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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std" docName="draft-ietf-bess-evpn-mh-split-horizon-11" number="9746" ipr="trust200902" consensus="true" submissionType="IETF" updates="8365, 7432">
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  <front>
    <title abbrev="EVPN MH Split Horizon Extensions">BGP EVPN Multi-Homing Multihoming
    Extensions for Split Horizon Filtering</title>
    <seriesInfo name="RFC" value="9746"/>
    <author fullname="Jorge Rabadan" initials="J." role="editor" surname="Rabadan">
      <organization>Nokia</organization>
      <address>
        <postal>
          <street>520 Almanor Avenue</street>
          <city>Sunnyvale</city>
          <region>CA</region>
          <code>94085</code>

          <country>USA</country>
          <country>United States of America</country>
        </postal>
        <email>jorge.rabadan@nokia.com</email>
      </address>
    </author>
    <author fullname="Kiran Nagaraj" initials="K." surname="Nagaraj">
      <organization>Nokia</organization>
      <address>
        <postal>
          <street>520 Almanor Avenue</street>
          <city>Sunnyvale</city>
          <region>CA</region>
          <code>94085</code>

          <country>USA</country>
          <country>United States of America</country>
        </postal>
        <email>kiran.nagaraj@nokia.com</email>
      </address>
    </author>
    <author fullname="Wen Lin" initials="W." surname="Lin">
      <organization abbrev="Juniper">Juniper Networks</organization>
      <address>
        <email>wlin@juniper.net</email>
      </address>
    </author>
    <author fullname="Ali Sajassi" initials="A." surname="Sajassi">
      <organization abbrev="Cisco">Cisco Systems, Inc.</organization>
      <address>
        <email>sajassi@cisco.com</email>
      </address>
    </author>
    <date day="16" month="August" year="2024"/>

    <workgroup>BESS Workgroup</workgroup> month="February" year="2025"/>
    <area>RTG</area>
    <workgroup>bess</workgroup>

<keyword>EVPN Multihoming</keyword>
<keyword>split-horizon filtering</keyword>
<keyword>split horizon filtering</keyword>
<keyword>local bias</keyword>
<keyword>ESI</keyword>
<keyword>encapsulations</keyword>
<keyword>SHT</keyword>

    <abstract>
      <t>Ethernet
      <t>An Ethernet Virtual Private Network (EVPN) is commonly used with
      Network Virtualization Overlay (NVO) tunnels, tunnels as well as with MPLS and
      Segment Routing (SR) tunnels. The multi-homing multihoming procedures in EVPN may
      vary based on the type of tunnel used within the EVPN Broadcast
      Domain. Specifically, there are two multi-homing Split Horizon multihoming split-horizon procedures
      designed to prevent looped frames on multi-homed multihomed Customer Edge (CE)
      devices: the ESI Ethernet Segment Identifier (ESI) Label-based procedure and
      the Local Bias local-bias procedure. The ESI Label-based
      Split Horizon split-horizon procedure is
      applied to MPLS-based tunnels, tunnels such as MPLSoUDP, MPLS over UDP (MPLSoUDP), while
      the Local Bias local-bias procedure is used for other tunnels, tunnels such as VXLAN.</t> Virtual
      eXtensible Local Area Network (VXLAN) tunnels.</t>

      <t>Current specifications do not allow operators to choose which Split
      Horizon split-horizon procedure to use for tunnel encapsulations that support both
      methods. Examples of tunnels that may support both procedures include
      MPLSoGRE,
      MPLSoUDP, GENEVE, MPLS over GRE (MPLSoGRE), Generic Network Virtualization
      Encapsulation (Geneve), and SRv6. Segment Routing over IPv6 (SRv6)
      tunnels. This document updates the EVPN
      multi-homing multihoming procedures described
      in RFC 8365 RFCs 7432 and RFC 7432, 8365, enabling operators to select the Split Horizon split-horizon
      procedure that meets their specific requirements.</t>
    </abstract>
  </front>
  <middle>
    <section anchor="sect-1" title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>Ethernet Virtual Private Networks (EVPN) (EVPNs) are commonly used with the
      following tunnel encapsulations:</t>

      <t><list style="symbols">
      <ul spacing="normal">
        <li>
          <t>Network Virtualization Overlay (NVO) tunnels, where the EVPN
          procedures are specified in <xref target="RFC8365"/>. target="RFC8365"
          format="default"/>.  MPLSoGRE <xref
          target="RFC4023"/>, target="RFC4023"
          format="default"/>, MPLSoUDP <xref target="RFC7510"/>, GENEVE target="RFC7510"
          format="default"/>, Geneve <xref
          target="RFC8926"/> target="RFC8926" format="default"/>,
          or VXLAN <xref target="RFC7348"/> target="RFC7348" format="default"/> tunnels are
          considered NVO tunnels.</t>
        </li>

        <li>
          <t>MPLS and Segment Routing with over MPLS data plane (SR-MPLS), (SR-MPLS) tunnels, where the
          relevant EVPN procedures are specified in <xref
          target="RFC7432"/>. Segment Routing with MPLS data plane target="RFC7432"
          format="default"/>. SR-MPLS tunneling is specified in <xref target="RFC8660"/>.</t>
          target="RFC8660" format="default"/>.</t>
        </li>
        <li>
          <t>Segment Routing with over IPv6 data plane (SRv6), (SRv6) tunnels, where the relevant EVPN
          procedures are specified in <xref target="RFC9252"/>. target="RFC9252"
          format="default"/>. SRv6 is specified in <xref target="RFC8402"/><xref target="RFC8754"/>.</t>
        </list></t>

      <t>Split Horizon, in target="RFC8402"
          format="default"/> and <xref target="RFC8754"
          format="default"/>.</t>
        </li>
      </ul>
      <t>In this document, the term "split horizon" follows the definition in <xref
      target="RFC7432"/>.
      target="RFC7432" format="default"/>. Split Horizon horizon refers to the EVPN
      multihoming procedure that prevents a PE Provider Edge (PE) from sending a
      frame back to a multihomed Customer Edge (CE) when that CE originated
      the frame in the first place.</t>

      <t>EVPN multihoming procedures may vary depending on the type of tunnel
      utilized within the EVPN Broadcast Domain. Specifically, there are two
      multihoming Split Horizon split-horizon procedures employed to prevent looped frames
      on multihomed CE devices: the ESI Label-based procedure and the Local
      Bias local-bias procedure.</t>

      <t>The ESI Label-based Split Horizon split-horizon procedure is used for MPLS or
      MPLS-over-X
      MPLS over X (MPLSoX) tunnels, such as MPLS-over-UDP, MPLSoUDP, and its procedures
      are detailed in <xref target="RFC7432"/>. target="RFC7432" format="default"/>. Conversely, the Local Bias local-bias
      procedure is used for IP-based tunnels, such as VXLAN tunnels, and it is
      described in <xref target="RFC8365"/>. </t> target="RFC8365" format="default"/>.</t>
      <section anchor="sect-1.1" title="Conventions numbered="true" toc="default">
        <name>Conventions and Terminology">
        <t>The Terminology</name>
        <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
        "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
    "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>",
    "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
    "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and
        "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are to be
    interpreted as described in BCP
        14 BCP&nbsp;14 <xref target="RFC2119"/> <xref
    target="RFC8174"/> when, and only when, they appear in all capitals, as
    shown here.</t>

        <t><list style="symbols">
            <t>AC: Attachment Circuit.</t>

            <t>A-D here.
        </t>

<dl spacing="normal">

            <dt>AC:</dt>
	    <dd>Attachment Circuit</dd>

            <dt>A-D per ES route: refers to the EVPN Ethernet Auto-Discovery route:</dt>
	    <dd>Auto-Discovery per ES Ethernet Segment
            route (as defined in <xref target="RFC7432"/>.</t>

            <t>Arg.FE2: refers target="RFC7432" format="default"/>).</dd>

            <dt>Arg.FE2:</dt>
	    <dd>Refers to the ESI filtering argument used for Split
            Horizon split horizon as
	    specified in <xref target="RFC9252"/>.</t>

            <t>Broadcast Domain (BD): target="RFC9252" format="default"/>.</dd>

            <dt>BD:</dt>
	    <dd>Broadcast Domain. Refers to an emulated Ethernet, such that
	    two systems on the same BD will receive each other's broadcast,
            unknown and multicast BUM
	    traffic. In this document, BD also refers to the instantiation of
	    a Broadcast Domain BD on an EVPN PE. An EVPN PE can be attached to one or multiple
	    BDs of the same tenant.</t>

            <t>BUM: Broadcast, tenant.</dd>

            <dt>BUM:</dt>
	    <dd>Broadcast, Unknown unicast Unicast, and Multicast traffic.</t>

            <t>Designated Forwarder (DF): as Multicast</dd>

	    <dt>CE:</dt><dd>Customer Edge</dd>

            <dt>DF:</dt>
	    <dd>Designated Forwarder. As defined in <xref
            target="RFC7432"/>,
            target="RFC7432" format="default"/>, an ES may be multihomed
            (attached to more than one PE). An ES may also contain multiple BDs,
            BDs of one or more EVIs.  For each such EVI, one of the PEs
            attached to the segment becomes that EVI's DF for that
            segment. Since a BD may belong to only one EVI, we can speak
            unambiguously of the BD's DF for a given
            segment.</t>

            <t>ES and ESI: Ethernet segment.</dd>

            <dt>ES:</dt>
	    <dd>Ethernet Segment</dd>

	    <dt>ESI:</dt>
	    <dd>Ethernet Segment and Ethernet Segment
            Identifier.</t>

            <t>EVI: EVPN Instance</t>

            <t>EVI-RT: EVI Identifier</dd>

            <dt>EVI:</dt>
	    <dd>EVPN Instance</dd>

            <dt>EVI-RT:</dt><dd>EVI Route Target. A Refers to a group of NVEs attached to the same
            EVI that will share the same EVI-RT.</t>

            <t>GENEVE: Generic EVI-RT.</dd>

            <dt>Geneve:</dt><dd>Generic Network Virtualization Encapsulation, Encapsulation
            <xref
            target="RFC8926"/> (<xref target="IANA-BGP-TUNNEL-ENCAP"/> target="RFC8926" format="default"/> (see tunnel type 19).</t>

            <t>MPLS 19 in <xref
            target="TUNNEL-ENCAP" format="default"/>).</dd>

            <dt>MPLS tunnels and non-MPLS NVO tunnels: refer tunnels:</dt><dd>Refers to Multi-Protocol
            Multiprotocol Label Switching (or the absence of it) Network
            Virtualization Overlay tunnels. Network Virtualization Overlay NVO tunnels use an IP
            encapsulation for overlay frames, where the source IP address
            identifies the ingress NVE and the destination IP address identifies the
            egress NVE.</t>

            <t>MPLSoUDP: Multi-Protocol NVE.</dd>

            <dt>MPLSoUDP:</dt><dd>Multiprotocol Label Switching over User
            Datagram
            Protocol, Protocol <xref target="RFC7510"/> (<xref
            target="IANA-BGP-TUNNEL-ENCAP"/> target="RFC7510" format="default"/> (see
            tunnel type 13).</t>

            <t>MPLSoGRE: Multi-Protocol 13 in <xref target="TUNNEL-ENCAP"
            format="default"/>).</dd>

            <dt>MPLSoGRE:</dt><dd>Multiprotocol Label Switching over Generic
            Network
            Encapsulation, Encapsulation <xref target="RFC4023"/> (<xref
            target="IANA-BGP-TUNNEL-ENCAP"/> target="RFC4023" format="default"/>
            (see tunnel type 11).</t>

            <t>MPLSoX: refers 11 in <xref target="TUNNEL-ENCAP"
            format="default"/>).</dd>

            <dt>MPLSoX:</dt><dd>Refers to MPLS over any IP encapsulation. Examples are
            MPLS-over-UDP encapsulation, for
            example, MPLSoUDP or MPLS-over-GRE.</t>

            <t>NVE: Network MPLSoGRE.</dd>

            <dt>NVE:</dt><dd>Network Virtualization Edge device.</t>

            <t>NVGRE: Network Edge</dd>

            <dt>NVGRE:</dt><dd>Network Virtualization Using Generic Routing
            Encapsulation,
            Encapsulation <xref target="RFC7637"/> (<xref
            target="IANA-BGP-TUNNEL-ENCAP"/> target="RFC7637" format="default"/> (see
            tunnel type 9).</t>

            <t>VXLAN: Virtual 9 in <xref target="TUNNEL-ENCAP"
            format="default"/>).</dd>

	    <dt>PE:</dt><dd>Provider Edge</dd>

	    <dt>RTs:</dt><dd>Route Targets</dd>

            <dt>VXLAN:</dt><dd>Virtual eXtensible Local Area Network, Network <xref
            target="RFC7348"/> (<xref target="IANA-BGP-TUNNEL-ENCAP"/>
            target="RFC7348" format="default"/> (see tunnel type 8).</t>

            <t>VXLAN-GPE: VXLAN 8 in <xref
            target="TUNNEL-ENCAP" format="default"/>).</dd>

            <dt>VXLAN-GPE:</dt><dd>VXLAN Generic Protocol Extension, Extension <xref
            target="I-D.ietf-nvo3-vxlan-gpe"/> (<xref
            target="IANA-BGP-TUNNEL-ENCAP"/>
            target="I-D.ietf-nvo3-vxlan-gpe" format="default"/> (see tunnel
            type 12).</t>

            <t>SHT: Split Horizon Type, it refers 12 in <xref target="TUNNEL-ENCAP"
            format="default"/>).</dd>

            <dt>SHT:</dt><dd>Split-Horizon Type. Refers to the Split Horizon split-horizon method
            that a PE intends to use and advertises in an A-D per ES
            route.</t>

            <t>SRv6: Segment
            route.</dd>

            <dt>SRv6:</dt><dd>Segment Routing with an over IPv6 data plane, (see <xref target="RFC8402"
            format="default"/> and <xref
            target="RFC8402"/><xref target="RFC8754"/>.</t>
          </list></t> target="RFC8754" format="default"/>).</dd>
	    <dt>TLV:</dt><dd>Type-Length-Value</dd>

        </dl>

        <t>This document also assumes familiarity with the terminology of
        <xref target="RFC7432"/> target="RFC7432" format="default"/> and <xref target="RFC8365"/>.</t> target="RFC8365"
        format="default"/>.</t>
      </section>
      <section title="Split Horizon numbered="true" toc="default">
        <name>Split-Horizon Filtering and Tunnel Encapsulations"> Encapsulations</name>

        <t>EVPN supports two Split Horizon Filtering split-horizon filtering mechanisms:</t>

        <t><list style="symbols">
        <ol type="1" spacing="normal">
          <li>
           <t>ESI Label based Split Horizon Label-based split-horizon filtering <xref
            target="RFC7432"/><vspace blankLines="1"/>When target="RFC7432"
           format="default"/>:</t>

	   <t>When EVPN is employed for MPLS transport tunnels, an MPLS label
	   facilitates Split
            Horizon split-horizon filtering to support All-Active
	   multihoming. The ingress
            Network Virtualization Edge (NVE) NVE device appends a label corresponding to the source Ethernet Segment Identifier (ESI ESI
       (the ESI label) during packet encapsulation.  The egress NVE verifies
       the ESI label when attempting to forward a multi-destination frame
       through a local Ethernet Segment (ES) ES interface. If the ESI label matches the site
       identifier (ESI) (the ESI) associated with that ES interface, then the packet is not forwarded. This mechanism effectively prevents forwarding loops for BUM traffic. <vspace
            blankLines="1"/>The ESI </t>

            <t>ESI Label Split Horizon split-horizon filtering should also
            be utilized with Single-Active multihoming to prevent transient
            loops for in-flight packets when the egress NVE assumes the role
            of Designated Forwarder DF for an ES.</t>

            <t>Local Bias
          </li>
          <li>
            <t>Local-bias filtering <xref target="RFC8365"/><vspace
            blankLines="1"/>Since target="RFC8365" format="default"/>:</t>
            <t>Since IP tunnels, tunnels such as VXLAN or NVGRE, NVGRE do not
            support the ESI label or any MPLS label, an alternative Split
            Horizon split-horizon filtering procedure must be implemented for All-Active
            multihoming. This mechanism, known as Local Bias, local bias, relies on the
            source IP address of the tunnel to determine whether to forward
            BUM traffic to a local Ethernet Segment (ES) ES interface at the
            egress Network Virtualization Edge (NVE). <vspace
            blankLines="1"/>In NVE.</t>
            <t>In summary and as specified in <xref
            target="RFC8365"/>, target="RFC8365"
            format="default"/>, each NVE tracks the IP address(es) of other
            NVEs with which it shares multihomed ESs. Upon receiving a BUM
            frame encapsulated in an IP tunnel, the egress NVE inspects the
            source IP address in the tunnel header, which identifies the
            ingress NVE. The egress NVE then filters out the frame on all
            local interfaces connected to ESs that are shared with the ingress
            NVE. <vspace blankLines="1"/>Due
            NVE.</t>

            <t>Due to this behavior at the egress NVE, the ingress NVE is
            required to perform local replication to all directly attached
            ESs, regardless of the Designated Forwarder DF election state, for all BUM traffic
            ingressing from the access
            Attachment Circuits (ACs). ACs. This local replication at the
            ingress NVE is the basis for the term Local Bias. <vspace
            blankLines="1"/>Local Bias "local bias".</t>

            <t>Local bias is not suitable for Single-Active multihoming, as
            the ingress NVE deactivates the ACs for which it is not the Designated Forwarder.
            DF. Consequently, local replication to non-Designated Forwarder non-DF ACs cannot occur,
            leading to transient in-flight BUM packets to be being looped back to
            the originating site by newly elected Designated Forwarder DF egress NVEs.</t>
          </list></t>
          </li>
        </ol>

        <t><xref target="RFC8365"/> target="RFC8365" format="default"/> specifies that Local Bias local bias
        is exclusively utilized for IP tunnels, while ESI Label-based Split Horizon split horizon is employed for IP-based MPLS tunnels. However, IP-based MPLS tunnels,
        tunnels such as MPLS over GRE (MPLSoGRE) MPLSoGRE or MPLS over UDP (MPLSoUDP), MPLSoUDP are also categorized as IP
        tunnels and have the potential to support both procedures. These
        tunnels are capable of carrying ESI labels and also utilize a tunnel
        IP header in which the source IP address identifies the ingress Network Virtualization Edge (NVE).</t>
        NVE.</t>

        <t>Similarly, certain IP tunnels - (those that include an identifier for
        the source Ethernet Segment (ES) ES in the tunnel header - header) may also potentially support either
        procedure. Examples of such tunnels include
        GENEVE Geneve and SRv6.:</t>

        <t><list style="symbols"> SRv6:</t>
        <ul spacing="normal">
          <li>
            <t>In a GENEVE Geneve tunnel, the source IP address identifies the
            ingress NVE therefore NVE; therefore, local bias is possible. Also, <xref
            target="I-D.ietf-bess-evpn-geneve"/> section Section 4.1
            of <xref target="I-D.ietf-bess-evpn-geneve" format="default"/>
            defines an Ethernet option TLV (Type Length Value) Type-Length-Value (TLV) to encode an
            ESI label value.</t>
          </li>
          <li>
            <t>In an SRv6 tunnel, the source IP address identifies the ingress
            NVE. By default, and as outlined in <xref target="RFC9252"/>, target="RFC9252"
            format="default"/>, the ingress PE adds specific information to
            the SRv6 packet to enable the egress PE to identify the source ES
            of the BUM packet. This information is the ESI filtering argument
            (Arg.FE2) (see <xref
            target="RFC9252"/> (section 6.1.1) <xref target="RFC8986"/>
            (section 4.12) target="RFC9252" sectionFormat="of"
            section="6.1.1"/> and <xref target="RFC8986" sectionFormat="of"
            section="4.12"/>) of the service Segment Identifier (SID) received
            on an A-D per ES route from the egress PE.</t>
          </list></t>
          </li>
        </ul>

        <t><xref target="Tunnel"/> target="Tunnel" format="default"/> presents various tunnel
        encapsulations along with their supported and default Split Horizon split-horizon
        methods. For
        GENEVE, Geneve, the default Split Horizon Type (SHT) SHT is contingent upon the
        negotiation of the Ethernet Option with the Source ID TLV. In the case
        of SRv6, the default SHT is specified as ESI Label filtering in the
        table, as its behavior is analogous to that of ESI Label filtering. In
        this document, ESI "ESI Label filtering filtering" refers to the Split Horizon split-horizon
        filtering based on the presence of a source Ethernet Segment (ES) ES
        identifier in the tunnel header.</t>

        <t>This document classifies the tunnel encapsulations used by EVPN
        into:<list style="numbers">
        into:</t>

        <ol spacing="normal" type="1"><li>
            <t>IP-based MPLS tunnels</t>

            <t>(SR-)MPLS tunnels, that is, MPLS
          </li>
          <li>
            <t>MPLS and Segment Routing with MPLS
            data plane SR-MPLS tunnels</t>
          </li>
          <li>
            <t>IP tunnels</t>
          </li>
          <li>
            <t>SRv6 tunnels</t>
          </list></t>
          </li>
        </ol>

        <t><xref target="Tunnel"/> target="Tunnel" format="default"/> lists the encapsulations
        supported by this document. Any tunnel encapsulation not listed in
        <xref
        target="Tunnel"/>) target="Tunnel" format="default"/> is out of scope. Tunnel
        encapsulations used by EVPN can be categorized into one of the four
        encapsulation groups mentioned above and support Split Horizon split-horizon
        filtering based on the following rules:</t>

        <t><list style="symbols">

        <ul spacing="normal">
          <li>
            <t>IP-based MPLS tunnels and SRv6 tunnels are capable of
            supporting both Split Horizon split-horizon filtering methods.</t>

            <t>(SR-)MPLS
          </li>
          <li>
            <t>MPLS and SR-MPLS tunnels only support ESI Label-based Split Horizon
            filtering</t> split-horizon filtering.</t>
          </li>
          <li>
            <t>IP tunnels support Local Bias Split Horizon local-bias split-horizon filtering and may
            also support ESI Label-based Split Horizon split-horizon filtering, provided
            they incorporate a mechanism to identify the source ESI in the
            header.</t>
          </list></t>

        <texttable
          </li>

        </ul>
        <table align="left" anchor="Tunnel" style="all"
                   title="Tunnel anchor="Tunnel">
          <name>Tunnel Encapsulations and Split Horizon Types">
          <ttcol>Tunnel Encapsulation</ttcol>

          <ttcol>Default Split Horizon Split-Horizon Types</name>
          <thead>
            <tr>
              <th align="left">Tunnel Encapsulation</th>
              <th align="left">Default Split-Horizon Type (SHT)</ttcol>

          <ttcol>Supports (SHT)</th>
              <th align="left">Supports Local Bias</ttcol>

          <ttcol>Supports ESI Label</ttcol>

          <c>MPLSoGRE Bias</th>
              <th align="left">Supports ESI Label</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="left">MPLSoGRE (IP-based MPLS)</c>

          <c>ESI Label filtering</c>

          <c>Yes</c>

          <c>Yes</c>

          <c>MPLSoUDP MPLS)</td>
              <td align="left">ESI Label filtering</td>
              <td align="left">Yes</td>
              <td align="left">Yes</td>
            </tr>
            <tr>
              <td align="left">MPLSoUDP (IP-based MPLS)</c>

          <c>ESI Label filtering</c>

          <c>Yes</c>

          <c>Yes</c>

          <c>(SR-)MPLS</c>

          <c>ESI Label filtering</c>

          <c>No</c>

          <c>Yes</c>

          <c>VXLAN MPLS)</td>
              <td align="left">ESI Label filtering</td>
              <td align="left">Yes</td>
              <td align="left">Yes</td>
            </tr>
            <tr>
              <td align="left">MPLS and SR-MPLS</td>
              <td align="left">ESI Label filtering</td>
              <td align="left">No</td>
              <td align="left">Yes</td>
            </tr>
            <tr>
              <td align="left">VXLAN (IP tunnels)</c>

          <c>Local Bias</c>

          <c>Yes</c>

          <c>No</c>

          <c>NVGRE tunnels)</td>
              <td align="left">Local Bias</td>
              <td align="left">Yes</td>
              <td align="left">No</td>
            </tr>
            <tr>
              <td align="left">NVGRE (IP tunnels)</c>

          <c>Local Bias</c>

          <c>Yes</c>

          <c>No</c>

          <c>VXLAN-GPE tunnels)</td>
              <td align="left">Local Bias</td>
              <td align="left">Yes</td>
              <td align="left">No</td>
            </tr>
            <tr>
              <td align="left">VXLAN-GPE (IP tunnels)</c>

          <c>Local Bias</c>

          <c>Yes</c>

          <c>No</c>

          <c>GENEVE tunnels)</td>
              <td align="left">Local Bias</td>
              <td align="left">Yes</td>
              <td align="left">No</td>
            </tr>
            <tr>
              <td align="left">Geneve (IP tunnels)</c>

          <c>Local tunnels)</td>
              <td align="left">Local Bias (no (if no ESI Lb), ESI Label (if ESI lb)</c>

          <c>Yes</c>

          <c>Yes</c>

          <c>SRv6</c>

          <c>ESI Label filtering</c>

          <c>Yes</c>

          <c>Yes</c>
        </texttable> lb)</td>
              <td align="left">Yes</td>
              <td align="left">Yes</td>
            </tr>
            <tr>
              <td align="left">SRv6</td>
              <td align="left">ESI Label filtering</td>
              <td align="left">Yes</td>
              <td align="left">Yes</td>
            </tr>
          </tbody>
        </table>
        <t>The ESI Label method is applicable for both All-Active and
        Single-Active configurations, whereas the Local Bias local-bias method is
        suitable only for All-Active configurations. Moreover, the ESI Label
        method is effective across different network domains, while Local Bias local bias
        is constrained to networks where there is no change in the next hop
        between the NVEs attached to the same ES. Nonetheless, some operators
        favor the Local Bias local-bias method due to its simplification of the
        encapsulation process, reduced resource consumption on NVEs, and the
        fact that the ingress NVE always forwards traffic locally to other
        interfaces, thereby decreasing the delay in reaching multihomed
        hosts.</t>
        <t>This document extends the EVPN multihoming procedures to allow
        operators to select the preferred Split Horizon split-horizon method for a given NVO
        tunnel according to their specific requirements. The choice between
        Local Bias
        local bias and ESI Label Split Horizon split horizon is now allowed (by
        configuration) for tunnel encapsulations that support both methods,
        and this selection is advertised along with the EVPN A-D per ES route.
        IP tunnels that do not support both methods, such as VXLAN or NVGRE,
        will continue to adhere to the procedures specified in <xref
        target="RFC8365"/>.
        target="RFC8365" format="default"/>. Note that this document does not
        modify the Local
        Bias local bias or the ESI Label Split Horizon split-horizon procedures
        themselves, just focuses on the signaling and selection of the Split Horizon split-horizon method to apply by the multihomed NVEs. </t>
      </section>
    </section>
    <section anchor="sect-2" title="BGP numbered="true" toc="default">
      <name>BGP EVPN Extensions"> Extensions</name>
      <t>Extensions to EVPN are required to enable NVEs to advertise their
      preferred Split Horizon split-horizon method for a given ES. <xref
      target="esi-label-extended-community"/>
      target="esi-label-extended-community" format="default"/> illustrates the
      ESI Label extended community (<xref target="RFC7432"/> Section 7.5), target="RFC7432" sectionFormat="of"
      section="7.5"/>), which is consistently advertised alongside the EVPN
      A-D per ES route. All NVEs connected to an ES advertise an A-D per ES
      route for that ES, including the extended community, which communicates
      information regarding the multihoming mode (either All-Active or
      Single-Active) and, if necessary, specifies the ESI Label to be
      utilized.</t>
      <figure anchor="esi-label-extended-community"
              title="ESI anchor="esi-label-extended-community">
        <name>ESI Label extended community">
        <artwork><![CDATA[ Extended Community</name>
        <artwork name="" type="" align="left" alt=""><![CDATA[
                     1                   2                   3
 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=0x01 | Flags(1 octet)|  Reserved=0   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Reserved=0   |          ESI Label                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
      </figure>

      <t><xref target="RFC7432"/> target="RFC7432" format="default"/> defines the low-order bit
      of the Flags octet (bit 0) as the "Single-Active" bit:</t>

      <t><list style="symbols">
      <ul spacing="normal">
        <li>
          <t>A value of 0 means that the multihomed ES is operating in
          All-Active multihoming redundancy mode.</t>
        </li>
        <li>
          <t>A value of 1 means that the multihomed ES is operating in
          Single-Active multihoming redundancy mode.</t>
        </list><xref target="sect-5"/>
        </li>
      </ul>
      <t><xref target="sect-5" format="default"/> establishes a registry for the Flags
      octet, designating the "Single-Active" bit as the low-order bit of the
      newly defined multihoming redundancy mode Multihoming Redundancy Mode field.</t>
      <section anchor="sect-2.1" title="The Split Horizon Type"> numbered="true" toc="default">
        <name>The Split-Horizon Type</name>
        <t><xref target="RFC8365"/> target="RFC8365" format="default"/> does not include any
        explicit indication regarding the Split Horizon split-horizon method in the A-D per Ethernet Segment
        (ES)
        ES route. In this document, the Split Horizon split-horizon procedure defined in
        <xref target="RFC8365"/> (section 8.3.1) target="RFC8365" sectionFormat="of" section="8.3.1"/> is
        considered the default behavior, presuming that Local Bias local bias is employed
        exclusively for IP tunnels, while ESI Label-based Split Horizon split horizon is
        used for IP-based MPLS tunnels. This document specifies that the two
        high-order bits in the Flags octet (bits 6 and 7) constitute the "Split Horizon "Split-Horizon Type" (SHT) or "SHT"
        field, where:</t>

        <figure>
          <artwork><![CDATA[

        <artwork name="" type="" align="left" alt="">
<![CDATA[
 7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|SHT|       |RED|
+-+-+-+-+-+-+-+-+
RED = "Multihoming Redundancy Mode" field (section 5) (see Table 2)

SHT bit 7 6
-----------
        0 0  --> Default SHT
                 Backwards compatible with [RFC8365] and [RFC7432]
        0 1  --> Local Bias
        1 0  --> ESI Label based Label-based filtering
        1 1  --> reserved for future use Unassigned
]]></artwork>
        </figure>

        <t><list style="symbols">
        <ul spacing="normal">
          <li>
            <t>SHT = 00 is backwards compatible with <xref target="RFC8365"/> target="RFC8365" format="default"/>
            and <xref target="RFC7432"/>, target="RFC7432" format="default"/>, and indicates that the advertising
            NVE intends to use the default or built-in SHT. The default SHT is
            shown in <xref target="Tunnel"/> target="Tunnel" format="default"/> for each encapsulation. An egress
            NVE that follows the <xref target="RFC8365"/> target="RFC8365" format="default"/> behavior and does
            not support this specification will ignore the SHT bits (which is
            equivalent to process processing them as a value of 00).</t>
          </li>
          <li>
            <t>SHT = 01 indicates that the advertising NVE intends to use
            Local Bias
            local-bias procedures in the ES for which the AD per-ES route is
            advertised.</t>
          </li>
          <li>
            <t>SHT = 10 indicates that the advertising NVE intends to use the
            ESI Label based Split Horizon Label-based split-horizon method procedures in the ES for
            which the AD per-ES route is advertised.</t>
          </li>
          <li>
            <t>SHT = 11 is a reserved value, for future use.</t>
          </list></t>
          </li>
        </ul>
      </section>
      <section anchor="sect-2.2"
               title="Use numbered="true" toc="default">
        <name>Use of the Split Horizon Split-Horizon Type In in A-D Per per ES Routes"> Routes</name>
        <t>The following behavior is observed:</t>

        <t><list style="symbols">
        <ul spacing="normal">
          <li>
            <t>An SHT value of 01 or 10 MUST NOT <bcp14>MUST NOT</bcp14> be used with
            encapsulations that support only one SHT in <xref target="Tunnel"/>, target="Tunnel"
            format="default"/>, and MAY <bcp14>MAY</bcp14> be used by
            encapsulations that support the two SHTs in <xref
            target="Tunnel"/>.</t> target="Tunnel"
            format="default"/>.</t>
          </li>
          <li>
            <t>An SHT value different from than 00 expresses the intent to use a
            specific Split Horizon split-horizon method, but does not reflect the actual
            operational SHT used by the advertising NVE, unless all the NVEs
            attached to the ES advertise the same SHT.</t>
          </li>
          <li>
            <t>In case of an inconsistency in the SHT value advertised by the
            NVEs attached to the same ES for a given EVI, all the NVEs MUST <bcp14>MUST</bcp14>
            revert to the behavior in <xref target="RFC8365"/> behavior, target="RFC8365" format="default"/> and use the
            default SHT in <xref target="Tunnel"/>, target="Tunnel" format="default"/>, irrespective of the
            advertised SHT.</t>
          </li>
          <li>
            <t>An SHT different from than 00 MUST NOT <bcp14>MUST NOT</bcp14> be set if the Single-Active "Single-Active"
            bit is set. A received A-D per ES route where Single-Active the "Single-Active" and
            SHT bits are different from than zero MUST <bcp14>MUST</bcp14> follow the treat-as-withdraw
            behavior in <xref target="RFC7606"/>.</t> target="RFC7606" format="default"/>.</t>
          </li>
          <li>
            <t>The SHT MUST <bcp14>MUST</bcp14> have the same value in each Ethernet A-D per ES
            route that an NVE advertises for a given ES and a given
            encapsulation (see <xref target="sect-3"/> target="sect-3" format="default"/> for NVEs supporting
            multiple encapsulations).</t>
          </list></t>
          </li>
        </ul>
        <t>As an example, egress NVEs that support IP-based MPLS tunnels, such
        as MPLSoGRE or MPLSoUDP, will advertise A-D per ES routes for the ES
        along with the BGP Encapsulation extended community, Extended Community, as defined in
        <xref target="RFC9012"/>. target="RFC9012" format="default"/>. This extended community indicates the
        encapsulation type (MPLSoGRE or MPLSoUDP) and may use the SHT value of
        01 or 10 to signify the intent to use Local Bias local bias or the ESI Label,
        respectively.</t>

        <t>An egress NVE MUST NOT <bcp14>MUST NOT</bcp14> use an SHT value other than 00 when
        advertising an A-D per ES route with <xref target="RFC9012"/> Tunnel the following tunnel encapsulation types of from <xref target="RFC9012" format="default"/>:
        VXLAN (type 8), NVGRE (type 9), MPLS (type 10),
        or no BGP tunnel encapsulation extended community Tunnel Encapsulation Extended Community at all. In all these
        cases, it is presumed that there is no choice for the Split Horizon split-horizon
        method; therefore, the SHT value MUST <bcp14>MUST</bcp14> be set to 00. If a route with
        any of the mentioned encapsulation options is received and has an SHT
        value different from than 00, it SHOULD <bcp14>SHOULD</bcp14> apply the treat-as-withdraw
        behavior, per <xref target="RFC7606"/>.</t> target="RFC7606" format="default"/>.</t>

<t>An egress NVE advertising A-D per ES route(s) for an ES with GENEVE Geneve
        encapsulation (<xref target="RFC9012"/>, Tunnel (tunnel encapsulation type 19, 19 in  the BGP Tunnel Encapsulation attribute <xref target="I-D.ietf-bess-evpn-geneve"/>) MAY target="RFC9012" format="default"/>) <bcp14>MAY</bcp14> use an SHT value of 01 or 10. A
        value of 01 indicates the intent to use Local Bias, local bias, regardless of the
        presence of an Ethernet option TLV with a non-zero Source-ID, as
        described in <xref target="I-D.ietf-bess-evpn-geneve"/>. target="I-D.ietf-bess-evpn-geneve"
        format="default"/>.  A value of 10 indicates the intent to use ESI
        Label-based Split
        Horizon, split horizon, and it is only valid if an Ethernet option
        TLV with a non-zero Source-ID is present. A value of 00 indicates the
        default behavior outlined in <xref target="Tunnel"/>, target="Tunnel" format="default"/>,
        which is to use Local Bias if: a)
        no ESI-Label local bias if:</t>

	<ol type="a">
	  <li>no ESI Label is present in the Ethernet option TLV, or b) if there </li>
	  <li>there is no Ethernet option TLV. Otherwise, TLV.</li>
	</ol>
	<t>Otherwise, the ESI Label Split Horizon split-horizon method is applied.</t>
        <t>These procedures assume a single encapsulation supported in the
        egress NVE. <xref target="sect-3"/> target="sect-3" format="default"/> describes additional procedures
        for NVEs supporting multiple encapsulations.</t>
      </section>
      <section anchor="sect-2.3" title="ESI numbered="true" toc="default">
        <name>The ESI Label Value In in A-D Per per ES Routes"> Routes</name>
        <t>This document also updates <xref target="RFC8365"/> target="RFC8365" format="default"/> regarding the
        value that is advertised in the ESI Label field of the ESI Label
        extended community, as follows:</t>

        <t><list style="symbols">
        <ul spacing="normal">
          <li>
            <t>The A-D per ES route(s) for an ES MAY <bcp14>MAY</bcp14> have an ESI Label value
            of zero if the SHT value is 01. <xref target="sect-2.2"/> target="sect-2.2" format="default"/>
            specifies the scenarios where the SHT can be 01. An ESI Label
            value of zero eliminates the need to allocate labels in cases
            where they are not utilized, such as in the Local Bias local-bias method.</t>
          </li>
          <li>
            <t>The A-D per ES route(s) for an ES MAY <bcp14>MAY</bcp14> have an ESI Label value
            of zero for VXLAN or NVGRE encapsulations.</t>
          </list></t>
          </li>
        </ul>
      </section>

      <section anchor="sect-2.4"
               title="Backwards numbered="true" toc="default">
        <name>Backwards Compatibility With RFC8365 NVEs"> with NVEs from RFC 8365</name>
        <t>As discussed in <xref target="sect-2.2"/> target="sect-2.2" format="default"/>, this
        specification is backwards compatible with the Split Horizon split-horizon filtering
        behavior in <xref target="RFC8365"/> target="RFC8365" format="default"/> and a
        non-upgraded NVE can be attached to the same ES as other NVEs
        supporting this specification.</t>
        <t>An NVE maintains an administrative SHT value for an Ethernet
        Segment (ES), ES, which is
        advertised alongside the A-D per ES route, and an operational SHT
        value, which is the one actually used regardless of what the NVE has
        advertised. The administrative SHT matches the operational SHT if all
        the NVEs attached to the ES have the same administrative SHT.</t>
        <t>This document assumes that an implementation of <xref
        target="RFC7432"/> target="RFC7432" format="default"/> or <xref target="RFC8365"/> target="RFC8365" format="default"/> that does not support
        the specifications in this document will ignore the values of all the
        Flags in the ESI Label extended community, except for the
        Single-Active
        "Single-Active" bit. Based on this assumption, a non-upgraded NVE will
        disregard any SHT value other than 00. If an upgraded NVE receives at
        least one A-D per ES route for the ES with an SHT value of 00, it MUST <bcp14>MUST</bcp14>
        revert its operational SHT to the default Split Horizon split-horizon method, as
        described in <xref target="Tunnel"/>, target="Tunnel" format="default"/>, irrespective of its
        administrative SHT.</t>

        <t>For instance, consider an NVE attached to ES N that receives two
        A-D per ES routes for N from different NVEs, NVE1 and NVE2. If the
        route from NVE1 has an SHT value of 00 and the one from NVE2 has an
        SHT value of 01, the NVE MUST <bcp14>MUST</bcp14> use the default Split Horizon split-horizon method
        specified in <xref target="Tunnel"/> target="Tunnel" format="default"/> as its operational SHT,
        regardless of its administrative SHT.</t>
        <t>All NVEs attached to an ES with an operational SHT value of 10 MUST <bcp14>MUST</bcp14>
        advertise a valid, non-zero ESI Label. If the operational SHT value is
        01, the ESI Label MAY <bcp14>MAY</bcp14> be zero. If the operational SHT value is 00, the
        ESI Label may be zero only if the default encapsulation supports Local
        Bias local
        bias exclusively, and the NVEs do not require the presence of a valid,
        non-zero ESI Label.</t>
        <t>If an NVE changes its operational SHT value from 01 (Local Bias) to
        00 (Default SHT) due to the presence of a new non-upgraded NVE in the
        ES, and it previously advertised a zero ESI Label, it MUST <bcp14>MUST</bcp14> send an
        update with a valid, non-zero ESI Label, unless all the non-upgraded
        NVEs in the ES support only Local Bias. local bias. For example, consider NVE1 and
        NVE2 using MPLSoUDP as encapsulation, attached to the same Ethernet
        Segment ES1, and advertising an SHT value of 01 (Local Bias) with a
        zero ESI Label value. Suppose NVE3, which does not support this
        specification, joins ES1 and advertises an SHT value of 00 (default).
        Upon receiving NVE3's A-D per ES route, NVE1 and NVE2 MUST <bcp14>MUST</bcp14> update
        their A-D per ES routes for ES1 to include a valid, non-zero ESI Label
        value. The assumption here is that NVE3 only supports the default ESI
        Label-based Split Horizon split-horizon filtering.</t>
      </section>
    </section>
    <section anchor="sect-3"
             title="Procedures numbered="true" toc="default">
      <name>Procedures for NVEs Supporting Multiple Encapsulations"> Encapsulations</name>
      <t>As specified in <xref target="RFC8365"/>, target="RFC8365" format="default"/>, an NVE
      that supports multiple data plane encapsulations (e.g., VXLAN, NVGRE,
      MPLS, MPLSoUDP,
      GENEVE) Geneve) must indicate all supported encapsulations using
      BGP Encapsulation extended communities as defined in <xref
      target="RFC9012"/>
      target="RFC9012" format="default"/> for all EVPN routes. This section
      provides clarification on the multihoming Split Horizon split-horizon behavior for
      NVEs that advertise and receive multiple BGP Encapsulation extended
      communities along with the A-D per ES routes. This section uses the
      notation {x, y} (more than two encapsulations is possible too) to denote
      the encapsulations advertised in BGP Encapsulation extended communities
      (or the BGP Tunnel Encapsulation Attribute), where x and y represent
      different encapsulation values. When GENEVE Geneve is one of the
      encapsulations, the tunnel type is indicated in either a BGP
      Encapsulation extended community or a BGP Tunnel Encapsulation Attribute. </t>
      Attribute.</t>

      <t>It is important to note that an NVE MAY <bcp14>MAY</bcp14> advertise multiple A-D per ES
      routes for the same ES, rather than a single route, with each route
      conveying a set of Route Targets (RT). (RTs). The total set of Route Targets RTs
      associated with a given ES is referred to as the RT-set for that ES.
      Each of the EVIs represented in the RT-set will have its RT included in
      one, and only one, A-D per ES route for the ES. When multiple A-D per ES
      routes are advertised for the same ES, each route must have a distinct
      Route Distinguisher.</t>

      <t>As per <xref target="RFC8365"/>, target="RFC8365" format="default"/>, an NVE that advertises multiple
      encapsulations in the A-D per ES route(s) for an ES MUST <bcp14>MUST</bcp14> advertise
      encapsulations that use the same Split Horizon split-horizon filtering method in the
      same route. For example:</t>

      <t><list style="symbols">
      <ul spacing="normal">
        <li>
          <t>An A-D per ES route for ES-x may be advertised with {VXLAN,
          NVGRE} encapsulations.</t>
        </li>
        <li>
          <t>An A-D per ES route for ES-y may be advertised with {MPLS,
          MPLSoUDP, MPLSoGRE} encapsulations (or a subset).</t>

          <t>But
        </li>
        <li>
          <t>However, an A-D per ES route for ES-z MUST NOT <bcp14>MUST NOT</bcp14> be advertised with
          {MPLS, VXLAN} encapsulations.</t>
        </list></t>
        </li>
      </ul>
      <t>This document extends the described behavior as follows:</t>

      <t><list style="letters">
      <ol spacing="normal" type="a"><li>
          <t>An A-D per ES route for ES-x may be advertised with multiple
          encapsulations, some of which support a single Split Horizon split-horizon method.
          In this case, the Split Horizon Type (SHT) SHT value MUST <bcp14>MUST</bcp14> be 00. For instance,
          encapsulations such as {VXLAN, NVGRE}, {VXLAN, GENEVE}, Geneve}, or {MPLS,
          MPLSoGRE, MPLSoUDP} can be advertised in an A-D per ES route.  In
          all these cases, the SHT value MUST <bcp14>MUST</bcp14> be 00 and the behavior
          treat-as-withdraw behavior <xref target="RFC7606"/> target="RFC7606" format="default"/>
          is applied in case of any other value.</t>
        </li>
        <li>
          <t>An A-D per ES route for ES-y may be advertised with multiple
          encapsulations that all support both Split Horizon split-horizon methods. In this
          case, the SHT value MAY <bcp14>MAY</bcp14> be 01 if the preferred method is Local Bias, local bias,
          or 10 if the ESI Label-based method is desired. For example,
          encapsulations such as {MPLSoGRE, MPLSoUDP, GENEVE} Geneve} (or a subset)
          MAY
          <bcp14>MAY</bcp14> be advertised in an A-D per ES route with an SHT value of 01.
          The ESI Label value in this case MAY <bcp14>MAY</bcp14> be zero.</t>
        </li>
        <li>
          <t>If ES-z with an RT-set composed of (RT1, RT2, RT3.. RTn) supports
          multiple encapsulations requiring different Split Horizon split-horizon methods, a
          distinct A-D per ES route (or group of routes) per Split Horizon split-horizon
          method MUST <bcp14>MUST</bcp14> be advertised. For example, consider an ES-z with n
          Route Targets (RTs) where:<list style="symbols"> RTs, where:</t>
          <ul spacing="normal">
            <li>
              <t>the EVIs corresponding to (RT1..RTi) support VXLAN,</t>
            </li>
            <li>
              <t>the ones for (RTi+1..RTm) (with i&lt;m) support MPLSoUDP with
              Local Bias,</t>

              <t>and the
              local bias, and</t>
            </li>
            <li>
              <t>the ones for (RTm+1..RTn) (with m&lt;n) support GENEVE Geneve
              with ESI Label based Split Horizon.</t>
            </list>In Label-based split horizon.</t>
            </li>
          </ul>
          <t>In this scenario, three groups of A-D per ES routes MUST <bcp14>MUST</bcp14> be
          advertised for ES-z:<list style="symbols"> ES-z:</t>
          <ul spacing="normal">
            <li>
              <t>A-D per ES route group 1, including (RT1..RTi), (RT1..RTi) with
              encapsulation {VXLAN}, {VXLAN} and an SHT value of 00. The ESI Label MAY <bcp14>MAY</bcp14>
              be zero.</t>
            </li>
            <li>
              <t>A-D per ES route group 2, including (RTi+1..RTm), (RTi+1..RTm) with
              encapsulation {MPLSoUDP}, {MPLSoUDP} and an SHT value of 01. The ESI Label
              MAY
              <bcp14>MAY</bcp14> be zero.</t>
            </li>
            <li>
              <t>A-D per ES route group 3, including (RTm+1..RTn), (RTm+1..RTn) with
              encapsulation {GENEVE}, {Geneve} and an SHT value of 10. The ESI Label
              MUST
              <bcp14>MUST</bcp14> have a valid, non-zero value, and the Ethernet option as
              defined in <xref target="RFC8926"/> MUST target="RFC8926" format="default"/> <bcp14>MUST</bcp14> be advertised.</t>
            </list></t>
        </list></t>
            </li>
          </ul>
        </li>
      </ol>
      <t>As per <xref target="RFC8365"/>, target="RFC8365" format="default"/>, it is the responsibility of the
      operator of a given EVI to ensure that all of the NVEs within that EVI
      support a common encapsulation. Failure to meet this condition may
      result in service disruption or failure.</t>
    </section>
    <section title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>All the security considerations described in <xref target="RFC7432"/> target="RFC7432" format="default"/>
      are applicable to this document.</t>

      <t>Additionally, this document modifies the procedures for Split Horizon split-horizon
      filtering as outlined in <xref target="RFC8365"/>, target="RFC8365" format="default"/>,
      offering operators a choice between Local Bias local bias and ESI Label-based
      filtering for tunnels that support both methods. Misconfiguration of the
      desired Split Horizon Type
      (SHT) SHT could lead to forwarding behaviors that differ from the
      intended configuration. Apart from this risk, this document describes
      procedures to ensure that all Provider Edge (PE) PE devices or Network Virtualization
      Edges (NVEs) NVEs connected to the same Ethernet Segment (ES)
      ES agree on a common SHT method, with a fallback to a default behavior
      in case of a mismatch in the SHT bits being advertised by any two PEs or
      NVEs in the
      Ethernet Segment. ES. Consequently, unauthorized changes to the SHT
      configuration by an attacker on a single PE or NVE of the Ethernet
      Segment ES should not
      cause traffic disruption (as long as the SHT value is valid as per this
      document) but may result in alterations to forwarding behavior.</t>
    </section>
    <section anchor="sect-5" title="IANA Considerations">
      <t>This document creates a registry called numbered="true" toc="default">
      <name>IANA Considerations</name>

      <t>Per this document, IANA has created the "EVPN ESI Label Extended Community Flags" registry for the 1-octet Flags field in the ESI Label Extended
      Community <xref target="RFC7432"/>, target="RFC7432" format="default"/>, as follows:</t>

      <texttable>
        <ttcol>Bit Position</ttcol>

        <ttcol>Name</ttcol>

        <ttcol>Reference</ttcol>

        <c>0-1</c>

        <c>Multihoming
      <table align="center">
        <thead>
          <tr>
            <th align="left">Bit Position</th>
            <th align="left">Name</th>
            <th align="left">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="left">0-1</td>
            <td align="left">Multihoming Redundancy Mode</c>

        <c><xref target="RFC7432"/></c>

        <c>2-5</c>

        <c>Unassigned</c>

        <c/>

        <c>6-7</c>

        <c>Split Horizon Type</c>

        <c>This Document</c>
      </texttable>

      <t>This document Mode</td>
            <td align="left">
              <xref target="RFC7432" format="default"/></td>
          </tr>
          <tr>
            <td align="left">2-5</td>
            <td align="left">Unassigned</td>
            <td align="left"/>
          </tr>
          <tr>
            <td align="left">6-7</td>
            <td align="left">Split-Horizon Type</td>
            <td align="left">RFC 9746</td>
          </tr>
        </tbody>
      </table>

      <t>IANA has also creates a registry for created the "Multihoming Redundancy
      Mode" registry for the related field of the EVPN "EVPN ESI Label Extended Community Flags. This Flags". The registry is called "Multihoming has been populated with the following initial values:
      </t>
      <table align="center">
        <thead>
          <tr>
            <th align="left">Value</th>
            <th align="left">Multihoming Redundancy Mode" and is initialized as
      follows:</t>

      <texttable>
        <ttcol>Value</ttcol>

        <ttcol>Multihoming redundancy mode</ttcol>

        <ttcol>Reference</ttcol>

        <c>00</c>

        <c>All-Active mode</c>

        <c><xref target="RFC7432"/></c>

        <c>01</c>

        <c>Single-Active mode</c>

        <c><xref target="RFC7432"/></c>

        <c>10</c>

        <c>Unassigned</c>

        <c/>

        <c>11</c>

        <c>Unassigned</c>

        <c/>
      </texttable> Mode</th>
            <th align="left">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="left">00</td>
            <td align="left">All-Active</td>
            <td align="left">
              <xref target="RFC7432" format="default"/></td>
          </tr>
          <tr>
            <td align="left">01</td>
            <td align="left">Single-Active</td>
            <td align="left">
              <xref target="RFC7432" format="default"/></td>
          </tr>
          <tr>
            <td align="left">10</td>
            <td align="left">Unassigned</td>
            <td align="left"/>
          </tr>
          <tr>
            <td align="left">11</td>
            <td align="left">Unassigned</td>
            <td align="left"/>
          </tr>
        </tbody>
      </table>
      <t>Finally, a third IANA has created the "Split-Horizon Type" registry for the "Split Horizon Type" related field of the
      EVPN
      "EVPN ESI Label Extended Community Flags is created by this document too.
      This Flags". The registry is called "Split Horizon Type" and is initialized as
      follows:</t>

      <texttable>
        <ttcol>Value</ttcol>

        <ttcol>Split Horizon has been populated with the following initial values:</t>
      <table align="center">
        <thead>
          <tr>
            <th align="left">Value</th>
            <th align="left">Split-Horizon Type value</ttcol>

        <ttcol>Reference</ttcol>

        <c>00</c>

        <c>Default SHT</c>

        <c>This document</c>

        <c>01</c>

        <c>Local Bias</c>

        <c>This document</c>

        <c>10</c>

        <c>ESI Label based filtering</c>

        <c>This document</c>

        <c>11</c>

        <c>Unassigned</c>

        <c/>
      </texttable> Value</th>
            <th align="left">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="left">00</td>
            <td align="left">Default SHT</td>
            <td align="left">RFC 9746</td>
          </tr>
          <tr>
            <td align="left">01</td>
            <td align="left">Local Bias</td>
            <td align="left">RFC 9746</td>
          </tr>
          <tr>
            <td align="left">10</td>
            <td align="left">ESI Label-based filtering</td>
            <td align="left">RFC 9746</td>
          </tr>
          <tr>
            <td align="left">11</td>
            <td align="left">Unassigned</td>
            <td align="left"/>
          </tr>
        </tbody>
      </table>
      <t>New registrations in the "EVPN ESI Label Extended Community Flags",
      "Multihoming Redundancy Mode", and "Split Horizon "Split-Horizon Type" registries will
      be made through the "IETF Review" procedure defined in <xref
      target="RFC8126"/>.
      target="RFC8126" format="default"/>. These registries are located in the
      "Border Gateway Protocol (BGP) Extended Communities" registry group.</t>
    </section>

  </middle>
  <back>
    <displayreference target="I-D.ietf-bess-evpn-geneve" to="EVPN-GENEVE"/>
    <displayreference target="I-D.ietf-nvo3-vxlan-gpe" to="VXLAN-GPE"/>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7432.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8365.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9252.xml"/>
      </references>
      <references>
        <name>Informative References</name>

<!-- [I-D.ietf-bess-evpn-geneve] IESG state: I-D Exists as of 09/05/24-->
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.ietf-bess-evpn-geneve.xml"/>

        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7348.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4023.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7637.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7510.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8926.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9012.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7606.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8660.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8986.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8402.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8754.xml"/>

<!-- [I-D.ietf-nvo3-vxlan-gpe] IESG state: Expired as of 09/05/24 -->
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-ietf-nvo3-vxlan-gpe.xml"/>

<!-- [IANA-BGP-TUNNEL-ENCAP] -->
        <reference anchor="TUNNEL-ENCAP" target="https://www.iana.org/assignments/bgp-tunnel-encapsulation">
          <front>
            <title>Border Gateway Protocol (BGP) Tunnel Encapsulation</title>
            <author>
              <organization>IANA</organization>
            </author>
            <date/>
          </front>
        </reference>
      </references>
    </references>
    <section anchor="sect-6" title="Acknowledgments"> anchor="Acknowledgments" numbered="false" toc="default">
      <name>Acknowledgments</name>
      <t>The authors would like to thank Anoop Ghanwani, Gyan Mishra and
      Jeffrey Zhang <contact fullname="Anoop Ghanwani"/>,
      <contact fullname="Gyan Mishra"/>, and <contact fullname="Jeffrey
      Zhang"/> for their review and useful comments. Thanks to Gunter van <contact
      fullname="Gunter Van de Velde Velde"/> and Sue Hares <contact fullname="Sue Hares"/> as
      well, for their thorough review.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      &RFC2119;

      &RFC8174;

      &RFC8126;

      &RFC7432;

      &RFC8365;

      &RFC9252;
    </references>

    <references title="Informative References">
      &I-D.ietf-bess-evpn-geneve;

      &RFC7348;

      &RFC4023;

      &RFC7637;

      &RFC7510;

      &RFC8926;

      &RFC9012;

      &RFC7606;

      &RFC8660;

      &RFC8986;

      &RFC8402;

      &RFC8754;

      &I-D.ietf-nvo3-vxlan-gpe;

      <reference anchor="IANA-BGP-TUNNEL-ENCAP"
                 target="https://www.iana.org/assignments/bgp-tunnel-encapsulation/bgp-tunnel-encapsulation.xhtml#tunnel-types">
        <front>
          <title>Border Gateway Protocol (BGP) Tunnel Encapsulation</title>

          <author fullname="IANA">
            <organization/>
          </author>

          <date/>
        </front>
      </reference>
    </references>

  </back>
</rfc>