Propagation of ARP/ND Flags in an Ethernet Virtual Private Network (EVPN)
RFC 9047

Document Type RFC - Proposed Standard (June 2021; No errata)
Authors Jorge Rabadan  , Senthil Sathappan  , Kiran Nagaraj  , Wen Lin 
Last updated 2021-06-22
Replaces draft-snr-bess-evpn-na-flags
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Internet Engineering Task Force (IETF)                   J. Rabadan, Ed.
Request for Comments: 9047                                  S. Sathappan
Category: Standards Track                                     K. Nagaraj
ISSN: 2070-1721                                                    Nokia
                                                                  W. Lin
                                                                 Juniper
                                                               June 2021

   Propagation of ARP/ND Flags in an Ethernet Virtual Private Network
                                 (EVPN)

Abstract

   This document defines an Extended Community that is advertised along
   with an Ethernet Virtual Private Network (EVPN) Media Access Control
   (MAC) / IP Advertisement route and carries information relevant to
   the Address Resolution Protocol (ARP) / Neighbor Discovery (ND)
   resolution so that an EVPN Provider Edge (PE) implementing a proxy-
   ARP/ND function in broadcast domains (BDs) or an ARP/ND function on
   Integrated Routing and Bridging (IRB) interfaces can reply to ARP
   Requests or Neighbor Solicitation (NS) messages with the correct
   information.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9047.

Copyright Notice

   Copyright (c) 2021 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
   (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 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
     1.1.  Terminology and Conventions
   2.  The EVPN ARP/ND Extended Community
   3.  Use of the EVPN ARP/ND Extended Community
     3.1.  Transmission of the EVPN ARP/ND Extended Community
     3.2.  Reception of the EVPN ARP/ND Extended Community
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   An EVPN MAC/IP Advertisement route can optionally carry IPv4 or IPv6
   addresses associated with a MAC address.  Remote PE routers can use
   this information to populate their ARP or ND tables on IRB interfaces
   or their proxy-ARP/ND tables in BDs.  PEs can then reply locally (act
   as an ARP/ND proxy, as per [RFC7432]) to IPv4 ARP Requests and IPv6
   Neighbor Solicitation messages and reduce or suppress the flooding
   produced by the address resolution procedure.  However, the
   information conveyed in the EVPN MAC/IP Advertisement route may not
   be enough for the remote PE to reply to local ARP or ND requests.
   For example, if a PE learns an IPv6 address and MAC address
   combination ND entry via EVPN (denoted by IPv6->MAC), the PE would
   not know if that particular IPv6->MAC pair belongs to a router or a
   host or if that address is an anycast address, as this information is
   not carried in the EVPN MAC/IP Advertisement routes.

   This document defines an Extended Community that is advertised along
   with an EVPN MAC/IP Advertisement route and carries information
   relevant to the ARP/ND resolution so that an EVPN PE implementing a
   proxy-ARP/ND function can reply to ARP Requests or Neighbor
   Solicitations with the correct information.  In particular, the flags
   defined in [RFC4861] can now be conveyed along with a MAC/IP
   Advertisement route so that an egress EVPN PE can issue Neighbor
   Advertisement (NA) messages with the correct flag information.

   The flags are carried in the EVPN Address Resolution Protocol and
   Neighbor Discovery (ARP/ND) Extended Community, as described in the
   following sections.

1.1.  Terminology and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   EVPN:   Ethernet Virtual Private Networks, as in [RFC7432]

   BD:     Broadcast Domain, also described in [RFC7432]

   ARP:    Address Resolution Protocol

   ND:     Neighbor Discovery protocol, specified in [RFC4861]

   PE:     Provider Edge router

   CE:     Customer Edge router

   IRB:    Integrated Routing and Bridging interface

   Proxy-ARP/ND:  A function on the EVPN PEs by which received ARP
           Requests or NS messages are replied to locally by the PE,
           without the need to flood the requests to remote PEs in the
           BD.  In order to reply to ARP Requests or NS messages, the PE
           does a lookup on an ARP/ND table, which is a collection of
           IP->MAC entries learned by the PE.

   IP->MAC:  An IP address and MAC address combination that represents a
           given host and is added to an ARP table or ND table.  This
           document uses IP->MAC generically for IPv4 and IPv6
           addresses.  When something is specific to IPv4, the document
           will use IPv4->MAC; likewise, IPv6->MAC will be used when
           something is specific to IPv6 entries only.

   Familiarity with the terminology in [RFC4861] and [RFC7432] is
   expected.

2.  The EVPN ARP/ND Extended Community

   This document defines a transitive EVPN Extended Community (Type
   field value of 0x06) with a Sub-Type of 0x08, as allocated by IANA.
   It is advertised along with EVPN MAC/IP Advertisement routes that
   carry an IPv4 or IPv6 address.

     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Type=0x06     | Sub-Type=0x08 |Flags (1 octet)| Reserved=0    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Reserved=0                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Flags field:

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |       |I| |O|R|
    +-+-+-+-+-+-+-+-+

   The following flags are defined in the Flags field, the third octet
   of the Extended Community:

   R:   Router flag (corresponds to Bit 23 of the Extended Community)

        Bit 7 of the Flags field is defined as the "Router flag".  When
        set, the R flag indicates that the IPv6->MAC pair advertised in
        the MAC/IP Advertisement route, along with the Extended
        Community, belongs to an IPv6 router.  If the R flag is zero,
        the IPv6->MAC pair belongs to a host.  The receiving PE
        implementing the ND function will use this information in
        Neighbor Advertisement messages for the associated IPv6 address.
        This flag has no meaning for ARP IPv4->MAC entries and MUST be
        ignored when the Extended Community is received with an EVPN
        MAC/IP Advertisement route for an IPv4->MAC pair.

   O:   Override flag (corresponds to Bit 22 of the Extended Community)

        Bit 6 of the Flags field is defined as the "Override flag".  An
        egress PE will normally advertise IPv6->MAC pairs with the O
        flag set, and only when IPv6 "anycast" is enabled in the BD or
        interface will the PE send an IPv6->MAC pair with the O flag =
        0.  The ingress PE will install the ND entry with the received O
        flag and will always use this O flag value when replying to a
        Neighbor Solicitation for the IPv6 address.  Similarly to the
        Router Flag, the Override flag has no meaning for ARP IPv4->MAC
        entries and MUST be ignored when the Extended Community is
        received with an EVPN MAC/IP Advertisement route for an
        IPv4->MAC pair.

   I:   Immutable ARP/ND Binding flag (corresponds to Bit 20 of the
        Extended Community)

        Bit 4 of the Flags field is defined as the "Immutable ARP/ND
        Binding flag".  When set, the egress PE indicates that the
        IP->MAC pair that was sent in an EVPN MAC/IP Advertisement route
        (along with the Extended Community) is a configured ARP/ND
        entry.  In this case, the IP address in the EVPN MAC/IP
        Advertisement route can only be bound together with the MAC
        address specified in the same route, and not with any other MAC
        addresses received in a different route without the I flag set.

   Bits 0-3 and 5 are not assigned by this document.  They MUST be set
   to zero and ignored on receipt.

   The reserved fields are set to 0 and ignored by the receiver.

3.  Use of the EVPN ARP/ND Extended Community

   This section describes the relevant procedures when advertising and
   processing the EVPN ARP/ND Extended Community.  In all the procedures
   below, a "PE" must be interpreted as a "PE that supports the proxy-
   ARP/ND (introduced by [RFC7432]) and implements the propagation of
   the ARP/ND flags that this document specifies".

3.1.  Transmission of the EVPN ARP/ND Extended Community

   When an IP->MAC entry is not learned via EVPN, a PE may learn IP->MAC
   pairs in the management plane (this will create static entries in the
   ARP/ND or proxy-ARP/ND table) or by snooping ARP or NA messages
   coming from the CE (this will create dynamic entries).  Those static
   and dynamic IP->MAC entries will be advertised in EVPN MAC/IP
   Advertisement routes that use the EVPN ARP/ND Extended Community as
   follows:

   *  Advertised MAC/IP Advertisement routes for IPv6->MAC entries MUST
      include one (and only one) ARP/ND Extended Community with the R
      and O flag values associated with the entry.  Those flag values
      are either dynamically learned (from NA messages) or configured in
      case of static entries.

   *  MAC/IP Advertisement routes for IPv4->MAC entries MAY include one
      ARP/ND Extended Community.  If the EVPN ARP/ND Extended Community
      is advertised along with an EVPN IPv4/MAC Advertisement route, the
      R and O flags SHOULD be set to zero.

   *  If an IP->MAC pair is static (it has been configured), the
      corresponding MAC/IP Advertisement route MUST be sent along with
      an ARP/ND Extended Community with the I flag set.

   *  This Extended Community does not change the procedures described
      in [RFC7432].  Specifically, the procedures for advertising the
      MAC Mobility Extended Community along with the MAC/IP
      Advertisement route are not changed.

3.2.  Reception of the EVPN ARP/ND Extended Community

   In addition to the procedures specified in [RFC7432], a PE receiving
   a MAC/IP Advertisement route will process the EVPN ARP/ND Extended
   Community as follows:

   *  Only one EVPN ARP/ND Extended Community is expected to be received
      along with an EVPN MAC/IP Advertisement route.  If more than one
      ARP/ND Extended Community is received, the PE MUST consider only
      the first one on the list for processing purposes and MUST NOT
      propagate the rest of the ARP/ND Extended Communities.

   *  The R, O, and I flags MUST be ignored if they are advertised along
      with an EVPN MAC/IP Advertisement route that does not contain an
      IP (IPv4 or IPv6) address.  Otherwise, they are processed as
      follows.

   *  R and O flag processing:

      -  If the EVPN MAC/IP Advertisement route contains an IPv6 address
         and the EVPN ARP/ND Extended Community, the PE MUST add the R
         and O flag values to the ND entry in the ND or proxy-ND table
         and propagate the value of the R and O flags from the ARP/ND
         Extended Community to the Neighbor Advertisements when replying
         to a solicitation for the IPv6 address.

      -  If no EVPN ARP/ND Extended Community is received along with the
         route, the PE will add the default R and O flags to the entry.
         The default R flag SHOULD be an administrative choice.  The
         default O flag SHOULD be 1.

      -  A PE MUST ignore the received R and O flags for an EVPN MAC/IP
         Advertisement route that contains an IPv4->MAC pair.

   *  I flag processing:

      -  A PE receiving an EVPN MAC/IP Advertisement route containing an
         IP->MAC and the I flag set SHOULD install the IP->MAC entry in
         the ARP/ND or proxy-ARP/ND table as an "immutable binding".
         This immutable binding entry will override an existing non-
         immutable binding for the same IP->MAC.  The absence of the
         EVPN ARP/ND Extended Community in a MAC/IP Advertisement route
         indicates that the IP->MAC entry is not an "immutable binding".

      -  Receiving multiple EVPN MAC/IP Advertisement routes with the I
         flag set to 1 for the same IP but a different MAC address is
         considered a misconfiguration or a transient error condition.
         If this happens in the network, a PE receiving multiple routes
         (with the I flag set to 1 for the same IP and a different MAC
         address) SHOULD update the IP->MAC entry with the latest
         received information.  Note that if a configured IP1->MAC1
         changes to point to a new MAC address, i.e., IP1->MAC2, the
         EVPN MAC/IP Advertisement route for IP1->MAC1 will be withdrawn
         before the EVPN MAC/IP Advertisement route for IP1->MAC2 is
         advertised.

      -  A PE originating an EVPN MAC/IP Advertisement route for
         IP1->MAC1 with the I flag set to 1 MAY also originate the route
         with the "Sticky/static flag" set (in the MAC Mobility Extended
         Community).  In such a case, the IP1->MAC1 binding is not only
         immutable but it cannot move as well.  Even so, if an update
         for the same immutable and static IP1->MAC1 is received from a
         different PE, one of the two routes will be selected.  This is
         analogous to the case described in Section 15.2 of [RFC7432]
         when two MAC/IP routes with the static flag set are received,
         and the PE likewise MUST alert the operator of such a
         situation.

   In a situation where a host (with an IP->MAC that is configured as
   immutable binding in the attached PE) is allowed to move between PEs
   (that is, the associated MAC is non-static), PEs can receive multiple
   MAC/IP Advertisement routes for the same IP->MAC.  In such
   situations, MAC mobility procedures as in [RFC7432] dictate the
   reachability of the MAC.

   As an example of the use of the I flag, consider PE1, PE2, and PE3
   attached to the same BD.  PE1 originates an EVPN MAC/IP Advertisement
   route for IP1->MAC1 with the I flag set to 1 later on, PE2 also
   originates an EVPN MAC/IP Advertisement route IP1->MAC1 with a higher
   sequence number and the I flag set to 1.  Then all the EVPN PEs
   attached to the same BD SHOULD retain their IP1->MAC1 ARP/ND binding
   but update MAC1's forwarding destination to PE2.  For some reason, if
   PE3 originates an EVPN MAC/IP Advertisement route for IP1->MAC2 with
   the I flag set to 0 (even with a higher sequence number), then the
   EVPN PEs in the BD will not update their IP1->MAC1 ARP/ND bindings
   since IP1 is bound to MAC1 (MAC2 SHOULD still be programmed in the
   Layer 2 BDs).  This is considered a misconfiguration in PE3.

   When the I flag is set to 1, a given IP is assumed to be always bound
   to the same MAC address; therefore, the mobility procedures described
   in [EXTENDED-MOBILITY] for "Host IP move to a new MAC" will not
   apply.

4.  Security Considerations

   The same security considerations described in [RFC7432] apply to this
   document.  In general, it is worth noting that the use of proxy-ARP/
   ND in EVPN BDs may add some security risks.  Attackers can make use
   of ARP/ND messages to create state in all the PEs attached to the
   same BD as the attacker and exhaust resources in those PEs.
   Therefore, additional security mechanisms may be needed.  Some
   examples of such additional security mechanisms are limiting the
   number of proxy-ARP/ND entries per BD and/or per port or closely
   monitoring the rate at which hosts create dynamic proxy-ARP/ND
   entries.

   In addition, this document adds pieces of information that impact the
   way ARP/ND entries are installed in ARP/ND and/or proxy-ARP/ND tables
   and, therefore, impacts the resolution protocols for IPv4 and IPv6
   addresses.  For instance, if a given IPv6->MAC binding is configured
   with the wrong R or O flags (intentionally or not) on a given PE, the
   rest of the PEs attached to the same BD will install the wrong
   information for the IPv6->MAC.  This will cause all the PEs in the BD
   to reply to Neighbor Solicitations for the IPv6 with NA messages
   containing the wrong R and O flags.  For example, as specified in
   [RFC4861], the receiver of an NA message with O not set will not
   update its existing cache entry for the IP->MAC; hence, the
   communication between the owner of the IP address and the receiver of
   the NA message with the wrong O flag will fail.  Similarly, the
   receiver of an NA message with the wrong R flag may update its
   Default Router List by incorrectly adding or removing an entry, which
   could, for example, lead to sending traffic to a node that is not a
   router, causing the traffic to be dropped.

   The I flag, or Immutable ARP/ND Binding flag, is a useful security
   tool, allowing an operator to ensure a given IP address is always
   bound to the same MAC and that information is distributed to all the
   PEs attached to the same BD.  ARP/ND spoofing attacks, in which a
   malicious host injects Gratuitous ARPs or unsolicited NAs for that IP
   address with a different MAC address, will not succeed in programming
   the ARP/ND and proxy-ARP/ND tables and therefore the spoofer will not
   receive the traffic.

5.  IANA Considerations

   IANA has changed the name for Sub-Type Value 0x08 in the "EVPN
   Extended Community Sub-Types" registry [IANA-BGP-EXT-COMM] to the
   following:

        +================+===========================+===========+
        | Sub-Type Value | Name                      | Reference |
        +================+===========================+===========+
        | 0x08           | ARP/ND Extended Community | RFC 9047  |
        +----------------+---------------------------+-----------+

          Table 1: Updated Value in the "EVPN Extended Community
                           Sub-Types" Registry

   IANA has created the "ARP/ND Extended Community Flags" registry,
   where the following initial allocations have been made:

     +===============+===================================+===========+
     | Flag Position | Name                              | Reference |
     +===============+===================================+===========+
     | 0-3           | Unassigned                        |           |
     +---------------+-----------------------------------+-----------+
     | 4             | Immutable ARP/ND Binding Flag (I) | RFC 9047  |
     +---------------+-----------------------------------+-----------+
     | 5             | Unassigned                        |           |
     +---------------+-----------------------------------+-----------+
     | 6             | Override Flag (O)                 | RFC 9047  |
     +---------------+-----------------------------------+-----------+
     | 7             | Router Flag (R)                   | RFC 9047  |
     +---------------+-----------------------------------+-----------+

         Table 2: Initial Values of the "ARP/ND Extended Community
                              Flags" Registry

   The registration policy for this registry is Standards Action
   [RFC8126].  This registry is located in the "Border Gateway Protocol
   (BGP) Extended Communities" registry [IANA-BGP-EXT-COMM].

   Note that the flag position 5 is left unassigned and not used in this
   specification since it was previously requested by
   [EVPN-IP-MAC-PROXY].

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [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>.

   [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>.

6.2.  Informative References

   [EVPN-IP-MAC-PROXY]
              Bickhart, R., Lin, W., Drake, J., Rabadan, J., and A. Lo,
              "Proxy IP->MAC Advertisement in EVPNs", Work in Progress,
              Internet-Draft, draft-rbickhart-evpn-ip-mac-proxy-adv-01,
              24 January 2020, <https://tools.ietf.org/html/draft-
              rbickhart-evpn-ip-mac-proxy-adv-01>.

   [EXTENDED-MOBILITY]
              Malhotra, N., Ed., Sajassi, A., Pattekar, A., Lingala, A.,
              Rabadan, J., and J. Drake, "Extended Mobility Procedures
              for EVPN-IRB", Work in Progress, Internet-Draft, draft-
              ietf-bess-evpn-irb-extended-mobility-05, 15 March 2021,
              <https://tools.ietf.org/html/draft-ietf-bess-evpn-irb-
              extended-mobility-05>.

   [IANA-BGP-EXT-COMM]
              IANA, "Border Gateway Protocol (BGP) Extended
              Communities", <https://www.iana.org/assignments/bgp-
              extended-communities>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Acknowledgments

   The authors would like to thank Ali Sajassi for his feedback.

Authors' Addresses

   Jorge Rabadan (editor)
   Nokia
   777 Middlefield Road
   Mountain View, CA 94043
   United States of America

   Email: jorge.rabadan@nokia.com

   Senthil Sathappan
   Nokia
   701 E. Middlefield Road
   Mountain View, CA 94043
   United States of America

   Email: senthil.sathappan@nokia.com

   Kiran Nagaraj
   Nokia
   701 E. Middlefield Road
   Mountain View, CA 94043
   United States of America

   Email: kiran.nagaraj@nokia.com

   Wen Lin
   Juniper Networks

   Email: wlin@juniper.net