E-TREE Support in EVPN & PBB-EVPN
draft-ietf-bess-evpn-etree-13

BESS Workgroup                                           A. Sajassi, Ed.
INTERNET-DRAFT                                                  S. Salam
Intended Status: Standards Track                                   Cisco
Updates: 7385                                                   J. Drake
                                                                 Juniper
                                                               J. Uttaro
                                                                     ATT
                                                              S. Boutros
                                                                  VMware
                                                              J. Rabadan
                                                                   Nokia
                                                                        
Expires: February 28, 2018                               August 28, 2017

                   E-TREE Support in EVPN & PBB-EVPN
                     draft-ietf-bess-evpn-etree-13 

Abstract

   The Metro Ethernet 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 RFC7387 ("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 RFC7432, BGP MPLS Based Ethernet VPN (EVPN), with
   some extensions and how such a solution can offer a more efficient
   implementation of these functions than that of RFC7796, E-Tree
   Support in Virtual Private LAN Service (VPLS). This document makes
   use of the most significant bit of the "Tunnel Type" field (in PMSI
   Tunnel Attribute) governed by the IANA registry created by RFC7385,
   and hence updates RFC7385 accordingly.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
 

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   time.  It is inappropriate to use Internet-Drafts as reference
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   The list of current Internet-Drafts can be accessed at
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   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1  Specification of Requirements . . . . . . . . . . . . . . .  4
     1.2  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  4
   2  E-Tree Scenarios  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1 Scenario 1: Leaf or Root Site(s) per PE  . . . . . . . . . .  5
     2.2 Scenario 2: Leaf or Root Site(s) per AC  . . . . . . . . . .  6
     2.3 Scenario 3: Leaf or Root Site(s) per MAC Address . . . . . .  8
   3 Operation for EVPN . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1 Known Unicast Traffic  . . . . . . . . . . . . . . . . . . .  9
     3.2 Broadcast, Unkonwn, and Multicast (BUM) Traffic  . . . . . . 10
       3.2.1 BUM Traffic Originated from a Single-homed Site on a
             Leaf AC  . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.2 BUM Traffic Originated from a Single-homed Site on a
             Root AC  . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.3 BUM Traffic Originated from a Multi-homed Site on a 
             Leaf AC  . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.4 BUM Traffic Originated from a Multi-homed Site on a 
             Root AC  . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.3 E-Tree Traffic Flows for EVPN  . . . . . . . . . . . . . . . 12
 

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       3.3.1 E-Tree with MAC Learning . . . . . . . . . . . . . . . . 12
       3.3.2 E-Tree without MAC Learning  . . . . . . . . . . . . . . 13
   4 Operation for PBB-EVPN . . . . . . . . . . . . . . . . . . . . . 13
     4.1 Known Unicast Traffic  . . . . . . . . . . . . . . . . . . . 14
     4.2 Broadcast, Unkonwn, and Multicast (BUM) Traffic  . . . . . . 14
     4.3 E-Tree without MAC Learning  . . . . . . . . . . . . . . . . 15
   5 BGP Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1 E-Tree Extended Community  . . . . . . . . . . . . . . . . . 15
     5.2 PMSI Tunnel Attribute  . . . . . . . . . . . . . . . . . . . 17
   6  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . 18
   7  Security Considerations . . . . . . . . . . . . . . . . . . . . 18
   8  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 18
     8.1 Considerations for PMSI Tunnel Types . . . . . . . . . . . . 18
   9  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1  Normative References  . . . . . . . . . . . . . . . . . . . 19
     9.2  Informative References  . . . . . . . . . . . . . . . . . . 20
   Appendix-A . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21

 

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1  Introduction

   The Metro Ethernet 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 Circuits (AC) (e.g., an Ethernet tag but may also be
   represented by a MAC address) is labeled as either a Root or a Leaf
   site. 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 sits. 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. The document identifies the functional
   components of an overall solution to emulate E-Tree services in MPLS
   networks in addition to multipoint-to-multipoint Ethernet LAN (E-LAN)
   services specified in [RFC7432] and [RFC7623]. 

   [RFC7432] defines EVPN, a solution for multipoint L2VPN services with
   advanced multi-homing capabilities, using BGP for distributing
   customer/client MAC address reach-ability information over the
   MPLS/IP network. [RFC7623] combines the functionality of EVPN with
   [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 (PBB-)EVPN (i.e.,
   [RFC7432] and [RFC7623]) with some extensions and how such a solution
   can offer a more efficient implementation of these functions than
   that of RFC7796, E-Tree Support in Virtual Private LAN Service
   (VPLS). Since this document specifies a solution based on [RFC7432],
   it requires the readers to have the knowledge of [RFC7432] as
   prerequisite. This document makes use of the most significant bit of
   the "Tunnel Type" field (in PMSI Tunnel Attribute) governed by the
   IANA registry created by RFC7385, and hence updates RFC7385
   accordingly. Section 2 discusses E-Tree scenarios. Section 3 and 4
   describe E-Tree solutions for EVPN and PBB-EVPN respectively, and
   section 5 covers BGP encoding for E-Tree solutions. 

1.1  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [KEYWORDS].

1.2  Terminology

 

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   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 [802.1Q].

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

   CE: 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.

   Ethernet Segment (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'.

   Ethernet Segment Identifier (ESI): A unique non-zero identifier that
   identifies an Ethernet segment is called an 'Ethernet Segment
   Identifier'.

   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.

2  E-Tree Scenarios

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

   - Either Leaf or Root site(s) per PE

   - Either Leaf or Root site(s) per Attachment Circuit (AC)

   - Either Leaf or Root site(s) per MAC address

2.1 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
 

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   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 the communications among Leaf PEs. To prevent the
   communications 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 topology constrain in EVPN, two BGP
   Route-Targets (RTs) are used for every EVPN Instance (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 Root site(s) imports both Root and Leaf RTs, whereas a PE with
   Leaf site(s) only imports the Root RT. 

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

2.2 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). 

 

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

                              Figure 2: Scenario 2

   In this scenario, just like the previous scenario (in section 2.1),
   two Route Targets (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 and thus 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 apply the proper
   ingress filtering. 

   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
   Root or Leaf indication before advertisements 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)

   B) To allow for a single RT 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 5.1.

   Approach (A) would require the same data plane enhancements as
   approach (B) if MAC-VRF/bridge tables used per broadcast domain
   (e.g., VLAN) are to remain consistent with [RFC7432] (section 6). 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 and thus is not recommended.  

   Given that both approaches (A) and (B) would require exact 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 constrain
   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
 

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   Leaf/Root RTs will be used to constrain MAC advertisements and they
   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.   

   For this scenario, if for a given EVI, 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 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. 

2.3 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 and a PE may receive traffic from both Root AND Leaf
   sites on a single Attachment Circuit (AC) of 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. This scenario is considered in this document for EVPN
   service with only known unicast traffic because the Designated
   Forwarding (DF) filtering per [RFC7432] would not be compatible with
   the required egress filtering - i.e., Broadcast, Unknown, and
   Multicast (BUM) traffic is not supported in this scenario and it is
   dropped by the ingress PE.

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

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

                              Figure 3: Scenario 3

 

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   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. 

3 Operation for EVPN

   [RFC7432] defines the notion of 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 know unicast traffic, additional extensions to [RFC7432]
   is needed (i.e., a new BGP Extended Community for leaf indication
   described in section 5.1) in order to enable ingress filtering as
   described in detail in the following sections. 

3.1 Known Unicast Traffic

   Since in EVPN, MAC learning is performed in control plane via
   advertisement of BGP routes, the filtering needed by 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 MPLS/IP core to be filtered at the egress PE as 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 flag indicates that the MAC address is associated with a root
   site. This scheme applies to all scenarios described in section 2. 

   Tagging MAC addresses with a Leaf indication enables remote PEs to
   perform ingress filtering for known unicast traffic - i.e., on the
   ingress PE, the MAC destination address lookup yields, in addition to
   the forwarding adjacency, a flag which indicates whether the target
   MAC is associated with a Leaf site or not. 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 situation where MAC moves are allowed among Leaf and Root sites
   (e.g., non-static MAC), PEs can receive multiple MAC/IP
   advertisements routes for the same MAC address with different
   Leaf/Root indications (and possibly different ESIs for multi-homing
   scenarios). In such situations, MAC mobility procedures (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. 
 

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   To support the above ingress filtering functionality, a new E-Tree
   Extended Community with a Leaf indication flag is introduced [section
   5.1]. This new Extended Community MUST be advertised with MAC/IP
   Advertisement route learned from a Leaf site. Besides MAC/IP
   Advertisement route, no other EVPN routes are required to carry this
   new extended community.

3.2 Broadcast, Unkonwn, and Multicast (BUM) Traffic

   In this specification, the support for filtering BUM (Broadcast,
   Unknown, and Multicast) traffic does not include ingress filtering
   because it is not possible to do so, due to the multi-destination
   nature of BUM traffic. 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 when
   they originated from a Leaf AC - i.e., to be tagged with a Leaf label
   as specified in section 5.1. This Leaf label allows for disposition
   PE (e.g., egress PE) to perform the necessary egress filtering
   function in data-plane similar to ESI label in [RFC7432]. The
   allocation of the Leaf label is on a per PE basis (e.g., independent
   of ESI and EVI) as descried in the following sections. 

   The Leaf label can be upstream assigned for P2MP LSP or downstream
   assigned for ingress replication tunnels. The main difference between
   downstream and upstream assigned Leaf label is that in case of
   downstream assigned not all egress PE devices need to receive the
   label in MPLS encapsulated BUM packets just like ESI label for
   ingress replication procedures defined in [RFC7432].

   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). Other mechanisms
   for identifying root or Leaf sites such the use of source MAC address
   of the receiving frame are optional. The scenarios below are
   described in context of Root/Leaf AC; however, they can be extended
   to Root/Leaf MAC address if needed. 

 

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3.2.1 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 (Section 5.1) indicating 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 ES's on that PE. This Leaf label is
   advertised to other PE devices, using the E-Tree Extended Community
   (section 5.1) along with an Ethernet A-D per ES route with ESI of
   zero and a set of Route Targets (RTs) corresponding to all EVIs on
   the PE where each EVI has at least one Leaf site. Multiple Ethernet
   A-D per ES routes will need to be advertised if the number of Route
   Targets (RTs) that need to be carried exceed the limit on a single
   route per [RFC7432]. The ESI for the Ethernet A-D per 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.  

3.2.2 BUM Traffic Originated from a Single-homed Site on a Root AC

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

3.2.3 BUM Traffic Originated from a Multi-homed Site on a Leaf AC

   In this scenario, it is assumed that while different ACs (VLANs) on
   the same ES could have 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 - ie, the service
   is EVPN L2 and not EVPN IRB [EVPN-IRB]. IRB use cases described in
   [EVPN-IRB] 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 Leaf label described in
   section 3.2.1. 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 Ethernet Segment.  

3.2.4 BUM Traffic Originated from a Multi-homed Site on a Root AC

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

 

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3.3 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 & Leaf
        - Known unicast traffic from Leaf to Root
        - 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.

3.3.1 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 Ethernet Segments, 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 Ethernet Segments, and advertise
   reachability in EVPN MAC/IP Advertisement Routes. For scenarios where
   two different RTs are used per EVI (one to designate Root site and
   another to designate 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 A-D routes for aliasing (in
   the case of multi-homed segments) and 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 (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 describes
 

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   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
   transmit direction from itself to Leaf PEs and at the same time it
   wants to support ingress-replication tunnels in receive direction,
   the Root PE can signal it efficiently by using a new composite tunnel
   type defined in section 5.2.  This new composite tunnel type is
   advertised by the root PE to simultaneously indicate a P2MP tunnel in
   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., mLDP or RSVP-
   TE) originated at the Leaf PEs may be used and thus there will be no
   need to use the modified PMSI tunnel attribute and the composite
   tunnel type values defined in section 5.2.

3.3.2 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. 

   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 5.2 should be used.

4 Operation for PBB-EVPN

   In PBB-EVPN, the PE advertises a Root/Leaf indication along with each
   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 [5.1] is
   advertised with each EVPN MAC/IP Advertisement route.

   In the case where a multi-homed Ethernet Segment has both Root and
   Leaf sites attached, two B-MAC addresses are advertised: one B-MAC
 

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   address is per ES as specified in [RFC7623] and implicitly denoting 
   Root, and the other B-MAC address is per PE and explicitly denoting
   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 "Leaf-indication" flag
   is set. In such multi-homing scenarios where an Ethernet Segment has
   both Root and Leaf ACs, it is assumed that While different ACs
   (VLANs) on the same ES could have 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 - ie, the
   service is L2 and not IRB. 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 Ethernet Segment. The mechanism by which the PE identifies
   whether a given frame originated from a Root or Leaf site on the
   segment is based on 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 multi-homed segments.

4.1 Known Unicast Traffic

   For known unicast traffic, the PEs perform ingress filtering: On the
   ingress PE, the C-MAC destination address lookup yields, in addition
   to the target B-MAC address and forwarding adjacency, a flag which
   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, and if both are a Leaf, then the packet is not forwarded.

4.2 Broadcast, Unkonwn, and Multicast (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
 

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   advertised B-MAC is a Leaf, and the local Ethernet Segment 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 Ethernet Segment is a Leaf, then the source B-MAC
   address is removed from the B-MAC list corresponding to the local
   Ethernet Segment 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 baseline [RFC7623] for an
   ESI and does not require any additional flags in the data-plane.

   Just as in section 3.2, the PE places all Leaf Ethernet Segments 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.

4.3 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 4.2 without the need for C-MAC learning (in data-plane) in I-
   component (C-bridge table) of PBB-EVPN PEs (at both ingress and
   egress PEs).  

5 BGP Encoding

   This document defines a new BGP Extended Community for EVPN.

5.1 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
 

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

   This document defines the following flags:

        + Leaf-Indication (L)

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

   When this Extended Community (EC) is advertised along with MAC/IP
   Advertisement route (for known unicast traffic) per section 3.1, the
   Leaf-Indication flag MUST be set to one and Leaf Label 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 SHOULD ignore
   Leaf Label and only processes Leaf-Indication flag. A value of zero
   for Leaf-Indication flag is invalid when sent along with MAC/IP
   advertisement route and an error should be logged.  

   When this EC is advertised along with Ethernet A-D per ES route (with
   ESI of zero) for BUM traffic to enable egress filtering on
   disposition PEs per sections 3.2.1 and 3.2.3, the Leaf Label MUST be
   set to a valid MPLS label (i.e., non-reserved assigned MPLS label
   [RFC3032]) and the Leaf-Indication flag SHOULD be set to zero. The
   receiving PE SHOULD ignore the Leaf-Indication flag. A non-valid MPLS
   label when sent along with the Ethernet A-D per ES route, should be
   ignored and logged as an error. 

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

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5.2 PMSI Tunnel Attribute

   [RFC6514] defines 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)   |
         +---------------------------------+

         Figure 5: PMSI Tunnel Attribute 

   This document defines a new Composite tunnel type by introducing a
   new 'Composite Tunnel' bit in the Tunnel Type field and adding a MPLS
   label to the Tunnel Identifier field of PMSI Tunnel attribute as
   detailed below. This document uses all other remaining fields per
   existing definition. Composite tunnel type is advertised by the root
   PE to simultaneously indicate a non-(ingress replication) tunnel
   (e.g., P2MP tunnel) in transmit direction and an ingress-replication
   tunnel in the receive direction for the BUM traffic. 

   When receiver ingress-replication label is 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 would
   treat the tunnel type as an undefined tunnel type and would 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 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, and a
   PE that receives a PMSI Tunnel attribute with such information,
   considers it as malformed and it SHOULD treat this Update as though
 

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   all the routes contained in this Update had been withdrawn per
   section 5 of [RFC6514].  

6  Acknowledgement

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

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 additional
   security check by allowing sites (or ACs) of an EVPN instance to be
   designated as "Root" or "Leaf" and preventing any traffic exchange
   among "Leaf" sites of that VPN through ingress filtering for known
   unicast traffic and egress filtering for BUM traffic.   

8  IANA Considerations

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

         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 Tunnel (PMSI Tunnel) Tunnel Types"
   registry in the "Border Gateway Protocol (BGP) Parameters" registry
   needs to be updated to reflect the use of the most significant bit as
 

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   "Composite Tunnel" bit (section 5.2).

   For this purpose, this document updates [RFC7385].

   The registry is to be updated, by removing the entries for 0xFB-0xFE
   and 0x0F, and replacing them by:

   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 0x00 to 0x7A is IETF Review
   [RFC8126]. The range for experimental use is now 0x7B-0x7E, and value
   in this range are not to be assigned. The status of 0x7F may only be
   changed through Standards Action [RFC8126].  

9  References

9.1  Normative References

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

   [RFC8126] Cotton et al, "Guidelines for Writing an IANA
   Considerations Section in RFCs", June, 2017.

   [RFC7387] Key et al., "A Framework for E-Tree Service over MPLS
   Network", October 2014.

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

   [RFC7432] Sajassi et al., "BGP MPLS Based Ethernet VPN", February,
   2015.

   [RFC7623] Sajassi et al., "Provider Backbone Bridging Combined with
   Ethernet VPN (PBB-EVPN)", September, 2015.

   [RFC7385] Andersson et al., "IANA Registry for P-Multicast Service
   Interface (PMSI) Tunnel Type Code Points", October, 2014.

 

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   [RFC7153] Rosen et al., "IANA Registries for BGP Extended
   Communities",  March, 2014.

   [RFC6514] Aggarwal et al., "BGP Encodings and Procedures for
   Multicast in MPLS/BGP IP VPNs",  February, 2012.

   [RFC4360] Sangli et al., "BGP Extended Communities Attribute",
   February, 2006.

9.2  Informative References

   [RFC4360] S. Sangli et al, "BGP Extended Communities Attribute", 
   February, 2006.

   [RFC3032] E. Rosen et al, "MPLS Label Stack Encoding",  January 2001.

   [RFC7796] Y. Jiang et al, "Ethernet-Tree (E-Tree) Support in Virtual
   Private LAN Service (VPLS)", March 2016.

   [EVPN-IRB] A. Sajassi et al, "Integrated Routing and Bridging in
   EVPN", draft-ietf-bess-evpn-inter-subnet-forwarding-03, February 8,
   2017.

   [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.

Appendix-A

   When two MAC-VRFs (two bridge tables per VLANs) 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 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 either require two lookups
   be performed per MAC address in each direction in case of a miss, or
   duplicating many MAC addresses between the two bridge tables
   belonging to the same VLAN (same E-Tree instance). 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 Root bridge
   table and locally learned MAC addresses from Root ACs need to be
   duplicated onto 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
 

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   dataplane performance inefficiencies in some platforms and thus this
   document introduces the coloring as described in section 2.2 and
   detailed in section 3.1.

Contributors

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

   Wim Henderickx
   Nokia

   Aldrin Isaac
   Wen Lin
   Juniper

Authors' Addresses

   Ali Sajassi
   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
 

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   Email: jorge.rabadan@nokia.com 

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