Ethernet-Tree (E-Tree) Support in Ethernet VPN (EVPN) and Provider Backbone Bridging EVPN (PBB-EVPN)
RFC 8317

Internet Engineering Task Force (IETF)                   A. Sajassi, Ed.
Request for Comments: 8317                                      S. Salam
Updates: 7385                                                      Cisco
Category: Standards Track                                       J. Drake
ISSN: 2070-1721                                                  Juniper
                                                               J. Uttaro
                                                                     ATT
                                                              S. Boutros
                                                                  VMware
                                                              J. Rabadan
                                                                   Nokia
                                                            January 2018

       Ethernet-Tree (E-Tree) Support in Ethernet VPN (EVPN) and
               Provider Backbone Bridging EVPN (PBB-EVPN)

Abstract

   The MEF Forum (MEF) has defined a rooted-multipoint Ethernet service
   known as Ethernet-Tree (E-Tree).  A solution framework for supporting
   this service in MPLS networks is described in RFC 7387, "A Framework
   for Ethernet-Tree (E-Tree) Service over a Multiprotocol Label
   Switching (MPLS) Network".  This document discusses how those
   functional requirements can be met with a solution based on RFC 7432,
   "BGP MPLS Based Ethernet VPN (EVPN)", with some extensions and a
   description of how such a solution can offer a more efficient
   implementation of these functions than that of RFC 7796,
   "Ethernet-Tree (E-Tree) Support in Virtual Private LAN Service
   (VPLS)".  This document makes use of the most significant bit of the
   Tunnel Type field (in the P-Multicast Service Interface (PMSI) Tunnel
   attribute) governed by the IANA registry created by RFC 7385; hence,
   it updates RFC 7385 accordingly.

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/rfc8317.

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Copyright Notice

   Copyright (c) 2018 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Specification of Requirements . . . . . . . . . . . . . .   5
     2.2.  Terms and Abbreviations . . . . . . . . . . . . . . . . .   5
   3.  E-Tree Scenarios  . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Scenario 1: Leaf or Root Site(s) per PE . . . . . . . . .   6
     3.2.  Scenario 2: Leaf or Root Site(s) per AC . . . . . . . . .   7
     3.3.  Scenario 3: Leaf or Root Site(s) per MAC Address  . . . .   8
   4.  Operation for EVPN  . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Known Unicast Traffic . . . . . . . . . . . . . . . . . .   9
     4.2.  BUM Traffic . . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  BUM Traffic Originated from a Single-Homed Site on a
               Leaf AC . . . . . . . . . . . . . . . . . . . . . . .  11
       4.2.2.  BUM Traffic Originated from a Single-Homed Site on a
               Root AC . . . . . . . . . . . . . . . . . . . . . . .  11
       4.2.3.  BUM Traffic Originated from a Multihomed Site on a
               Leaf AC . . . . . . . . . . . . . . . . . . . . . . .  12
       4.2.4.  BUM Traffic Originated from a Multihomed Site on a
               Root AC . . . . . . . . . . . . . . . . . . . . . . .  12
     4.3.  E-Tree Traffic Flows for EVPN . . . . . . . . . . . . . .  12
       4.3.1.  E-Tree with MAC Learning  . . . . . . . . . . . . . .  13
       4.3.2.  E-Tree without MAC Learning . . . . . . . . . . . . .  14
   5.  Operation for PBB-EVPN  . . . . . . . . . . . . . . . . . . .  14
     5.1.  Known Unicast Traffic . . . . . . . . . . . . . . . . . .  15
     5.2.  BUM Traffic . . . . . . . . . . . . . . . . . . . . . . .  15
     5.3.  E-Tree without MAC Learning . . . . . . . . . . . . . . .  16
   6.  BGP Encoding  . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  E-Tree Extended Community . . . . . . . . . . . . . . . .  16
     6.2.  PMSI Tunnel Attribute . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Considerations for PMSI Tunnel Types  . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Appendix A.  Multiple Bridge Tables per E-Tree Service Instance .  22
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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1.  Introduction

   The MEF Forum (MEF) has defined a rooted-multipoint Ethernet service
   known as Ethernet-Tree (E-Tree) [MEF6.1].  In an E-Tree service, a
   customer site that is typically represented by an Attachment Circuit
   (AC) (e.g., an 802.1Q VLAN tag [IEEE.802.1Q]), is labeled as either a
   Root or a Leaf site.  A customer site may also be represented by a
   Media Access Control (MAC) address along with a VLAN tag.  Root sites
   can communicate with all other customer sites (both Root and Leaf
   sites).  However, Leaf sites can communicate with Root sites but not
   with other Leaf sites.  In this document, unless explicitly mentioned
   otherwise, a site is always represented by an AC.

   [RFC7387] describes a solution framework for supporting E-Tree
   service in MPLS networks.  This document identifies the functional
   components of an overall solution to emulate E-Tree services in MPLS
   networks and supplements the multipoint-to-multipoint Ethernet LAN
   (E-LAN) services specified in [RFC7432] and [RFC7623].

   [RFC7432] defines EVPN, a solution for multipoint Layer 2 Virtual
   Private Network (L2VPN) services with advanced multihoming
   capabilities that uses BGP for distributing customer/client MAC
   address reachability information over the MPLS/IP network.  [RFC7623]
   combines the functionality of EVPN with [IEEE.802.1ah] Provider
   Backbone Bridging (PBB) for MAC address scalability.

   This document discusses how the functional requirements for E-Tree
   service can be met with a solution based on EVPN [RFC7432] and
   PBB-EVPN [RFC7623] with some extensions to their procedures and BGP
   attributes.  Such a solution based on PBB-EVPN or EVPN can offer a
   more efficient implementation of these functions than that of
   [RFC7796], "Ethernet-Tree (E-Tree) Support in Virtual Private LAN
   Service (VPLS)".  This efficiency is achieved by performing filtering
   of unicast traffic at the ingress Provider Edge (PE) nodes as opposed
   to egress filtering where the traffic is sent through the network and
   gets filtered and discarded at the egress PE nodes.  The details of
   this ingress filtering are described in Section 4.1.  Since this
   document specifies a solution based on [RFC7432], the knowledge of
   that document is a prerequisite.  This document makes use of the most
   significant bit of the Tunnel Type field (in the PMSI Tunnel
   attribute) governed by the IANA registry created by [RFC7385]; hence,
   it updates [RFC7385] accordingly.  Section 3 discusses E-Tree
   scenarios, Sections 4 and 5 describe E-Tree solutions for EVPN and
   PBB-EVPN (respectively), and Section 6 covers BGP encoding for E-Tree
   solutions.

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2.  Terminology

2.1.  Specification of Requirements

   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.

2.2.  Terms and Abbreviations

   Broadcast Domain:  In a bridged network, the broadcast domain
      corresponds to a Virtual LAN (VLAN), where a VLAN is typically
      represented by a single VLAN ID (VID) but can be represented by
      several VIDs where Shared VLAN Learning (SVL) is used per
      [IEEE.802.1ah].

   Bridge Table:  An instantiation of a broadcast domain on a MAC-VRF.

   CE:  A Customer Edge device, e.g., a host, router, or switch.

   EVI:  An EVPN Instance spanning the Provider Edge (PE) devices
      participating in that EVPN.

   MAC-VRF:  A Virtual Routing and Forwarding table for Media Access
      Control (MAC) addresses on a PE.

   ES:  When a customer site (device or network) is connected to one or
      more PEs via a set of Ethernet links, then that set of links is
      referred to as an "Ethernet Segment".

   ESI:  An Ethernet Segment Identifier is a unique non-zero identifier
      that identifies an ES.

   Ethernet Tag:  An Ethernet Tag identifies a particular broadcast
      domain, e.g., a VLAN.  An EVPN instance consists of one or more
      broadcast domains.

   P2MP:  Point-to-Multipoint.

   PE:  Provider Edge device.

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3.  E-Tree Scenarios

   This document categorizes E-Tree scenarios into the following three
   categories, depending on the nature of the Root/Leaf site
   association:

   Scenario 1:  either Leaf or Root site(s) per PE;

   Scenario 2:  either Leaf or Root site(s) per Attachment Circuit (AC);
                or,

   Scenario 3:  either Leaf or Root site(s) per MAC address.

3.1.  Scenario 1: Leaf or Root Site(s) per PE

   In this scenario, a PE may receive traffic from either Root ACs or
   Leaf ACs for a given MAC-VRF/bridge table, but not both.  In other
   words, a given EVPN Instance (EVI) on a Provider Edge (PE) device is
   either associated with Root(s) or Leaf(s).  The PE may have both Root
   and Leaf ACs, albeit for different EVIs.

                   +---------+            +---------+
                   |   PE1   |            |   PE2   |
    +---+          |  +---+  |  +------+  |  +---+  |            +---+
    |CE1+---AC1----+--+   |  |  | MPLS |  |  |   +--+----AC2-----+CE2|
    +---+  (Root)  |  |MAC|  |  |  /IP |  |  |MAC|  |   (Leaf)   +---+
                   |  |VRF|  |  |      |  |  |VRF|  |
                   |  |   |  |  |      |  |  |   |  |            +---+
                   |  |   |  |  |      |  |  |   +--+----AC3-----+CE3|
                   |  +---+  |  +------+  |  +---+  |   (Leaf)   +---+
                   +---------+            +---------+

                           Figure 1: Scenario 1

   In this scenario, tailored BGP Route Target (RT) import/export
   policies among the PEs belonging to the same EVI can be used to
   prevent communication among Leaf PEs.  To prevent communication among
   Leaf ACs connected to the same PE and belonging to the same EVI,
   split-horizon filtering is used to block traffic from one Leaf AC to
   another Leaf AC on a MAC-VRF for a given E-Tree EVI.  The purpose of
   this topology constraint is to avoid having PEs with only Leaf sites
   importing and processing BGP MAC routes from each other.  To support
   such a topology constraint in EVPN, two BGP RTs are used for every
   EVI: one RT is associated with the Root sites (Root ACs) and the
   other is associated with the Leaf sites (Leaf ACs).  On a per-EVI
   basis, every PE exports the single RT associated with its type of
   site(s).  Furthermore, a PE with a Root site(s) imports both Root and
   Leaf RTs, whereas a PE with a Leaf site(s) only imports the Root RT.

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   For this scenario, if it is desired to use only a single RT per EVI
   (just like E-LAN services in [RFC7432]), then approach B in Scenario
   2 (described below) needs to be used.

3.2.  Scenario 2: Leaf or Root Site(s) per AC

   In this scenario, a PE can receive traffic from both Root ACs and
   Leaf ACs for a given EVI.  In other words, a given EVI on a PE can be
   associated with both Root(s) and Leaf(s).

                     +---------+            +---------+
                     |   PE1   |            |   PE2   |
    +---+            |  +---+  |  +------+  |  +---+  |        +---+
    |CE1+-----AC1----+--+   |  |  |      |  |  |   +--+---AC2--+CE2|
    +---+    (Leaf)  |  |MAC|  |  | MPLS |  |  |MAC|  | (Leaf) +---+
                     |  |VRF|  |  |  /IP |  |  |VRF|  |
                     |  |   |  |  |      |  |  |   |  |        +---+
                     |  |   |  |  |      |  |  |   +--+---AC3--+CE3|
                     |  +---+  |  +------+  |  +---+  | (Root) +---+
                     +---------+            +---------+

                           Figure 2: Scenario 2

   In this scenario, (as in Scenario 1 Section 3.1), two RTs (one for
   Root and another for Leaf) can be used.  However, the difference is
   that on a PE with both Root and Leaf ACs, all remote MAC routes are
   imported; thus, in order to apply the proper ingress filtering, there
   needs to be a way to differentiate remote MAC routes associated with
   Leaf ACs versus the ones associated with Root ACs.

   In order to recognize the association of a destination MAC address to
   a Leaf or Root AC and, thus, support ingress filtering on the ingress
   PE with both Leaf and Root ACs, MAC addresses need to be colored with
   a Root or Leaf-Indication before advertising to other PEs.  There are
   two approaches for such coloring:

   (A)  to always use two RTs (one to designate Leaf RT and another for
        Root RT), or

   (B)  to allow for a single RT to be used per EVI, just like
        [RFC7432], and, thus, color MAC addresses via a "color" flag in
        a new extended community as detailed in Section 6.1.

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   Approach A would require the same data-plane enhancements as approach
   B if MAC-VRF and bridge tables used per VLAN are to remain consistent
   with Section 6 of [RFC7432].  In order to avoid data-plane
   enhancements for approach A, multiple bridge tables per VLAN may be
   considered; however, this has major drawbacks (as described in
   Appendix A); thus, it is not recommended.

   Given that both approaches A and B would require the same data-plane
   enhancements, approach B is chosen here in order to allow for RT
   usage consistent with baseline EVPN [RFC7432] and for better
   generality.  It should be noted that if one wants to use RT
   constraints in order to avoid MAC advertisements associated with a
   Leaf AC to PEs with only Leaf ACs, then two RTs (one for Root and
   another for Leaf) can still be used with approach B; however, in such
   applications, Leaf/Root RTs will be used to constrain MAC
   advertisements and are not used to color the MAC routes for ingress
   filtering (i.e., in approach B, the coloring is always done via the
   new extended community).

   If, for a given EVI, a significant number of PEs have both Leaf and
   Root sites attached (even though they may start as Root-only or Leaf-
   only PEs), then a single RT per EVI should be used.  The reason for
   such a recommendation is to alleviate the configuration overhead
   associated with using two RTs per EVI at the expense of having some
   unwanted MAC addresses on the Leaf-only PEs.

3.3.  Scenario 3: Leaf or Root Site(s) per MAC Address

   In this scenario, a customer Root or Leaf site is represented by a
   MAC address on an AC and a PE may receive traffic from both Root and
   Leaf sites on that AC for an EVI.  This scenario is not covered in
   either [RFC7387] or [MEF6.1]; however, it is covered in this document
   for the sake of completeness.  In this scenario, since an AC carries
   traffic from both Root and Leaf sites, the granularity at which Root
   or Leaf sites are identified is on a per-MAC-address basis.  This
   scenario is considered in this document for EVPN service with only
   known unicast traffic because the Designated Forwarder (DF) filtering
   per [RFC7432] would not be compatible with the required egress
   filtering; that is, Broadcast, Unknown Unicast, and Multicast (BUM)
   traffic is not supported in this scenario; it is dropped by the
   ingress PE.

   For this scenario, the approach B in Scenario 2 is used in order to
   allow for single RT usage by service providers.

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                     +---------+            +---------+
                     |   PE1   |            |   PE2   |
    +---+            |  +---+  |  +------+  |  +---+  |            +---+
    |CE1+-----AC1----+--+   |  |  |      |  |  |   +--+-----AC2----+CE2|
    +---+    (Root)  |  | E |  |  | MPLS |  |  | E |  | (Leaf/Root)+---+
                     |  | V |  |  |  /IP |  |  | V |  |
                     |  | I |  |  |      |  |  | I |  |            +---+
                     |  |   |  |  |      |  |  |   +--+-----AC3----+CE3|
                     |  +---+  |  +------+  |  +---+  |   (Leaf)   +---+
                     +---------+            +---------+

                           Figure 3: Scenario 3

   In conclusion, the approach B in scenario 2 is the recommended
   approach across all the above three scenarios, and the corresponding
   solution is detailed in the following sections.

4.  Operation for EVPN

   [RFC7432] defines the notion of the Ethernet Segment Identifier (ESI)
   MPLS label used for split-horizon filtering of BUM traffic at the
   egress PE.  Such egress filtering capabilities can be leveraged in
   provision of E-Tree services, as it will be seen shortly for BUM
   traffic.  For known unicast traffic, additional extensions to
   [RFC7432] are needed (i.e., a new BGP extended community for Leaf-
   Indication described in Section 6.1) in order to enable ingress
   filtering as described in detail in the following sections.

4.1.  Known Unicast Traffic

   In EVPN, MAC learning is performed in the control plane via
   advertisement of BGP routes.  Because of this, the filtering needed
   by an E-Tree service for known unicast traffic can be performed at
   the ingress PE, thus providing very efficient filtering and avoiding
   sending known unicast traffic over the MPLS/IP core to be filtered at
   the egress PE, as is done in traditional E-Tree solutions (i.e.,
   E-Tree for VPLS [RFC7796]).

   To provide such ingress filtering for known unicast traffic, a PE
   MUST indicate to other PEs what kind of sites (Root or Leaf) its MAC
   addresses are associated with.  This is done by advertising a Leaf-
   Indication flag (via an extended community) along with each of its
   MAC/IP Advertisement routes learned from a Leaf site.  The lack of
   such a flag indicates that the MAC address is associated with a Root
   site.  This scheme applies to all scenarios described in Section 3.

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   Tagging MAC addresses with a Leaf-Indication enables remote PEs to
   perform ingress filtering for known unicast traffic; that is, on the
   ingress PE, the MAC destination address lookup yields (in addition to
   the forwarding adjacency) a flag that indicates whether or not the
   target MAC is associated with a Leaf site.  The ingress PE cross-
   checks this flag with the status of the originating AC, and if both
   are Leafs, then the packet is not forwarded.

   In a situation where MAC moves are allowed among Leaf and Root sites
   (e.g., non-static MAC), PEs can receive multiple MAC/IP Advertisement
   routes for the same MAC address with different Root or Leaf-
   Indications (and possibly different ESIs for multihoming scenarios).
   In such situations, MAC mobility procedures (see Section 15 of
   [RFC7432]) take precedence to first identify the location of the MAC
   before associating that MAC with a Root or a Leaf site.

   To support the above ingress filtering functionality, a new E-Tree
   extended community with a Leaf-Indication flag is introduced (see
   Section 6.1).  This new extended community MUST be advertised with
   MAC/IP Advertisement routes learned from a Leaf site.  Besides MAC/IP
   Advertisement routes, no other EVPN routes are required to carry this
   new extended community for the purpose of known unicast traffic.

4.2.  BUM Traffic

   This specification does not provide support for filtering Broadcast,
   Unknown Unicast, and Multicast (BUM) traffic on the ingress PE; due
   to the multidestination nature of BUM traffic, it is not possible to
   perform filtering of the same on the ingress PE.  As such, the
   solution relies on egress filtering.  In order to apply the proper
   egress filtering, which varies based on whether a packet is sent from
   a Leaf AC or a Root AC, the MPLS-encapsulated frames MUST be tagged
   with an indication of when they originated from a Leaf AC (i.e., to
   be tagged with a Leaf label as specified in Section 6.1).  This Leaf
   label allows for disposition PE (e.g., egress PE) to perform the
   necessary egress filtering function in a data plane similar to the
   ESI label in [RFC7432].  The allocation of the Leaf label is on a
   per-PE basis (e.g., independent of ESI and EVI) as described in the
   following sections.

   The Leaf label can be upstream assigned for Point-to-Multipoint
   (P2MP) Label Switched Path (LSP) or downstream assigned for Ingress
   Replication tunnels.  The main difference between a downstream- and
   upstream-assigned Leaf label is that, in the case of downstream-
   assigned Leaf labels, not all egress PE devices need to receive the
   label in MPLS-encapsulated BUM packets, just like the ESI label for
   Ingress Replication procedures defined in [RFC7432].

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   On the ingress PE, the PE needs to place all its Leaf ACs for a given
   bridge domain in a single split-horizon group in order to prevent
   intra-PE forwarding among its Leaf ACs.  This intra-PE split-horizon
   filtering applies to BUM traffic as well as known unicast traffic.

   There are four scenarios to consider as follows.  In all these
   scenarios, the ingress PE imposes the right MPLS label associated
   with the originated Ethernet Segment (ES) depending on whether the
   Ethernet frame originated from a Root or a Leaf site on that Ethernet
   Segment (ESI label or Leaf label).  The mechanism by which the PE
   identifies whether a given frame originated from a Root or a Leaf
   site on the segment is based on the AC identifier for that segment
   (e.g., Ethernet Tag of the frame for 802.1Q frames [IEEE.802.1Q]).
   Other mechanisms for identifying Root or Leaf sites, such as the use
   of the source MAC address of the receiving frame, are optional.  The
   scenarios below are described in context of a Root/Leaf AC, however,
   they can be extended to the Root/Leaf MAC address if needed.

4.2.1.  BUM Traffic Originated from a Single-Homed Site on a Leaf AC

   In this scenario, the ingress PE adds a Leaf label advertised using
   the E-Tree extended community (see Section 6.1), which indicates a
   Leaf site.  This Leaf label, used for single-homing scenarios, is not
   on a per-ES basis but rather on a per PE basis (i.e., a single Leaf
   MPLS label is used for all single-homed ESs on that PE).  This Leaf
   label is advertised to other PE devices using the E-Tree extended
   community (see Section 6.1) along with an Ethernet Auto-Discovery per
   ES (EAD-ES) route with an ESI of zero and a set of RTs corresponding
   to all EVIs on the PE where each EVI has at least one Leaf site.
   Multiple EAD-ES routes will need to be advertised if the number of
   RTs that need to be carried exceed the limit on a single route per
   [RFC7432].  The ESI for the EAD-ES route is set to zero to indicate
   single-homed sites.

   When a PE receives this special Leaf label in the data path, it
   blocks the packet if the destination AC is of type Leaf; otherwise,
   it forwards the packet.

4.2.2.  BUM Traffic Originated from a Single-Homed Site on a Root AC

   In this scenario, the ingress PE does not add any ESI or Leaf labels
   and it operates per the procedures in [RFC7432].

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4.2.3.  BUM Traffic Originated from a Multihomed Site on a Leaf AC

   In this scenario, it is assumed that while different ACs (VLANs) on
   the same ES could have a different Root/Leaf designation (some being
   Roots and some being Leafs), the same VLAN does have the same Root/
   Leaf designation on all PEs on the same ES.  Furthermore, it is
   assumed that there is no forwarding among subnets (i.e., the service
   is EVPN L2 and not EVPN Integrated Routing and Bridging (IRB)
   [EVPN-INTEGRATED]).  IRB use cases described in [EVPN-INTEGRATED] are
   outside the scope of this document.

   In this scenario, if a multicast or broadcast packet is originated
   from a Leaf AC, then it only needs to carry a Leaf label as described
   in Section 4.2.1.  This label is sufficient in providing the
   necessary egress filtering of BUM traffic from getting sent to Leaf
   ACs, including the Leaf AC on the same ES.

4.2.4.  BUM Traffic Originated from a Multihomed Site on a Root AC

   In this scenario, both the ingress and egress PE devices follow the
   procedure defined in [RFC7432] for adding and/or processing an ESI
   MPLS label; that is, existing procedures for BUM traffic in [RFC7432]
   are sufficient and there is no need to add a Leaf label.

4.3.  E-Tree Traffic Flows for EVPN

   Per [RFC7387], a generic E-Tree service supports all of the following
   traffic flows:

   -  known unicast traffic from Root to Roots & Leafs

   -  known unicast traffic from Leaf to Roots

   -  BUM traffic from Root to Roots & Leafs

   -  BUM traffic from Leaf to Roots

   A particular E-Tree service may need to support all of the above
   types of flows or only a select subset, depending on the target
   application.  In the case where only multicast and broadcast flows
   need to be supported, the L2VPN PEs can avoid performing any MAC
   learning function.

   The following subsections will describe the operation of EVPN to
   support E-Tree service with and without MAC learning.

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4.3.1.  E-Tree with MAC Learning

   The PEs implementing an E-Tree service must perform MAC learning when
   unicast traffic flows must be supported among Root and Leaf sites.
   In this case, the PE(s) with Root sites performs MAC learning in the
   data path over the ESs and advertises reachability in EVPN MAC/IP
   Advertisement routes.  These routes will be imported by all PEs for
   that EVI (i.e., PEs that have Leaf sites as well as PEs that have
   Root sites).  Similarly, the PEs with Leaf sites perform MAC learning
   in the data path over their ESs and advertise reachability in EVPN
   MAC/IP Advertisement routes.  For scenarios where two different RTs
   are used per EVI (one to designate a Root site and another to
   designate a Leaf site), the MAC/IP Advertisement routes are imported
   only by PEs with at least one Root site in the EVI (i.e., a PE with
   only Leaf sites will not import these routes).  PEs with Root and/or
   Leaf sites may use the Ethernet Auto-Discovery per EVI (EAD-EVI)
   routes for aliasing (in the case of multihomed segments) and EAD-ES
   routes for mass MAC withdrawal per [RFC7432].

   To support multicast/broadcast from Root to Leaf sites, either a P2MP
   tree rooted at the PE(s) with the Root site(s) (e.g., Root PEs) or
   Ingress Replication can be used (see Section 16 of [RFC7432]).  The
   multicast tunnels are set up through the exchange of the EVPN
   Inclusive Multicast route, as defined in [RFC7432].

   To support multicast/broadcast from Leaf to Root sites, either
   Ingress Replication tunnels from each Leaf PE or a P2MP tree rooted
   at each Leaf PE can be used.  The following two paragraphs describe
   when each of these tunneling schemes can be used and how to signal
   them.

   When there are only a few Root PEs with small amount of multicast/
   broadcast traffic from Leaf PEs toward Root PEs, then Ingress
   Replication tunnels from Leaf PEs toward Root PEs should be
   sufficient.  Therefore, if a Root PE needs to support a P2MP tunnel
   in the transmit direction from itself to Leaf PEs, and, at the same
   time, it wants to support Ingress Replication tunnels in the receive
   direction, the Root PE can signal it efficiently by using a new
   composite tunnel type defined in Section 6.2.  This new composite
   tunnel type is advertised by the Root PE to simultaneously indicate a
   P2MP tunnel in the transmit direction and an Ingress Replication
   tunnel in the receive direction for the BUM traffic.

   If the number of Root PEs is large, P2MP tunnels (e.g., Multipoint
   LDP (mLDP) or RSVP-TE) originated at the Leaf PEs may be used; thus,
   there will be no need to use the modified PMSI Tunnel attribute and
   the composite tunnel type values defined in Section 6.2.

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4.3.2.  E-Tree without MAC Learning

   The PEs implementing an E-Tree service need not perform MAC learning
   when the traffic flows between Root and Leaf sites are mainly
   multicast or broadcast.  In this case, the PEs do not exchange EVPN
   MAC/IP Advertisement routes.  Instead, the Inclusive Multicast
   Ethernet Tag route is used to support BUM traffic.  In such
   scenarios, the small amount of unicast traffic (if any) is sent as
   part of BUM traffic.

   The fields of this route are populated per the procedures defined in
   [RFC7432], and the multicast tunnel setup criteria are as described
   in the previous section.

   Just as in the previous section, if the number of Root PEs are only a
   few and, thus, Ingress Replication is desired from Leaf PEs to these
   Root PEs, then the modified PMSI attribute and the composite tunnel
   type values defined in Section 6.2 should be used.

5.  Operation for PBB-EVPN

   In PBB-EVPN, the PE advertises a Root or Leaf-Indication along with
   each Backbone MAC (B-MAC) Advertisement route to indicate whether the
   associated B-MAC address corresponds to a Root or a Leaf site.  Just
   like the EVPN case, the new E-Tree extended community defined in
   Section 6.1 is advertised with each EVPN MAC/IP Advertisement route.

   In the case where a multihomed ES has both Root and Leaf sites
   attached, two B-MAC addresses are advertised: one B-MAC address is
   per ES (as specified in [RFC7623]) and implicitly denotes Root, and
   the other B-MAC address is per PE and explicitly denotes Leaf.  The
   former B-MAC address is not advertised with the E-Tree extended
   community, but the latter B-MAC denoting Leaf is advertised with the
   new E-Tree extended community where a "Leaf-indication" flag is set.
   In multihoming scenarios where an ES has both Root and Leaf ACs, it
   is assumed that while different ACs (VLANs) on the same ES could have
   a different Root/Leaf designation (some being Roots and some being
   Leafs), the same VLAN does have the same Root/Leaf designation on all
   PEs on the same ES.  Furthermore, it is assumed that there is no
   forwarding among subnets (i.e., the service is L2 and not IRB).  An
   IRB use case is outside the scope of this document.

   The ingress PE uses the right B-MAC source address depending on
   whether the Ethernet frame originated from the Root or Leaf AC on
   that ES.  The mechanism by which the PE identifies whether a given
   frame originated from a Root or Leaf site on the segment is based on

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   the Ethernet Tag associated with the frame.  Other mechanisms of
   identification, beyond the Ethernet Tag, are outside the scope of
   this document.

   Furthermore, a PE advertises two special global B-MAC addresses, one
   for Root and another for Leaf, and tags the Leaf one as such in the
   MAC Advertisement route.  These B-MAC addresses are used as source
   addresses for traffic originating from single-homed segments.  The
   B-MAC address used for indicating Leaf sites can be the same for both
   single-homed and multihomed segments.

5.1.  Known Unicast Traffic

   For known unicast traffic, the PEs perform ingress filtering: on the
   ingress PE, the Customer/Client MAC (C-MAC) [RFC7623] destination
   address lookup yields, in addition to the target B-MAC address and
   forwarding adjacency, a flag that indicates whether the target B-MAC
   is associated with a Root or a Leaf site.  The ingress PE also checks
   the status of the originating site; if both are Leafs, then the
   packet is not forwarded.

5.2.  BUM Traffic

   For BUM traffic, the PEs must perform egress filtering.  When a PE
   receives an EVPN MAC/IP Advertisement route (which will be used as a
   source B-MAC for BUM traffic), it updates its egress filtering (based
   on the source B-MAC address) as follows:

   -  If the EVPN MAC/IP Advertisement route indicates that the
      advertised B-MAC is a Leaf, and the local ES is a Leaf as well,
      then the source B-MAC address is added to its B-MAC list used for
      egress filtering (i.e., to block traffic from that B-MAC address).
      Otherwise, the B-MAC filtering list is not updated.

   -  If the EVPN MAC/IP Advertisement route indicates that the
      advertised B-MAC has changed its designation from a Leaf to a
      Root, and the local ES is a Leaf, then the source B-MAC address is
      removed from the B-MAC list corresponding to the local ES used for
      egress filtering (i.e., to unblock traffic from that B-MAC
      address).

   When the egress PE receives the packet, it examines the B-MAC source
   address to check whether it should filter or forward the frame.  Note
   that this uses the same filtering logic as the split-horizon
   filtering described in Section 6.2.1.3 of [RFC7623] and does not
   require any additional flags in the data plane.

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   Just as in Section 4.2, the PE places all Leaf ESs of a given bridge
   domain in a single split-horizon group in order to prevent intra-PE
   forwarding among Leaf segments.  This split-horizon function applies
   to BUM traffic as well as known unicast traffic.

5.3.  E-Tree without MAC Learning

   In scenarios where the traffic of interest is only multicast and/or
   broadcast, the PEs implementing an E-Tree service do not need to do
   any MAC learning.  In such scenarios, the filtering must be performed
   on egress PEs.  For PBB-EVPN, the handling of such traffic is per
   Section 5.2 without the need for C-MAC learning (in the data plane)
   in the I-component (C-bridge table) of PBB-EVPN PEs (at both ingress
   and egress PEs).

6.  BGP Encoding

   This document defines a new BGP extended community for EVPN.

6.1.  E-Tree Extended Community

   This extended community is a new transitive extended community
   [RFC4360] having a Type field value of 0x06 (EVPN) and the Sub-Type
   0x05.  It is used for Leaf-Indication of known unicast and BUM
   traffic.  It indicates that the frame is originated from a Leaf site.

   The E-Tree extended community is encoded as an 8-octet value as
   follows:

        0                   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=0x05 | Flags(1 Octet)|  Reserved=0   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Reserved=0   |           Leaf Label                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: E-Tree Extended Community

   The Flags field has the following format:

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |     MBZ     |L|     (MBZ = MUST Be Zero)
         +-+-+-+-+-+-+-+-+

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   This document defines the following flags:

      + Leaf-Indication (L)

   A value of one indicates a Leaf AC/site.  The rest of the flag bits
   are reserved and should be set to zero.

   When this extended community is advertised along with the MAC/IP
   Advertisement route (for known unicast traffic) per Section 4.1, the
   Leaf-Indication flag MUST be set to one and the Leaf label SHOULD be
   set to zero.  The receiving PE MUST ignore Leaf label and only
   process the Leaf-Indication flag.  A value of zero for the Leaf-
   Indication flag is invalid when sent along with a MAC/IP
   Advertisement route, and an error should be logged.

   When this extended community is advertised along with the EAD-ES
   route (with an ESI of zero) for BUM traffic to enable egress
   filtering on disposition PEs per Sections 4.2.1 and 4.2.3, the Leaf
   label MUST be set to a valid MPLS label (i.e., a non-reserved,
   assigned MPLS label [RFC3032]) and the Leaf-Indication flag SHOULD be
   set to zero.  The value of the 20-bit MPLS label is encoded in the
   high-order 20 bits of the Leaf label field.  The receiving PE MUST
   ignore the Leaf-Indication flag.  A non-valid MPLS label, when sent
   along with the EAD-ES route, should be ignored and logged as an
   error.

   The reserved bits SHOULD be set to zero by the transmitter and MUST
   be ignored by the receiver.

6.2.  PMSI Tunnel Attribute

   [RFC6514] defines the PMSI Tunnel attribute, which is an optional
   transitive attribute with the following format:

               +-------------------------------------------+
               | Flags (1 octet)                           |
               +-------------------------------------------+
               | Tunnel Type (1 octet)                     |
               +-------------------------------------------+
               | Ingress Replication MPLS Label (3 octets) |
               +-------------------------------------------+
               | Tunnel Identifier (variable)              |
               +-------------------------------------------+

                      Table 1: PMSI Tunnel Attribute

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   This document defines a new composite tunnel type by introducing a
   new 'composite tunnel' bit in the Tunnel Type field and adding an
   MPLS label to the Tunnel Identifier field of the PMSI Tunnel
   attribute, as detailed below.  All other fields remain as defined in
   [RFC6514].  Composite tunnel type is advertised by the Root PE to
   simultaneously indicate a non-Ingress-Replication tunnel (e.g., P2MP
   tunnel) in the transmit direction and an Ingress Replication tunnel
   in the receive direction for the BUM traffic.

   When receiver Ingress Replication labels are needed, the high-order
   bit of the Tunnel Type field (composite tunnel bit) is set while the
   remaining low-order seven bits indicate the Tunnel Type as before
   (for the existing Tunnel Types).  When this composite tunnel bit is
   set, the "tunnel identifier" field begins with a three-octet label,
   followed by the actual tunnel identifier for the transmit tunnel.
   PEs that don't understand the new meaning of the high-order bit treat
   the Tunnel Type as an undefined Tunnel Type and treat the PMSI Tunnel
   attribute as a malformed attribute [RFC6514].  That is why the
   composite tunnel bit is allocated in the Tunnel Type field rather
   than the Flags field.  For the PEs that do understand the new meaning
   of the high-order, if Ingress Replication is desired when sending BUM
   traffic, the PE will use the label in the Tunnel Identifier field
   when sending its BUM traffic.

   Using the composite tunnel bit for Tunnel Types 0x00 'no tunnel
   information present' and 0x06 'Ingress Replication' is invalid.  A PE
   that receives a PMSI Tunnel attribute with such information considers
   it malformed, and it SHOULD treat this Update as though all the
   routes contained in this Update had been withdrawn per Section 6 of
   [RFC6514].

7.  Security Considerations

   Since this document uses the EVPN constructs of [RFC7432] and
   [RFC7623], the same security considerations in these documents are
   also applicable here.  Furthermore, this document provides an
   additional security check by allowing sites (or ACs) of an EVPN
   instance to be designated as a "Root" or "Leaf" by the network
   operator / service provider and thus prevent any traffic exchange
   among "Leaf" sites of that VPN through ingress filtering for known
   unicast traffic and egress filtering for BUM traffic.  Since (by
   default and for the purpose of backward compatibility) an AC that
   doesn't have a Leaf designation is considered a Root AC, in order to
   avoid any traffic exchange among Leaf ACs, the operator SHOULD
   configure the AC with a proper role (Leaf or Root) before activating
   the AC.

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8.  IANA Considerations

   IANA has allocated sub-type value 5 in the "EVPN Extended Community
   Sub-Types" registry defined in [RFC7153] as follows:

         SUB-TYPE VALUE     NAME                        Reference
         --------------     -------------------------   -------------
         0x05               E-Tree Extended Community   This document

   This document creates a one-octet registry called "E-Tree Flags".
   New registrations will be made through the "RFC Required" procedure
   defined in [RFC8126].  Initial registrations are as follows:

         Bit               Name                         Reference
         ----              --------------               -------------
         0-6               Unassigned
         7                 Leaf-Indication              This document

8.1.  Considerations for PMSI Tunnel Types

   The "P-Multicast Service Interface (PMSI) Tunnel Types" registry in
   the "Border Gateway Protocol (BGP) Parameters" registry has been
   updated to reflect the use of the most significant bit as the
   "composite tunnel" bit (see Section 6.2).

   For this purpose, this document updates [RFC7385] by changing the
   previously unassigned values (i.e., 0x08 - 0xFA) as follows:

   Value          Meaning                            Reference
   ---------      -----------------------------      --------------
   0x0C-0x7A      Unassigned
   0x7B-0x7E      Experimental                       This Document
   0x7F           Reserved                           This Document
   0x80-0xFA      Reserved for Composite Tunnel      This Document
   0xFB-0xFE      Experimental                       [RFC7385]
   0xFF           Reserved                           [RFC7385]

   The allocation policy for values 0x08-0x7A is per IETF Review
   [RFC8126].  The range for "Experimental" has been expanded to include
   the previously assigned range of 0xFB-0xFE and the new range of
   0x7B-0x7E.  The values in these ranges are not to be assigned.  The
   value 0x7F, which is the mirror image of (0xFF), is reserved in this
   document.

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9.  References

9.1.  Normative References

   [MEF6.1]   MEF Forum, "Ethernet Services Definitions - Phase 2",
              MEF 6.1, April 2008, <https://mef.net/PDF_Documents/
              technical-specifications/MEF6-1.pdf>.

   [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>.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <https://www.rfc-editor.org/info/rfc4360>.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
              <https://www.rfc-editor.org/info/rfc6514>.

   [RFC7153]  Rosen, E. and Y. Rekhter, "IANA Registries for BGP
              Extended Communities", RFC 7153, DOI 10.17487/RFC7153,
              March 2014, <https://www.rfc-editor.org/info/rfc7153>.

   [RFC7385]  Andersson, L. and G. Swallow, "IANA Registry for
              P-Multicast Service Interface (PMSI) Tunnel Type Code
              Points", RFC 7385, DOI 10.17487/RFC7385, October 2014,
              <https://www.rfc-editor.org/info/rfc7385>.

   [RFC7387]  Key, R., Ed., Yong, L., Ed., Delord, S., Jounay, F., and
              L. Jin, "A Framework for Ethernet Tree (E-Tree) Service
              over a Multiprotocol Label Switching (MPLS) Network",
              RFC 7387, DOI 10.17487/RFC7387, October 2014,
              <https://www.rfc-editor.org/info/rfc7387>.

   [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>.

   [RFC7623]  Sajassi, A., Ed., Salam, S., Bitar, N., Isaac, A., and W.
              Henderickx, "Provider Backbone Bridging Combined with
              Ethernet VPN (PBB-EVPN)", RFC 7623, DOI 10.17487/RFC7623,
              September 2015, <https://www.rfc-editor.org/info/rfc7623>.

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   [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>.

   [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>.

9.2.  Informative References

   [EVPN-INTEGRATED]
              Sajassi, A., Salam, S., Thoria, S., Drake, J., Rabadan,
              J., and L. Yong, "Integrated Routing and Bridging in
              EVPN", Work in Progress, draft-ietf-bess-evpn-inter-
              subnet-forwarding-03, February 2017.

   [IEEE.802.1ah]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks - Media Access Control (MAC) Bridges and Virtual
              Bridged Local Area Networks", Clauses 25 and 26, IEEE
              Std 802.1Q, DOI 10.1109/IEEESTD.2011.6009146.

   [IEEE.802.1Q]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks - Bridges and Bridged Networks - Media Access
              Control (MAC) Bridges and Virtual Bridged Local Area
              Networks", IEEE Std 802.1Q,
              DOI 10.1109/IEEESTD.2011.6009146.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC7796]  Jiang, Y., Ed., Yong, L., and M. Paul, "Ethernet-Tree
              (E-Tree) Support in Virtual Private LAN Service (VPLS)",
              RFC 7796, DOI 10.17487/RFC7796, March 2016,
              <https://www.rfc-editor.org/info/rfc7796>.

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Appendix A.  Multiple Bridge Tables per E-Tree Service Instance

   When two MAC-VRFs (two bridge tables per VLAN) are used for an E-Tree
   service (one for Root ACs and another for Leaf ACs) on a given PE,
   then the following complications in a data-plane path can result.

   Maintaining two MAC-VRFs (two bridge tables) per VLAN (when both Leaf
   and Root ACs exists for that VLAN) would require either that two
   lookups be performed per MAC address in each direction in case of a
   miss or that the duplication of many MAC addresses between the two
   bridge tables belonging to the same VLAN (same E-Tree instance) be
   made.  Unless two lookups are made, duplication of MAC addresses
   would be needed for both locally learned and remotely learned MAC
   addresses.  Locally learned MAC addresses from Leaf ACs need to be
   duplicated onto a Root bridge table, and locally learned MAC
   addresses from Root ACs need to be duplicated onto a Leaf bridge
   table.  Remotely learned MAC addresses from Root ACs need to be
   copied onto both Root and Leaf bridge tables.  Because of potential
   inefficiencies associated with data-plane implementation of
   additional MAC lookup or duplication of MAC entries, this option is
   not believed to be implementable without data-plane performance
   inefficiencies in some platforms; thus, this document introduces the
   coloring as described in Section 3.2 and detailed in Section 4.1.

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Acknowledgements

   We would like to thank Eric Rosen, Jeffrey Zhang, Wen Lin, Aldrin
   Issac, Wim Henderickx, Dennis Cai, and Antoni Przygienda for their
   valuable comments and contributions.  The authors would also like to
   thank Thomas Morin for shepherding this document and providing
   valuable comments.

Authors' Addresses

   Ali Sajassi (editor)
   Cisco

   Email: sajassi@cisco.com

   Samer Salam
   Cisco

   Email: ssalam@cisco.com

   John Drake
   Juniper

   Email: jdrake@juniper.net

   Jim Uttaro
   AT&T

   Email: ju1738@att.com

   Sami Boutros
   VMware

   Email: sboutros@vmware.com

   Jorge Rabadan
   Nokia

   Email: jorge.rabadan@nokia.com

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