenant Systems (TS's) support one or more subnets and
   these TS's are associated with one or more NVEs. Therefore, besides
   the advertisement of MAC/IP addresses for each TS which can be multi-
   homed with All-Active redundancy mode, the associated NVE needs to
   also advertise the subnets statically configured on each TS.

   The main difference between this solution and the previous one is the
   additional advertisement corresponding to each subnet. These subnet
   advertisements are accomplished using EVPN IP Prefix route defined in
   [EVPN-PREFIX]. These subnet prefixes are advertised with the IP
   address of their associated TS (which is in overlay address space) as
   their next hop. The receiving NVEs perform recursive route resolution
   to resolve the subnet prefix with its associated ingress NVE so that
   they know which NVE to forward the packets to when they are destined
   for that subnet prefix.

   The advantage of this recursive route resolution is that when a TS
   moves from one NVE to another, there is no need to re-advertise any
   of the subnet prefixes for that TS. All it is needed is to advertise
   the IP/MAC addresses associated with the TS itself and exercise MAC
   mobility procedures for that TS. The recursive route resolution
   automatically takes care of the updates for the subnet prefixes of
   that TS.

   Figure below illustrates this scenario where a given tenant (e.g., an
   IP-VPN service) has three subnets represented by MAC-VRF1, MAC-VRF2,
   and MAC-VRF3 across two NVEs. There are four TS's associated with
   these three MAC-VRFs - i.e., TS1, TS5 are connected to MAC-VRF1 on
   NVE1, TS2 is connected to MAC-VRF2 on NVE1,  TS3 is connected to MAC-
   VRF3 on NVE2, and TS4 is connected to MAC-VRF1 on NVE2. TS1 has two
   subnet prefixes (SN1 and SN2) and TS3 has a single subnet prefix,
   SN3. The MAC-VRFs on each NVE are associated with their corresponding
   IP-VRF using their IRB interfaces. When TS4 and TS1 exchange intra-
   subnet traffic, only L2 forwarding (bridging) part of the IRB
   solution is used (i.e., the traffic only goes through their MAC-
   VRFs); however, when TS3 wants to forward traffic to SN1 or SN2
   sitting behind TS1 (inter-subnet traffic), then both bridging and
   routing parts of the IRB solution are exercised (i.e., the traffic
   goes through the corresponding MAC-VRFs and IP-VRFs). The following
   subsections describe the control and data planes operations for this
   IRB scenario in details.







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                             NVE1      +----------+
     SN1--+          +-------------+   |          |
          |--TS1-----|(MAC- \      |   |          |
     SN2--+ IP1/M1   | VRF1) \     |   |          |
                     |     (IP-VRF)|---|          |
                     |       /     |   |          |
             TS2-----|(MAC- /      |   |  MPLS/   |
            IP2/M2   | VRF2)       |   |  VxLAN/  |
                     +-------------+   |  NVGRE   |
                     +-------------+   |          |
     SN3--+--TS3-----|(MAC-\       |   |          |
            IP3/M3   | VRF3)\      |   |          |
                     |     (IP-VRF)|---|          |
                     |       /     |   |          |
             TS4-----|(MAC- /      |   |          |
            IP4/M4   | VRF1)       |   |          |
                     +-------------+   +----------+
                            NVE2


Figure 7: IRB forwarding on NVEs for Tenant Systems with configured subnets

5.2.1 Control Plane Operation

   Each NVE advertises a Route Type-5 (RT-5, IP Prefix Route defined in
   [EVPN-PREFIX]) for each of its subnet prefixes with the IP address of
   its TS as the next hop (gateway address field) as follow:

   - RD associated with the IP-VRF
   - ESI = 0
   - Ethernet Tag = 0;
   - IP Prefix Length = 32 or 128
   - IP Prefix = SNi
   - Gateway Address = IPi; IP address of TS
   - Label = 0

   This RT-5 is advertised with one or more Route Targets that have been
   configured as "export route targets" of the IP-VRF from which the
   route is originated.

   Each NVE also advertises an RT-2 (MAC/IP Advertisement Route) along
   with their associated Route Targets and Extended Communities for each
   of its TS's exactly as described in section 5.1.1.

   Upon receiving the RT-5 advertisement, the receiving NVE performs the
   following:

   - It uses the Route Target to identify the corresponding IP-VRF



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   - It imports the IP prefix into its corresponding IP-VRF that is
   configured with an import RT that is one of the RTs being carried by
   the RT-5 route along with the IP address of the associated TS as its
   next hop.

   When receiving the RT-2 advertisement, the receiving NVE imports
   MAC/IP addresses of the TS into the corresponding MAC-VRF and IP-VRF
   per section 5.1.1. When both routes exist, recursive route resolution
   is performed to resolve the IP prefix (received in RT-5) to its
   corresponding NVE's IP address (e.g., its BGP next hop). BGP next hop
   will be used as underlay tunnel destination address (e.g., VTEP DA
   for VxLAN encapsulation) and Router's MAC will be used as inner MAC
   for VxLAN encapsulation.


5.2.2 Data Plane Operation

   The following description of the data-plane operation describes just
   the logical functions and the actual implementation may differ. Lets
   consider data-plane operation when a host on SN1 sitting behind TS1
   wants to send traffic to a host sitting behind SN3 behind TS3.

   - TS1 send a packet with MAC DA corresponding to the MAC-VRF1 IRB
   interface of NVE1, and VLAN-tag corresponding to MAC-VRF1.

   - Upon receiving the packet, the ingress NVE1 uses VLAN-tag to
   identify the MAC-VRF1. It then looks up the MAC DA and forwards the
   frame to its IRB interface just like section 5.1.1.

   - The Ethernet header of the packet is stripped and the packet is fed
   to the IP-VRF; where, IP lookup is performed on the destination
   address. This lookup yields the fields needed for VxLAN encapsulation
   with NVE2's MAC address as the inner MAC DA, NVE'2 IP address as the
   VTEP DA, and the VNID. MAC SA is set to NVE1's MAC address and VTEP
   SA is set to NVE1's IP address.

   -  The packet is then encapsulated with the proper header based on
   the above info and is forwarded to the egress NVE (NVE2).

   - On the egress NVE (NVE2), assuming the packet is VxLAN
   encapsulated, the VxLAN and the inner Ethernet headers are removed
   and the resultant IP packet is fed to the IP-VRF associated with that
   the VNID.

   - Next, a lookup is performed based on IP DA (which is in SN3) in the
   associated IP-VRF of NVE2. The IP lookup yields the access-facing IRB
   interface over which the packet needs to be sent. Before sending the
   packet over this interface, the ARP table is consulted to get the



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   destination TS (TS3) MAC address.

   - The IP packet is encapsulated with an Ethernet header with the MAC
   SA set to that of the access-facing IRB interface of the egress NVE
   (NVE2) and the MAC DA is set to that of destination TS (TS3) MAC
   address. The packet is sent to the corresponding MAC-VRF3 and after a
   lookup of MAC DA, is forwarded to the destination TS (TS3) over the
   corresponding interface.

6  Inter-Subnet DCI Scenarios

   The inter-subnet DCI scenarios can be categorized into the following
   four categories. The last two scenarios, along with its corresponding
   solution, are described in [EVPN-IPVPN-INTEROP]. The first two
   scenarios are covered in this document.

   1. Switching among IP subnets in different DCs using EVPN without GW

   2. Switching among IP subnets in different DCs using EVPN with GW

   3. Switching among IP subnets spread across IP-VPN and EVPN networks
   with GW

   4. Switching among IP subnets spread across IP-VPN and EVPN networks
   without GW


   In the above scenario, the term "GW" refers to the case where a node
   situated at the WAN edge of the data center network behaves as a
   default gateway (GW) for all the destinations that are outside the
   data center. The absence of GW refers to the scenario where NVEs
   within a data center maintain individual (host) routes that are
   outside of the data center.

   In the case (3), the WAN edge node also performs route aggregation
   for all the destinations within its own data center, and acts as an
   interworking unit between EVPN and IP VPN (it implements both EVPN
   and IP-VPN functionality).













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                             +---+    Enterprise Site 1
                             |PE1|----- H1
                             +---+
                               /
                         ,---------.             Enterprise Site 2
                       ,'           `.    +---+
        ,---------.  /(    MPLS/IP    )---|PE2|-----  H2
       '   DCN 3   `./ `.   Core    ,'    +---+
        `-+------+'     `-+------+'
        __/__           / /      \ \
       :NVE4 :        +---+       \ \
       '-----'   ,----|GW |.       \ \
          |    ,'     +---+ `.      ,---------.
         TS6  (      DCN 1    )   ,'           `.
               `.           ,'   (      DCN 2    )
                 `-+------+'      `.           ,'
                   __/__            `-+------+'
                  :NVE1 :           __/__   __\__
                  '-----'          :NVE2 :  :NVE3 :
                   |  |            '-----'  '-----'
                  TS1 TS2            |  |      |
                                    TS3 TS4   TS5

                  Figure 8: Interoperability Use-Cases

   In what follows, we will describe scenarios 1 and 2 in more details.


6.1 Switching among IP subnets in different DCs without GW

   This case is similar to that of section 2.1 above albeit for the fact
   that the TS's belong to different data centers that are
   interconnected over a WAN (e.g. MPLS/IP PSN). The data centers in
   question here are seamlessly interconnected to the WAN, i.e., the WAN
   edge devices do not maintain any TS-specific addresses in the
   forwarding path - e.g., there is no WAN edge GW(s) between these DCs.

   As an example, consider TS3 and TS6 of Figure 2 above. Assume that
   connectivity is required between these two TS's where TS3 belongs to
   the SN3 whereas TS6 belongs to the SN6. NVE2 has an EVI3 associated
   with SN3 and NVE4 has an EVI6 associated with the SN6. Both SN3 and
   SN6 are part of the same IP-VRF.

   When an EVPN MAC advertisement route is received by a NVE, the IP
   address associated with the route is used to populate the IP-VRF
   table, whereas the MAC address associated with the route is used to
   populate both the MAC-VRF table, as well as the adjacency associated
   with the IP route in the IP-VRF table (i.e., ARP table).



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   When an Ethernet frame is received by an ingress NVE, it performs a
   lookup on the destination MAC address in the associated EVI. If the
   MAC address corresponds to its IRB Interface MAC address, the ingress
   NVE deduces that the packet MUST be inter-subnet routed. Hence, the
   ingress NVE performs an IP lookup in the associated IP-VRF table. The
   lookup identifies an adjacency that contains a MAC rewrite and in
   turn the next-hop (i.e. egress) Gateway to which the packet must be
   forwarded along with the associated MPLS label stack. The MAC rewrite
   holds the MAC address associated with the destination host (as
   populated by the EVPN MAC route), instead of the MAC address of the
   next-hop Gateway. The ingress NVE then rewrites the destination MAC
   address in the packet with the address specified in the adjacency. It
   also rewrites the source MAC address with its IRB Interface MAC
   address for the destination subnet. The ingress NVE, then, forwards
   the frame to the next-hop (i.e. egress) Gateway after encapsulating
   it with the MPLS label stack.

   Note that this label stack includes the LSP label as well as an EVPN
   label. The EVPN label could be either advertised by the ingress
   Gateway, if inter-AS option B is used, or advertised by the egress
   NVE, if inter-AS option C is used. When the MPLS encapsulated packet
   is received by the ingress Gateway, the processing again differs
   depending on whether inter-AS option B or option C is employed: in
   the former case, the ingress Gateway swaps the EVPN label in the
   packets with the EVPN label value received from the egress Gateway.
   In the latter case, the ingress Gateway does not modify the EVPN
   label and performs normal label switching on the LSP label.
   Similarly on the egress Gateway, for option B, the egress Gateway
   swaps the EVPN label with the value advertised by the egress NVE.
   Whereas, for option C, the egress Gateway does not modify the EVPN
   label, and performs normal label switching on the LSP label. When the
   MPLS encapsulated packet is received by the egress NVE, it uses the
   EVPN label to identify the bridge-domain table. It then performs a
   MAC lookup in that table, which yields the outbound interface to
   which the Ethernet frame must be forwarded. Figure 3 below depicts
   the packet flow.















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            NVE1            ASBR1          ASBR2           NVE2
      +------------+  +------------+  +------------+  +------------+
      |            |  |            |  |            |  |            |
      |(MAC - (IP  |  |    [LS]    |  |    [LS]    |  |(IP  - (MAC |
      | VRF)   VRF)|  |            |  |            |  | VRF)   VRF)|
      |  |     |   |  |    |  |    |  |    |  |    |  |       |  | |
      +------------+  +------------+  +------------+  +------------+
         ^     v           ^  V            ^  V               ^  V
         |     |           |  |            |  |               |  |
   TS1->-+     +-->--------+  +------------+  +---------------+  +->-TS2


  Figure 9: Inter-Subnet Forwarding Among EVPN NVEs in Different DCs
   without GW



6.2 Switching among IP subnets in different DCs with GW

   In this scenario, connectivity is required between TS's in different
   data centers, and those hosts belong to different IP subnets. What
   makes this case different from that of Section 2.2 is that at least
   one of the data centers has a gateway as the WAN edge switch. Because
   of that, the NVE's IP-VRF  within that data center need not maintain
   (host) routes to individual TS's outside of that data center.

   As an example, consider a tenant with TS1 and TS5 of Figure 2 above.
   Assume that connectivity is required between these two TS's where TS1
   belongs to the SN1 whereas TS5 belongs to the SN5. NVE3 has an EVI5
   associated with the SN5 and this EVI is represented by the MAC-VRF
   which is connected to the IP-VRF via an IRB interface. NVE1 has an
   EVI1 associated with the SN1 and this EVI is represented by the MAC-
   VRF which is connected to the IP-VRF representing the same tenant.
   Due to the gateway at the edge of DCN 1, NVE1's IP-VRF does not need
   to have the address of TS5 but instead it has a default route in its
   IP-VRF with the next-hop being the GW.

   In this scenario, the NVEs within a given data center do not have
   entries for the MAC/IP addresses of hosts in remote data centers.
   Rather, the NVEs have a default IP route pointing to the WAN gateway
   for each VRF. This is accomplished by the WAN gateway advertising for
   a given EVPN that spans multiple DC a default VPN-IP route that is
   imported by the NVEs of that VPN that are in the gateway's own DC.

   When an Ethernet frame is received by an ingress NVE, it performs a
   lookup on the destination MAC address in the associated MAC-VRF
   table. If the MAC address corresponds to the IRB Interface MAC
   address, the ingress NVE deduces that the packet MUST be inter-subnet



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   routed. Hence, the ingress NVE performs an IP lookup in the
   associated IP-VRF table. The lookup, in this case, matches the
   default host route which points to the local WAN gateway (GW1). The
   ingress NVE (NVE1) then rewrites the destination MAC address in the
   packet with the MAC address of core-facing IRB interface of GW1 (not
   shown in the figure) or it can rewrite it with the router's MAC
   address of GW1. It also rewrites the source MAC address with its own
   core-facing IRB Interface's MAC address for the destination subnet
   (i.e., the subnet between NVE1 and GW1) or it can rewrite it with its
   own router's MAC address of NVE1. The ingress NVE, then, forwards the
   frame to GW1  after encapsulating it with the MPLS label stack. Note
   that this label stack includes the LSP label as well as the label for
   default host route that was advertised by the local WAN gateway. When
   the MPLS encapsulated packet is received by GW1, it uses the default
   host route MPLS label to identify the core-facing MAC-VRF. It does a
   MAC-DA lookup and forwards the packet to the IP-VRF after stripping
   the Ethernet header. It then performs an IP lookup in that table. The
   lookup identifies an adjacency that contains a MAC rewrite and in
   turn the remote WAN gateway (GW2) to which the packet must be
   forwarded along with the associated MPLS label stack. The MAC rewrite
   holds the MAC address associated with the ultimate destination host
   (as populated by the EVPN MAC route). GW1 then rewrites the
   destination MAC address in the packet with the address specified in
   the adjacency. It also rewrites the source MAC address with the MAC
   address of its core-facing IRB interface (not shown in the figure) or
   its router's MAC address. GW1, then, forwards the frame to the GW2
   after encapsulating it with the MPLS label stack. Note that this
   label stack includes the LSP label as well as a EVPN label that was
   advertised by GW2. When the MPLS encapsulated packet is received by
   GW2, it uses the EVPN label to identify the destination MAC-VRF. It
   then performs a MAC-DA lookup and grabs the EVPN label advertised by
   NVE2 along with adjacencies info. It then encapsulates the packet
   with the corresponding label stack and forwards the packet to NVE2.
   It should be noted that no MAC header re-write is performed on GW2.
   This implies that both GW1 and GW2 need to keep the remote host MAC
   addresses along with the corresponding EVPN labels in their tables.
   The egress NVE (NVE2) then upon receiving the packet, performs a MAC
   lookup in the MAC-VRF (identified by the received EVPN label) to
   determine the outbound port to send the traffic on.

   Figure 4 below depicts the forwarding model.










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            NVE1            GW1             GW2            NVE2
      +------------+  +------------+  +------------+  +------------+
      |            |  |            |  |            |  |            |
      |(MAC - (IP  |  |(IP  - (MAC |  |    (MAC    |  |(IP  - (MAC |
      | VRF)   VRF)|  | VRF)   VRF)|  |     VRF)   |  | VRF)   VRF)|
      |  |     |   |  | |  |       |  |    |  |    |  |       |  | |
      +------------+  +------------+  +------------+  +------------+
         ^     v        ^  V               ^  V               ^  V
         |     |        |  |               |  |               |  |
   TS1->-+     +-->-----+  +---------------+  +---------------+  +->-TS2


  Figure 10: Inter-Subnet Forwarding Among EVPN NVEs in Different DCs
   with GW


7 TS Mobility

7.1 TS Mobility & Optimum Forwarding for TS Outbound Traffic

   Optimum forwarding for the TS outbound traffic, upon TS mobility, can
   be achieved using either the anycast default Gateway MAC and IP
   addresses, or using the address aliasing as discussed in [DC-
   MOBILITY].

7.2 TS Mobility & Optimum Forwarding for TS Inbound Traffic

   For optimum forwarding of the TS inbound traffic, upon TS mobility,
   all the NVEs and/or IP-VPN PEs need to know the up to date location
   of the TS. Two scenarios must be considered, as discussed next.

   In what follows, we use the following terminology:

   - source NVE refers to the NVE behind which the TS used to reside
   prior to the TS mobility event.

   - target NVE refers to the new NVE behind which the TS has moved
   after the mobility event.

7.2.1 Mobility without Route Aggregation

   In this scenario, when a target NVE detects that a MAC mobility event
   has occurred, it initiates the MAC mobility handshake in BGP as
   specified in section 5.1.3. The WAN Gateways, acting as ASBRs in this
   case, re-advertise the MAC route of the target NVE with the MAC
   Mobility extended community attribute unmodified. Because the WAN
   Gateway for a given data center re-advertises BGP routes received
   from the WAN into the data center, the source NVE will receive the



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   MAC Advertisement route of the target NVE (with the next hop
   attribute adjusted depending on which inter-AS option is employed).
   The source NVE will then withdraw its original MAC Advertisement
   route as a result of evaluating the Sequence Number field of the MAC
   Mobility extended community in the received MAC Advertisement route.
   This is per the procedures already defined in [EVPN].


8  Acknowledgements

   The authors would like to thank Sami Boutros and Jeffrey Zhang for
   their valuable comments.

9  Security Considerations

   The security considerations discussed in [EVPN] apply to this
   document.

10  IANA Considerations

   IANA has allocated a new transitive extended community Type of 0x06
   and Sub-Type of 0x03 for EVPN Router's MAC Extended Community.

11  References

11.1  Normative References

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


   [EVPN] Sajassi et al., "BGP MPLS Based Ethernet VPN", RFC 7432,
              February, 2015.

   [TUNNEL-ENCAP] Rosen et al., "The BGP Tunnel Encapsulation
              Attribute", draft-ietf-idr-tunnel-encaps-03, November
              2016.

   [EVPN-PREFIX] Rabadan et al., "IP Prefix Advertisement in EVPN",
              draft-ietf-bess-evpn-prefix-advertisement-03, September,
              2016.

11.2  Informative References


   [RFC7606]  Chen, E., Scudder, J., Mohapatra, P., and K. Patel,
   "Revised Error Handling for BGP UPDATE Messages", RFC 7606, August
   2015, <http://www.rfc-editor.org/info/rfc7606>.



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   [802.1Q] "IEEE Standard for Local and metropolitan area networks -
   Media Access Control (MAC) Bridges and Virtual Bridged Local Area
   Networks", IEEE Std 802.1Q(tm), 2014 Edition, November 2014.

   [EVPN-IPVPN-INTEROP] Sajassi et al., "EVPN Seamless Interoperability
   with IP-VPN", draft-sajassi-l2vpn-evpn-ipvpn-interop-01, work in
   progress, October, 2012.

   [DC-MOBILITY] Aggarwal et al., "Data Center Mobility based on
   BGP/MPLS, IP Routing and NHRP", draft-raggarwa-data-center-mobility-
   05.txt, work in progress, June, 2013.

12  Contributors

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

   Florin Balus
   Cisco

   Yakov Rekhter
   Juniper

   Wim Henderickx
   Nokia

   Linda Dunbar
   Huawei

   Dennis Cai
   Alibaba

Authors' Addresses


   Ali Sajassi (Editor)
   Cisco
   Email: sajassi@cisco.com


   Samer Salam
   Cisco
   Email: sslam@cisco.com


   Samir Thoria
   Cisco
   Email: sthoria@cisco.com



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   John E. Drake
   Juniper Networks
   Email: jdrake@juniper.net


   Lucy Yong
   Huawei Technologies
   Email: lucy.yong@huawei.com


   Jorge Rabadan
   Nokia
   Email: jorge.rabadan@nokia.com






































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