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Versions: 01 02 03 draft-ietf-bess-evpn-overlay

L2VPN Workgroup                                      A. Sajassi (Editor)
INTERNET-DRAFT                                                     Cisco
Intended Status: Standards Track
                                                       J. Drake (Editor)
Y. Rekhter                                                       Juniper
R. Shekhar
B. Schliesser                                                Nabil Bitar
Juniper                                                          Verizon

S. Salam                                                    Aldrin Isaac
K. Patel                                                       Bloomberg
D. Rao
S. Thoria                                                   James Uttaro
Cisco                                                               AT&T

L. Yong                                                    W. Henderickx
Huawei                                                    Alcatel-Lucent

D. Cai
S. Sinha
Cisco

Expires: December 18, 2014                                 June 18, 2014


         A Network Virtualization Overlay Solution using EVPN
                    draft-sd-l2vpn-evpn-overlay-03


Abstract

   This document describes how EVPN can be used as an 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: MPLS over GRE, VXLAN,
   and NVGRE.

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



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   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


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 . . . . . . . . . . . . . . . . .  5
   3  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4 EVPN Features  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   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 . . . . . .  9
       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  . . . . . . . . . . . . . . . . . . . . . . . 12
   6  EVPN with Multiple Data Plane Encapsulations  . . . . . . . . . 13
   7  NVE Residing in Hypervisor  . . . . . . . . . . . . . . . . . . 13
     7.1 Impact on EVPN BGP Routes & Attributes for VXLAN/NVGRE
         Encapsulation  . . . . . . . . . . . . . . . . . . . . . . . 14
     7.2 Impact on EVPN Procedures for VXLAN/NVGRE Encapsulation  . . 14



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   8  NVE Residing in ToR Switch  . . . . . . . . . . . . . . . . . . 15
     8.1  EVPN Multi-Homing Features  . . . . . . . . . . . . . . . . 15
       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 . . . . . . . . . . . 17
     8.3 Impact on EVPN Procedures  . . . . . . . . . . . . . . . . . 17
       8.3.1 Split Horizon  . . . . . . . . . . . . . . . . . . . . . 18
       8.3.2 Aliasing and Backup-Path . . . . . . . . . . . . . . . . 19
   9 Support for Multicast  . . . . . . . . . . . . . . . . . . . . . 19
   10 Inter-AS  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   11  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 21
   12  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   13  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   14  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     14.1  Normative References . . . . . . . . . . . . . . . . . . . 22
     14.2  Informative References . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23































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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). 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 ToR device

   Note that the use of EVPN as an NVO solution does not necessarily
   mandate that the BGP control-plane be running on the NVE. For such
   scenarios, it is still possible to leverage the EVPN solution by
   using XMPP, or alternative mechanisms, to extend the control-plane to
   the NVE as discussed in [L3VPN-ENDSYSTEMS].

   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,



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   and any impact that the encapsulation has on those features.


2  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [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 across the PEs participating in that
   EVPN

   MAC-VRF:  A Virtual Routing and Forwarding table for MAC addresses on
   a PE for an EVI

   Ethernet Segment Identifier (ESI):  If a CE is multi-homed to two or
   more PEs, the set of Ethernet links that attaches the CE to the PEs
   is an 'Ethernet segment'.   Ethernet segments MUST have a unique non-
   zero identifier, the '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. Ethernet tag(s) are assigned to the broadcast
   domains of a given EVPN instance by the provider of that EVPN, and
   each PE in that EVPN instance performs a mapping between broadcast
   domain identifier(s) understood by each of its attached CEs and the
   corresponding Ethernet tag.

   Single-Active Multihoming: When a device or a network is multihomed
   to a group of two or more PEs and when only a single PE in such a
   redundancy group can forward traffic to/from the multihomed device or
   network for a given VLAN, such multihoming is referred to as "Single-
   Active"

   All-Active Multihoming: When a device is multihomed to a group of two



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   or more PEs and when all PEs in such redundancy group can forward
   traffic to/from the multihomed device or network for a given VLAN,
   such multihoming is referred to as "All-Active".

4 EVPN Features

   EVPN was originally designed to support the requirements detailed in
   [EVPN-REQ] 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 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.  This set of links
   is termed an Ethernet Segment (ES).

   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 domain
   (for upto 4094 such bridge domains). All these bridge domains can
   also be mapped onto a single EVI (in case of VLAN-aware bundle



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   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 &
   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.

   11) 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 NVEs, VXLAN Tunnel End
   Point (VTEPs) in VXLAN and Network Virtualization Endpoint (NVEs) in
   NVGRE. Both of these technologies include the identifier of the
   specific NVO instance, Virtual Network Identifier (VNI) in VXLAN and
   Virtual Subnet Identifier (VSID), NVGRE, in each packet.

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

   [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 [VXLAN]. In this scenario, the VTEP does not include an
   inner VLAN tag on frame encapsulation, and discards decapsulated
   frames with an inner VLAN tag. This mode of operation in [VXLAN] maps
   to VLAN Based Service in [EVPN], 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 either map to VLAN Based Service or VLAN Bundle
   Service in [EVPN] because inner VLAN tag is not used for lookup by
   the disposition PE when performing VXLAN decapsulation as described
   in section 6 of [VXLAN].



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

   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. 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 VNI or VSID are defined as 24-bit globally unique values,
   there are scenarios in which it is desirable to use a locally
   significant value for VNI or VSID, 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 or VSIDs 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 or VSID
   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 / VSIDs 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



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5.1.1.2 Data Center Interconnect without Gateway

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


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

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

           Figure 2: Data Center Interconnect without Gateway


5.1.2 Virtual Identifiers to EVI Mapping

   When the EVPN control plane is used in conjunction with VXLAN or
   NVGRE, 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 or VSID is
   mapped to a unique EVI. As such, a BGP RD and RT is needed per VNI /
   VSID on every VTEP. 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 VTEPs that are
   interested in a given VNI or VSID. The disadvantage of this model may
   be the provisioning overhead if RD and RT are not derived
   automatically from VNI or VSID.

   In this option, the MAC-VRF table is identified by the RT in the
   control plane and by the VNI or VSID for the data-plane. In this



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   option, the specific the MAC-VRF table corresponds to only a single
   bridge domain (e.g., a single subnet).

   2. Option 2: Multiple Subnets per EVI

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

   In this option the MAC-VRF table is identified by the RT in the
   control plane and a specific bridge domain for that MAC-VRF is
   identified by the <RT, Ethernet Tag ID> in the control plane. In this
   option, the VNI/VSID in the data-plane is sufficient to identify a
   specific bridge domain - e.g., no need to do a lookup based on
   VNI/VSID field and Ethernet Tag ID fields to identify a bridge
   domain.



5.1.2.1 Auto Derivation of RT

   When the option of a single VNI or VSID 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 derived easily as
   described in [EVPN] 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 (or VSIDs), 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 and VSIDs are administered independently which
   means VNI and VSID 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 3  0
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++
    |              AS #             |A| TYPE| D-ID  | Service Instance ID|



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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++



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

   - Three least significant bytes of the local admin field of the RT is
   set to the VNI or 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 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 or VSID by the egress VTEP or NVE, respectively, with
   the difference being that an MPLS label has local significance while
   a VNI or VSID typically has global significance.  Accordingly, and
   specifically to support the option of locally assigned VNIs, the MPLS
   label field in the MAC Advertisement, Ethernet AD per EVI, and
   Inclusive Multicast Ethernet Tag routes is used to carry the VNI or
   VSID.  For the balance of this memo, the MPLS label field will be
   referred to as the VNI/VSID field. The VNI/VSID field is used for
   both locally and globally assigned VNIs or VSIDs.

   For the VNI based mode (a single VNI per EVI), the Ethernet Tag field



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   in the MAC Advertisement, Ethernet AD per EVI, and Inclusive
   Multicast route MUST be set to zero just as in the VLAN Based service
   in [EVPN].  For the VNI bundle mode (multiple VNIs per EVI with a
   single bridge domain), the Ethernet Tag field in the MAC
   Advertisement, Ethernet AD per EVI, and Inclusive Multicast Ethernet
   Tag routes MUST be set to zero just as in the VLAN Bundle service in
   [EVPN].

   For the VNI-aware bundle mode (multiple VNIs per EVI each with its
   own bridge domain), the Ethernet Tag field in the MAC Advertisement,
   Ethernet AD per EVI, and Inclusive Multicast route MUST identify a
   bridge domain within an EVI and the set of Ethernet Tags for that EVI
   needs to be configured consistently on all PEs within that EVI.  The
   value advertised in the Ethernet Tag field MAY be a VNI as long as it
   matches the existing semantics of the Ethernet Tag, i.e., it
   identifies a bridge domain within an EVI 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 [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. Four new values will be defined
   to extend the list of encapsulation types defined in [RFC5512]:

        + TBD (IANA assigned) - VXLAN Encapsulation
        + TBD (IANA assigned) - NVGRE Encapsulation
        + TBD (IANA assigned) - MPLS Encapsulation
        + TBD (IANA assigned) - MPLS in GRE Encapsulation

   If the BGP Encapsulation extended community is not present, then the
   default MPLS encapsulation or a statically configured encapsulation
   is assumed.

   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 [EVPN].


5.2 MPLS over GRE

   The EVPN data-plane is modeled as an EVPN MPLS client layer sitting
   over an MPLS PSN tunnel. Some of the EVPN functions (split-horizon,
   aliasing and repair-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



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   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 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.


6  EVPN with Multiple Data Plane Encapsulations

   The use of the BGP Encapsulation extended community allows each PE in
   a given EVI to know each of the encapsulations supported by each of
   the other PEs in that EVI.  I.e., each of the PEs in a given EVI may
   support multiple data plane encapsulations.  An ingress PE can send a
   frame to an egress PE only if the set of encapsulations advertised by
   the egress PE in the subject MAC Advertisement or Per EVI Ethernet AD
   route, forms a non-empty intersection with the set of encapsulations
   supported by the ingress PE, and it is at the discretion of the
   ingress PE 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
   statically configured encapsulation is assumed.)

   If BGP Encapsulation extended community is not present, then the
   default MPLS encapsulation (or statically configured encapsulation)
   is used. However, if this attribute is present, then an ingress PE
   can send a frame to an egress PE only if the set of encapsulations
   advertised by the egress PE in the subject MAC Advertisement or Per
   EVI Ethernet AD route, forms a non-empty intersection with the set of
   encapsulations supported by the ingress PE, and it is at the
   discretion of the ingress PE which encapsulation to choose from this
   intersection.

   An ingress node that uses shared multicast trees for sending
   broadcast or multicast frames MUST 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 PEs in that EVI support at least one common
   encapsulation. If this condition is violated, it could result in
   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



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   When a PE and its CEs are co-located in the same physical device,
   e.g., when the PE resides in a server and the CEs are its VMs, the
   links between them are virtual and they typically share fate;  i.e.,
   the subject CEs 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 need not be "visible" to any other PEs, and thus
   does not require any specific protocol mechanisms.  The most common
   case of this is when the NVE resides in 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

   When the VXLAN VNI or NVGRE VSID is assumed to be a global value, one
   might question the need for the Route Distinguisher (RD) in the EVPN
   routes. In the scenario where all data centers are under a single
   administrative domain, and there is a single global VNI/VSID space,
   the RD MAY be set to zero in the EVPN routes. However, in the
   scenario 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 [EVPN].
   In other words, whenever there is more than one administrative domain
   for global VNI or VSID, then a non-zero RD MUST be used, or whenever
   the VNI or VSID value have local significance, then a non-zero RD
   MUST be used. It is recommend to use a non-zero RD 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 the set of eight :

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

   However, as noted in section 8.6 of [EVPN] in order to enable a
   single-homed ingress PE to take advantage of fast convergence,
   aliasing, and backup-path when interacting with multi-homed egress
   PEs attached to a given Ethernet segment, a single-homed ingress PE
   SHOULD be able to receive and process Ethernet AD per ES and Ethernet
   AD per EVI routes."


7.2 Impact on EVPN Procedures for VXLAN/NVGRE Encapsulation



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   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 [EVPN].

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

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

   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 [EVPN].

   However, as noted in section 8.6 of [EVPN] in order to enable a
   single-homed ingress PE to take advantage of fast convergence,
   aliasing, and back-up path when interacting with multi-homed egress
   PEs attached to a given Ethernet segment, a single-homed ingress PE
   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 [EVPN].

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 in the hypervisor, as discussed in
   Section 5, as far as the required EVPN functionality.

   [EVPN] 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
   encapsulation (such as VXLAN or NVGRE) and what modifications are
   required.

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 [EVPN].



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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 adjacencies to point to the backup NVE(s).

8.1.3 Split-Horizon

   If a CE that is multi-homed to two or more NVEs on an Ethernet
   segment ES1 operating in all-active redundancy mode sends a
   multicast, broadcast or unknown unicast packet to a one of these
   NVEs, then that NVE will forward that packet to all of the other PEs
   in that EVI including the other NVEs attached to ES1 and those NVEs
   MUST drop the packet and not forward back to the originating CE.
   This is termed '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
   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.



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   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 CE 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.


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 [EVPN] are used. As discussed
   in Section 3.1.3, the VSID or VNI is carried in the VNI/VSID field in
   the MAC Advertisement, Ethernet AD per EVI, and Inclusive Multicast
   Ethernet Tag routes.



8.3 Impact on EVPN Procedures

   Two cases need to be examined here, depending on whether the NVEs are



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   operating in Active/Standby or in All-Active redundancy.

   First, let's consider the case of Active/Standby redundancy, 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 or VSID. In
   this case, the Split-Horizon and Aliasing functions are not required
   but other functions such as multi-homed Ethernet segment auto-
   discovery, fast convergence and mass withdraw, repair path, and DF
   election are required. In this case, the impact of the use of the
   VXLAN/NVGRE encapsulation on the EVPN procedures is when the Backup-
   Path function is supported, as discussed next:

   In EVPN, the NVEs connected to a multi-homed site using
   Active/Standby redundancy optionally advertise a VPN label, in the
   Ethernet A-D Route per EVI, used to send traffic to the backup NVE in
   the case where the primary NVE fails. In the case where VXLAN or
   NVGRE encapsulation is used, some alternative means that does not
   rely on MPLS labels is required to support Backup-Path. This is
   discussed in Section 4.3.2 below. If the Backup-Path function is not
   used, then the VXLAN/NVGRE encapsulation would have no impact on the
   EVPN procedures.

   Second, let's consider the case of All-Active redundancy. In this
   case, out of the EVPN multi-homing features listed in section 4.1,
   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
   active/active multi-homing where an ingress ToR switch adds a label
   corresponding to the site of origin (aka ESI MPLS Label) when
   encapsulating the packet. The egress ToR switch checks the ESI MPLS
   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 MPLS
   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



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   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 [EVPN] 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,
   two different Ethernet AD routes are used for this purpose. The one
   used for Aliasing has a VPN scope and carries a VPN label but the one
   used for Backup-Path has Ethernet segment scope and doesn't carry any
   VPN specific info (e.g., Ethernet Tag and MPLS label are set to
   zero). The same two routes are used when VXLAN or NVGRE encapsulation
   is used with the difference that when Ethernet AD route is used for
   Aliasing with VPN scope, the Ethernet Tag field is set to VNI or VSID
   to indicate VPN scope (and MPLS field may be set to a VPN label if
   needed).

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 VXLAN
   VNI or NVGRE VSID. The Ethernet Tag field of this route is used to
   encode the VNI for VLXAN or VSID for NVGRE. 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 3.1.1. The following
   tunnel types as defined in [RFC6514] can be used in the PMSI tunnel
   attribute for VXLAN/NVGRE:

         + 3 - PIM-SSM Tree



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

   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 CEs 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 or VSID.
   Whereas, in the latter, a multicast tree is shared among multiple
   VNIs or VSIDs. This is done by having the NVEs advertise multiple
   Inclusive Multicast routes with different VNI or VSID encoded in the
   Ethernet Tag field, but with the same tunnel identifier encoded in
   the PMSI Tunnel attribute.


10 Inter-AS

   For inter-AS operation, two scenarios must be considered:

   - Scenario 1: The tunnel endpoint IP addresses are public
   - Scenario 2: The tunnel endpoint IP addresses are private

   In the first scenario, inter-AS operation is straight-forward and
   follows existing BGP inter-AS procedures. However, in the first
   scenario where the tunnel endpoint IP addresses are public, there may
   be security concern regarding the distribution of these addresses
   among different ASes. This security concern is one of the main
   reasons for having the so called inter-AS "option-B" in MPLS VPN
   solutions such as EVPN.

   The second scenario is more challenging, because the absence of the
   MPLS client layer from the VXLAN encapsulation creates a situation
   where the ASBR has no fully qualified indication within the tunnel
   header as to where the tunnel endpoint resides. To elaborate on this,
   recall that with MPLS, the client layer labels (i.e. the VPN labels)
   are downstream assigned. As such, this label implicitly has a
   connotation of the tunnel endpoint, and it is sufficient for the ASBR
   to look up the client layer label in order to identify the label
   translation required as well as the tunnel endpoint to which a given
   packet is being destined. With the VXLAN encapsulation, the VNI is
   globally assigned and hence is shared among all endpoints. The
   destination IP address is the only field which identifies the tunnel
   endpoint in the tunnel header, and this address is privately managed
   by every data center network. Since the tunnel address is allocated



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   out of a private address pool, then we either need to do a lookup
   based on VTEP IP address in context of a VRF (e.g., use IP-VPN) or
   terminate the VXLAN tunnel and do a lookup based on the tenant's MAC
   address to identify the egress tunnel on the ASBR. This effectively
   mandates that the ASBR to either run another overlay solution such as
   IP-VPN over MPLS/IP core network or to be aware of the MAC addresses
   of all VMs in its local AS, at the very least.

   If VNIs/VSIDs have local significance, then the inter-AS operation
   can be simplified to that of MPLS and thus MPLS inter-AS option B and
   C can be leveraged in here.  That's why the use of local significance
   VNIs/VSIDs (e.g., MPLS labels) are recommended for inter-AS operation
   of DC networks without gateways.


11  Acknowledgement

   The authors would like to thank David Smith, John Mullooly, Thomas
   Nadeau for their valuable comments and feedback.

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]. The security considerations from those 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



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   the scope of this document.

13  IANA Considerations


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.",
              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.

14.2  Informative References

   [EVPN-REQ] Sajassi et al., "Requirements for Ethernet VPN (EVPN)",
   draft-ietf-l2vpn-evpn-req-01.txt, work in progress, October 21, 2012.

   [NVGRE]   Sridhavan, M., et al., "NVGRE: Network Virtualization using
   Generic Routing Encapsulation", draft-sridharan-virtualization-nvgre-
   01.txt, July 8, 2012.

   [VXLAN] Dutt, D., et al, "VXLAN: A Framework for Overlaying
   Virtualized Layer 2 Networks over Layer 3 Networks", draft-
   mahalingam-dutt-dcops-vxlan-02.txt,  August 22, 2012.

   [EVPN] Sajassi et al., "BGP MPLS Based Ethernet VPN", draft-ietf-
   l2vpn-evpn-02.txt, work in progress, February, 2012.

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

   [L3VPN-ENDSYSTEMS] Marques et al., "BGP-signaled End-system IP/VPNs",
   draft-ietf-l3vpn-end-system, work in progress, October 2012.



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

Authors' Addresses


   Ali Sajassi
   Cisco
   Email: sajassi@cisco.com


   John Drake
   Juniper Networks
   Email: jdrake@juniper.net


   Nabil Bitar
   Verizon Communications
   Email : nabil.n.bitar@verizon.com


   Aldrin Isaac
   Bloomberg
   Email: aisaac71@bloomberg.net


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


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


   Ravi Shekhar
   Juniper Networks
   Email: rshekhar@juniper.net


   Samer Salam
   Cisco
   Email: ssalam@cisco.com


   Keyur Patel



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   Cisco
   Email: Keyupate@cisco.com


   Dhananjaya Rao
   Cisco
   Email: dhrao@cisco.com


   Samir Thoria
   Cisco
   Email: sthoria@cisco.com







































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