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SAVI in an EVPN network
draft-levyabegnoli-bess-evpn-savi-00

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Authors Eric Levy-Abegnoli , Pascal Thubert , Ratko Kovacina
Last updated 2023-07-10
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draft-levyabegnoli-bess-evpn-savi-00
BESS                                                    E. Levy-Abegnoli
Internet-Draft                                                P. Thubert
Intended status: Informational                               R. Kovacina
Expires: 11 January 2024                                   Cisco Systems
                                                            10 July 2023

                        SAVI in an EVPN network
                  draft-levyabegnoli-bess-evpn-savi-00

Abstract

   Source Address Validation procedures have been specified in the SAVI
   Working Group and provide a set of mechanisms and state machines to
   verify Source Address ownership.  The main mechanisms are described
   in [RFC6620] and [RFC7513].

   [RFC7432] and furthermore [RFC9161] specify how an EVPN network could
   learn and distribute IP addresses.  [RFC9161] describes a mechanism
   by which the PE can proxy some ND messages based on this information.

   In this document, we review how these two sets of specifications and
   underlying mechanisms can interact to provide Source Address
   Validation in an EVPN network.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 January 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Deployment models . . . . . . . . . . . . . . . . . . . . . .   3
   4.  DHCP assigned addresses . . . . . . . . . . . . . . . . . . .   5
     4.1.  SAVI-DHCP background  . . . . . . . . . . . . . . . . . .   5
     4.2.  Interactions between RFC7513 and EVPN . . . . . . . . . .   6
   5.  SLAAC and other IPv6 addresses  . . . . . . . . . . . . . . .   6
     5.1.  SAVI-FCFS background  . . . . . . . . . . . . . . . . . .   6
     5.2.  Interactions between SAVI-FCFS and EVPN . . . . . . . . .   6
   6.  Enhanced SAVI/EVPN interactions . . . . . . . . . . . . . . .   8
     6.1.  Address not found . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Address found in BGP/EVPN table, remote not active  . . .   9
     6.3.  Address found in BGP/EVPN table, remote active  . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   12. Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   [RFC6620] describes a mechanism that provides Source Address
   Validation Improvements (SAVI) for IPv6 networks based the First Come
   First Serve (FCFS) principle, applicable to any type of IPv6
   addresses, including those assigned through IPv6 [RFC4291][RFC8200]
   Neighbor Discovery (ND) [RFC4861] Stateless Address Autoconfiguration
   (SLAAC) [RFC4862].  According to that specification, an IPv6 entry
   freshly snooped on a SAVI device needs to reach the “VALID“ state
   before traffic sourced with it can be forwarded.

   Another SAVI specification, [RFC7513], describes a similar mechanism
   for addresses assigned by DHCPv6/DHCPv4 server.  Again, traffic
   sourced which such addresses can only be forwarded when the address
   state is “BOUND”.

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   Section 10 of "BGP MPLS-Based Ethernet VPN" [RFC7432](EVPN) indicates
   that a Provider Edge (PE) router may learn IP addresses and advertise
   them to other PEs.  EVPN allows PEs to execute a proxy ARP/ND
   function that is further detailed in "Operational Aspects of Proxy
   ARP/ND in Ethernet Virtual Private Network" [RFC9161].  According to
   section 3.2 of [RFC9161], IPv6 addresses should be learnt by snooping
   Neighbor Advertisements (NA), then advertised in an EVPN MAC/IP
   Advertisement route, and finally used on remote PEs to perform said
   proxy ARP/ND function.

   Assuming one would want to perform Source Address Validation in an
   EVPN network, two models can be deployed:

   1.  the SAVI function runs on SAVI switches external to PEs

   2.  the SAVI function is co-located with the PEs

   These two models are reviewed in Section 3.  Corresponding
   interactions between SAVI and EVPN are reviewed as well in this
   document.

2.  Acronyms

   This document uses the following abbreviations:

   CE:  Customer Edge (router)
   DAD:  Duplicate Address Detection
   NA:  Neighbor Advertisement (message)
   ND:  Neighbor Discovery (protocol)
   NDP:  Neighbor Discovery Protocol
   NS:  Neighbor Solicitation (message)
   PE:  Provider Edge (router)
   VLAN:  Virtual Local Area Network
   VxLAN:  Virtual Extensible LAN
   EVPN:  Ethernet Virtual Private Network
   VTEP:  VXLAN Tunnel EndPoint (node)
   BGP:  Border Gateway Protocol

3.  Deployment models

   In model 1, a SAVI device is inserted between client (host or CE) and
   PE (or VTEP).  This first model is straight forward and does not
   require any additional specification.  It is presented in Figure 1.

   The SAVI devices operate as a verification perimeter between
   untrusted clients (CEs or Hosts) and PEs (or VTEPs).  As specified in
   [RFC6620] and [RFC7513], only validated addresses can inject traffic
   over the trusted perimeter.  The mechanisms to validate addresses are

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   specified in these two RFCs.  Note that IPv6 DAD NS (IP source is set
   to the unspecified address, see section section-2.5.2 of [RFC4291] ),
   used by SAVI for validation, can still be forwarded by EVPN but are
   not used by EVPN proxy for learning.  Therefore, as long as an
   address is not validated by SAVI, it remains unknown by EVPN.

                                protection perimeter
             +- - - - - - - - - - - - - - - - - - - - - - -+
             |                                             |
             |                trusted perimeter            |
             |            +- - - - - - - - - -+            |
             |            |                   |            |
             |            |    L3 NETWORK     |            |
             |            |  VTEP        VTEP |            |
   +-------+ | +------+  ++------+     +------++  +------+ | +-------+
   |       | | | SAVI |  |       |     |       |  | SAVI | | |       |
   |HOST/CE| | |      |  |EVPN/PE|     |EVPN/PE|  |      | | |HOST/CE|
   |       | | |device|  |       |     |       |  |device| | |       |
   +-------+ | +------+  ++------+     +------++  +------+ | +-------+
             |            |                   |            |
             |            |                   |            |
             |            |                   |            |
             |            |                   |            |
             |            +- - - - - - - - - -+            |
             |                                             |
             |                                             |
             +- - - - - - - - - - - - - - - - - - - - - - -+

                        Figure 1: Deployment model 1

   In model 2, the SAVI device and the PE/VTEP are colapsed into the
   same box.  SAVI device is referenced as a "SAVI instance".  This
   model is presented in Figure 2.

   Model 2 is fundamentally the same as the model 1.  SAVI instance acts
   as a Security wall between untrusted clients and the VTEPs, and only
   allows validated sources to inject traffic beyond the VTEP.  The
   injection happens over an API between SAVI instance and EVPN rather
   than over a physical wire.  Furthermore, only validated sources are
   learnt per section 3.2 of [RFC9161].

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                                protection perimeter
             +- - - - - - - -  - - - -  - - - - - -- - -- - +
             |                                              |
             |                trusted perimeter             |
             |            +- - - -- - - - - - -+            |
             |            |                    |            |
             |            |    L3 NETWORK      |            |
             |    VTEP/PE |                    | VTEP/PE    |
   +-------+ | +--------+-+------+      +------+-+--------+ | +-------+
   |       | | |SAVI    |        |      |        | SAVI   | | |       |
   |HOST/CE| | |       API  EVPN |      | EVPN  API       | | |HOST/CE|
   |       | | |instance|        |      |        |instance| | |       |
   +-------+ | +--------+-+------+      +------+-+--------+ | +-------+
             |            |                    |            |
             |            |                    |            |
             |            |                    |            |
             |            |                    |            |
             |            +- - - - ---- - - - -+            |
             |                                              |
             |                                              |
             +- - - - - - - - - - - -   - - - - - - - - - - +
                                protection perimeter

                        Figure 2: Deployment model 2

   In this model however, it is possible and even desirable to leverage
   the knowledge of remote IP entries stored in the VTEP BGP tables to
   make SAVI validation process more efficient.  This is described in
   section Section 6

   The next section review interactions between SAVI and EVPN for
   different type of addresses

4.  DHCP assigned addresses

4.1.  SAVI-DHCP background

   The "SAVI Solution for DHCP" [RFC7513] specification describes a
   mechanism that provides source address validation for IPv6 networks
   where addresses are assigned by DHCPv6 server.  The address
   validation is achieved by snooping DHCPv6 address assignments, which
   is known as DHCP Snooping, or validating discovered address with the
   DHCP server which is described as Data Snooping process.  Both
   processes are described in detail in [RFC7513].

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   During DHCP Snooping process and according to associated state
   machine, an IP entry freshly snooped on a SAVI device progresses from
   NO_BIND to BOUND.

4.2.  Interactions between RFC7513 and EVPN

   The whole DHCP address assignment procedure is performed using IPv6
   Link-Local addresses, which are expected to be VALID prior to the
   beginning of this process.  The DHCP server or relay can then be
   located anywhere in the EVPN network without the DHCP messages being
   blocked by SAVI.  EVPN itself does not learn from DHCP, so until the
   address is assigned, DAD completed and the host have sent an NA
   (response to a lookup or unsolicited NA), EVPN is not aware of the
   assigned address.  After the NA is sent, the address can be learnt by
   EVPN, stored locally by the EVPN proxy-ND, and distributed by BGP per
   [RFC9161].

5.  SLAAC and other IPv6 addresses

5.1.  SAVI-FCFS background

   According to [RFC6620], any address, used as the source of a packet
   or showing up as a target of a Neighbor Advertisement (NA) or a
   target of a DAD Neighbor Solicitation (NS), which is not locally
   known and validated, is treated as a TENTATIVE address and a DAD NS
   (Duplicate Address Detection Neighbor Solicitation) message with this
   address in the target field is originated by the SAVI device and
   broadcasted to any validated or trusted port.  A response received
   implies a duplication and/or IP theft and on the contrary, no
   response allow to progress the state of the address from TENTATIVE to
   VALID.

5.2.  Interactions between SAVI-FCFS and EVPN

   Main interactions are listed below:

   *  Sources not validated by SAVI are not learnt nor distributed by
      EVPN: SAVI does not allow NA to be forwarded (model 1) or signaled
      (model 2) to EVPN.

   *  During the validation process for a Source, SAVI (ingress)
      originates a broadcast DAD, with target set to "Source" and
      forwards (model 1) or signals (model 2) it to EVPN, which delivers
      it to remote SAVI devices (over the wire or an API).  Only SAVI
      devices that have a local validated entry or a trusted interface
      forward the DAD to enable the target to defend itself.

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   *  In case a response is sent by one of the DAD recipients, it is a
      broadcast NA (to "all-nodes"), processed by SAVI (egress).  The
      source is expected to be a SAVI validated source, and as such,
      delivered from (egress) SAVI to EVPN for delivery to all VTEPs and
      finally to (ingress) SAVI to complete the validation process of
      the target "Source".

   *  The first NA sent by this Source triggers EVPN proxy learning and
      EVPN distribution to all VTEPs per [RFC9161].

   The flow below shows two sequences, A and B, with A where Host does
   not respond to DAD sent by SAVI and B where host does respond.
   Sequence goes like this for A:

   1.  Data sourced with S is received on the ingress SAVI instance.  An
       entry for S is created in TENTATIVE state.

   2.  A DAD is built with target=S and broadcasted to all SAVI
       instances over the EVPN network.

   3.  Only instances that have the entry "S" in VALID state or trusted
       ports forward the DAD.

   In the absence of a response from HOST, the state of the "S" entry at
   the ingress SAVI progresses to VALID.

   For B, after step 3:

   4.  HOST responds to DAD with an NA.

   Upon receiving the NA, the ingress SAVI instance move the entry to
   NO_BIND and does not allow traffic from this source to be forwarded.

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                                  layer3 network
                                +-----------+
                   VTEP         |           |      VTEP
   +------+      +--------+-----+--+      +-+------+--------+  +-------+
   |      |      |        |        |      |        |        |  |       |
   | HOST |      |  SAVI  |   EVPN |      | EVPN   |  SAVI  |  |HOST   |
   |      |      |        |        |      |        |        |  |       |
   +----+-+      +----+---+---+-+--+      +-+--+---+---+----+  +---+---+
        |             |       | |           |  |       |           |
        |             |       | +-----------+  |       |           |
       (1)data, src=S |       |                |      S in VALID   |
        +------------>|       |                |      state        |
        |             |       |                |       |           |
        |        S: TENTATIVE |                |       |           |
        |             |       |(2)DAD NS, target=S     |           |
        |     +-      +-------+---------------+------->|(3)DAD NS  |
        |     |       |       |               |        +---------->|
        |     |     timeout   |               |        |           |
        |     |       |       |               |        |           |
        | A   |       |       |               |        |           |
        |     |  S: VALID     |               |        |           |
        |     |       |       |               |        |           |
        |     |       |       |               |        |           |
        |     +-      |       |               |        |           |
        |             |       |(2)DAD NS, target=S     |           |
        |     +-      +-------+---------------+------->|(3)DAD NS  |
        |     |       |       |               |        +---------->|
        |     |       |       |(4)NA to all_nodes(4)   |           |
        | B   |       |<------+---------------+--------+-----------+
        |     |       |       |               |        |           |
        |     |  S: NO_BIND   |               |        |           |
        |     +-      |       |               |        |           |
        |             |       |               |        |           |
        |             |       |               |        |           |

                            Figure 3: FCFS-SAVI

6.  Enhanced SAVI/EVPN interactions

   In model 2 where SAVI is co-located with BGP and EVPN proxy table,
   upon starting the validation process for an address, SAVI MAY perform
   a lookup into EVPN/BGP proxy table, where entry can be 1) not found,
   2) local or 3) remote.

   If the entry is local, this is an error as it is expected only VALID
   entries should be learnt by EVPN ND proxy.  The entry in the proxy
   table MUST be deleted.

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   Other cases are described in the next sections

   As described in [RFC6620], when validating a source address that is
   not known in the local SAVI table, a multicast DAD NS message is sent
   to all remote SAVI instances to check for the presence of this
   address on any of these instances.  This process can be optimized by
   leveraging the content of EVPN / BGP tables on VTEPs and "unicast-
   forward" the DAD to the known address owner, if any, more in
   [RFC6085].

   Depending on the presence of the address in the BGP table and if
   present, whether the address is active or not, there are three
   possible scenarios as described below.

   1.  Address not found in BGP/EVPN table.

   2.  Address found in BGP/EVPN table, remote not active.

   3.  Address found in BGP/EVPN table, remote active.

6.1.  Address not found

   When the source address that is being validated by SAVI process is
   not present in the BGP / EVPN table, it means that is was not learned
   on any of the remote VTEPs which indicates that none of the remote
   SAVI devices have the entry for that source address in VALID state.
   Based on the validation process described in [RFC6620], in such
   scenario the multicast NS DAD MAY NOT be forwarded over the EVPN
   network and the SAVI entry MAY move directly to VALID state.

   Note that to prevent race condition where a host is VALID in the
   local SAVI instance but not yet present in the EVPN proxy table, SAVI
   MAY signal the entry to EVPN as soon as it becomes VALID.

6.2.  Address found in BGP/EVPN table, remote not active

   When the source address that is being validated by SAVI process is
   present in the BGP / EVPN table, then the multicast NS DAD sent by
   SAVI can be "unicast-forwarded" per section 3.4.  "Unicast-Forward
   Sub-function" of [RFC9161].

   If the address is no longer active on the remote location, there will
   be no response and the validation process will follow the steps as
   specified in [RFC6620] to move the entry to VALID state.  Once the
   entry is in VALID state, the traffic received from that address will
   be forwarded into EVPN network and the IP address will be learned by
   the EVPN.  This is shown in Figure 4

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   SAVI instance MAY signal the entry to EVPN as soon as it becomes
   VALID.

   This would be an example of host mobility use case.

                        local VTEP                  remote VTEP
            + - - - - -  - -  - - - - - - +  +--- - - - - - - +
   +------+ | +---------+     +---------+ |  | +-----+ +----+ | +------+
   |      | | | local   |     |         | |  | |     | |    | | |      |
   |Client| | | SAVI    |<API>|  EVPN   | |  | |EVPN | |SAVI| | |Client|
   +------+ | +---------+     +---------+ |  | +-----+ +----+ | +------+
      |     + - - | - - - - - - -  |  - - +  +--- - --- - | --+    |
      |  src=IP   |                |             |    IP in VALID  |
      +---------->+                |             |    state        |
      |           |                |             |        |        |
      |           | Lookup EVPN/BGP|             |        |        |
      |           | for IP         |             |        |        |
      |           +--------------->+             |        |        |
      |           | IP FOUND, MAC<>IP            |        |        |
      |           <----------------+             |        |        |
      |           |                |             |        |        |
      |           |        unicast NS DAD to MAC |        |        |
      |           +--------------------------------------> ------->|
      |           |                |             |        |        |
      |           |No response     |             |      No response
      |           |entry moved to VALID          |        |
      |           +                |             |        |
      | NA is allowed to reach EVPN|             |        |
      +--------------------------->+--------------------->
      +                            | IP learned  |   IP in NO_BIND
      |                            | in BGP      |   state
                                   +------------->

                 Figure 4: Remote not responding - Mobility

6.3.  Address found in BGP/EVPN table, remote active

   As in previous section, local SAVI sends the multicast NS DAD to the
   known (by EVPN, as a remote entry) target but this time, the target
   responds with a multicast NA.  The validation process will follow the
   steps as specified in [RFC6620] and move the entry to NO_BIND state.
   Any NA received on this local SAVI instance from the validating port
   will not be signalled to EVPN and will not be learnt by EVPN ND
   proxy.  This is a IP duplication / theft use case and should be
   signalled through logging or by triggering an alarm.

   This is shown in Figure 5

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                       local VTEP                  remote VTEP
            + - - - - -  - -  - - - - - - +  +--- - - - - - - +
   +------+ | +---------+     +---------+ |  | +-----+ +----+ | +------+
   |      | | | local   |     |         | |  | |     | |    | | |      |
   |Client| | | SAVI    |<API>|  EVPN   | |  | |EVPN | |SAVI| | |Client|
   +------+ | +---------+     +---------+ |  | +-----+ +----+ | +------+
      |     + - - | - - - - - - -  |  - - +  +--- - --- - |  -+    |
      |  src=IP   |                |             |    IP in VALID  |
      +---------->+                |             |    state        |
      |           |                |             |        |        |
      |           | Lookup EVPN/BGP|             |        |        |
      |           | for IP         |             |        |        |
      |           +--------------->+             |        |        |
      |           | IP FOUND, MAC<>IP            |        |        |
      |           <----------------+             |        |        |
      |           |                |             |        |        |
      |           |        unicast NS DAD to MAC |        |        |
      |           +--------------------------------------> ------> |
      |           |                |             |        |    NA  |
      |           |                |             | allowed|<-------|
      |           |<--------------------------------------|        |
      |  Entry in NO_BIND state    |             |        |        |
      |           |                |             |        |        |
      | NA, target=IP              |             |        |        |
      |---------->|                |             |        |        |
      |  Traffic from src/target IP|             |        |        |
      |  is DROPPED and not visible|             |        |        |
      |   to EVPN/BGP              |             |        |        |

                     Figure 5: Remote active - IP theft

7.  Security Considerations

8.  IANA Considerations

   This specification does not require IANA action.

9.  Contributors

10.  Acknowledgments

11.  Normative References

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

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   [RFC6620]  Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
              SAVI: First-Come, First-Served Source Address Validation
              Improvement for Locally Assigned IPv6 Addresses",
              RFC 6620, DOI 10.17487/RFC6620, May 2012,
              <https://www.rfc-editor.org/info/rfc6620>.

   [RFC6085]  Gundavelli, S., Townsley, M., Troan, O., and W. Dec,
              "Address Mapping of IPv6 Multicast Packets on Ethernet",
              RFC 6085, DOI 10.17487/RFC6085, January 2011,
              <https://www.rfc-editor.org/info/rfc6085>.

   [RFC7513]  Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
              Validation Improvement (SAVI) Solution for DHCP",
              RFC 7513, DOI 10.17487/RFC7513, May 2015,
              <https://www.rfc-editor.org/info/rfc7513>.

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

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC9161]  Rabadan, J., Ed., Sathappan, S., Nagaraj, K., Hankins, G.,
              and T. King, "Operational Aspects of Proxy ARP/ND in
              Ethernet Virtual Private Networks", RFC 9161,
              DOI 10.17487/RFC9161, January 2022,
              <https://www.rfc-editor.org/info/rfc9161>.

12.  Informative References

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

Authors' Addresses

   Eric Levy-Abegnoli
   Cisco Systems, Inc
   Emerald Square, Batiment C

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Internet-Draft           SAVI in an EVPN network               July 2023

   rue Evariste Galois
   06410 BIOT - Sophia Antipolis
   France
   Phone: +33 497 23 26 20
   Email: elevyabe@cisco.com

   Pascal Thubert
   Cisco Systems, Inc
   Emerald Square, Batiment C
   rue Evariste Galois
   06410 BIOT - Sophia Antipolis
   France
   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

   Ratko Kovacina
   Cisco Systems, Inc
   2000 Innovation Dr
   Kanata, ON K2K 3E8, Canada  ON K2K 3E8
   Canada
   Phone: +1 613 254 4545
   Email: rkovacin@cisco.com

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