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Versions: (draft-sd-l2vpn-evpn-overlay) 00 01 02 03 04 05 06 07 08

L2VPN Workgroup                                      A. Sajassi (Editor)
INTERNET-DRAFT                                                     Cisco
Intended Status: Standards Track                       J. Drake (Editor)
                                                                 Juniper
                                                                N. Bitar
                                                                   Nokia
                                                              R. Shekhar
                                                                 Juniper
                                                               J. Uttaro
                                                                    AT&T
                                                           W. Henderickx
                                                                   Nokia

Expires: June 1, 2017                                   December 1, 2016


         A Network Virtualization Overlay Solution using EVPN
                    draft-ietf-bess-evpn-overlay-07


Abstract

   This document describes how Ethernet VPN (EVPN) [RFC7432] can be used
   as an Network Virtualization Overlay (NVO) solution and explores the
   various tunnel encapsulation options over IP  and their impact on the
   EVPN control-plane and procedures. In particular, the following
   encapsulation options are analyzed: VXLAN, NVGRE, and MPLS over GRE.

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) 2012 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  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2  Specification of Requirements . . . . . . . . . . . . . . . . .  4
   3  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4 EVPN Features  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5 Encapsulation Options for EVPN Overlays  . . . . . . . . . . . .  7
     5.1 VXLAN/NVGRE Encapsulation  . . . . . . . . . . . . . . . . .  7
       5.1.1 Virtual Identifiers Scope  . . . . . . . . . . . . . . .  8
         5.1.1.1 Data Center Interconnect with Gateway  . . . . . . .  8
         5.1.1.2 Data Center Interconnect without Gateway . . . . . .  8
       5.1.2 Virtual Identifiers to EVI Mapping . . . . . . . . . . .  9
         5.1.2.1 Auto Derivation of RT  . . . . . . . . . . . . . . . 10
       5.1.3  Constructing EVPN BGP Routes  . . . . . . . . . . . . . 11
     5.2 MPLS over GRE  . . . . . . . . . . . . . . . . . . . . . . . 13
   6  EVPN with Multiple Data Plane Encapsulations  . . . . . . . . . 13
   7  NVE Residing in Hypervisor  . . . . . . . . . . . . . . . . . . 14
     7.1 Impact on EVPN BGP Routes & Attributes for VXLAN/NVGRE
         Encapsulation  . . . . . . . . . . . . . . . . . . . . . . . 14
     7.2 Impact on EVPN Procedures for VXLAN/NVGRE Encapsulation  . . 15
   8  NVE Residing in ToR Switch  . . . . . . . . . . . . . . . . . . 15
     8.1  EVPN Multi-Homing Features  . . . . . . . . . . . . . . . . 16
       8.1.1 Multi-homed Ethernet Segment Auto-Discovery  . . . . . . 16
       8.1.2 Fast Convergence and Mass Withdraw . . . . . . . . . . . 16
       8.1.3 Split-Horizon  . . . . . . . . . . . . . . . . . . . . . 16
       8.1.4 Aliasing and Backup-Path . . . . . . . . . . . . . . . . 16
       8.1.5 DF Election  . . . . . . . . . . . . . . . . . . . . . . 17
     8.2 Impact on EVPN BGP Routes & Attributes . . . . . . . . . . . 18
     8.3 Impact on EVPN Procedures  . . . . . . . . . . . . . . . . . 18
       8.3.1 Split Horizon  . . . . . . . . . . . . . . . . . . . . . 18
       8.3.2 Aliasing and Backup-Path . . . . . . . . . . . . . . . . 19



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       8.3.3 Unknown Unicast Traffic Designation  . . . . . . . . . . 19
   9 Support for Multicast  . . . . . . . . . . . . . . . . . . . . . 20
   10 Data Center Interconnections - DCI  . . . . . . . . . . . . . . 21
     10.1 DCI using GWs . . . . . . . . . . . . . . . . . . . . . . . 21
     10.2 DCI using ASBRs . . . . . . . . . . . . . . . . . . . . . . 22
       10.2.1 ASBR Functionality with NVEs in Hypervisors . . . . . . 23
       10.2.2 ASBR Functionality with NVEs in TORs  . . . . . . . . . 23
   11  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 25
   12  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   13  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   14  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     14.1  Normative References . . . . . . . . . . . . . . . . . . . 26
     14.2  Informative References . . . . . . . . . . . . . . . . . . 27
   Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28




































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1  Introduction

   In the context of this document, a Network Virtualization Overlay
   (NVO) is a solution to address the requirements of a multi-tenant
   data center, especially one with virtualized hosts, e.g., Virtual
   Machines (VMs) or virtual workloads. The key requirements of such a
   solution, as described in [Problem-Statement], are:

   - Isolation of network traffic per tenant

   - Support for a large number of tenants (tens or hundreds of
   thousands)

   - Extending L2 connectivity among different VMs belonging to a given
   tenant segment (subnet) across different PODs within a data center or
   between different data centers

   - Allowing a given VM to move between different physical points of
   attachment within a given L2 segment


   The underlay network for NVO solutions is assumed to provide IP
   connectivity between NVO endpoints (NVEs).

   This document describes how Ethernet VPN (EVPN) can be used as an NVO
   solution and explores applicability of EVPN functions and procedures.
   In particular, it describes the various tunnel encapsulation options
   for EVPN over IP, and their impact on the EVPN control-plane and
   procedures for two main scenarios:

   a) when the NVE resides in the hypervisor, and
   b) when the NVE resides in a Top of Rack (ToR) device

   The possible encapsulation options for EVPN overlays that are
   analyzed in this document are:

   - VXLAN and NVGRE
   - MPLS over GRE

   Before getting into the description of the different encapsulation
   options for EVPN over IP, it is important to highlight the EVPN
   solution's main features, how those features are currently supported,
   and any impact that the encapsulation has on those features.


2  Specification of Requirements

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



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3  Terminology

   NVO: Network Virtualization Overlay

   NVE: Network Virtualization Endpoint

   VNI:  Virtual Network Identifier (for VXLAN)

   VSID: Virtual Subnet Identifier (for NVGRE)

   EVPN: Ethernet VPN

   EVI: An EVPN instance spanning the Provider Edge (PE) devices
   participating in that EVPN.

   MAC-VRF: A Virtual Routing and Forwarding table for Media Access
   Control (MAC) addresses on a PE.

   Ethernet Segment (ES): When a customer site (device or network) is
   connected to one or more PEs via a set of Ethernet links, then that
   set of links is referred to as an 'Ethernet segment'.

   Ethernet Segment Identifier (ESI): A unique non-zero identifier that
   identifies an Ethernet segment is called an 'Ethernet Segment
   Identifier'.

   Ethernet Tag: An Ethernet tag identifies a particular broadcast
   domain, e.g., a VLAN.  An EVPN instance consists of one or more
   broadcast domains.

   PE: Provider Edge device.

   Single-Active Redundancy Mode: When only a single PE, among all the
   PEs attached to an Ethernet segment, is allowed to forward traffic
   to/from that Ethernet segment for a given VLAN, then the Ethernet
   segment is defined to be operating in Single-Active redundancy mode.

   All-Active Redundancy Mode: When all PEs attached to an Ethernet
   segment are allowed to forward known unicast traffic to/from that
   Ethernet segment for a given VLAN, then the Ethernet segment is
   defined to be operating in All-Active redundancy mode.


4 EVPN Features



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   EVPN was originally designed to support the requirements detailed in
   [RFC7209] and therefore has the following attributes which directly
   address control plane scaling and ease of deployment issues.

   1)  Control plane traffic is distributed with BGP and Broadcast and
   Multicast traffic is sent using a shared multicast tree or with
   ingress replication.

   2)  Control plane learning is used for MAC (and IP) addresses instead
   of data plane learning. The latter requires the flooding of unknown
   unicast and ARP frames; whereas, the former does not require any
   flooding.

   3) Route Reflector is used to reduce a full mesh of BGP sessions
   among PE devices to a single BGP session between a PE and the RR.
   Furthermore, RR hierarchy can be leveraged to scale the number of BGP
   routes on the RR.

   4)  Auto-discovery via BGP is used to discover PE devices
   participating in a given VPN, PE devices participating in a given
   redundancy group, tunnel encapsulation types, multicast tunnel type,
   multicast members, etc.

   5)  All-Active multihoming is used.  This allows a given customer
   device (CE) to have multiple links to multiple PEs, and traffic
   to/from that CE fully utilizes all of these links.

   6)  When a link between a CE and a PE fails, the PEs for that EVI are
   notified of the failure via the withdrawal of a single EVPN route.
   This allows those PEs to remove the withdrawing PE as a next hop for
   every MAC address associated with the failed link.  This is termed
   'mass withdrawal'.

   7)  BGP route filtering and constrained route distribution are
   leveraged to ensure that the control plane traffic for a given EVI is
   only distributed to the PEs in that EVI.

   8) When a 802.1Q interface is used between a CE and a PE, each of the
   VLAN ID (VID) on that interface can be mapped onto a bridge table
   (for upto 4094 such bridge tables). All these bridge tables may be
   mapped onto a single MAC-VRF (in case of VLAN-aware bundle service).

   9)  VM Mobility mechanisms ensure that all PEs in a given EVI know
   the ES with which a given VM, as identified by its MAC and IP
   addresses, is currently associated.

   10)  Route Targets are used to allow the operator (or customer) to
   define a spectrum of logical network topologies including mesh, hub &



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   spoke, and extranets (e.g., a VPN whose sites are owned by different
   enterprises), without the need for proprietary software or the aid of
   other virtual or physical devices.

   Because the design goal for NVO is millions of instances per common
   physical infrastructure, the scaling properties of the control plane
   for NVO are extremely important.   EVPN and the extensions described
   herein, are designed with this level of scalability in mind.


5 Encapsulation Options for EVPN Overlays

5.1 VXLAN/NVGRE Encapsulation

   Both VXLAN and NVGRE are examples of technologies that provide a data
   plane encapsulation which is used to transport a packet over the
   common physical IP infrastructure between Network Virtualization
   Edges (NVEs) - e.g., VXLAN Tunnel End Points (VTEPs) in VXLAN
   network. Both of these technologies include the identifier of the
   specific NVO instance, Virtual Network Identifier (VNI) in VXLAN and
   Virtual Subnet Identifier (VSID) in NVGRE, in each packet. In the
   remainder of this document we use VNI as the representation for NVO
   instance with the understanding that VSID can equally be used if the
   encapsulation is NVGRE unless it is stated otherwise.

   Note that a Provider Edge (PE) is equivalent to a NVE/VTEP.

   VXLAN encapsulation is based on UDP, with an 8-byte header following
   the UDP header. VXLAN provides a 24-bit VNI, which typically provides
   a one-to-one mapping to the tenant VLAN ID, as described in
   [RFC7348]. In this scenario, the ingress VTEP does not include an
   inner VLAN tag on the encapsulated frame, and the egress VTEP
   discards the frames with an inner VLAN tag. This mode of operation in
   [RFC7348] maps to VLAN Based Service in [RFC7432], where a tenant
   VLAN ID gets mapped to an EVPN instance (EVI).

   VXLAN also provides an option of including an inner VLAN tag in the
   encapsulated frame, if explicitly configured at the VTEP. This mode
   of operation can map to VLAN Bundle Service in [RFC7432] because all
   the tenant's tagged frames map to a single bridge table / MAC-VRF,
   and the inner VLAN tag is not used for lookup by the disposition PE
   when performing VXLAN decapsulation as described in section 6 of
   [RFC7348].

   [NVGRE] encapsulation is based on [GRE] and it mandates the inclusion
   of the optional GRE Key field which carries the VSID. There is a one-
   to-one mapping between the VSID and the tenant VLAN ID, as described
   in [NVGRE] and the inclusion of an inner VLAN tag is prohibited. This



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   mode of operation in [NVGRE] maps to VLAN Based Service in
   [RFC7432].

   As described in the next section there is no change to the encoding
   of EVPN routes to support VXLAN or NVGRE encapsulation except for the
   use of BGP Encapsulation extended community to indicate the
   encapsulation type (e.g., VxLAN or NVGRE). However, there is
   potential impact to the EVPN procedures depending on where the NVE is
   located (i.e., in hypervisor or TOR) and whether multi-homing
   capabilities are required.

5.1.1 Virtual Identifiers Scope

   Although VNIs are defined as 24-bit globally unique values, there are
   scenarios in which it is desirable to use a locally significant value
   for VNI, especially in the context of data center interconnect:

5.1.1.1 Data Center Interconnect with Gateway

   In the case where NVEs in different data centers need to be
   interconnected, and the NVEs need to use VNIs as a globally unique
   identifiers within a data center, then a Gateway needs to be employed
   at the edge of the data center network. This is because the Gateway
   will provide the functionality of translating the VNI when crossing
   network boundaries, which may align with operator span of control
   boundaries. As an example, consider the network of Figure 1 below.
   Assume there are three network operators: one for each of the DC1,
   DC2 and WAN networks. The Gateways at the edge of the data centers
   are responsible for translating the VNIs between the values used in
   each of the data center networks and the values used in the WAN.

                             +--------------+
                             |              |
           +---------+       |     WAN      |       +---------+
   +----+  |        +---+  +----+        +----+  +---+        |  +----+
   |NVE1|--|        |   |  |WAN |        |WAN |  |   |        |--|NVE3|
   +----+  |IP      |GW |--|Edge|        |Edge|--|GW | IP     |  +----+
   +----+  |Fabric  +---+  +----+        +----+  +---+ Fabric |  +----+
   |NVE2|--|         |       |              |       |         |--|NVE4|
   +----+  +---------+       +--------------+       +---------+  +----+

   |<------ DC 1 ------>                          <------ DC2  ------>|

            Figure 1: Data Center Interconnect with Gateway

5.1.1.2 Data Center Interconnect without Gateway

   In the case where NVEs in different data centers need to be



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   interconnected, and the NVEs need to use locally assigned VNIs (e.g.,
   similar to MPLS labels), then there may be no need to employ Gateways
   at the edge of the data center network. More specifically, the VNI
   value that is used by the transmitting NVE is allocated by the NVE
   that is receiving the traffic (in other words, this is similar to
   "downstream assigned" MPLS label). This allows the VNI space to be
   decoupled between different data center networks without the need for
   a dedicated Gateway at the edge of the data centers. This topics is
   covered in section 10.2.


                              +--------------+
                              |              |
              +---------+     |     WAN      |    +---------+
      +----+  |         |   +----+        +----+  |         |  +----+
      |NVE1|--|         |   |ASBR|        |ASBR|  |         |--|NVE3|
      +----+  |IP Fabric|---|    |        |    |--|IP Fabric|  +----+
      +----+  |         |   +----+        +----+  |         |  +----+
      |NVE2|--|         |     |              |    |         |--|NVE4|
      +----+  +---------+     +--------------+    +---------+  +----+

      |<------ DC 1 ----->                        <---- DC2  ------>|

              Figure 2: Data Center Interconnect with ASBR


5.1.2 Virtual Identifiers to EVI Mapping

   When the EVPN control plane is used in conjunction with VXLAN (or
   NVGRE encapsulation), two options for mapping the VXLAN VNI (or NVGRE
   VSID) to an EVI are possible:

   1. Option 1: Single Subnet per EVI

   In this option, a single subnet represented by a VNI is mapped to a
   unique EVI. This corresponds to the VLAN Based service in [RFC7432],
   where a tenant VLAN ID gets mapped to an EVPN instance (EVI). As
   such, a BGP RD and RT is needed per VNI on every NVE. The advantage
   of this model is that it allows the BGP RT constraint mechanisms to
   be used in order to limit the propagation and import of routes to
   only the NVEs that are interested in a given VNI. The disadvantage of
   this model may be the provisioning overhead if RD and RT are not
   derived automatically from VNI.

   In this option, the MAC-VRF table is identified by the RT in the
   control plane and by the VNI in the data-plane. In this option, the
   specific MAC-VRF table corresponds to only a single bridge table.




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   2. Option 2: Multiple Subnets per EVI

   In this option, multiple subnets each represented by a unique VNI are
   mapped to a single EVI. For example, if a tenant has multiple
   segments/subnets each represented by a VNI, then all the VNIs for
   that tenant are mapped to a single EVI - e.g., the EVI in this case
   represents the tenant and not a subnet . This corresponds to the
   VLAN-aware bundle service in [RFC7432]. The advantage of this model
   is that it doesn't require the provisioning of RD/RT per VNI.
   However, this is a moot point if option 1 with auto-derivation is
   used. The disadvantage of this model is that routes would be imported
   by NVEs that may not be interested in a given VNI.

   In this option the MAC-VRF table is identified by the RT in the
   control plane and a specific bridge table for that MAC-VRF is
   identified by the <RT, Ethernet Tag ID> in the control plane. In this
   option, the VNI in the data-plane is sufficient to identify a
   specific bridge table.



5.1.2.1 Auto Derivation of RT

   When the option of a single VNI per EVI is used, it is important to
   auto-derive RT for EVPN BGP routes in order to simplify configuration
   for data center operations. RD can be auto generated as described in
   [RFC7432] and RT can be auto-derived as described next.

   Since a gateway PE as depicted in figure-1 participates in both the
   DCN and WAN BGP sessions, it is important that when RT values are
   auto-derived for VNIs, there is no conflict in RT spaces between DCN
   and WAN networks assuming that both are operating within the same AS.
   Also, there can be scenarios where both VXLAN and NVGRE
   encapsulations may be needed within the same DCN and their
   corresponding VNIs are administered independently which means VNI
   spaces can overlap. In order to ensure that no such conflict in RT
   spaces arises, RT values for DCNs are auto-derived as follow:

   0                   1                   2                   3      4
   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 2  0
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---+
   |              AS #             |A| TYPE| D-ID  |Service Instance ID|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---+



   - 2 bytes of global admin field of the RT is set to the AS number.




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   - Three least significant bytes of the local admin field of the RT is
   set to the VNI, VSID, I-SID, or VID.

   - The most significant bit of the local admin field of the RT is set
   as follow:
        0: auto-derived
        1: manually-derived

   - The next 3 bits of the most significant byte of the local admin
   field of the RT identifies the space in which the other 3 bytes are
   defined. The following spaces are defined:
        0 : VID
        1 : VXLAN
        2 : NVGRE
        3 : I-SID
        4 : EVI
        5 : dual-VID

   - The remaining 4 bits of the most significant byte of the local
   admin field of the RT identifies the domain-id. The default value of
   domain-id is zero indicating that only a single numbering space exist
   for a given technology. However, if there are more than one number
   space exist for a given technology (e.g., overlapping VXLAN spaces),
   then each of the number spaces need to be identify by their
   corresponding domain-id starting from 1.


5.1.3  Constructing EVPN BGP Routes

   In EVPN, an MPLS label for instance identifying forwarding table is
   distributed by the egress PE via the EVPN control plane and is placed
   in the MPLS header of a given packet by the ingress PE. This label is
   used upon receipt of that packet by the egress PE for disposition of
   that packet. This is very similar to the use of the VNI by the egress
   NVE, with the difference being that an MPLS label has local
   significance while a VNI typically has global significance.
   Accordingly, and specifically to support the option of locally-
   assigned VNIs, the MPLS Label1 field in the MAC/IP Advertisement
   route, the MPLS label field in the Ethernet AD per EVI route, and the
   MPLS label field in the PMSI Tunnel Attribute of the Inclusive
   Multicast Ethernet Tag (IMET) route are used to carry the VNI. For
   the balance of this memo, the MPLS label field will be referred to as
   the VNI field. The VNI field is used for both local and global VNIs,
   and for either case the entire 24-bit field is used to encode the VNI
   value.

   For the VLAN-based service (a single VNI per MAC-VRF), the Ethernet
   Tag field in the MAC/IP Advertisement, Ethernet AD per EVI, and IMET



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   route MUST be set to zero just as in the VLAN Based service in
   [RFC7432].

   For the VLAN-aware bundle service (multiple VNIs per MAC-VRF with
   each VNI associated with its own bridge table), the Ethernet Tag
   field in the MAC Advertisement, Ethernet AD per EVI, and IMET route
   MUST identify a bridge table within a MAC-VRF and the set of Ethernet
   Tags for that EVI needs to be configured consistently on all PEs
   within that EVI.  For locally-assigned VNIs, the value advertised in
   the Ethernet Tag field MUST be set to a VID just as in the VLAN-aware
   bundle service in [RFC7432]. Such setting must be done consistently
   on all PE devices participating in that EVI within a given domain.
   For global VNIs, the value advertised in the Ethernet Tag field
   SHOULD be set to a VNI as long as it matches the existing semantics
   of the Ethernet Tag, i.e., it identifies a bridge table within a MAC-
   VRF and the set of VNIs are configured consistently on each PE in
   that EVI.

   In order to indicate that which type of data plane encapsulation
   (i.e., VXLAN, NVGRE, MPLS, or MPLS in GRE) is to be used, the BGP
   Encapsulation extended community defined in [TUNNEL-ENCAP] and
   [RFC5512] is included with all EVPN routes (i.e. MAC Advertisement,
   Ethernet AD per EVI, Ethernet AD per ESI, Inclusive Multicast
   Ethernet Tag, and Ethernet Segment) advertised by an egress PE. Five
   new values have been assigned by IANA to extend the list of
   encapsulation types defined in [TUNNEL-ENCAP] and they are listed in
   section 13.

   The MPLS encapsulation tunnel type, listed in section 13, is needed
   in order to distinguish between an advertising node that only
   supports non-MPLS encapsulations and one that supports MPLS and non-
   MPLS encapsulations.  An  advertising node that only supports MPLS
   encapsulation does not need to advertise any encapsulation tunnel
   types;  i.e.,  if the BGP Encapsulation extended community is not
   present, then either MPLS encapsulation or a statically configured
   encapsulation is assumed.

   The Ethernet Segment and Ethernet AD per ESI routes MAY be advertised
   with multiple encapsulation types as long as they use the same EVPN
   multi-homing procedures - e.g., the mix of VXLAN and NVGRE
   encapsulation types is a valid one but not the mix of VXLAN and MPLS
   encapsulation types.

   The Next Hop field of the MP_REACH_NLRI attribute of the route MUST
   be set to the IPv4 or IPv6 address of the NVE. The remaining fields
   in each route are set as per [RFC7432].

   Note that the procedure defined here to use the MPLS Label field to



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   carry the VNI in the presence of a Tunnel Encapsulation Extended
   Community specifying the use of a VNI, is aligned with the procedures
   described in section 8.2.2.2 of [tunnel-encap] ("When a Valid VNI has
   not been Signaled").

5.2 MPLS over GRE

   The EVPN data-plane is modeled as an EVPN MPLS client layer sitting
   over an MPLS PSN-tunnel server layer. Some of the EVPN functions
   (split-horizon, aliasing, and backup-path) are tied to the MPLS
   client layer. If MPLS over GRE encapsulation is used, then the EVPN
   MPLS client layer can be carried over an IP PSN tunnel transparently.
   Therefore, there is no impact to the EVPN procedures and associated
   data-plane operation.

   The existing standards for MPLS over GRE encapsulation as defined by
   [RFC4023] can be used for this purpose; however, when it is used in
   conjunction with EVPN the GRE key field SHOULD be present, and SHOULD
   be used to provide a 32-bit entropy field. The Checksum and Sequence
   Number fields are not needed and their corresponding C and S bits
   MUST be set to zero. A PE capable of supporting this encapsulation,
   should advertise its EVPN routes along with the Tunnel Encapsulation
   extended community indicating MPLS over GRE encapsulation, as
   described in previous section.


6  EVPN with Multiple Data Plane Encapsulations

   The use of the BGP Encapsulation extended community per [TUNNEL-
   ENCAP] and [RFC5512] allows each NVE in a given EVI to know each of
   the encapsulations supported by each of the other NVEs in that EVI.
   i.e., each of the NVEs in a given EVI may support multiple data plane
   encapsulations.  An ingress NVE can send a frame to an egress NVE
   only if the set of encapsulations advertised by the egress NVE forms
   a non-empty intersection with the set of encapsulations supported by
   the ingress NVE, and it is at the discretion of the ingress NVE which
   encapsulation to choose from this intersection.   (As noted in
   section 5.1.3, if the BGP Encapsulation extended community is not
   present, then the default MPLS encapsulation or a locally configured
   encapsulation is assumed.)

   An ingress node that uses shared multicast trees for sending
   broadcast or multicast frames MAY maintain distinct trees for each
   different encapsulation type.

   It is the responsibility of the operator of a given EVI to ensure
   that all of the NVEs in that EVI support at least one common
   encapsulation. If this condition is violated, it could result in



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   service disruption or failure.  The use of the BGP Encapsulation
   extended community provides a method to detect when this condition is
   violated but the actions to be taken are at the discretion of the
   operator and are outside the scope of this document.

7  NVE Residing in Hypervisor

   When a NVE and its hosts/VMs are co-located in the same physical
   device, e.g., when they reside in a server, the links between them
   are virtual and they typically share fate;  i.e., the subject
   hosts/VMs are typically not multi-homed or if they are multi-homed,
   the multi-homing is a purely local matter to the server hosting the
   VM and the NVEs, and need not be "visible" to any other NVEs residing
   on other servers, and thus does not require any specific protocol
   mechanisms.  The most common case of this is when the NVE resides on
   the hypervisor.

   In the sub-sections that follow, we will discuss the impact on EVPN
   procedures for the case when the NVE resides on the hypervisor and
   the VXLAN (or NVGRE) encapsulation is used.

7.1 Impact on EVPN BGP Routes & Attributes for VXLAN/NVGRE Encapsulation

   In scenarios where different groups of data centers are under
   different administrative domains, and these data centers are
   connected via one or more backbone core providers as described in
   [NOV3-Framework], the RD must be a unique value per EVI or per NVE as
   described in [RFC7432]. In other words, whenever there is more than
   one administrative domain for global VNI, then a unique RD MUST be
   used, or whenever the VNI value have local significance, then a
   unique RD MUST be used. Therefore, it is recommend to use a unique RD
   as described in [RFC7432] at all time.

   When the NVEs reside on the hypervisor, the EVPN BGP routes and
   attributes associated with multi-homing are no longer required. This
   reduces the required routes and attributes to the following subset of
   four out of eight:

   - MAC/IP Advertisement Route
   - Inclusive Multicast Ethernet Tag Route
   - MAC Mobility Extended Community
   - Default Gateway Extended Community

   However, as noted in section 8.6 of [RFC7432] in order to enable a
   single-homing ingress NVE to take advantage of fast convergence,
   aliasing, and backup-path when interacting with multi-homed egress
   NVEs attached to a given Ethernet segment, the single-homing ingress
   NVE SHOULD be able to receive and process Ethernet AD per ES and



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   Ethernet AD per EVI routes.


7.2 Impact on EVPN Procedures for VXLAN/NVGRE Encapsulation

   When the NVEs reside on the hypervisors, the EVPN procedures
   associated with multi-homing are no longer required. This limits the
   procedures on the NVE to the following subset of the EVPN procedures:

   1. Local learning of MAC addresses received from the VMs per section
   10.1 of [RFC7432].

   2. Advertising locally learned MAC addresses in BGP using the MAC/IP
   Advertisement routes.

   3. Performing remote learning using BGP per Section 10.2 of
   [RFC7432].

   4. Discovering other NVEs and constructing the multicast tunnels
   using the Inclusive Multicast Ethernet Tag routes.

   5. Handling MAC address mobility events per the procedures of Section
   16 in [RFC7432].

   However, as noted in section 8.6 of [RFC7432] in order to enable a
   single-homing ingress NVE to take advantage of fast convergence,
   aliasing, and back-up path when interacting with multi-homed egress
   NVEs attached to a given Ethernet segment, a single-homing ingress
   NVE SHOULD implement the ingress node processing of Ethernet AD per
   ES and Ethernet AD per EVI routes as defined in sections 8.2 Fast
   Convergence and 8.4 Aliasing and Backup-Path of [RFC7432].

8  NVE Residing in ToR Switch

   In this section, we discuss the scenario where the NVEs reside in the
   Top of Rack (ToR) switches AND the servers (where VMs are residing)
   are multi-homed to these ToR switches. The multi-homing may operate
   in All-Active or Single-Active redundancy mode. If the servers are
   single-homed to the ToR switches, then the scenario becomes similar
   to that where the NVE resides on the hypervisor, as discussed in
   Section 7, as far as the required EVPN functionality are concerned.

   [RFC7432] defines a set of BGP routes, attributes and procedures to
   support multi-homing. We first describe these functions and
   procedures, then discuss which of these are impacted by the VxLAN
   (or NVGRE) encapsulation and what modifications are required. As it
   will be seen later in this section, the only EVPN procedure that is
   impacted by IP underlay tunnels is that of split-horizon filtering



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   for multi-homed Ethernet Segments described in section 8.3.1.

8.1  EVPN Multi-Homing Features

   In this section, we will recap the multi-homing features of EVPN to
   highlight the encapsulation dependencies. The section only describes
   the features and functions at a high-level. For more details, the
   reader is to refer to [RFC7432].

8.1.1 Multi-homed Ethernet Segment Auto-Discovery

   EVPN NVEs (or PEs) connected to the same Ethernet Segment (e.g. the
   same server via LAG) can automatically discover each other with
   minimal to no configuration through the exchange of BGP routes.

8.1.2 Fast Convergence and Mass Withdraw

   EVPN defines a mechanism to efficiently and quickly signal, to remote
   NVEs, the need to update their forwarding tables upon the occurrence
   of a failure in connectivity to an Ethernet segment (e.g., a link or
   a port failure). This is done by having each NVE advertise an
   Ethernet A-D Route per Ethernet segment for each locally attached
   segment. Upon a failure in connectivity to the attached segment, the
   NVE withdraws the corresponding Ethernet A-D route. This triggers all
   NVEs that receive the withdrawal to update their next-hop adjacencies
   for all MAC addresses associated with the Ethernet segment in
   question. If no other NVE had advertised an Ethernet A-D route for
   the same segment, then the NVE that received the withdrawal simply
   invalidates the MAC entries for that segment. Otherwise, the NVE
   updates the next-hop adjacency list accordingly.

8.1.3 Split-Horizon

   If a server is multi-homed to two or more NVEs (represented by an
   Ethernet segment ES1) and operating in an all-active redundancy mode,
   sends a BUM packet (ie,  Broadcast, Unknown unicast, or Multicast) to
   one of these NVEs, then it is important to ensure the packet is not
   looped back to the server via another NVE connected to this server.
   The filtering mechanism on the NVE to prevent such loop and packet
   duplication is called "split horizon filtering'.


8.1.4 Aliasing and Backup-Path

   In the case where a station is multi-homed to multiple NVEs, it is
   possible that only a single NVE learns a set of the MAC addresses
   associated with traffic transmitted by the station. This leads to a
   situation where remote NVEs receive MAC advertisement routes, for



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   these addresses, from a single NVE even though multiple NVEs are
   connected to the multi-homed station. As a result, the remote NVEs
   are not able to effectively load-balance traffic among the NVEs
   connected to the multi-homed Ethernet segment. This could be the
   case, for e.g. when the NVEs perform data-path learning on the
   access, and the load-balancing function on the station hashes traffic
   from a given source MAC address to a single NVE. Another scenario
   where this occurs is when the NVEs rely on control plane learning on
   the access (e.g. using ARP), since ARP traffic will be hashed to a
   single link in the LAG.

   To alleviate this issue, EVPN introduces the concept of Aliasing.
   This refers to the ability of an NVE to signal that it has
   reachability to a given locally attached Ethernet segment, even when
   it has learnt no MAC addresses from that segment. The Ethernet A-D
   route per EVI is used to that end. Remote NVEs which receive MAC
   advertisement routes with non-zero ESI SHOULD consider the MAC
   address as reachable via all NVEs that advertise reachability to the
   relevant Segment using Ethernet A-D routes with the same ESI and with
   the Single-Active flag reset.

   Backup-Path is a closely related function, albeit it applies to the
   case where the redundancy mode is Single-Active. In this case, the
   NVE signals that it has reachability to a given locally attached
   Ethernet Segment using the Ethernet A-D route as well. Remote NVEs
   which receive the MAC advertisement routes, with non-zero ESI, SHOULD
   consider the MAC address as reachable via the advertising NVE.
   Furthermore, the remote NVEs SHOULD install a Backup-Path, for said
   MAC, to the NVE which had advertised reachability to the relevant
   Segment using an Ethernet A-D route with the same ESI and with the
   Single-Active flag set.


8.1.5 DF Election

   If a host is multi-homed to two or more NVEs on an Ethernet segment
   operating in all-active redundancy mode, then for a given EVI only
   one of these NVEs, termed the Designated Forwarder (DF) is
   responsible for sending it broadcast, multicast, and, if configured
   for that EVI, unknown unicast frames.

   This is required in order to prevent duplicate delivery of multi-
   destination frames to a multi-homed host or VM, in case of all-active
   redundancy.

   In NVEs where .1Q tagged frames are received from hosts, the DF
   election SHOULD BE performed based on host VLAN IDs (VIDs) per
   section 8.5 of [RFC 7432]. Furthermore, multi-homing PEs of a given



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   Ethernet Segment MAY perform DF election using configured IDs such as
   VNI, EVI, normalized VIDs, and etc. as along the IDs are configured
   consistently across the multi-homing PEs.

   In GWs where VxLAN encapsulated frames are received, the DF election
   is performed on VNIs. Again, it is assumed that for a given Ethernet
   Segment, VNIs are unique and consistent (e.g., no duplicate VNIs
   exist).


8.2 Impact on EVPN BGP Routes & Attributes

   Since multi-homing is supported in this scenario, then the entire set
   of BGP routes and attributes defined in [RFC7432] are used. The
   setting of the Ethernet Tag field in the MAC Advertisement, Ethernet
   AD per EVI, and Inclusive Multicast routes follows that of section
   5.1.3. Furthermore, the setting of the VNI field in the MAC
   Advertisement and Ethernet AD per EVI routes follows that of section
   5.1.3.


8.3 Impact on EVPN Procedures

   Two cases need to be examined here, depending on whether the NVEs are
   operating in Single-Active or in All-Active redundancy mode.

   First, lets consider the case of Single-Active redundancy mode, where
   the hosts are multi-homed to a set of NVEs, however, only a single
   NVE is active at a given point of time for a given VNI. In this case,
   the aliasing is not required and the split-horizon filtering may not
   be required, but other functions such as multi-homed Ethernet segment
   auto-discovery, fast convergence and mass withdraw, backup path, and
   DF election are required.

   Second, let's consider the case of All-Active redundancy mode. In
   this case, out of all the EVPN multi-homing features listed in
   section 8.1, the use of the VXLAN or NVGRE encapsulation impacts the
   split-horizon and aliasing features, since those two rely on the MPLS
   client layer. Given that this MPLS client layer is absent with these
   types of encapsulations, alternative procedures and mechanisms are
   needed to provide the required functions. Those are discussed in
   detail next.

8.3.1 Split Horizon

   In EVPN, an MPLS label is used for split-horizon filtering to support
   All-Active multi-homing where an ingress NVE adds a label
   corresponding to the site of origin (aka ESI Label) when



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   encapsulating the packet. The egress NVE checks the ESI label when
   attempting to forward a multi-destination frame out an interface, and
   if the label corresponds to the same site identifier (ESI) associated
   with that interface, the packet gets dropped. This prevents the
   occurrence of forwarding loops.

   Since the VXLAN or NVGRE encapsulation does not include this ESI
   label, other means of performing the split-horizon filtering function
   MUST be devised. The following approach is recommended for split-
   horizon filtering when VXLAN (or NVGRE) encapsulation is used.

   Every NVE track the IP address(es) associated with the other NVE(s)
   with which it has shared multi-homed Ethernet Segments. When the NVE
   receives a multi-destination frame from the overlay network, it
   examines the source IP address in the tunnel header (which
   corresponds to the ingress NVE) and filters out the frame on all
   local interfaces connected to Ethernet Segments that are shared with
   the ingress NVE. With this approach, it is required that the ingress
   NVE performs replication locally to all directly attached Ethernet
   Segments (regardless of the DF Election state) for all flooded
   traffic ingress from the access interfaces (i.e. from the hosts).
   This approach is referred to as "Local Bias", and has the advantage
   that only a single IP address needs to be used per NVE for split-
   horizon filtering, as opposed to requiring an IP address per Ethernet
   Segment per NVE.

   In order to prevent unhealthy interactions between the split horizon
   procedures defined in [RFC7432] and the local bias procedures
   described in this document, a mix of MPLS over GRE encapsulations on
   the one hand and VXLAN/NVGRE encapsulations on the other on a given
   Ethernet Segment is prohibited.

8.3.2 Aliasing and Backup-Path

   The Aliasing and the Backup-Path procedures for VXLAN/NVGRE
   encapsulation is very similar to the ones for MPLS. In case of MPLS,
   Ethernet A-D route per EVI is used for Aliasing when the
   corresponding Ethernet Segment operates in All-Active multi-homing,
   and the same route is used for Backup-Path when the corresponding
   Ethernet Segment operates in Single-Active multi-homing. In case of
   VxLAN/NVGRE, the same route is used for the Aliasing and the Backup-
   Path with the difference that the Ethernet Tag and VNI fields in
   Ethernet A-D per EVI route is set as described in section 5.1.3.

8.3.3 Unknown Unicast Traffic Designation

   In EVPN, when an ingress PE uses ingress replication to flood unknown
   unicast traffic to egress PEs, the ingress PE uses a different EVPN



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   MPLS label (from the one used for known unicast traffic) to identify
   such BUM traffic. The egress PEs use this label to identify such BUM
   traffic and thus apply DF filtering for All-Active multi-homed sites.
   In absence of unknown unicast traffic designation and in presence of
   enabling unknown unicast flooding, there can be transient duplicate
   traffic to All-Active multi-homed sites under the following
   condition: the host MAC address is learned by the egress PE(s) and
   advertised to the ingress PE; however, the MAC advertisement has not
   been received or processed by the ingress PE, resulting in the host
   MAC address to be unknown on the ingress PE but be known on the
   egress PE(s). Therefore, when a packet destined to that host MAC
   address arrives on the ingress PE, it floods it via ingress
   replication to all the egress PE(s) and since they are known to the
   egress PE(s), multiple copies is sent to the All-Active multi-homed
   site. It should be noted that such transient packet duplication only
   happens when a) the destination host is multi-homed via All-Active
   redundancy mode, b) flooding of unknown unicast is enabled in the
   network, c) ingress replication is used, and d) traffic for the
   destination host is arrived on the ingress PE before it learns the
   host MAC address via BGP EVPN advertisement. In order to prevent such
   occurrence of packet duplication (however low probability that may
   be), the ingress PE MAY use a flag-bit in the VxLAN header to
   indicate BUM traffic type. Bit 6 of flag field in the VxLAN header is
   used for this purpose per section 3.1 of [VXLAN-GPE].


9 Support for Multicast

   The E-VPN Inclusive Multicast BGP route is used to discover the
   multicast tunnels among the endpoints associated with a given EVI
   (e.g., given VNI) for VLAN-based service and a given <EVI,VLAN> for
   VLAN-aware bundle service. All fields of this route is set as
   described in section 5.1.3. The Originating router's IP address field
   is set to the NVE's IP address. This route is tagged with the PMSI
   Tunnel attribute, which is used to encode the type of multicast
   tunnel to be used as well as the multicast tunnel identifier. The
   tunnel encapsulation is encoded by adding the BGP Encapsulation
   extended community as per section 5.1.1. For example, the PMSI Tunnel
   attribute may indicate the multicast tunnel is of type PIM-SM;
   whereas, the BGP Encapsulation extended community may indicate the
   encapsulation for that tunnel is of type VxLAN. The following tunnel
   types as defined in [RFC6514] can be used in the PMSI tunnel
   attribute for VXLAN/NVGRE:

         + 3 - PIM-SSM Tree
         + 4 - PIM-SM Tree
         + 5 - BIDIR-PIM Tree
         + 6 - Ingress Replication



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   Except for Ingress Replication, this multicast tunnel is used by the
   PE originating the route for sending multicast traffic to other PEs,
   and is used by PEs that receive this route for receiving the traffic
   originated by hosts connected to the PE that originated the route.

   In the scenario where the multicast tunnel is a tree, both the
   Inclusive as well as the Aggregate Inclusive variants may be used. In
   the former case, a multicast tree is dedicated to a VNI. Whereas, in
   the latter, a multicast tree is shared among multiple VNIs. For VNI-
   based service, the Aggregate Inclusive mode is accomplished by having
   the NVEs advertise multiple IMET routes with different Route Targets
   (one per VNI) but with the same tunnel identifier encoded in the PMSI
   tunnel attribute. For VNI-aware bundle service, the Aggregate
   Inclusive mode is accomplished by having the NVEs advertise multiple
   IMET routes with different VNI encoded in the Ethernet Tag field, but
   with the same tunnel identifier encoded in the PMSI Tunnel attribute.


10 Data Center Interconnections - DCI

   For DCI, the following two main scenarios are considered when
   connecting data centers running evpn-overlay (as described here) over
   MPLS/IP core network:

   - Scenario 1: DCI using GWs
   - Scenario 2: DCI using ASBRs

   The following two subsections describe the operations for each of
   these scenarios.

10.1 DCI using GWs

   This is the typical scenario for interconnecting data centers over
   WAN. In this scenario, EVPN routes are terminated and processed in
   each GW and MAC/IP routes are always re-advertised from DC to WAN but
   from WAN to DC, they are not re-advertised if unknown MAC address
   (and default IP address) are utilized in NVEs. In this scenario, each
   GW maintains a MAC-VRF (and/or IP-VRF) for each EVI. The main
   advantage of this approach is that NVEs do not need to maintain MAC
   and IP addresses from any remote data centers when default IP route
   and unknown MAC routes are used - i.e., they only need to maintain
   routes that are local to their own DC. When default IP route and
   unknown MAC route are used, any unknown IP and MAC packets from NVEs
   are forwarded to the GWs where all the VPN MAC and IP routes are
   maintained. This approach reduces the size of MAC-VRF and IP-VRF
   significantly at NVEs. Furthermore, it results in a faster
   convergence time upon a link or NVE failure in a multi-homed network
   or device redundancy scenario, because the failure related BGP routes



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   (such as mass withdraw message) do not need to get propagated all the
   way to the remote NVEs in the remote DCs. This approach is described
   in details in section 3.4 of [DCI-EVPN-OVERLAY].

10.2 DCI using ASBRs

   This approach can be considered as the opposite of the first approach
   and it favors simplification at DCI devices over NVEs such that
   larger MAC-VRF (and IP-VRF) tables are need to be maintained on NVEs;
   whereas, DCI devices don't need to maintain any MAC (and IP)
   forwarding tables. Furthermore, DCI devices do not need to terminate
   and processed routes related to multi-homing but rather to relay
   these messages for the establishment of an end-to-end LSP path. In
   other words, DCI devices in this approach operate similar to ASBRs
   for inter-AS options B. This requires locally assigned VNIs to be
   used just like downstream assigned MPLS VPN label where for all
   practical purposes the VNIs function like 24-bit VPN labels. This
   approach is equally applicable to data centers (or Carrier Ethernet
   networks) with MPLS encapsulation.

   In inter-AS option B, when ASBR receives an EVPN route from its DC
   over iBGP and re-advertises it to other ASBRs, it re-advertises the
   EVPN route by re-writing the BGP next-hops to itself, thus losing the
   identity of the PE that originated the advertisement. This re-write
   of BGP next-hop impacts the EVPN Mass Withdraw route (Ethernet A-D
   per ES) and its procedure adversely. However, it does not impact EVPN
   Aliasing mechanism/procedure because when the Aliasing routes (Ether
   A-D per EVI) are advertised, the receiving PE first resolves a MAC
   address for a given EVI into its corresponding <ES,EVI> and
   subsequently, it resolves the <ES,EVI> into multiple paths (and their
   associated next hops) via which the <ES,EVI> is reachable. Since
   Aliasing and MAC routes are both advertised per EVI basis and they
   use the same RD and RT (per EVI), the receiving PE can associate them
   together on a per BGP path basis (e.g., per originating PE) and thus
   perform recursive route resolution - e.g., a MAC is reachable via an
   <ES,EVI> which in turn, is reachable via a set of BGP paths, thus the
   MAC is reachable via the set of BGP paths. Since on a per EVI basis,
   the association of MAC routes and the corresponding Aliasing route is
   fixed and determined by the same RD and RT, there is no ambiguity
   when the BGP next hop for these routes is re-written as these routes
   pass through ASBRs - i.e., the receiving PE may receive multiple
   Aliasing routes for the same EVI from a single next hop (a single
   ASBR), and it can still create multiple paths toward that <ES, EVI>.

   However, when the BGP next hop address corresponding to the
   originating PE is re-written, the association between the Mass
   Withdraw route (Ether A-D per ES) and its corresponding MAC routes
   cannot be made based on their RDs and RTs because the RD for Mass



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   Withdraw route is different than the one for the MAC routes.
   Therefore, the functionality needed at the ASBRs and the receiving
   PEs depends on whether the Mass Withdraw route is originated and
   whether there is a need to handle route resolution ambiguity for this
   route. The following two subsections describe the functionality
   needed by the ASBRs and the receiving PEs depending on whether the
   NVEs reside in a Hypervisors or in TORs.

10.2.1 ASBR Functionality with NVEs in Hypervisors

   When NVEs reside in hypervisors as described in section 7.1, there is
   no multi-homing and thus there is no need for the originating NVE to
   send Ethernet A-D per ES or Ethernet A-D per EVI routes. However, as
   noted in section 7, in order to enable a single-homing ingress NVE to
   take advantage of fast convergence, aliasing, and backup-path when
   interacting with multi-homing egress NVEs attached to a given
   Ethernet segment, the single-homing NVE SHOULD be able to receive and
   process Ethernet AD per ES and Ethernet AD per EVI routes. The
   handling of these routes are described in the next section.

10.2.2 ASBR Functionality with NVEs in TORs

   When NVEs reside in TORs and operate in multi-homing redundancy mode,
   then as described in section 8, there is a need for the originating
   NVE to send Ethernet A-D per ES route(s) (used for mass withdraw) and
   Ethernet A-D per EVI routes (used for aliasing). As described above,
   the re-write of BGP next-hop by ASBRs creates ambiguities when
   Ethernet A-D per ES routes are received by the remote NVE in a
   different ASBR because the receiving NVE cannot associated that route
   with the MAC/IP routes of that Ethernet Segment advertised by the
   same originating NVE. This ambiguity inhibits the function of mass-
   withdraw per ES by the receiving NVE in a different AS.

   As an example consider a scenario where CE is multi-homed to PE1 and
   PE2 where these PEs are connected via ASBR1 and then ASBR2 to the
   remote PE3. Furthermore, consider that PE1 receives M1 from CE1 but
   not PE2. Therefore, PE1 advertises Eth A-D per ES1, Eth A-D per EVI1,
   and M1; whereas, PE2 only advertises Eth A-D per ES1 and Eth A-D per
   EVI1. ASBR1 receives all these five advertisements and passes them to
   ASBR2 (with itself as the BGP next hop). ASBR2, in turn, passes them
   to the remote PE3 with itself as the BGP next hop. PE3 receives these
   five routes where all of them have the same BGP next-hop (i.e.,
   ASBR2). Furthermore, the two Ether A-D per ES routes received by PE3
   have the same info - i.e., same ESI and the same BGP next hop.
   Although both of these routes are maintained by the BGP process in
   PE3 (because they have different RDs and thus treated as different
   BGP routes), information from only one of them is used in the L2
   routing table (L2 RIB).



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                      PE1
                     /   \
                    CE     ASBR1---ASBR2---PE3
                     \   /
                      PE2

                      Figure 1: Inter-AS Option B


   Now, when the AC between the PE2 and the CE fails and PE2 sends NLRI
   withdrawal for Ether A-D per ES route and this withdrawal gets
   propagated and received by the PE3, the BGP process in PE3 removes
   the corresponding BGP route; however, it doesn't remove the
   associated info (namely ESI and BGP next hop) from the L2 routing
   table (L2 RIB) because it still has the other Ether A-D per ES route
   (originated from PE1) with the same info. That is why the mass-
   withdraw mechanism does not work when doing DCI with inter-AS option
   B. However, as described previoulsy, the aliasing function works and
   so does "mass-withdraw per EVI" (which is associated with withdrawing
   the EVPN route associated with Aliasing - i.e., Ether A-D per EVI
   route).

   In the above example, the PE3 receives two Aliasing routes with the
   same BGP next hop (ASBR2) but different RDs. One of the Alias route
   has the same RD as the advertised MAC route (M1). PE3 follows the
   route resolution procedure specified in [RFC7432] upon receiving the
   two Aliasing route - ie, it resolves M1 to <ES, EVI1> and
   subsequently it resolves <ES,EVI1> to a BGP path list with two paths
   along with the corresponding VNIs/MPLS labels (one associated with
   PE1 and the other associated with PE2). It should be noted that even
   though both paths are advertised by the same BGP next hop (ASRB2),
   the receiving PE3 can handle them properly. Therefore, M1 is
   reachable via two paths. This creates two end-to-end LSPs, from PE3
   to PE1 and from PE3 to PE2, for M1 such that when PE3 wants to
   forward traffic destined to M1, it can load balanced between the two
   LSPs. Although route resolution for Aliasing routes with the same BGP
   next hop is not explicitly mentioned in [RFC7432], this is the
   expected operation and thus it is elaborated here.

   When the AC between the PE2 and the CE fails and PE2 sends NLRI
   withdrawal for Ether A-D per EVI routes and these withdrawals get
   propagated and received by the PE3, the PE3 removes the Aliasing
   route and updates the path list - ie, it removes the path
   corresponding to the PE2. Therefore, all the corresponding MAC routes
   for that <ES,EVI> that point to that path list will now have the
   updated path list with a single path associated with PE1. This action
   can be considered as the mass-withdraw at the per-EVI level. The
   mass-withdraw at per-EVI level has longer convergence time than the



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   mass-withdraw at per-ES level; however, it is much faster than the
   convergence time when the withdraw is done on a per-MAC basis.

   If a PE becomes detached from a given ES, then in addition to
   withdrawing its previously advertised Ethernet AD Per ES routes, it
   MUST also withdraw its previously advertised Ethernet AD Per EVI
   routes for that ES.  For a remote PE that is separated from the
   withdrawing PE by one or more EVPN inter-AS option B ASBRs, the
   withdrawal of the Ethernet AD Per ES routes is not actionable.
   However, a remote PE is able to correlate a previously advertised
   Ethernet AD Per EVI route with any MAC/IP Advertisement routes also
   advertised by the withdrawing PE for that <ES, EVI, BD>.  Hence, when
   it receives the withdrawal of an Ethernet AD Per EVI route, it SHOULD
   remove the withdrawing PE as a next-hop for all MAC addresses
   associated with that <ES, EVI, BD>.

   In the previous example, when the AC between PE2 and the CE fails,
   PE2 will withdraw its Ethernet AD Per ES and Per EVI routes.  When
   PE3  receives the withdrawal of an Ethernet AD Per EVI route, it
   removes PE2 as a valid next-hop for all MAC addresses associated with
   the  corresponding <ES, EVI, BD>.  Therefore, all the MAC next-hops
   for that <ES,EVI, BD> will now have a single next-hop, viz the LSP to
   PE1.

   In summary, it can be seen that aliasing (and backup path)
   functionality should work as is for inter-AS option B without
   requiring any addition functionality in ASBRs or PEs. However, the
   mass-withdraw functionality falls back from per-ES mode to per-EVI
   mode for inter-AS option B - i.e., PEs receiving mass-withdraw route
   from the same AS take action on Ether A-D per ES route; whereas, PEs
   receiving mass-withdraw route from different AS take action on Ether
   A-D per EVI route.



11  Acknowledgement

   The authors would like to thank Aldrin Isaac, David Smith, John
   Mullooly, Thomas Nadeau for their valuable comments and feedback. The
   authors would also like to thank Jakob Heitz for his contribution on
   section 10.2.

12  Security Considerations

   This document uses IP-based tunnel technologies to support data
   plane transport.  Consequently, the security considerations of those
   tunnel technologies apply.  This document defines support for VXLAN
   and NVGRE encapsulations. The security considerations from those



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   documents as well as [RFC4301] apply to the data plane aspects of
   this document.

   As with [RFC5512], any modification of the information that is used
   to form encapsulation headers, to choose a tunnel type, or to choose
   a particular tunnel for a particular payload type may lead to user
   data packets getting misrouted, misdelivered, and/or dropped.

   More broadly, the security considerations for the transport of IP
   reachability information using BGP are discussed in [RFC4271] and
   [RFC4272], and are equally applicable for the extensions described
   in this document.

   If the integrity of the BGP session is not itself protected, then an
   imposter could mount a denial-of-service attack by establishing
   numerous BGP sessions and forcing an IPsec SA to be created for each
   one.  However, as such an imposter could wreak havoc on the entire
   routing system, this particular sort of attack is probably not of
   any special importance.

   It should be noted that a BGP session may itself be transported over
   an IPsec tunnel.  Such IPsec tunnels can provide additional security
   to a BGP session.  The management of such IPsec tunnels is outside
   the scope of this document.

13  IANA Considerations

   IANA has allocated the following BGP Tunnel Encapsulation Attribute
   Tunnel Types:

   8    VXLAN Encapsulation
   9    NVGRE Encapsulation
   10   MPLS Encapsulation
   11   MPLS in GRE Encapsulation
   12   VXLAN GPE Encapsulation

14  References

14.1  Normative References

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


   [RFC4271]  Y. Rekhter, Ed., T. Li, Ed., S. Hares, Ed., "A Border
              Gateway Protocol 4 (BGP-4)", January 2006.

   [RFC4272]  S. Murphy, "BGP Security Vulnerabilities Analysis.",



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              January 2006.

   [RFC4301]   S. Kent, K. Seo., "Security Architecture for the
              Internet Protocol.", December 2005.

   [RFC5512]  Mohapatra, P. and E. Rosen, "The BGP Encapsulation
              Subsequent Address Family Identifier (SAFI) and the BGP
              Tunnel Encapsulation Attribute", RFC 5512, April 2009.

   [RFC7432] Sajassi et al., "BGP MPLS Based Ethernet VPN",  RFC 7432,
              February 2014


14.2  Informative References

   [RFC7209] Sajassi et al., "Requirements for Ethernet VPN (EVPN)", RFC
   7209, May 2014

   [RFC7348] Mahalingam, M., et al, "VXLAN: A Framework for Overlaying
   Virtualized Layer 2 Networks over Layer 3 Networks", RFC 7348, August
   2014

   [NVGRE]   Garg, P., et al., "NVGRE: Network Virtualization using
   Generic Routing Encapsulation", draft-sridharan-virtualization-nvgre-
   07.txt, November 11, 2014

   [Problem-Statement] Narten et al., "Problem Statement: Overlays for
   Network Virtualization", draft-ietf-nvo3-overlay-problem-statement-
   01, September 2012.

   [NOV3-FRWK] Lasserre et al., "Framework for DC Network
   Virtualization", draft-ietf-nvo3-framework-01.txt, work in progress,
   October 2012.

   [DCI-EVPN-OVERLAY] Rabadan et al., "Interconnect Solution for EVPN
   Overlay networks", draft-ietf-bess-dci-evpn-overlay-02, work in
   progress, February 29, 2016.

   [TUNNEL-ENCAP] Rosen et al., "The BGP Tunnel Encapsulation
   Attribute", draft-ietf-idr-tunnel-encaps-02, work in progress, May
   31, 2016.

   [VXLAN-GPE] Maino et al., "Generic Protocol Extension for VXLAN",
   draft-ietf-nvo3-vxlan-gpe-03, work in progress October 25, 2016.

Contributors

   S. Salam K. Patel D. Rao S. Thoria D. Cai Cisco



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   Y. Rekhter A. Issac Wen Lin Nischal Sheth Juniper

   L. Yong Huawei


Authors' Addresses


   Ali Sajassi
   Cisco
   Email: sajassi@cisco.com


   John Drake
   Juniper Networks
   Email: jdrake@juniper.net


   Nabil Bitar
   Nokia
   Email : nabil.bitar@nokia.com


   R. Shekhar
   Juniper
   Email: rshekhar@juniper.net


   James Uttaro
   AT&T
   Email: uttaro@att.com


   Wim Henderickx
   Alcatel-Lucent
   e-mail: wim.henderickx@nokia.com















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