Operational Aspects of Proxy-ARP/ND in Ethernet Virtual Private Networks

Document Type Active Internet-Draft (bess WG)
Authors Jorge Rabadan  , Senthil Sathappan  , Kiran Nagaraj  , Greg Hankins  , Thomas King 
Last updated 2021-06-08
Replaces draft-snr-bess-evpn-proxy-arp-nd
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BESS Workgroup                                           J. Rabadan, Ed.
Internet-Draft                                              S. Sathappan
Updates: 7432 (if approved)                                   K. Nagaraj
Intended status: Standards Track                              G. Hankins
Expires: December 10, 2021                                         Nokia
                                                                 T. King
                                                            June 8, 2021

Operational Aspects of Proxy-ARP/ND in Ethernet Virtual Private Networks


   This document describes the Ethernet Virtual Private Networks (EVPN)
   Proxy-ARP/ND function, augmented by the capability of the ARP/ND
   Extended Community.  From that perspective this document updates the
   EVPN specification to provide more comprehensive documentation of the
   operation of the Proxy-ARP/ND function.  The EVPN Proxy-ARP/ND
   function and the ARP/ND Extended Community help operators of Internet
   Exchange Points, Data Centers, and other networks deal with IPv4 and
   IPv6 address resolution issues associated with large Broadcast
   Domains by reducing and even suppressing the flooding produced by
   address resolution in the 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 December 10, 2021.

Copyright Notice

   Copyright (c) 2021 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The Data Center Use-Case  . . . . . . . . . . . . . . . .   5
     2.2.  The Internet Exchange Point Use-Case  . . . . . . . . . .   5
   3.  Solution Description  . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Proxy-ARP/ND Sub-Functions  . . . . . . . . . . . . . . .   8
     3.2.  Learning Sub-Function . . . . . . . . . . . . . . . . . .   9
       3.2.1.  Proxy-ND and the NA Flags . . . . . . . . . . . . . .  11
     3.3.  Reply Sub-Function  . . . . . . . . . . . . . . . . . . .  12
     3.4.  Unicast-forward Sub-Function  . . . . . . . . . . . . . .  13
     3.5.  Maintenance Sub-Function  . . . . . . . . . . . . . . . .  14
     3.6.  Flood (to Remote PEs) Handling  . . . . . . . . . . . . .  15
     3.7.  Duplicate IP Detection  . . . . . . . . . . . . . . . . .  16
   4.  Solution Benefits . . . . . . . . . . . . . . . . . . . . . .  18
   5.  Deployment Scenarios  . . . . . . . . . . . . . . . . . . . .  19
     5.1.  All Dynamic Learning  . . . . . . . . . . . . . . . . . .  19
     5.2.  Dynamic Learning with Proxy-ARP/ND  . . . . . . . . . . .  20
     5.3.  Hybrid Dynamic Learning and Static Provisioning with
           Proxy-ARP/ND  . . . . . . . . . . . . . . . . . . . . . .  20
     5.4.  All Static Provisioning with Proxy-ARP/ND . . . . . . . .  20
     5.5.  Example of Deployment in Internet Exchange Points . . . .  21
     5.6.  Example of Deployment in Data Centers . . . . . . . . . .  22
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  24
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     10.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

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   BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   ARP: Address Resolution Protocol.

   AS-MAC: Anti-spoofing MAC.  It is a especial MAC configured on all
   the PEs attached to the same BD and used for the Duplicate IP
   Detection procedures.

   BD: Broadcast Domain.

   BUM: Broadcast, Unknown unicast and Multicast layer-2 traffic.

   CE: Customer Edge router.

   DAD: Duplicate Address Detection, as per [RFC4861].

   DC: Data Center.

   EVI: EVPN Instance.

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

   GARP: Gratuitous ARP message.

   IP->MAC: an IP address associated to a MAC address.  IP->MAC entries
   are programmed in Proxy-ARP/ND tables and may be of three different
   types: dynamic, static or EVPN-learned.

   IXP: Internet eXchange Point.

   IXP-LAN: the IXP's large Broadcast Domain to where Internet routers
   are connected.

   LAG: Link Aggregation Group.

   MAC or IP DA: MAC or IP Destination Address.

   MAC or IP SA: MAC or IP Source Address.

   ND: Neighbor Discovery Protocol.

   NS: Neighbor Solicitation message.

   NA: Neighbor Advertisement.

   NUD: Neighbor Unreachability Detection, as per [RFC4861].

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   O Flag: Override Flag in NA messages, as per [RFC4861].

   PE: Provider Edge router.

   R Flag: Router Flag in NA messages, as per [RFC4861].

   RT2: EVPN Route type 2 or EVPN MAC/IP Advertisement route, as per

   S Flag: Solicited Flag in NA messages, as per [RFC4861].

   SN-multicast address: Solicited-Node IPv6 multicast address used by
   NS messages.

   TLLA: Target Link Layer Address, as per [RFC4861].

   VPLS: Virtual Private LAN Service.

   This document assumes familiarity with the terminology used in

2.  Introduction

   As specified in [RFC7432] the IP Address field in the Ethernet
   Virtual Private Networks (EVPN) MAC/IP Advertisement route may
   optionally carry one of the IP addresses associated with the MAC
   address.  A PE may learn local IP->MAC pairs and advertise them in
   EVPN MAC/IP Advertisement routes.  Remote PEs importing those routes
   in the same Broadcast Domain (BD) may add those IP->MAC pairs to
   their Proxy-ARP/ND tables and reply to local ARP requests or Neighbor
   Solicitations (or 'unicast-forward' those packets to the owner MAC),
   reducing and even suppressing in some cases the flooding in the EVPN

   EVPN and its associated Proxy-ARP/ND function are extremely useful in
   DCs or Internet Exchange Points (IXPs) with large broadcast domains,
   where the amount of ARP/ND flooded traffic causes issues on connected
   routers and CEs.  [RFC6820] describes the address resolution problems
   in large DC networks.

   This document describes the Proxy-ARP/ND function in [RFC7432]
   networks, augmented by the capability of the ARP/ND Extended
   Community [I-D.ietf-bess-evpn-na-flags].  From that perspective this
   document updates [RFC7432].

   Proxy-ARP/ND may be implemented to help IXPs, DCs and other operators
   deal with the issues derived from address resolution in large
   broadcast domains.

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2.1.  The Data Center Use-Case

   As described in [RFC6820] the IPv4 and IPv6 address resolution can
   create a lot of issues in large DCs.  In particular, the issues
   created by the IPv4 Address Resolution Protocol procedures may be

   On one hand, ARP Requests use broadcast MAC addresses, therefore any
   Tenant System in a large Broadcast Domain will see a large amount of
   ARP traffic, which is not addressed to most of the receivers.

   On the other hand, the flooding issue becomes even worse if some
   Tenant Systems disappear from the broadcast domain, since some
   implementations will persistently retry sending ARP Requests.  As
   [RFC6820] states, there are no clear requirements for retransmitting
   ARP Requests in the absence of replies, hence an implementation may
   choose to keep retrying endlessly even if there are no replies.

   The amount of flooding that address resolution creates can be
   mitigated by the use of EVPN and its Proxy-ARP/ND function.

2.2.  The Internet Exchange Point Use-Case

   The implementation described in this document is especially useful in
   IXP networks.

   A typical IXP provides access to a large layer-2 Broadcast Domain for
   peering purposes (referred to as 'the peering network'), where
   (hundreds of) Internet routers are connected.  We refer to these
   Internet routers as Customer Edge (CE) devices in this section.
   Because of the requirement to connect all routers to a single layer-2
   network the peering networks use IPv4 addresses in length ranges from
   /21 to /24 (and even bigger for IPv6), which can create very large
   broadcast domains.  This peering network is transparent to the CEs,
   and therefore, floods any ARP request or NS messages to all the CEs
   in the network.  Gratuitous ARP and NA messages are flooded to all
   the CEs too.

   In these IXP networks, most of the CEs are typically peering routers
   and roughly all the BUM traffic is originated by the ARP and ND
   address resolution procedures.  This ARP/ND BUM traffic causes
   significant data volumes that reach every single router in the
   peering network.  Since the ARP/ND messages are processed in "slow
   path" software processors and they take high priority in the routers,
   heavy loads of ARP/ND traffic can cause some routers to run out of
   resources.  CEs disappearing from the network may cause address
   resolution explosions that can make a router with limited processing
   power fail to keep BGP sessions running.

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   The issue might be better in IPv6 routers if MLD-snooping was
   enabled, since ND uses SN-multicast address in NS messages; however,
   ARP uses broadcast and has to be processed by all the routers in the
   network.  Some routers may also be configured to broadcast periodic
   GARPs [RFC5227].  The amount of ARP/ND flooded traffic grows
   exponentially with the number of IXP participants, therefore the
   issue can only grow worse as new CEs are added.

   In order to deal with this issue, IXPs have developed certain
   solutions over the past years.  While these solutions may mitigate
   the issues of address resolution in large broadcasts domains, EVPN
   provides new more efficient possibilities to IXPs.  EVPN and its
   Proxy-ARP/ND function may help solve the issue in a distributed and
   scalable way, fully integrated with the PE network.

3.  Solution Description

   Figure 1 illustrates an example EVPN network where the Proxy-ARP/ND
   function is enabled.

   IP1/M1          +----------------------------+   |IP1->M1 EVPN|
    GARP --->Proxy-ARP/ND                       |   |IP2->M2 EVPN|
   +---+      +--------+   RT2(IP1/M1)          |   |IP3->M3 sta |
   |CE1+------|   BD1  |    ------>      +------+---|IP4->M4 dyn |
   +---+      +--------+                 |          +------------+
                  PE1                    | +--------+ Who has IP1?
                   |           EVPN      | |   BD1  | <-----  +---+
                   |           EVI1      | |        | ----->  |CE3|
   IP2/M2          |                     | |        | IP1->M1 +---+
    GARP  --->Proxy-ARP/ND               | +--------+   |    IP3/M3
     +---+    +--------+   RT2(IP2/M2)   |              |
     |CE2+----|   BD1  |    ------>      +--------------+
     +---+    +--------+                       PE3|    +---+
                  PE2                           | +----+CE4|
                   +----------------------------+      +---+
                                                 <---IP4/M4 GARP

                  Figure 1: Proxy-ARP/ND network example

   When the Proxy-ARP/ND function is enabled in a BD (Broadcast Domain)
   of the EVPN PEs, each PE creates a Proxy table specific to that BD
   that can contain three types of Proxy-ARP/ND entries:

   a.  Dynamic entries: learned by snooping CE's ARP and ND messages.
       For instance, IP4->M4 in Figure 1.

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   b.  Static entries: provisioned on the PE by the management system.
       For instance, IP3->M3 in Figure 1.

   c.  EVPN-learned entries: learned from the IP/MAC information encoded
       in the received RT2's coming from remote PEs.  For instance,
       IP1->M1 and IP2->M2 in Figure 1.

   As a high level example, the operation of the EVPN Proxy-ARP/ND
   function in the network of Figure 1 is described below.  In this
   example we assume IP1, IP2 and IP3 are IPv4 addresses:

   1.  Proxy-ARP/ND is enabled in BD1 of PE1, PE2 and PE3.

   2.  The PEs start adding dynamic, static and EVPN-learned entries to
       their Proxy tables:

       A.  PE3 adds IP1->M1 and IP2->M2 based on the EVPN routes
           received from PE1 and PE2.  Those entries were previously
           learned as dynamic entries in PE1 and PE2 respectively, and
           advertised in BGP EVPN.
       B.  PE3 adds IP4->M4 as dynamic.  This entry is learned by
           snooping the corresponding ARP messages sent by CE4.
       C.  An operator also provisions the static entry IP3->M3.

   3.  When CE3 sends an ARP Request asking for the MAC address of IP1,
       PE3 will:

       A.  Intercept the ARP Request and perform a Proxy-ARP lookup for
       B.  If the lookup is successful (as in Figure 1), PE3 will send
           an ARP Reply with IP1->M1.  The ARP Request will not be
           flooded to the EVPN network or any other local CEs.
       C.  If the lookup is not successful, PE3 will flood the ARP
           Request in the EVPN network and the other local CEs.

   In the same example, if we assume IP1, IP2, IP3 and IP4 are now IPv6
   addresses and Proxy-ARP/ND is enabled in BD1:

   1.  PEs will start adding entries in a similar way as for IPv4,
       however there are some differences:

       A.  IP1->M1 and IP2->M2 are learned as dynamic entries in PE1 and
           PE2 respectively, by snooping NA messages and not by snooping
           NS messages.  In the IPv4 case, any ARP frame can be snooped
           to learn the dynamic Proxy-ARP entry.  When learning the
           dynamic entries, the R and O Flags contained in the snooped
           NA messages will be added to the Proxy-ND entries too.

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       B.  PE1 and PE2 will advertise those entries in EVPN MAC/IP
           Advertisement routes, including the corresponding learned R
           and O Flags in the ARP/ND Extended Community.

       C.  PE3 also adds IP4->M4 as dynamic, after snooping an NA
           message sent by CE4.

   2.  When CE3 sends an NS message asking for the MAC address of IP1,
       PE3 behaves as in the IPv4 example, by intercepting the NS, doing
       a lookup on the IP and replying with an NA if the lookup is
       successful.  If it is successful the NS is not flooded to the
       EVPN PEs or any other local CEs.

   3.  If the lookup is not successful, PE3 will flood the NS to remote
       EVPN PEs attached to the same BD and the other local CEs as in
       the IPv4 case.

   As PE3 learns more and more host entries in the Proxy-ARP/ND table,
   the flooding of ARP Request messages among PEs is reduced and in some
   cases it can even be suppressed.  In a network where most of the
   participant CEs are not moving between PEs and they advertise their
   presence with GARPs or unsolicited-NA messages, the ARP/ND flooding
   among PEs, as well as the unknown unicast flooding, can practically
   be suppressed.  In an EVPN-based IXP network, where all the entries
   are Static, the ARP/ND flooding among PEs is in fact totally

   In a network where CEs move between PEs, the Proxy-ARP/ND function
   relies on the CE signaling its new location via GARP or unsolicited-
   NA messages so that tables are immediately updated.  If a CE moves
   "silently", that is, without issuing any GARP or NA message upon
   getting attached to the destination PE, the mechanisms described in
   Section 3.5 make sure that the Proxy-ARP/ND tables are eventually

3.1.  Proxy-ARP/ND Sub-Functions

   The Proxy-ARP/ND function can be structured in six sub-functions or

   1.  Learning sub-function

   2.  Reply sub-function

   3.  Unicast-forward sub-function

   4.  Maintenance sub-function

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   5.  Flood handling sub-function

   6.  Duplicate IP detection sub-function

   A Proxy-ARP/ND implementation MUST at least support the Learning,
   Reply, Maintenance and Duplicate IP detection sub-functions.  The
   following sections describe each individual sub-function.

3.2.  Learning Sub-Function

   A Proxy-ARP/ND implementation in an EVPN BD MUST support dynamic and
   EVPN-learned entries, and SHOULD support static entries.

   Static entries are provisioned from the management plane.  A static
   entry is configured on the PE attached to the host using the IP
   address in that entry.  The provisioned static IP->MAC entry MUST be
   advertised in EVPN with an ARP/ND Extended Community where the
   Immutable ARP/ND Binding Flag (I) is set to 1, as per
   [I-D.ietf-bess-evpn-na-flags].  When the I flag in the ARP/ND
   Extended Community is 1, the advertising PE indicates that the IP
   address must not be associated to a MAC, other than the one included
   in the EVPN MAC/IP Advertisement route.  The advertisement of I=1 in
   the ARP/ND Extended Community is compatible with any value of the
   Sticky bit (S) or Sequence Number in the [RFC7432] MAC Mobility
   Extended Community.  Note that the I bit in the ARP/ND Extended
   Community refers to the immutable configured association between the
   IP and the MAC address in the IP->MAC binding, whereas the S bit in
   the MAC Mobility Extended Community refers to the fact that the
   advertised MAC address is not subject to the [RFC7432] mobility

   An entry may associate a configured static IP to a list of potential
   MACs, i.e. IP1->(MAC1,MAC2..MACN).  Until a frame (including local
   ARP/NA message) is received from the CE, the PE will not advertise
   any IP1->MAC in EVPN.  Upon receiving traffic from the CE, the PE
   will check that the source MAC, E.g., MAC1, is included in the list
   of allowed MACs.  Only in that case, the PE will activate the
   IP1->MAC1 and advertise only that IP1 and MAC1 in an EVPN MAC/IP
   Advertisement route.

   The PE MUST create EVPN-learned entries from the received valid EVPN
   MAC/IP Advertisement routes containing a MAC and IP address.

   Dynamic entries are learned in different ways depending on whether
   the entry contains an IPv4 or IPv6 address:

   a.  Proxy-ARP dynamic entries:

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          The PE MUST snoop all ARP packets (that is, all frames with
          Ethertype 0x0806) received from the CEs attached to the BD in
          order to learn dynamic entries.  ARP packets received from
          remote EVPN PEs attached to the same BD are not snooped.  The
          Learning function will add the Sender MAC and Sender IP of the
          snooped ARP packet to the Proxy-ARP table.  Note that a MAC or
          an IP address with value 0 SHOULD NOT be learned.

   b.  Proxy-ND dynamic entries:

          The PE MUST snoop the NA messages (Ethertype 0x86dd, ICMPv6
          type 136) received from the CEs attached to the BD and learn
          dynamic entries from the Target Address and TLLA information.
          NA messages received from remote EVPN PEs are not snooped.  A
          PE implementing Proxy-ND as in this document MUST NOT create
          dynamic IP->MAC entries from NS messages, since they don't
          contain the R Flag required by the Proxy-ND reply function.
          See Section 3.2.1 for more information about the R Flag.

          This document specifies an "anycast" capability that can be
          configured for the proxy-ND function of the PE, and affects
          how dynamic Proxy-ND entries are learned based on the O Flag
          of the snooped NA messages.  If the O Flag is zero in the
          received NA message, the IP->MAC SHOULD only be learned in
          case the IPv6 "anycast" capability is enabled in the BD.
          Irrespective, an NA message with O Flag = 0 will be normally
          forwarded by the PE based on a MAC DA lookup.

   The following procedure associated to the Learning sub-function is

   o  When a new Proxy-ARP/ND EVPN or static active entry is learned (or
      provisioned), the PE SHOULD send a GARP or unsolicited-NA message
      to all the connected access CEs.  The PE SHOULD send a GARP or
      unsolicited-NA message for dynamic entries only if the ARP/NA
      message that previously created the entry on the PE was NOT
      flooded to all the local connected CEs before.  This GARP/
      unsolicited-NA message makes sure the CE ARP/ND caches are updated
      even if the ARP/NS/NA messages from CEs connected to remote PEs
      are not flooded in the EVPN network.

   Note that if a Static entry is provisioned with the same IP as an
   existing EVPN-learned or Dynamic entry, the Static entry takes

   In case of a PE reboot, the static and EVPN entries will be re-added
   as soon as the PE is back online and receives all the EVPN routes for
   the BD.  However, the dynamic entries will be gone.  Due to that

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   reason, new NS and ARP Requests will be flooded by the PE to remote
   PEs and dynamic entries gradually re-learned again.

3.2.1.  Proxy-ND and the NA Flags

   [RFC4861] describes the use of the R Flag in IPv6 address resolution:

   o  Nodes capable of routing IPv6 packets must reply to NS messages
      with NA messages where the R Flag is set (R Flag=1).

   o  Hosts that are not able to route IPv6 packets must indicate that
      inability by replying with NA messages that contain R Flag=0.

   The use of the R Flag in NA messages has an impact on how hosts
   select their default gateways when sending packets off-link, as per

   o  Hosts build a Default Router List based on the received RAs and
      NAs with R Flag=1.  Each cache entry has an IsRouter flag, which
      must be set for received RAs and is set based on the R flag in the
      received NAs.  A host can choose one or more Default Routers when
      sending packets off-link.

   o  In those cases where the IsRouter flag changes from TRUE to FALSE
      as a result of a NA update, the node must remove that router from
      the Default Router List and update the Destination Cache entries
      for all destinations using that neighbor as a router, as specified
      in [RFC4861] section 7.3.3.  This is needed to detect when a node
      that is used as a router stops forwarding packets due to being
      configured as a host.

   The R Flag and O Flag for a Proxy-ARP/ND entry will be learned in the
   following ways:

   o  The R Flag information SHOULD be added to the Static entries by
      the management interface.  The O Flag information MAY also be
      added by the management interface.  If the R and O Flags are not
      configured, the default value is 1.

   o  Dynamic entries SHOULD learn the R Flag and MAY learn the O Flag
      from the snooped NA messages used to learn the IP->MAC itself.

   o  EVPN-learned entries SHOULD learn the R Flag and MAY learn the O
      Flag from the ARP/ND Extended Community
      [I-D.ietf-bess-evpn-na-flags] received from EVPN along with the
      RT2 used to learn the IP->MAC itself.  If no ARP/ND Extended
      Community is received, the PE will add a configured R Flag/O Flag
      to the entry.  These configured R and O Flags MAY be an

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      administrative choice with a default value of 1.  The
      configuration of this administrative choice provides a backwards
      compatible option with EVPN PEs that follow [RFC7432] but do not
      support this specification.

   Note that, typically, IP->MAC entries with O=0 will not be learned,
   and therefore the Proxy-ND function will reply to NS messages with NA
   messages that contain O=1.  However, this document allows the
   configuration of the "anycast" capability in the BD where the Proxy-
   ND function is enabled.  If "anycast" is enabled in the BD and an NA
   message with O=0 is received, the associated IP->MAC entry will be
   learned with O=0.  If this "anycast" capability is enabled in the BD,
   Duplicate IP Detection must be disabled so that the PE is able to
   learn the same IP mapped to different MACs in the same Proxy-ND
   table.  If the "anycast" capability is disabled, NA messages with O
   Flag = 0 will not create a Proxy-ND entry (although they will be
   forwarded normally), hence no EVPN advertisement with ARP/ND Extended
   Community will be generated.

3.3.  Reply Sub-Function

   This sub-function will reply to address resolution requests/
   solicitations upon successful lookup in the Proxy-ARP/ND table for a
   given IP address.  The following considerations should be taken into
   account, assuming that the ARP Request/NS lookup hits a Proxy-ARP/ND
   entry IP1->MAC1:

   a.  When replying to ARP Request or NS messages:

       -  the PE SHOULD use the Proxy-ARP/ND entry MAC address MAC1 as
          MAC SA.  This is RECOMMENDED so that the resolved MAC can be
          learned in the MAC FIB of potential layer-2 switches sitting
          between the PE and the CE requesting the address resolution.

       -  for an ARP reply, the PE MUST use the Proxy-ARP entry IP1 and
          MAC1 addresses in the Sender Protocol Address and Hardware
          Address fields, respectively.

       -  for an NA message in response to an address resolution NS or
          DAD NS, the PE MUST use IP1 as the IP SA and Target Address.
          M1 MUST be used as the Target Link Local Address (TLLA).

   b.  A PE SHOULD NOT reply to a request/solicitation received on the
       same attachment circuit over which the IP->MAC is learned.  In
       this case the requester and the requested IP are assumed to be
       connected to the same layer-2 CE/access network linked to the
       PE's attachment circuit, and therefore the requested IP owner
       will receive the request directly.

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   c.  A PE SHOULD reply to broadcast/multicast address resolution
       messages, that is, ARP-Request, NS messages as well as DAD NS
       messages.  A PE SHOULD NOT reply to unicast address resolution
       requests (for instance, NUD NS messages).

   d.  When replying to an NS, a PE SHOULD set the Flags in the NA
       messages as follows:

       -  The R-bit is set as it was learned for the IP->MAC entry in
          the NA messages that created the entry (see Section 3.2.1).

       -  The S Flag will be set/unset as per [RFC4861].

       -  The O Flag will be set in all the NA messages issued by the
          PE, except in the case the BD is configured with the "anycast"
          capability and the entry was previously learned with O=0.  If
          "anycast" is enabled and there are more than one MAC for a
          given IP in the Proxy-ND table, the PE will reply to NS
          messages with as many NA responses as 'anycast' entries there
          are in the Proxy-ND table.

   e.  A PE MUST NOT reply to ARP probes received from the CEs.  An ARP
       probe is an ARP request constructed with an all-zero sender IP
       address that may be used by hosts for IPv4 Address Conflict
       Detection [RFC5227].

   f.  For Proxy-ARP, a PE MUST only reply to ARP-Request with the
       format specified in [RFC0826].  For Proxy-ND, a PE MUST reply to
       NS messages with the format and options specified in [RFC4861],
       and MAY reply to NS messages containing other options.  Received
       NS messages with unknown options MAY be forwarded (as unicast
       packets) to the owner of the requested IP (assuming the MAC is
       known in the Proxy-ARP/ND table and BD).  An administrative
       choice to control the behavior for received NS messages with
       unknown options ('unicast-forward', 'discard' or 'forward') MAY
       be supported.  The 'forward' option implies flooding the NS
       message based on the MAC DA.  The 'unicast-forward' option is
       described in Section 3.4.  If 'discard' is available, the
       operator should assess if flooding NS unknown options may be a
       security risk for the EVPN BD (and is so, enable 'discard'), or
       if, on the contrary, not forwarding NS unknown options may
       disrupt connectivity.

3.4.  Unicast-forward Sub-Function

   As discussed in Section 3.3, in some cases the operator may want to
   'unicast-forward' certain ARP-Request and NS messages as opposed to
   reply to them.  The implementation of a 'unicast-forward' function is

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   RECOMMENDED.  This option can be enabled with one of the following

   a.  unicast-forward always

   b.  unicast-forward unknown-options

   If 'unicast-forward always' is enabled, the PE will perform a Proxy-
   ARP/ND table lookup and in case of a hit, the PE will forward the
   packet to the owner of the MAC found in the Proxy-ARP/ND table.  This
   is irrespective of the options carried in the ARP/ND packet.  This
   option provides total transparency in the BD and yet reduces the
   amount of flooding significantly.

   If 'unicast-forward unknown-options' is enabled, upon a successful
   Proxy-ARP/ND lookup, the PE will perform a 'unicast-forward' action
   only if the ARP-Request or NS messages carry unknown options, as
   explained in Section 3.3.  The 'unicast-forward unknown-options'
   configuration allows the support of new applications using ARP/ND in
   the BD while still reducing the flooding.

   Irrespective of the enabled option, if there is no successful Proxy-
   ARP/ND lookup, the unknown ARP-Request/NS will be flooded in the
   context of the BD, as per Section 3.6.

3.5.  Maintenance Sub-Function

   The Proxy-ARP/ND tables SHOULD follow a number of maintenance
   procedures so that the dynamic IP->MAC entries are kept if the owner
   is active and flushed (and the associated RT2 withdrawn) if the owner
   is no longer in the network.  The following procedures are

   a.  Age-time

       A dynamic Proxy-ARP/ND entry MUST be flushed out of the table if
       the IP->MAC has not been refreshed within a given age-time.  The
       entry is refreshed if an ARP or NA message is received for the
       same IP->MAC entry.  The age-time is an administrative option and
       its value should be carefully chosen depending on the specific
       use case: in IXP networks (where the CE routers are fairly
       static) the age-time may normally be longer than in DC networks
       (where mobility is required).

   b.  Send-refresh option

       The PE MAY send periodic refresh messages (ARP/ND "probes") to
       the owners of the dynamic Proxy-ARP/ND entries, so that the

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       entries can be refreshed before they age out.  The owner of the
       IP->MAC entry would reply to the ARP/ND probe and the
       corresponding entry age-time reset.  The periodic send-refresh
       timer is an administrative option and is RECOMMENDED to be a
       third of the age-time or a half of the age-time in scaled

       An ARP refresh issued by the PE will be an ARP-Request message
       with the Sender's IP = 0 sent from the PE's MAC SA.  If the PE
       has an IP address in the subnet, for instance on an Integrated
       Routing and Bridging (IRB) interface, then it MAY use it as a
       source for the ARP request (instead of Sender's IP = 0).  An ND
       refresh will be a NS message issued from the PE's MAC SA and a
       Link Local Address associated to the PE's MAC.

       The refresh request messages SHOULD be sent only for dynamic
       entries and not for static or EVPN-learned entries.  Even though
       the refresh request messages are broadcast or multicast, the PE
       SHOULD only send the message to the attachment circuit associated
       to the MAC in the IP->MAC entry.

   The age-time and send-refresh options are used in EVPN networks to
   avoid unnecessary EVPN RT2 withdrawals: if refresh messages are sent
   before the corresponding BD FIB and Proxy-ARP/ND age-time for a given
   entry expires, inactive but existing hosts will reply, refreshing the
   entry and therefore avoiding unnecessary EVPN MAC/IP Advertisement
   withdrawals in EVPN.  Both entries (MAC in the BD and IP->MAC in
   Proxy-ARP/ND) are reset when the owner replies to the ARP/ND probe.
   If there is no response to the ARP/ND probe, the MAC and IP->MAC
   entries will be legitimately flushed and the RT2s withdrawn.

3.6.  Flood (to Remote PEs) Handling

   The Proxy-ARP/ND function implicitly helps reducing the flooding of
   ARP Request and NS messages to remote PEs in an EVPN network.
   However, in certain use cases, the flooding of ARP/NS/NA messages
   (and even the unknown unicast flooding) to remote PEs can be
   suppressed completely in an EVPN network.

   For instance, in an IXP network, since all the participant CEs are
   well known and will not move to a different PE, the IP->MAC entries
   for the local CEs may be all provisioned on the PEs by a management
   system.  Assuming the entries for the CEs are all provisioned on the
   local PE, a given Proxy-ARP/ND table will only contain static and
   EVPN-learned entries.  In this case, the operator may choose to
   suppress the flooding of ARP/NS/NA from the local PE to the remote
   PEs completely.

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   The flooding may also be suppressed completely in IXP networks with
   dynamic Proxy-ARP/ND entries assuming that all the CEs are directly
   connected to the PEs and they all advertise their presence with a
   GARP/unsolicited-NA when they connect to the network.  If any of
   those two assumptions is not true and any of the PEs may not learn
   all the local Proxy-ARP/ND entries, flooding of the ARP/NS/NA
   messages from the local PE to the remote PEs SHOULD NOT be
   suppressed, or the address resolution process for some CEs will not
   be completed.

   In networks where fast mobility is expected (DC use case), it is NOT
   RECOMMENDED to suppress the flooding of unknown ARP-Requests/NS or
   GARPs/unsolicited-NAs.  Unknown ARP-Requests/NS refer to those ARP-
   Request/NS messages for which the Proxy-ARP/ND lookups for the
   requested IPs do not succeed.

   In order to give the operator the choice to suppress/allow the
   flooding to remote PEs, a PE MAY support administrative options to
   individually suppress/allow the flooding of:

   o  Unknown ARP-Request and NS messages.

   o  GARP and unsolicited-NA messages.

   The operator will use these options based on the expected behavior on
   the CEs.

3.7.  Duplicate IP Detection

   The Proxy-ARP/ND function MUST support duplicate IP detection as per
   this section so that ARP/ND-spoofing attacks or duplicate IPs due to
   human errors can be detected.  For IPv6 addresses, CEs will continue
   to carry out the DAD procedures as per [RFC4862].  The solution
   described in this section is an additional security mechanism carried
   out by the PEs that guarantees IPv6 address moves between PEs are
   legit and not the result of an attack.  [RFC6957] describes a
   solution for IPv6 Duplicate Address Detection Proxy, however, it is
   defined for point-to-multipoint topologies with a split-horizon
   forwarding, where the 'CEs' have no direct communication within the
   same L2 link and therefore it is not suitable for EVPN Broadcast
   Domains.  In addition, the solution described in this section
   includes the use of the AS-MAC for additional security.

   ARP/ND spoofing is a technique whereby an attacker sends "fake" ARP/
   ND messages onto a broadcast domain.  Generally the aim is to
   associate the attacker's MAC address with the IP address of another
   host causing any traffic meant for that IP address to be sent to the
   attacker instead.

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   The distributed nature of EVPN and Proxy-ARP/ND allows the easy
   detection of duplicated IPs in the network, in a similar way to the
   MAC duplication detection function supported by [RFC7432] for MAC

   Duplicate IP detection monitors "IP-moves" in the Proxy-ARP/ND table
   in the following way:

   a.  When an existing active IP1->MAC1 entry is modified, a PE starts
       an M-second timer (default value of M=180), and if it detects N
       IP moves before the timer expires (default value of N=5), it
       concludes that a duplicate IP situation has occurred.  An IP move
       is considered when, for instance, IP1->MAC1 is replaced by
       IP1->MAC2 in the Proxy-ARP/ND table.  Static IP->MAC entries,
       that is, locally provisioned or EVPN-learned entries with I=1 in
       the ARP/ND Extended Community, are not subject to this procedure.
       Static entries MUST NOT be overridden by dynamic Proxy-ARP/ND

   b.  In order to detect the duplicate IP faster, the PE SHOULD send a
       Confirm message to the former owner of the IP.  A Confirm message
       is a unicast ARP-Request/NS message sent by the PE to the MAC
       addresses that previously owned the IP, when the MAC changes in
       the Proxy-ARP/ND table.  The Confirm message uses a sender's IP in case of ARP (if the PE has an IP address in the subnet
       then it MAY use it) and an IPv6 Link Local Address in case of NS.
       If the PE does not receive an answer within a given time, the new
       entry will be confirmed and activated.  The default RECOMMENDED
       time to receive the confirmation is 30 seconds.  In case of
       spoofing, for instance, if IP1->MAC1 moves to IP1->MAC2, the PE
       may send a unicast ARP-Request/NS message for IP1 with MAC DA=
       MAC1 and MAC SA= PE's MAC.  This will force the legitimate owner
       to respond if the move to MAC2 was spoofed, and make the PE issue
       another Confirm message, this time to MAC DA= MAC2.  If both,
       legitimate owner and spoofer keep replying to the Confirm
       message, the PE will detect the duplicate IP within the M-second

       -  If the IP1->MAC1 pair was previously owned by the spoofer and
          the new IP1->MAC2 was from a valid CE, then the issued Confirm
          message would trigger a response from the spoofer.

       -  If it were the other way around, that is, IP1->MAC1 was
          previously owned by a valid CE, the Confirm message would
          trigger a response from the CE.

          Either way, if this process continues, then duplicate
          detection will kick in.

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   c.  Upon detecting a duplicate IP situation:

       1.  The entry in duplicate detected state cannot be updated with
           new dynamic or EVPN-learned entries for the same IP.  The
           operator MAY override the entry, though, with a static

       2.  The PE SHOULD alert the operator and stop responding to ARP/
           NS for the duplicate IP until a corrective action is taken.

       3.  Optionally the PE MAY associate an "anti-spoofing-mac" (AS-
           MAC) to the duplicate IP in the Proxy-ARP/ND table.  The PE
           will send a GARP/unsolicited-NA message with IP1->AS-MAC to
           the local CEs as well as an RT2 (with IP1->AS-MAC) to the
           remote PEs.  This will update the ARP/ND caches on all the
           CEs in the BD, and hence all the CEs in the BD will use the
           AS-MAC as MAC DA when sending traffic to IP1.  This procedure
           prevents the spoofer from attracting any traffic for IP1.
           Since the AS-MAC is a managed MAC address known by all the
           PEs in the BD, all the PEs MAY apply filters to drop and/or
           log any frame with MAC DA= AS-MAC.  The advertisement of the
           AS-MAC as a "black-hole MAC" (by using an indication in the
           RT2) that can be used directly in the BD to drop frames is
           for further study.

   d.  The duplicate IP situation will be cleared when a corrective
       action is taken by the operator, or alternatively after a HOLD-
       DOWN timer (default value of 540 seconds).

   The values of M, N and HOLD-DOWN timer SHOULD be a configurable
   administrative option to allow for the required flexibility in
   different scenarios.

   For Proxy-ND, the Duplicate IP Detection described in this section
   SHOULD only monitor IP moves for IP->MACs learned from NA messages
   with O Flag=1.  NA messages with O Flag=0 would not override the ND
   cache entries for an existing IP, and therefore the procedure in this
   section would not detect duplicate IPs.  This Duplicate IP Detection
   for IPv6 SHOULD be disabled when the IPv6 "anycast" capability is
   activated in a given BD.

4.  Solution Benefits

   The solution described in this document provides the following

   a.  The solution may suppress completely the flooding of the ARP/ND
       messages in the EVPN network, assuming that all the CE IP->MAC

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       addresses local to the PEs are known or provisioned on the PEs
       from a management system.  Note that in this case, the unknown
       unicast flooded traffic can also be suppressed, since all the
       expected unicast traffic will be destined to known MAC addresses
       in the PE BDs.

   b.  The solution reduces significantly the flooding of the ARP/ND
       messages in the EVPN network, assuming that some or all the CE
       IP->MAC addresses are learned on the data plane by snooping ARP/
       ND messages issued by the CEs.

   c.  The solution provides a way to refresh periodically the CE
       IP->MAC entries learned through the data plane, so that the
       IP->MAC entries are not withdrawn by EVPN when they age out
       unless the CE is not active anymore.  This option helps reducing
       the EVPN control plane overhead in a network with active CEs that
       do not send packets frequently.

   d.  Provides a mechanism to detect duplicate IP addresses and avoid
       ARP/ND-spoof attacks or the effects of duplicate addresses due to
       human errors.

5.  Deployment Scenarios

   Four deployment scenarios with different levels of ARP/ND control are
   available to operators using this solution, depending on their
   requirements to manage ARP/ND: all dynamic learning, all dynamic
   learning with Proxy-ARP/ND, hybrid dynamic learning and static
   provisioning with Proxy-ARP/ND, and all static provisioning with

5.1.  All Dynamic Learning

   In this scenario for minimum security and mitigation, EVPN is
   deployed in the BD with the Proxy-ARP/ND function shutdown.  PEs do
   not intercept ARP/ND requests and flood all requests issued by the
   CEs, as a conventional layer-2 network among those CEs would do.
   While no ARP/ND mitigation is used in this scenario, the operator can
   still take advantage of EVPN features such as control plane learning
   and all-active multihoming in the peering network.

   Although this option does not require any of the procedures described
   in this document, it is added as baseline/default option for
   completeness.  This option is equivalent to VPLS as far as ARP/ND is
   concerned.  The options described in Section 5.2, Section 5.3 and
   Section 5.4 are only possible in EVPN networks in combination with
   their Proxy-ARP/ND capabilities.

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5.2.  Dynamic Learning with Proxy-ARP/ND

   This scenario minimizes flooding while enabling dynamic learning of
   IP->MAC entries.  The Proxy-ARP/ND function is enabled in the BDs of
   the EVPN PEs, so that the PEs snoop ARP/ND messages issued by the CEs
   and respond to CE ARP-requests/NS messages.

   PEs will flood requests if the entry is not in their Proxy table.
   Any unknown source IP->MAC entries will be learned and advertised in
   EVPN, and traffic to unknown entries is discarded at the ingress PE.

   This scenario makes use of the Learning, Reply and Maintenance sub-
   functions, with an optional use of the Unicast-forward and Duplicate
   IP detection sub-functions.  The Flood handling sub-function uses
   default flooding for unknown ARP-Request/NS messages.

5.3.  Hybrid Dynamic Learning and Static Provisioning with Proxy-ARP/ND

   Some IXPs and other operators want to protect particular hosts on the
   BD while allowing dynamic learning of CE addresses.  For example, an
   operator may want to configure static IP->MAC entries for management
   and infrastructure hosts that provide critical services.  In this
   scenario, static entries are provisioned from the management plane
   for protected IP->MAC addresses, and dynamic learning with Proxy-ARP/
   ND is enabled as described in Section 5.2 on the BD.

   This scenario makes use of the same sub-functions as in Section 5.2,
   but adding static entries added by the Learning sub-function.

5.4.  All Static Provisioning with Proxy-ARP/ND

   For a solution that maximizes security and eliminates flooding and
   unknown unicast in the peering network, all IP->MAC entries are
   provisioned from the management plane.  The Proxy-ARP/ND function is
   enabled in the BDs of the EVPN PEs, so that the PEs intercept and
   respond to CE requests.  Dynamic learning and ARP/ND snooping is
   disabled so that ARP-Requests and NS to unknown IPs are discarded at
   the ingress PE.  This scenario provides an operator the most control
   over IP->MAC entries and allows an operator to manage all entries
   from a management system.

   In this scenario, the Learning sub-function is limited to static
   entries, the Maintenance sub-function will not require any procedures
   due to the static entries, and the Flood handling sub-function will
   completely suppress Unknown ARP-Requests/NS messages as well as GARP
   and unsolicited-NA messages.

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5.5.  Example of Deployment in Internet Exchange Points

   Nowadays, almost all IXPs install some security rules in order to
   protect the peering network (BD).  These rules are often called port
   security.  Port security summarizes different operational steps that
   limit the access to the IXP-LAN and the customer router, and controls
   the kind of traffic that the routers are allowed to exchange (e.g.,
   Ethernet, IPv4, IPv6).  Due to this, the deployment scenario as
   described in Section 5.4 "All Static Provisioning with Proxy-ARP/ND"
   is the predominant scenario for IXPs.

   In addition to the "All Static Provisioning" behavior, in IXP
   networks it is recommended to configure the Reply Sub-Function to
   'discard' ARP-Requests/NS messages with unrecognized options.

   At IXPs, customers usually follow a certain operational life-cycle.
   For each step of the operational life-cycle specific operational
   procedures are executed.

   The following describes the operational procedures that are needed to
   guarantee port security throughout the life-cycle of a customer with
   focus on EVPN features:

   1.  A new customer is connected the first time to the IXP:

       Before the connection between the customer router and the IXP-LAN
       is activated, the MAC of the router is allow-listed on the IXP's
       switch port.  All other MAC addresses are blocked.  Pre-defined
       IPv4 and IPv6 addresses of the IXP peering network space are
       configured at the customer router.  The IP->MAC static entries
       (IPv4 and IPv6) are configured in the management system of the
       IXP for the customer's port in order to support Proxy-ARP/ND.

       In case a customer uses multiple ports aggregated to a single
       logical port (LAG) some vendors randomly select the MAC address
       of the LAG from the different MAC addresses assigned to the
       ports.  In this case the static entry will be used associated to
       a list of allowed MACs.

   2.  Replacement of customer router:

       If a customer router is about to be replaced, the new MAC
       address(es) must be installed in the management system besides
       the MAC address(es) of the currently connected router.  This
       allows the customer to replace the router without any active
       involvement of the IXP operator.  For this, static entries are
       also used.  After the replacement takes place, the MAC
       address(es) of the replaced router can be removed.

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   3.  Decommissioning a customer router

       If a customer router is decommissioned, the router is
       disconnected from the IXP PE.  Right after that, the MAC
       address(es) of the router and IP->MAC bindings can be removed
       from the management system.

5.6.  Example of Deployment in Data Centers

   DCs normally have different requirements than IXPs in terms of Proxy-
   ARP/ND.  Some differences are listed below:

   a.  The required mobility in virtualized DCs makes the "Dynamic
       Learning" or "Hybrid Dynamic and Static Provisioning" models more
       appropriate than the "All Static Provisioning" model.

   b.  IPv6 'anycast' may be required in DCs, while it is typically not
       a requirement in IXP networks.  Therefore if the DC needs IPv6
       anycast addresses, the "anycast" capability will be explicitly
       enabled in the Proxy-ND function, hence the Proxy-ND sub-
       functions modified accordingly.  For instance, if IPv6 'anycast'
       is enabled in the Proxy-ND function, the Duplicate IP Detection
       procedure in Section 3.7 must be disabled.

   c.  DCs may require special options on ARP/ND as opposed to the
       address resolution function, which is the only one typically
       required in IXPs.  Based on that, the Reply Sub-function may be
       modified to forward or discard unknown options.

6.  Security Considerations

   The security considerations of [RFC7432] and
   [I-D.ietf-bess-evpn-na-flags] apply to this document too.  Note that
   EVPN does not inherently provide cryptographic protection (including
   confidentiality protection).

   The procedures in this document reduce the amount of ARP/ND message
   flooding, which in itself provides a protection to "slow path"
   software processors of routers and Tenant Systems in large BDs.  The
   ARP/ND requests that are replied by the Proxy-ARP/ND function (hence
   not flooded) are normally targeted to existing hosts in the BD.  ARP/
   ND requests targeted to absent hosts are still normally flooded;
   however, the suppression of Unknown ARP-Requests and NS messages
   described in Section 3.6 can provide an additional level of security
   against ARP-Requests/NS messages issued to non-existing hosts.

   While the unicast-forward and/or flood suppression sub-functions
   provide an added security mechanism for the BD, they can also

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   increase the risk of blocking the service for a CE if the EVPN PEs
   cannot provide the ARP/ND resolution that the CE needs.

   The solution also provides protection against Denial Of Service
   attacks that use ARP/ND-spoofing as a first step.  The Duplicate IP
   Detection and the use of an AS-MAC as explained in Section 3.7
   protects the BD against ARP/ND spoofing.

   The Proxy-ARP/ND function specified in this document does not allow
   the learning of an IP address mapped to multiple MAC addresses in the
   same table, unless the "anycast" capability is enabled (and only in
   case of Proxy-ND).  When "anycast" is enabled in the Proxy-ND
   function, the number of allowed entries for the same IP address MUST
   be limited by the operator to prevent DoS attacks that attempt to
   fill the Proxy-ND table with a significant number of entries for the
   same IP.

   The document provides some examples and guidelines that can be used
   by IXPs in their EVPN BDs.  When EVPN and its associated Proxy-ARP/ND
   function are used in IXP networks, they provide ARP/ND security and
   mitigation.  IXPs must still employ additional security mechanisms
   that protect the peering network as per the established BCPs such as
   the ones described in [Euro-IX-BCP].  For example, IXPs should
   disable all unneeded control protocols, and block unwanted protocols
   from CEs so that only IPv4, ARP and IPv6 Ethertypes are permitted on
   the peering network.  In addition, port security features and ACLs
   can provide an additional level of security.

   Finally, it is worth noting that the Proxy-ARP/ND solution in this
   document will not work if there is a mechanism securing ARP/ND
   exchanges among CEs, because the PE is not able to secure the
   "proxied" ND messages.

7.  IANA Considerations

   No IANA considerations.

8.  Acknowledgments

   The authors want to thank Ranganathan Boovaraghavan, Sriram
   Venkateswaran, Manish Krishnan, Seshagiri Venugopal, Tony Przygienda,
   Robert Raszuk and Iftekhar Hussain for their review and
   contributions.  Thank you to Oliver Knapp as well, for his detailed

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9.  Contributors

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

   Wim Henderickx

   Daniel Melzer
   DE-CIX Management GmbH

   Erik Nordmark

10.  References

10.1.  Normative References

   [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>.

   [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,

   [RFC0826]  Plummer, D., "An Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,

   [RFC5227]  Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
              DOI 10.17487/RFC5227, July 2008,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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              Rabadan, J., Sathappan, S., Nagaraj, K., and W. Lin,
              "Propagation of ARP/ND Flags in EVPN", draft-ietf-bess-
              evpn-na-flags-09 (work in progress), December 2020.

10.2.  Informative References

              Euro-IX, "European Internet Exchange Association Best
              Practises -

   [RFC6820]  Narten, T., Karir, M., and I. Foo, "Address Resolution
              Problems in Large Data Center Networks", RFC 6820,
              DOI 10.17487/RFC6820, January 2013,

   [RFC6957]  Costa, F., Combes, J-M., Ed., Pougnard, X., and H. Li,
              "Duplicate Address Detection Proxy", RFC 6957,
              DOI 10.17487/RFC6957, June 2013,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

Authors' Addresses

   Jorge Rabadan (editor)
   777 Middlefield Road
   Mountain View, CA  94043

   Email: jorge.rabadan@nokia.com

   Senthil Sathappan
   701 E. Middlefield Road
   Mountain View, CA 94043 USA

   Email: senthil.sathappan@nokia.com

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   Kiran Nagaraj
   701 E. Middlefield Road
   Mountain View, CA 94043 USA

   Email: kiran.nagaraj@nokia.com

   Greg Hankins

   Email: greg.hankins@nokia.com

   Thomas King
   DE-CIX Management GmbH

   Email: thomas.king@de-cix.net

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