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IETF RFC 7387
Last modified on Thursday, October 23rd, 2014
 Permanent link to RFC 7387
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Internet Engineering Task Force (IETF) R. Key, Ed.
Request for Comments: 7387
Category: Informational L. Yong, Ed.
ISSN: 2070-1721 Huawei
S. Delord
Telstra
F. Jounay
Orange CH
L. Jin
October 2014
A Framework for Ethernet Tree (E-Tree) Service
over a Multiprotocol Label Switching (MPLS) Network
 Abstract
This document describes an Ethernet-Tree (E-Tree) solution framework
for supporting the Metro Ethernet Forum (MEF) E-Tree service over a
Multiprotocol Label Switching (MPLS) network. The objective is to
provide a simple and effective approach to emulate E-Tree services in
addition to Ethernet LAN (E-LAN) services on an existing MPLS
network.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7387.
Key, et al. Informational PAGE 1 
RFC 7387 E-Tree Framework October 2014
Copyright Notice
Copyright (c) 2014 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................3
2. Overview ........................................................4
2.1. Ethernet Bridge Network ....................................4
2.2. MEF Multipoint Ethernet Services: E-LAN and E-Tree .........4
2.3. IETF L2VPN .................................................5
2.3.1. Virtual Private LAN Service (VPLS) ..................5
2.3.2. Ethernet VPN (EVPN) .................................5
2.3.3. Virtual Private Multicast Service (VPMS) ............6
3. E-Tree Architecture Reference Model .............................6
4. E-Tree Use Cases ................................................8
5. L2VPN Gaps for Emulating MEF E-Tree Service .....................9
5.1. No Differentiation on AC Role ..............................9
5.2. No AC Role Indication or Advertisement .....................9
5.3. Other Issues ...............................................9
6. Security Considerations ........................................10
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................11
Acknowledgments ...................................................12
Contributors ......................................................12
Authors' Addresses ................................................13
Key, et al. Informational PAGE 2
RFC 7387 E-Tree Framework October 2014
1. Introduction
This document describes an Ethernet-Tree (E-Tree) solution framework
for supporting the Metro Ethernet Forum (MEF) E-Tree service over a
Multiprotocol Label Switching (MPLS) network. The objective is to
provide a simple and effective approach to emulate E-Tree services in
addition to Ethernet LAN (E-LAN) services on an existing MPLS
network.
This document extends the existing IETF-specified Layer 2 Virtual
Private Network (L2VPN) framework [RFC 4664] to provide the emulation
of E-Tree services over an MPLS network. It specifies the E-Tree
architecture reference model and describes the corresponding
functional components. It also points out the gaps and required
extension areas in existing L2VPN solutions such as Virtual Private
LAN Service (VPLS) [RFC 4761] [RFC 4762] and Ethernet Virtual Private
Network (EVPN) [EVPN] for supporting E-Tree services.
1.1. Terminology
This document adopts all the terminologies defined in RFC 4664
[RFC 4664], RFC 4761 [RFC 4761], and RFC 4762 [RFC 4762]. It also uses
the following terms:
Leaf Attachment Circuit (AC): An AC with Leaf role. An ingress
Ethernet frame at a Leaf AC (Ethernet frame arriving over an AC at
the Provider Edge (PE) of an MPLS network) can only be delivered
to one or more Root ACs in an E-Tree service instance. An ingress
Ethernet frame at a Leaf AC must not be delivered to any Leaf ACs
in the E-Tree service instance.
Root AC: An AC with Root role. An ingress Ethernet frame at a Root
AC can be delivered to one or more of the other ACs in the
associated E-Tree service instance.
E-Tree: An Ethernet VPN service in which each AC is assigned the role
of a Root or Leaf. The forwarding rules in an E-Tree are as
follows:
o The Root AC can communicate with other Root ACs and Leaf ACs.
o Leaf ACs can only communicate with Root ACs.
Key, et al. Informational PAGE 3
RFC 7387 E-Tree Framework October 2014
2. Overview
2.1. Ethernet Bridge Network
In this document, "Ethernet bridge network" refers to the Ethernet
bridge/switch network defined in IEEE 802.1Q [IEEE802.1Q]. In a
bridge network, a data frame is an Ethernet frame; data forwarding is
based on destination Media Access Control (MAC) address; MAC
reachability is learned in the data plane based on the source MAC
address and the port (or tagged port) on which the frame arrives; and
the MAC aging mechanism is used to remove inactive MAC addresses from
the MAC forwarding table on an Ethernet switch.
Data frames arriving at a switch may be destined to known unicast,
unknown unicast, multicast, or broadcast MAC destinations. Unknown
unicast, multicast, and broadcast frames are forwarded in a similar
way, i.e., to every port except the ingress port on which the frame
arrives. Multicast forwarding can be further constrained when using
multicast control protocol snooping or using multicast MAC
registration protocols [IEEE802.1Q].
An Ethernet host receiving an Ethernet frame checks the destination
address in the frame to decide whether it is the intended
destination.
2.2. MEF Multipoint Ethernet Services: E-LAN and E-Tree
MEF 6.1 [MEF6.1] defines two multipoint Ethernet Service types:
o E-LAN (Ethernet LAN), a multipoint-to-multipoint service
o E-Tree (Ethernet Tree), a rooted-multipoint service
The MEF defines User-Network Interface (UNI) in a multipoint service
as the Ethernet interface between Customer Equipment (CE) and a
Provider Edge (PE), i.e., the PE can send and receive Ethernet frames
to/from the CE. The MEF also defines UNI roles in a multipoint
service. One role is Root, and another is Leaf.
Note that MEF UNI in a service is equivalent to the Attachment
Circuit (AC) defined in L2VPN [RFC 4664]. The Root AC and Leaf AC
defined in this document are the same as the Root UNI and Leaf UNI as
defined in MEF 10.3 [MEF10.3]. The terms "Root AC" and "Leaf AC" are
used in the following MEF service description.
Key, et al. Informational PAGE 4
RFC 7387 E-Tree Framework October 2014
For an E-LAN service, all ACs have the Root role, which means that
any AC can communicate with other ACs in the service. The E-LAN
service defined by the MEF may be implemented by IETF L2VPN solutions
such as VPLS and EVPN [EVPN].
An E-Tree service has one or more Root ACs and at least two Leaf ACs.
An E-Tree service supports communication among the roots and between
a root and a leaf but prohibits communication among the leaves.
Existing IETF L2VPN solutions can't support the E-Tree service. This
document specifies the E-Tree architecture reference model that
supports the E-Tree service defined by the MEF [MEF6.1]. Section 4
will discuss different E-Tree use cases.
2.3. IETF L2VPN
2.3.1. Virtual Private LAN Service (VPLS)
VPLS [RFC 4761] [RFC 4762] is an L2VPN solution that provides
multipoint-to-multipoint Ethernet connectivity across IP/MPLS
networks. VPLS emulates traditional Ethernet Virtual LAN (VLAN)
services in MPLS networks and may support MEF E-LAN services.
A data frame in VPLS is an Ethernet frame. Data forwarding in a VPLS
instance is based on the destination MAC address and the VLAN on
which the frame arrives. MAC reachability learning is performed in
the data plane based on the source address and the AC or pseudowire
(PW) on which the frame arrives. MAC aging is the mechanism used to
remove inactive MAC addresses from a VPLS switching instance (VSI) on
a PE. VPLS supports forwarding for known unicast frames, as well as
unknown unicast, broadcast, and multicast Ethernet frames.
Many service providers have deployed VPLS in their networks to
provide L2VPN services to customers.
2.3.2. Ethernet VPN (EVPN)
Ethernet VPN [EVPN] is an enhanced L2VPN solution that emulates an
Ethernet LAN or virtual LAN(s) across MPLS networks.
EVPN supports active-active multihoming of CEs and uses the
Multiprotocol Border Gateway Protocol (MP-BGP) control plane to
advertise MAC address reachability from an ingress PE to egress PEs.
Thus, a PE learns MAC addresses that are reachable over local ACs in
the data plane and other MAC addresses reachable across the MPLS
network over remote ACs via the EVPN MP-BGP control plane. As a
result, EVPN aims to support large-scale L2VPN with better resiliency
compared to VPLS.
Key, et al. Informational PAGE 5
RFC 7387 E-Tree Framework October 2014
EVPN is a relatively new technique and is still under development in
the IETF L2VPN WG.
2.3.3. Virtual Private Multicast Service (VPMS)
VPMS [VPMS] is an L2VPN solution that provides point-to-multipoint
connectivity across MPLS networks and supports various attachment
circuit (AC) types, including Frame Relay, ATM, Ethernet, PPP, etc.
In the case of Ethernet ACs, VPMS provides single coverage of
receiver membership, i.e., there is no differentiation among
multicast groups in one VPN. The destination address in the Ethernet
frame is not used in data forwarding.
VPMS supports unidirectional point-to-multipoint transport from a
sender to multiple receivers and may support reverse transport in a
point-to-point manner.
3. E-Tree Architecture Reference Model
Figure 1 illustrates the E-Tree architecture reference model. Three
Provider Edges -- PE1, PE2, and PE3 -- are shown in the figure. Each
PE has a Virtual Service Instance (VSI) associated with an E-Tree
service instance. A CE attaches to the VSI on a PE via an AC. Each
AC must be configured with a Root or Leaf role. In Figure 1, AC1,
AC2, AC5, AC6, AC9, and AC10 are Root ACs; AC3, AC4, AC7, AC8, AC11,
and AC12 are Leaf ACs. This implies that a PE (local or remote)
processes the Ethernet frames from CE01, CE02, etc., as if they
originated from a Root AC, and it processes the Ethernet frames from
CE03, CE04, etc., as if they originated from a Leaf AC.
Under this architecture model, the forwarding rules among the ACs,
regardless of whether the sending AC and receiving AC are on the same
PE or on different PEs, are described as follows:
o An egress frame (the frame to be transmitted over an AC) at an AC
with Root role must be the result of an ingress frame at an AC
(the frame received at an AC) that has Root or Leaf role and is
attached to the same E-Tree service instance.
o An egress frame at the AC with Leaf role must be the result of an
ingress frame at an AC that has Root role and is attached to the
same E-Tree service instance.
Key, et al. Informational PAGE 6
RFC 7387 E-Tree Framework October 2014
<------------E-Tree----------->
PE1+---------+ +---------+PE2
+----+ | +---+ | | +---+ | +----+
|CE01+----AC1----+--+ | | | | +--+----AC5----+CE05|
+----+ (Root AC) | | V | | | | V | | (Root AC) +----+
+----+ | | | | | | | | +----+
|CE02+----AC2----+--+ | | | | +--+----AC6----+CE06|
+----+ (Root AC) | | S +--+---------+--+ S | | (Root AC) +----+
+----+ | | | | | | | | +----+
|CE03+----AC3----+--+ | | | | +--+----AC7----+CE07|
+----+ (Leaf AC) | | I | | | | I | | (Leaf AC) +----+
+----+ | | | | | | | | +----+
|CE04+----AC4----+--+ | | | | +--+----AC8----+CE08|
+----+ (Leaf AC) | +-+-+ | | +-+-+ | (Leaf AC) +----+
+----+----+ +----+----+
| MPLS Core |
| +----+----+
| | +-+-+ | +----+
| | | +--+----AC9----+CE09|
| | | V | | (Root AC) +----+
| | | | | +----+
| | | +--+----AC10---+CE10|
+--------------+--+ S | | (Root AC) +----+
| | | | +----+
| | +--+----AC11---+CE11|
| | I | | (Leaf AC) +----+
| | | | +----+
| | +--+----AC12---+CE12|
| +---+ | (Leaf AC) +----+
PE3 +---------+
<-------------E-Tree---------->
Figure 1: E-Tree Architecture Reference Model
These rules apply to all frame types, i.e., known unicast, unknown
unicast, broadcast, and multicast. For known unicast frames,
forwarding in a VSI context is based on the destination MAC address.
A VSI on a PE corresponds to an E-Tree service instance and maintains
a MAC forwarding table that is isolated from other VSI tables on the
PE. It also keeps track of local AC roles. The VSI receives a frame
from an AC or across the MPLS core, and it forwards the frame to
another AC over which the destination is reachable according to the
VSI forwarding table and forwarding rules described above. When the
target AC is on a remote PE, the VSI forwards the frame to the remote
PE over the MPLS core. Forwarding over the MPLS core will be
dependent on the E-Tree solution. For instance, a solution may adopt
PWs to mesh VSIs as in VPLS and to forward frames over VSIs subject
Key, et al. Informational PAGE 7
RFC 7387 E-Tree Framework October 2014
to the E-Tree forwarding rules. Alternatively, a solution may adopt
the EVPN forwarding paradigm constrained by the E-Tree forwarding
rules. Thus, solutions that satisfy the E-Tree requirements could be
extensions to VPLS and EVPN.
In most use cases, an E-Tree service has only a few Root ACs (root CE
sites) but many Leaf ACs (leaf CE sites). Furthermore, a PE may have
only Root ACs or only Leaf ACs. Figure 1 provides a general E-Tree
architecture model.
4. E-Tree Use Cases
Table 1 below presents some major use cases for E-Tree.
+---------------------------+--------------+------------+
| Use Case | Root AC | Leaf AC |
+---+---------------------------+--------------+------------+
| 1 | Hub & Spoke VPN | Hub Site | Spoke Site |
+---+---------------------------+--------------+------------+
| 2 | Wholesale Access | Customer's | Customer's |
| | | Interconnect | Subscriber |
+---+---------------------------+--------------+------------+
| 3 | Mobile Backhaul | RAN NC | RAN BS |
+---+---------------------------+--------------+------------+
| 4 | IEEE 1588 PTPv2 [IEEE1588]| PTP Server | PTP Client |
| | Clock Synchronization | | |
+---+---------------------------+--------------+------------+
| 5 | Internet Access | BNG Router | Subscriber |
| | Reference [TR-101] | | |
+---+---------------------------+--------------+------------+
| 6 | Broadcast Video | Video Source | Subscriber |
| | (unidirectional only) | | |
+---+---------------------------+--------------+------------+
| 7 | Broadcast/Multicast Video | Video Source | Subscriber |
| | plus Control Channel | | |
+---+---------------------------+--------------+------------+
| 8 | Device Management | Management | Managed |
| | | System | Device |
+---+---------------------------+--------------+------------+
Where:
RAN: Radio Access Network NC: Network Controller
BS: Base Station PTP: Precision Time Protocol
BNG: Broadband Network Gateway
Table 1: E-Tree Use Cases
Key, et al. Informational PAGE 8
RFC 7387 E-Tree Framework October 2014
Common to all use cases, direct Layer 2 leaf-to-leaf communication is
required to be prohibited. For mobile backhaul, this may not be
valid for Long Term Evolution (LTE) X2 interfaces; an LTE X2
interface [LTE] enables communication between two evolved Node Bs
(eNBs). E-Tree service is appropriate for such use cases.
Also common to the use cases mentioned above, there may be one or
multiple Root ACs in one E-Tree service. The need for multiple Root
ACs may be driven by a redundancy requirement or by having multiple
serving sites. Whether a particular E-Tree service needs to support
one or multiple Root ACs depends on the application.
5. L2VPN Gaps for Emulating MEF E-Tree Service
The MEF E-Tree service defines special forwarding rules that prohibit
forwarding Ethernet frames among leaves. This poses some challenges
to IETF L2VPN solutions such as VPLS and EVPN in emulating E-Tree
service over an MPLS network. There are two major issues described
in the following subsections.
5.1. No Differentiation on AC Role
IP/MPLS L2VPN architecture has no distinct roles on Attachment
Circuits (ACs) and supports any-to-any connectivity among all ACs.
It does not have any mechanism to support forwarding constraints
based on an AC role. However, the MEF E-Tree service defines two AC
roles -- Root and Leaf -- and defines the forwarding rules based on
the originating and receiving AC roles of a given frame.
5.2. No AC Role Indication or Advertisement
In an L2VPN, when a PE, say PE2, receives a frame from another PE,
say PE1, over the MPLS core, PE2 does not know if the frame from PE1
is originated from a Root AC or Leaf AC. This causes the forwarding
issue on PE2 because the E-Tree forwarding rules require that the
forwarder must know the role of the frame origin, i.e., from Root AC
or Leaf AC. This is specifically important when PE2 has both Root AC
and Leaf AC attached to the VSI. E-Tree forwarding rules apply to
all types of frames (known unicast destination, unknown unicast
destination, multicast, and broadcast).
5.3. Other Issues
Some desirable requirements for IETF E-Tree are specific to an
IP/MPLS L2VPN implementation such as Leaf-only PE. A Leaf-only PE is
a PE that only has Leaf AC(s) in an E-Tree service instance; thus,
other PEs on the same E-Tree service instance do not necessarily
forward the frames originated from a Leaf AC to the Leaf-only PE,
Key, et al. Informational PAGE 9
RFC 7387 E-Tree Framework October 2014
which may save some network resources. It is also desirable for an
E-Tree solution to work with existing PEs that support single-role
ACs, where the role is equivalent to the root in an E-Tree service.
These requirements are described in the E-Tree requirement document
[RFC 7152].
6. Security Considerations
An E-Tree service may be deployed for security reasons to prohibit
communication among sites (leaves). An E-Tree solution must enforce
E-Tree forwarding constraints. The solution must also guarantee that
Ethernet frames do not leak outside of the E-Tree service instance to
which they belong.
An E-Tree service prohibits communication among leaf sites but does
not have knowledge of higher-layer security constraints. Therefore,
in general, higher-layer applications cannot rely on E-Tree to
provide security protection unless all security constraints are fully
implemented by the E-Tree service.
Enhancing L2VPN for E-Tree services inherits the same security issues
described in the L2VPN framework document [RFC 4664]. These relate to
both control-plane and data-plane security issues that may arise in
the following areas:
o issues fully contained in the provider network
o issues fully contained in the customer network
o issues in the customer-provider interface network
The framework document has substantial discussions on the security
issues and potential solutions to address them. An E-Tree solution
must consider these issues and address them properly. VPLS [RFC 4761]
[RFC 4762] and/or EVPN [EVPN] will likely be candidate solutions for
an E-Tree service over an MPLS network. The security capabilities
built into those solutions will be naturally adopted when supporting
E-Tree. For details, see the Security Considerations sections in
[RFC 4761], [RFC 4762], and [EVPN].
Key, et al. Informational PAGE 10
RFC 7387 E-Tree Framework October 2014
7. References
7.1. Normative References
[MEF6.1] Metro Ethernet Forum, "Ethernet Services Definitions -
Phase 2", MEF 6.1, April 2008.
[MEF10.3] Metro Ethernet Forum, "Ethernet Service Attributes -
Phase 3", MEF 10.3, October 2013.
[RFC 4664] Andersson, L., Ed., and E. Rosen, Ed., "Framework for
Layer 2 Virtual Private Networks (L2VPNs)", RFC 4664,
September 2006,
<http://www.rfc-editor.org/info/RFC 4664>.
[RFC 4761] Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, January 2007,
<http://www.rfc-editor.org/info/RFC 4761>.
[RFC 4762] Lasserre, M., Ed., and V. Kompella, Ed., "Virtual
Private LAN Service (VPLS) Using Label Distribution
Protocol (LDP) Signaling", RFC 4762, January 2007,
<http://www.rfc-editor.org/info/RFC 4762>.
[RFC 7152] Key, R., Ed., DeLord, S., Jounay, F., Huang, L., Liu,
Z., and M. Paul, "Requirements for Metro Ethernet Forum
(MEF) Ethernet-Tree (E-Tree) Support in Layer 2 Virtual
Private Network (L2VPN)", RFC 7152, March 2014,
<http://www.rfc-editor.org/info/RFC 7152>.
7.2. Informative References
[IEEE802.1Q] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Media Access Control (MAC) Bridges and
Virtual Bridged Local Area", IEEE Std 802.1Q, 2011.
[IEEE1588] IEEE, "IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", IEEE Std 1588, July 2008.
[LTE] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA) and Evolved Universal Terrestrial Radio Access
Network (E-UTRAN)", 3GPP TS 36.300 v11.2.0, July 2012.
[TR-101] Broadband Forum, "Migration to Ethernet-Based Broadband
Aggregation", TR-101 Issue 2, July 2011.
Key, et al. Informational PAGE 11
RFC 7387 E-Tree Framework October 2014
[VPMS] Kamite, Y., Jounay, F., Niven-Jenkins, B., Brungard, D.,
and L. Jin, "Framework and Requirements for Virtual
Private Multicast Service (VPMS)", Work in Progress,
draft-ietf-l2vpn-vpms-frmwk-requirements-05,
October 2012.
[EVPN] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
and J. Uttaro, "BGP MPLS Based Ethernet VPN", Work in
Progress, draft-ietf-l2vpn-evpn-11, October 2014.
Acknowledgments
The authors would like to thank Nabil Bitar and Adrian Farrel for
their detailed review and suggestions.
Contributors
The following people contributed significantly to this document.
Yuji Kamite
NTT Communications Corporation
Granpark Tower
3-4-1 Shibaura, Minato-ku
Tokyo 108-8118, Japan
EMail: y.kamite@ntt.com
Wim Henderickx
Alcatel-Lucent
Copernicuslaan 50
2018 Antwerp, Belgium
EMail: wim.henderickx@alcatel-lucent.com
Key, et al. Informational PAGE 12
RFC 7387 E-Tree Framework October 2014
Authors' Addresses
Raymond Key (editor)
EMail: raymond.key@ieee.org
Lucy Yong (editor)
Huawei USA
EMail: lucy.yong@huawei.com
Simon Delord
Telstra
EMail: simon.delord@gmail.com
Frederic Jounay
Orange CH
4 rue caudray 1020 Renens
Switzerland
EMail: frederic.jounay@orange.ch
Lizhong Jin
EMail: lizho.jin@gmail.com
Key, et al. Informational PAGE 13
RFC TOTAL SIZE: 28205 bytes
PUBLICATION DATE: Thursday, October 23rd, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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