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rfc:rfc8317

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.

Sajassi, et al. Standards Track [Page 1] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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.

Sajassi, et al. Standards Track [Page 2] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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

Sajassi, et al. Standards Track [Page 3] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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.

Sajassi, et al. Standards Track [Page 4] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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.

Sajassi, et al. Standards Track [Page 5] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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.

Sajassi, et al. Standards Track [Page 6] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 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.

Sajassi, et al. Standards Track [Page 7] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 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.

Sajassi, et al. Standards Track [Page 8] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

                   +---------+            +---------+
                   |   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.

Sajassi, et al. Standards Track [Page 9] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 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].

Sajassi, et al. Standards Track [Page 10] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 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].

Sajassi, et al. Standards Track [Page 11] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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:
  1. known unicast traffic from Root to Roots & Leafs
  1. known unicast traffic from Leaf to Roots
  1. BUM traffic from Root to Roots & Leafs
  1. 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.

Sajassi, et al. Standards Track [Page 12] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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.

Sajassi, et al. Standards Track [Page 13] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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

Sajassi, et al. Standards Track [Page 14] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 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:
  1. 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.
  1. 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.

Sajassi, et al. Standards Track [Page 15] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 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)
       +-+-+-+-+-+-+-+-+

Sajassi, et al. Standards Track [Page 16] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 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

Sajassi, et al. Standards Track [Page 17] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 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.

Sajassi, et al. Standards Track [Page 18] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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.

Sajassi, et al. Standards Track [Page 19] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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>.

Sajassi, et al. Standards Track [Page 20] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

 [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>.

Sajassi, et al. Standards Track [Page 21] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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.

Sajassi, et al. Standards Track [Page 22] RFC 8317 E-Tree Support in EVPN and PBB-EVPN January 2018

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

Sajassi, et al. Standards Track [Page 23]

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