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rfc:rfc6246

Internet Engineering Task Force (IETF) A. Sajassi, Ed. Request for Comments: 6246 F. Brockners Category: Informational Cisco Systems ISSN: 2070-1721 D. Mohan, Ed.

                                                                Nortel
                                                            Y. Serbest
                                                                  AT&T
                                                             June 2011
        Virtual Private LAN Service (VPLS) Interoperability
                  with Customer Edge (CE) Bridges

Abstract

 One of the main motivations behind Virtual Private LAN Service (VPLS)
 is its ability to provide connectivity not only among customer
 routers and servers/hosts but also among customer IEEE bridges.  VPLS
 is expected to deliver the same level of service that current
 enterprise users are accustomed to from their own enterprise bridged
 networks or their Ethernet Service Providers.
 When customer edge (CE) devices are IEEE bridges, then there are
 certain issues and challenges that need to be accounted for in a VPLS
 network.  The majority of these issues have been addressed in the
 IEEE 802.1ad standard for provider bridges and they can be leveraged
 for VPLS networks.  This document extends the provider edge (PE)
 model described in RFC 4664 based on IEEE 802.1ad bridge module, and
 it illustrates a clear demarcation between the IEEE bridge module and
 IETF LAN emulation module.  By doing so, it shows that the majority
 of interoperability issues with CE bridges can be delegated to the
 802.1ad bridge module, thus removing the burden on the IETF LAN
 emulation module within a VPLS PE.

Sajassi, et al. Informational [Page 1] RFC 6246 VPLS Interop with CE Bridges June 2011

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/rfc6246.

Copyright Notice

 Copyright (c) 2011 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.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Sajassi, et al. Informational [Page 2] RFC 6246 VPLS Interop with CE Bridges June 2011

Table of Contents

 1. Introduction ....................................................3
    1.1. Conventions ................................................4
 2. Ethernet Service Instance .......................................4
 3. VPLS-Capable PE Model with Bridge Module ........................5
 4. Mandatory Issues ................................................8
    4.1. Service Mapping ............................................8
    4.2. CE Bridge Protocol Handling ...............................10
    4.3. Partial Mesh of Pseudowires ...............................11
    4.4. Multicast Traffic .........................................12
 5. Optional Issues ................................................13
    5.1. Customer Network Topology Changes .........................13
    5.2. Redundancy ................................................15
    5.3. MAC Address Learning ......................................16
 6. Interoperability with 802.1ad Networks .........................17
 7. Acknowledgments ................................................17
 8. Security Considerations ........................................17
 9. Normative References ...........................................18
 10. Informative References ........................................19

1. Introduction

 Virtual Private LAN Service (VPLS) is a LAN emulation service
 intended for providing connectivity between geographically dispersed
 customer sites across MANs/WANs (over MPLS/IP), as if they were
 connected using a LAN.  One of the main motivations behind VPLS is
 its ability to provide connectivity not only among customer routers
 and servers/hosts but also among IEEE customer bridges.  If only
 connectivity among customer IP routers/hosts is desired, then an IP-
 only LAN Service [IPLS] solution could be used.  The strength of the
 VPLS solution is that it can provide connectivity to both bridge and
 non-bridge types of CE devices.  VPLS is expected to deliver the same
 level of service that current enterprise users are accustomed to from
 their own enterprise bridged networks [802.1D] [802.1Q] today or the
 same level of service that they receive from their Ethernet Service
 Providers using IEEE 802.1ad-based networks [802.1ad] (or its
 predecessor, QinQ-based networks).
 When CE devices are IEEE bridges, then there are certain issues and
 challenges that need to be accounted for in a VPLS network.  The
 majority of these issues have been addressed in the IEEE 802.1ad
 standard for provider bridges and they can be leveraged for VPLS
 networks.  This document extends the PE model described in [RFC4664]
 based on the IEEE 802.1ad bridge module and illustrates a clear
 demarcation between IEEE bridge module and IETF LAN emulation module.
 By doing so, it describes that the majority of interoperability
 issues with CE bridges can be delegated to the 802.1ad bridge module,

Sajassi, et al. Informational [Page 3] RFC 6246 VPLS Interop with CE Bridges June 2011

 thus removing the burden on the IETF LAN emulation module within a
 VPLS PE.  This document discusses these issues and, wherever
 possible, suggests areas to be explored in rectifying these issues.
 The detailed solution specification for these issues is outside of
 the scope of this document.
 This document also discusses interoperability issues between VPLS and
 IEEE 802.1ad networks when the end-to-end service spans across both
 types of networks, as outlined in [RFC4762].
 This document categorizes the CE-bridge issues into two groups: 1)
 mandatory and 2) optional.  The issues in group (1) need to be
 addressed in order to ensure the proper operation of CE bridges.  The
 issues in group (2) would provide additional operational improvement
 and efficiency and may not be required for interoperability with CE
 bridges.  Sections 5 and 6 discuss these mandatory and optional
 issues, respectively.

1.1. Conventions

 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 [RFC2119].

2. Ethernet Service Instance

 Before starting the discussion of bridging issues, it is important to
 clarify the Ethernet Service definition.  The term VPLS has different
 meanings in different contexts.  In general, VPLS is used in the
 following contexts [RFC6136]: a) as an end-to-end bridged LAN service
 over one or more networks (one of which is an MPLS/IP network), b) as
 an MPLS/IP network supporting these bridged LAN services, and c) as
 (V)LAN emulation.  For better clarity, we differentiate between its
 usage as network versus service by using the terms VPLS network and
 VPLS instance, respectively.  Furthermore, we confine VPLS (both
 network and service) to only the portion of the end-to-end network
 that spans an MPLS/IP network.  For an end-to-end service (among
 different sites of a given customer), we use the term "Ethernet
 Service Instance" or ESI.
 We define the Ethernet Service Instance (ESI) as an association of
 two or more Attachment Circuits (ACs) over which an Ethernet service
 is offered to a given customer.  An AC can be either a User-Network
 Interface (UNI) or a Network-Network Interface (NNI); furthermore, it
 can be an Ethernet interface or a VLAN, it can be an ATM or Frame
 Relay Virtual Circuit, or it can be a PPP/HDLC (PPP/High-Level Data

Sajassi, et al. Informational [Page 4] RFC 6246 VPLS Interop with CE Bridges June 2011

 Link Control) interface.  If an ESI is associated with more than two
 ACs, then it is a multipoint ESI.  In this document, wherever the
 keyword ESI is used, it means multipoint ESI unless stated otherwise.
 An ESI can correspond to a VPLS instance if its associated ACs are
 only connected to a VPLS network, or an ESI can correspond to a
 Service VLAN if its associated ACs are only connected to a Provider-
 Bridged network [802.1ad].  Furthermore, an ESI can be associated
 with both a VPLS instance and a Service VLAN when considering an end-
 to-end service that spans across both VPLS and Provider-Bridged
 networks.  An ESI can span across different networks (e.g., IEEE
 802.1ad and VPLS) belonging to the same or different administrative
 domains.
 An ESI most often represents a customer or a specific service
 requested by a customer.  Since traffic isolation among different
 customers (or their associated services) is of paramount importance
 in service provider networks, its realization shall be done such that
 it provides a separate Media Access Control (MAC) address domain and
 broadcast domain per ESI.  A separate MAC address domain is provided
 by using a separate MAC forwarding table (e.g., Forwarding
 Information Base (FIB), also known as filtering database [802.1D])
 per ESI (for both VPLS and IEEE 802.1ad networks).  A separate
 broadcast domain is provided by using a full mesh of pseudowires per
 ESI over the IP/MPLS core in a VPLS network and/or a dedicated
 Service VLAN per ESI in an IEEE 802.1ad network.

3. VPLS-Capable PE Model with Bridge Module

 [RFC4664] defines three models for VPLS-capable PE (VPLS-PE), based
 on the bridging functionality that needs to be supported by the PE.
 If the CE devices can be routers/hosts or IEEE bridges, the second
 model from [RFC4664] is the most suitable, and it is both adequate to
 provide the VPLS level of service and consistent with the IEEE
 standards for Provider Bridges [802.1ad].  We briefly describe the
 second model and then expand upon this model to show its sub-
 components based on the [802.1ad] Provider Bridge model.
 As described in [RFC4664], the second model for VPLS-PE contains a
 single bridge module supporting all the VPLS instances on that PE ,
 where each VPLS instance is represented by a unique VLAN inside that
 bridge module (also known as a Service VLAN or S-VLAN).  The bridge
 module has a single "Emulated LAN" interface over which it
 communicates with all VPLS forwarders, and each VPLS instance is
 represented by a unique S-VLAN tag.  Each VPLS instance can consist
 of a set of pseudowires, and its associated forwarder can correspond
 to a single VLAN as depicted in Figure 1 below.  Thus, sometimes it
 is referred to as VLAN emulation.

Sajassi, et al. Informational [Page 5] RFC 6246 VPLS Interop with CE Bridges June 2011

    +----------------------------------------+
    |           VPLS-Capable PE Model        |
    |   +---------------+          +------+  |
    |   |               |          |VPLS-1|------------
    |   |               |=======+  |Fwdr  |------------ PWs
    |   |     Bridge    --------|---      |------------
    |   |               | SVID-1|  +------+  |
    |   |     Module    |       |     o      |
    |   |               |       |     o      |
    |   |   (802.1ad    |       |     o      |
    |   |    bridge)    |       |     o      |
    |   |               |       |     o      |
    |   |               | SVID-n|  +------+  |
    |   |               --------|---VPLS-n|-------------
    |   |               |=======+  | Fwdr |------------- PWs
    |   |               |   ^      |      |-------------
    |   +---------------+   |      +------+  |
    |                       |                |
    +-----------------------|----------------+
                            |
            LAN emulation (multi-access) interface
               Figure 1.  VPLS-Capable PE Model
 Customer frames associated with a given ESI carry the S-VLAN ID for
 that ESI over the LAN emulation interface.  The S-VLAN ID is stripped
 before transmitting the frames over the set of pseudowires (PWs)
 associated with that VPLS instance (assuming raw mode PWs are used as
 specified in [RFC4448]).
 The bridge module can itself consist of one or two sub-components,
 depending on the functionality that it needs to perform.  Figure 2
 depicts the model for the bridge module based on [802.1ad].

Sajassi, et al. Informational [Page 6] RFC 6246 VPLS Interop with CE Bridges June 2011

               +-------------------------------+
               |  802.1ad Bridge Module Model  |
               |                               |
    +---+  AC  |  +------+      +-----------+  |
    |CE |---------|C-VLAN|------|           |  |
    +---+      |  |bridge|------|           |  |
               |  +------+      |           |  |
               |     o          |   S-VLAN  |  |
               |     o          |           |  | ---> to VPLS Fwdr
               |     o          |   Bridge  |  |
    +---+  AC  |  +------+      |           |  |
    |CE |---------|C-VLAN|------|           |  |
    +---+      |  |bridge|------|           |  |
               |  +------+      |           |  |
    +---+  AC  |                |           |  |
    |CE |-----------------------|           |  |
    +---+      |                +-----------+  |
               +-------------------------------+
           Figure 2.  Model of the 802.1ad Bridge Module
 The S-VLAN bridge component is always required and it is responsible
 for tagging customer frames with S-VLAN tags in the ingress direction
 (from customer UNIs) and removing S-VLAN tags in the egress direction
 (toward customer UNIs).  It is also responsible for running the
 provider's bridge protocol -- such as Rapid Spanning Tree Protocol
 (RSTP), Multiple Spanning Tree Protocol (MSTP), Generic VLAN
 Registration Protocol (GVRP), GARP Multicast Registration Protocol
 (GMRP), etc. -- among provider bridges within a single administrative
 domain.
 The customer VLAN (C-VLAN) bridge component is required when the
 customer Attachment Circuits are VLANs (aka C-VLANs).  In such cases,
 the VPLS-capable PE needs to participate in some of the customer's
 bridging protocol such as RSTP and MSTP.  Such participation is
 required because a C-VLAN at one site can be mapped into a different
 C-VLAN at a different site or, in case of asymmetric mapping, a
 customer Ethernet port at one site can be mapped into a C-VLAN (or
 group of C-VLANs) at a different site.
 The C-VLAN bridge component does service selection and identification
 based on C-VLAN tags.  Each frame from the customer device is
 assigned to a C-VLAN and presented at one or more internal port-based
 interfaces, each supporting a single service instance that the
 customer desires to carry that C-VLAN.  Similarly, frames from the
 provider network are assigned to an internal interface or 'LAN' (e.g,
 between C-VLAN and S-VLAN components) on the basis of the S-VLAN tag.
 Since each internal interface supports a single service instance, the

Sajassi, et al. Informational [Page 7] RFC 6246 VPLS Interop with CE Bridges June 2011

 S-VLAN tag can be, and is, removed at this interface by the S-VLAN
 bridge component.  If multiple C-VLANs are supported by this service
 instance (e.g., via VLAN bundling or port-based service), then the
 frames will have already been tagged with C-VLAN tags.  If a single
 C-VLAN is supported by this service instance (e.g., VLAN-based), then
 the frames will not have been tagged with a C-VLAN tag since C-VLAN
 can be derived from the S-VLAN (e.g., one-to-one mapping).  The
 C-VLAN-aware bridge component applies a port VLAN ID (PVID) to
 untagged frames received on each internal 'LAN', allowing full
 control over the delivery of frames for each C-VLAN through the
 Customer UNI Port.

4. Mandatory Issues

4.1. Service Mapping

 Different Ethernet AC types can be associated with a single Ethernet
 Service Instance (ESI).  For example, an ESI can be associated with
 only physical Ethernet ports, VLANs, or a combination of the two
 (e.g., one end of the service could be associated with physical
 Ethernet ports and the other end could be associated with VLANs).  In
 [RFC4762], unqualified and qualified learning are used to refer to
 port-based and VLAN-based operation, respectively.  [RFC4762] does
 not describe the possible mappings between different types of
 Ethernet ACs (e.g., 802.1D, 802.1Q, or 802.1ad frames).  In general,
 the mapping of a customer port or VLAN to a given service instance is
 a local function performed by the local PE, and the service
 provisioning shall accommodate it.  In other words, there is no
 reason to restrict and limit an ESI to have only port-based ACs or to
 have only VLAN-based ACs.  [802.1ad] allows for each customer AC
 (either a physical port, a VLAN, or a group of VLANs) to be mapped
 independently to an ESI that provides better service offerings to
 enterprise customers.  For better and more flexible service offerings
 and for interoperability purposes between VPLS and 802.1ad networks,
 it is imperative that both networks offer the same capabilities in
 terms of customer ACs mapping to the customer service instance.
 The following table lists possible mappings that can exist between
 customer ACs and their associated ESIs.  As can be seen, there are
 several possible ways to perform such mappings.  In the first
 scenario, it is assumed that an Ethernet physical port only carries
 untagged traffic and all traffic is mapped to the corresponding
 service instance or ESI.  This is referred to as "port-based with
 untagged traffic".  In the second scenario, it is assumed that an
 Ethernet physical port carries both tagged and untagged traffic and
 all that traffic is mapped to the corresponding service instance or
 ESI.  This is referred to as "port-based with tagged and untagged
 traffic".  In the third scenario, it is assumed that only a single

Sajassi, et al. Informational [Page 8] RFC 6246 VPLS Interop with CE Bridges June 2011

 VLAN is mapped to the corresponding service instance or ESI.  This is
 referred to as "VLAN-based".  Finally, in the fourth scenario, it is
 assumed that a group of VLANs from the Ethernet physical interface is
 mapped to the corresponding service instance or ESI.  This is
 referred to as "VLAN bundling".
 ===================================================================
             Ethernet I/F & Associated Service Instance(s)
 -------------------------------------------------------------------
            Port-based       Port-based       VLAN-based    VLAN
            untagged         tagged &                       bundling
                             untagged
 -------------------------------------------------------------------
 Port-based    Y               N               Y(Note-1)    N
 untagged
 Port-based    N               Y               Y(Note-2)    Y
 tagged &
 untagged
 VLAN-based    Y(Note-1)       Y(Note-2)       Y            Y(Note-3)
 VLAN          N               Y               Y(Note-3)    Y
 Bundling
 ===================================================================
 Note-1: In this asymmetric mapping scenario, it is assumed that the
 CE device with "VLAN-based" AC is capable of supporting [802.1Q]
 frame format.
 Note-2: In this asymmetric mapping scenario, it is assumed that the
 CE device with "VLAN-based" AC can support [802.1ad] frame format
 because it will receive Ethernet frames with two tags, where the
 outer tag is an S-VLAN and the inner tag is a C-VLAN received from
 "port-based" AC.  One application example for such CE device is in a
 Broadband Remote Access Server (BRAS) for DSL aggregation over a
 Metro Ethernet network.
 Note-3: In this asymmetric mapping scenario, it is assumed that the
 CE device with "VLAN-based" AC can support the [802.1ad] frame format
 because it will receive Ethernet frames with two tags, where the
 outer tag is an S-VLAN and the inner tag is a C-VLAN received from
 "VLAN bundling" AC.

Sajassi, et al. Informational [Page 9] RFC 6246 VPLS Interop with CE Bridges June 2011

 If a PE uses an S-VLAN tag for a given ESI (either by adding an
 S-VLAN tag to customer traffic or by replacing a C-VLAN tag with a
 S-VLAN tag), then the frame format and EtherType for S-VLAN SHALL
 adhere to [802.1ad].
 As mentioned before, the mapping function between the customer AC and
 its associated ESI is a local function; thus, when the AC is a single
 customer VLAN, it is possible to map different customer VLANs at
 different sites to a single ESI without coordination among those
 sites.
 When a port-based mapping or a VLAN-bundling mapping is used, then
 the PE may use an additional S-VLAN tag to mark the customer traffic
 received over that AC as belonging to a given ESI.  If the PE uses
 the additional S-VLAN tag, then in the opposite direction the PE
 SHALL strip the S-VLAN tag before sending the customer frames over
 the same AC.  However, when VLAN-mapping mode is used at an AC and if
 the PE uses the S-VLAN tag locally, then if the Ethernet interface is
 a UNI, the tagged frames over this interface SHALL have a frame
 format based on [802.1Q].  In such a case, the PE SHALL translate the
 customer tag (C-VLAN) into the provider tag (S-VLAN) upon receiving a
 frame from the customer.  In the opposite direction, the PE SHALL
 translate from provider frame format (802.1ad) back to customer frame
 format (802.1Q).
 All the above asymmetric services can be supported via the PE model
 with the bridge module depicted in Figure 2 (based on [802.1ad]).

4.2. CE Bridge Protocol Handling

 When a VPLS-capable PE is connected to a CE bridge, then -- depending
 on the type of Attachment Circuit -- different protocol handling may
 be required by the bridge module of the PE.  [802.1ad] states that
 when a PE is connected to a CE bridge, then the service offered by
 the PE may appear to specific customer protocols running on the CE in
 one of the four ways:
    a) Transparent to the operation of the protocol among CEs of
       different sites using the service provided, appearing as an
       individual LAN without bridges;
    b) Discarding frames, acting as a non-participating barrier to the
       operation of the protocol;
    c) Peering, with a local protocol entity at the point of provider
       ingress and egress, participating in and terminating the
       operation of the protocol; or

Sajassi, et al. Informational [Page 10] RFC 6246 VPLS Interop with CE Bridges June 2011

    d) Participation in individual instances of customer protocols.
 All the above CE bridge protocol handling can be supported via the PE
 model with the bridge module depicted in Figure 2 (based on
 [802.1ad]).  For example, when an Attachment Circuit is port-based,
 then the bridge module of the PE can operate transparently with
 respect to the CE's RSTPs or MSTPs (and thus no C-VLAN component is
 required for that customer UNI).  However, when an Attachment Circuit
 is VLAN-based (either VLAN-based or VLAN bundling), then the bridge
 module of the PE needs to peer with the RSTPs or MSTPs running on the
 CE (and thus the C-VLAN bridge component is required).  In other
 words, when the AC is VLAN-based, then protocol peering between CE
 and PE devices may be needed.  There are also protocols that require
 peering but are independent from the type of Attachment Circuit.  An
 example of such protocol is the link aggregation protocol [802.1AX];
 however, this is a media-dependent protocol as its name implies.
 [802.1ad] reserves a block of 16 MAC addresses for the operation of
 C-VLAN and S-VLAN bridge components.  Also, it shows which of these
 reserved MAC addresses are only for C-VLAN bridge components, which
 are only for S-VLAN bridge components, and which apply to both C-VLAN
 and S-VLAN components.

4.3. Partial Mesh of Pseudowires

 A VPLS service depends on a full mesh of pseudowires, so a pseudowire
 failure reduces the underlying connectivity to a partial mesh, which
 can have adverse effects on the VPLS service.  If the CE devices
 belonging to an ESI are routers running link state routing protocols
 that use LAN procedures over that ESI, then a partial mesh of PWs can
 result in "black holing" traffic among the selected set of routers.
 And if the CE devices belonging to an ESI are IEEE bridges, then a
 partial mesh of PWs can cause broadcast storms in the customer and
 provider networks.  Furthermore, it can cause multiple copies of a
 single frame to be received by the CE and/or PE devices.  Therefore,
 it is of paramount importance to be able to detect PW failure and to
 take corrective action to prevent creation of partial mesh of PWs.
 When the PE model depicted in Figure 2 is used, then [802.1ag]
 procedures could be used for detection of partial mesh of PWs.
 [802.1ag] defines a set of procedures for fault detection,
 verification, isolation, and notification per ESI.
 The fault detection mechanism of [802.1ag] can be used to perform
 connectivity check among PEs belonging to a given VPLS instance.  It
 checks the integrity of a service instance end-to-end within an
 administrative domain, e.g., from one AC at one end of the network to
 another AC at the other end of the network.  Therefore, its path

Sajassi, et al. Informational [Page 11] RFC 6246 VPLS Interop with CE Bridges June 2011

 coverage includes the bridge module within a PE and it is not limited
 to just PWs.  Furthermore, [802.1ag] operates transparently over the
 full mesh of PWs for a given service instance since it operates at
 the Ethernet level (and not at the PW level).  It should be noted
 that since a PW consists of two unidirectional Label Switched Paths
 (LSPs), then one direction can fail independently of the other.  Even
 in this case, the procedures of [802.1ag] can provide a consistent
 view of the full mesh to the participating PEs by relying on remote
 defect indication (RDI).
 Another, less preferred, option is to define a procedure for
 detection of partial mesh; in this procedure, each PE keeps track of
 the status of its PW Endpoint Entities (EEs, e.g., VPLS forwarders)
 as well as the EEs reported by other PEs.  Therefore, upon a PW
 failure, the PE that detects the failure not only takes notice
 locally but also notifies other PEs belonging to that service
 instance so that all the participant PEs have a consistent view of
 the PW mesh.  Such a procedure is for the detection of partial mesh
 per service instance, and in turn it relies on additional procedure
 for PW failure detection such as Bidirectional Forward Detection
 (BFD) or Virtual Circuit Connectivity Verification (VCCV).  Given
 that there can be tens (or even hundreds) of thousands of PWs in a
 PE, there can be scalability issues with such fault
 detection/notification procedures.

4.4. Multicast Traffic

 VPLS follows a centralized model for multicast replication within an
 ESI.  VPLS relies on ingress replication.  The ingress PE replicates
 the multicast packet for each egress PE and sends it to the egress PE
 using point-to-point PW over a unicast tunnel.  VPLS operates on an
 overlay topology formed by the full mesh of pseudo-wires.  Thus,
 depending on the underlying topology, the same datagram can be sent
 multiple times down the same physical link.  VPLS currently does not
 offer any mechanisms to restrict the distribution of multicast or
 broadcast traffic of an ESI throughout the network, which causes an
 additional burden on the ingress PE through unnecessary packet
 replication.  This in turn causes additional load on the MPLS core
 network and additional processing at the receiving PE where
 extraneous multicast packets are discarded.
 One possible approach to delivering multicast more efficiently over a
 VPLS network is to include the use of IGMP snooping in order to send
 the packet only to the PEs that have receivers for that traffic,
 rather than to all the PEs in the VPLS instance.  If the customer
 bridge or its network has dual-home connectivity, then -- for proper
 operation of IGMP snooping -- the PE must generate a "General Query"
 over that customer's UNIs upon receiving a customer topology change

Sajassi, et al. Informational [Page 12] RFC 6246 VPLS Interop with CE Bridges June 2011

 notification as described in [RFC4541].  A "General Query" by the PE
 results the customer multicast MAC address(es) being properly
 registered at the PE when there are customer topology changes.  It
 should be noted that IGMP snooping provides a solution for IP
 multicast packets and is not applicable to general multicast data.
 Using the IGMP snooping as described, the ingress PE can select a
 subset of PWs for packet replication, thus avoiding sending multicast
 packets to the egress PEs that don't need them.  However, the
 replication is still performed by the ingress PE.  In order to avoid
 replication at the ingress PE, one may want to use multicast
 distribution trees (MDTs) in the provider core network; however, this
 brings some potential pitfalls.  If the MDT is used for all multicast
 traffic of a given customer, then this results in customer multicast
 and unicast traffic being forwarded on different PWs and even on a
 different physical topology within the provider network.  This is a
 serious issue for customer bridges because customer Bridge Protocol
 Data Units (BPDUs), which are multicast data, can take a different
 path through the network than the unicast data.  Situations might
 arise where either unicast OR multicast connectivity is lost.  If
 unicast connectivity is lost but multicast forwarding continues to
 work, the customer spanning tree would not take notice which results
 in loss of its unicast traffic.  Similarly, if multicast connectivity
 is lost, but unicast is working, then the customer spanning tree will
 activate the blocked port, which may result in a loop within the
 customer network.  Therefore, the MDT cannot be used for both
 customer multicast control and data traffic.  If it is used, it
 should only be limited to customer data traffic.  However, there can
 be a potential issue even when it is used for customer data traffic
 since the MDT doesn't fit the PE model described in Figure 1 (it
 operates independently from the full mesh of PWs that correspond to
 an S-VLAN).  It is also not clear how connectivity fault management
 (CFM) procedures (802.1ag) used for the ESI integrity check (e.g.,
 per service instance) can be applied to check the integrity of the
 customer multicast traffic over the provider MDT.  Because of these
 potential issues, the specific applications of the provider MDT to
 customer multicast traffic shall be documented and its limitations be
 clearly specified.

5. Optional Issues

5.1. Customer Network Topology Changes

 A single CE or a customer network can be connected to a provider
 network using more than one User-Network Interface (UNI).
 Furthermore, a single CE or a customer network can be connected to
 more than one provider network.  [RFC4665] provides some examples of
 such customer network connectivity; they are depicted in Figure 3

Sajassi, et al. Informational [Page 13] RFC 6246 VPLS Interop with CE Bridges June 2011

 below.  Such network topologies are designed to protect against the
 failure or removal of network components from the customer network,
 and it is assumed that the customer leverages the spanning tree
 protocol to protect against these cases.  Therefore, in such
 scenarios, it is important to flush customer MAC addresses in the
 provider network upon the customer topology change in order to avoid
 black-holing of customer frames.
                  +-----------                     +---------------
                  |                                |
 +------+     +------+            +------+     +------+
 |  CE  |-----|  PE  |            |  CE  |-----|  PE  |
 |device|     |device|            |device|     |device| SP network
 +------+\    +------+            +------+\    +------+
    |     \       |                  |     \       |
    |Back  \      |                  |Back  \      +---------------
    |door   \     |   SP network     |door   \     +---------------
    |link    \    |                  |link    \    |
 +------+     +------+            +------+     +------+
 |  CE  |     |  PE  |            |  CE  |     |  PE  |
 |device|-----|device|            |device|-----|device| SP network
 +------+     +------+            +------+     +------+
                  |                                |
                  +------------                    +---------------
                 (a)                                 (b)
  Figure 3.  Combination of Dual-Homing and Backdoor Links for
                              CE Devices
 The customer networks use their own instances of the spanning tree
 protocol to configure and partition their active topology so that the
 provider connectivity doesn't result in a data loop.  Reconfiguration
 of a customer's active topology can result in the apparent movement
 of customer end stations from the point of view of the PEs.  There
 are two methods for addressing this issue based on the provider
 bridge model depicted in Figure 1.  In the first method, the Topology
 Change Notification (TCN) message received from the CE device is
 translated into one or more out-of-band "MAC Address Withdrawal"
 messages as specified in [RFC4762].  In the second method, the TCN
 message received from the CE device is translated into one or more
 in-band "Flush" messages per [p802.1Qbe].  The second method is
 recommended because of ease of interoperability between the bridge
 and LAN emulation modules of the PE.

Sajassi, et al. Informational [Page 14] RFC 6246 VPLS Interop with CE Bridges June 2011

5.2. Redundancy

 [RFC4762] talks about dual-homing of a given Multi-Tenant Unit switch
 (MTU-s) to two PEs over a provider MPLS access network to provide
 protection against link and node failure.  For example, in case the
 primary PE fails or the connection to it fails, then the MTU-s uses
 the backup PWs to reroute the traffic to the backup PE.  Furthermore,
 it discusses the provision of redundancy when a provider Ethernet
 access network is used and how any arbitrary access network topology
 (not just hub-and-spoke) can be supported using the provider's MSTP
 protocol.  It also discusses how the provider MSTP for a given access
 network can be confined to that access network and operate
 independently from MSTP protocols running in other access networks.
 In both types of redundancy mechanism (Ethernet and MPLS access
 networks), only one PE is active for a given VPLS instance at any
 time.  In case of an Ethernet access network, core-facing PWs (for a
 VPLS instance) at the PE are blocked by the MSTP; whereas, in case of
 a MPLS access network, the access-facing PW is blocked at the MTU-s
 for a given VPLS instance.
  1. ———————–+ Provider +———————–

. Core .

                +------+    .            .    +------+
                |  PE  |======================|  PE  |
     Provider   |  (P) |---------\    /-------|  (P) |  Provider
     Access     +------+    .     \  /   .    +------+  Access
     Network                .      \/    .              Network
       (1)      +------+    .      /\    .    +------+     (2)
                |  PE  |----------/  \--------|  PE  |
                |  (B) |----------------------|  (B) |
                +------+    .            .    +------+
                            .            .
    ------------------------+            +-----------------------
                    Figure 4.  Bridge Module Model
 Figure 4 shows two provider access networks each with two PEs that
 are connected via a full mesh of PWs for a given VPLS instance.  As
 shown in the figure, only one PE in each access network serves as a
 Primary PE (P) for that VPLS instance and the other PE serves as the
 backup PE (B).  In this figure, each primary PE has two active PWs
 originating from it.  Therefore, when a multicast, broadcast, and
 unknown unicast frame arrives at the primary PE from the access
 network side, the PE replicates the frame over both PWs in the core
 even though it only needs to send the frame over a single PW (shown
 with "==" in Figure 4) to the primary PE on the other side.  This is
 an unnecessary replication of the customer frames and consumes core-

Sajassi, et al. Informational [Page 15] RFC 6246 VPLS Interop with CE Bridges June 2011

 network bandwidth (half of the frames get discarded at the receiving
 PE).  This issue is aggravated when there are more than two PEs per
 provider access network -- e.g., if there are three PEs or four PEs
 per access network, then 67% or 75%, respectively, of core-network
 bandwidth for multicast, broadcast, and unknown unicast are
 respectively wasted.
 Therefore, it is recommended to have a protocol among PEs that can
 disseminate the status of PWs (active or blocked) among themselves.
 Furthermore, it is recommended to have the protocol tied up with the
 redundancy mechanism such that (per VPLS instance) the status of
 active/backup PE gets reflected on the corresponding PWs emanating
 from that PE.
 The above discussion was centered on the inefficiency regarding
 packet replication over MPLS core networks for current VPLS
 redundancy mechanism.  Another important issue to consider is the
 interaction between customer and service provider redundancy
 mechanisms, especially when customer devices are IEEE bridges.  If
 CEs are IEEE bridges, then they can run RSTPs or MSTPs.  RSTP
 convergence and detection time is much faster than its predecessor
 (IEEE 802.1D STP, which is obsolete).  Therefore, if the provider
 network offers a VPLS redundancy mechanism, then it should provide
 transparency to the customer's network during a failure within its
 network, e.g., the failure detection and recovery time within the
 service provider network should be less than the one in the customer
 network.  If this is not the case, then a failure within the provider
 network can result in unnecessary switch-over and temporary
 flooding/loop within the customer's network that is dual-homed.

5.3. MAC Address Learning

 When customer devices are routers, servers, or hosts, then the number
 of MAC addresses per customer sites is very limited (most often one
 MAC address per CE).  However, when CEs are bridges, then there can
 be many customer MAC addresses (e.g., hundreds of MAC addresses)
 associated with each CE.
 [802.1ad] has devised a mechanism to alleviate MAC address learning
 within provider Ethernet networks that can equally be applied to VPLS
 networks.  This mechanism calls for disabling MAC address learning
 for an S-VLAN (or a service instance) within a provider bridge (or
 PE) when there is only one ingress and one egress port associated
 with that service instance on that PE.  In such cases, there is no
 need to learn customer MAC addresses on that PE since the path
 through that PE for that service instance is fixed.  For example, if
 a service instance is associated with four CEs at four different
 sites, then the maximum number of provider bridges (or PEs) that need

Sajassi, et al. Informational [Page 16] RFC 6246 VPLS Interop with CE Bridges June 2011

 to participate in that customer MAC address learning is only three,
 regardless of how many PEs are in the path of that service instance.
 This mechanism can reduce the number of MAC addresses learned in a
 hierarchical VPLS (H-VPLS) with QinQ access configuration.
 If the provider access network is of type Ethernet (e.g., IEEE
 802.1ad-based network), then the MSTP can be used to partition the
 access network into several loop-free spanning tree topologies where
 Ethernet service instances (S-VLANs) are distributed among these tree
 topologies.  Furthermore, GVRP can be used to limit the scope of each
 service instance to a subset of its associated tree topology (thus
 limiting the scope of customer MAC address learning to that sub-
 tree).  Finally, the MAC address disabling mechanism (described
 above) can be applied to that sub-tree to further limit the number of
 nodes (PEs) on that sub-tree that need to learn customer MAC
 addresses for that service instance.
 Furthermore, [802.1ah] provides the capability of encapsulating
 customers' MAC addresses within the provider MAC header.  A MTU-s
 capable of this functionality can significantly reduce the number of
 MAC addresses learned within the provider network for H-VPLS with
 QinQ access, as well as H-VPLS with MPLS access.

6. Interoperability with 802.1ad Networks

 [RFC4762] discusses H-VPLS provider-network topologies with both
 Ethernet [802.1ad] and MPLS access networks.  Therefore, it is
 important to ensure seamless interoperability between these two types
 of networks.
 Provider bridges as specified in [802.1ad] are intended to operate
 seamlessly with customer bridges and provide the required services.
 Therefore, if a PE is modeled based on Figures 1 and 2, which include
 a [802.1ad] bridge module, then it should operate seamlessly with
 Provider Bridges given that the issues discussed in this document
 have been taken into account.

7. Acknowledgments

 The authors would like to thank Norm Finn and Samer Salam for their
 comments and valuable feedback.

8. Security Considerations

 In addition to the security issues described in [RFC4762], the
 following considerations apply:

Sajassi, et al. Informational [Page 17] RFC 6246 VPLS Interop with CE Bridges June 2011

  1. When a CE that is a customer bridge is connected to the VPLS

network, it may be desirable to secure the end-to-end communication

   between the customer bridge nodes across the VPLS network.  This
   can be accomplished by running [802.1AE] MAC security between the
   C-VLAN components of the customer bridges.  In this case, the VPLS
   PEs must ensure transparent delivery of the encryption/security
   protocol datagrams using the Bridge Group Address [802.1ad].
  1. When a CE that is a customer bridge is connected to the VPLS

network, it may be desirable to secure the communication between

   the customer bridge and its directly connected PE.  If the PE is
   modeled to include a [802.1ad] bridge module, then this can be
   achieved by running MAC security between the customer bridge and
   the S-VLAN component of the VPLS PE as described in Section 7.7.2
   of [802.1AX].
  1. When an 802.1ad network is connected to a VPLS network, it is

possible to secure the NNI between the two networks using the

   procedures of [802.1AE] and [802.1AX] between the S-VLAN components
   of the Provider Edge Bridge and the attached VPLS PE, as long as
   the PE is modeled to include an [802.1ad] bridge module.

9. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4762]   Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
             LAN Service (VPLS) Using Label Distribution Protocol
             (LDP) Signaling", RFC 4762, January 2007.
 [802.1ad]   IEEE 802.1ad-2005, "Amendment to IEEE 802.1Q-2005. IEEE
             Standard for Local and Metropolitan Area Networks -
             Virtual Bridged Local Area Networks Revision-Amendment 4:
             Provider Bridges".
 [802.1AE]   IEEE 802.1AE-2006, "IEEE Standard for Local and
             Metropolitan Area Networks - Media Access Control (MAC)
             Security".
 [802.1ag]   IEEE 802.1ag-2007, "IEEE Standard for Local and
             Metropolitan Area Networks - Virtual Bridged Local Area
             Networks Amendment 5: Connectivity Fault Management".
 [802.1ah]   IEEE 802.1ah-2008, "IEEE Standard for Local and
             Metropolitan Area Networks - Virtual Bridged Local Area
             Networks Amendment 7: Provider Backbone Bridges".

Sajassi, et al. Informational [Page 18] RFC 6246 VPLS Interop with CE Bridges June 2011

 [802.1AX]   IEEE 802.1AX-2008 "IEEE Standard for Local and
             Metropolitan Area Networks - Link Aggregation".

10. Informative References

 [IPLS]      Shah, H., Rosen, E., Le Faucheur, F., and G. Heron, "IP-
             Only LAN Service (IPLS)", Work in Progress, February
             2010.
 [RFC4448]   Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
             "Encapsulation Methods for Transport of Ethernet over
             MPLS Networks", RFC 4448, April 2006.
 [RFC4541]   Christensen, M., Kimball, K., and F. Solensky,
             "Considerations for Internet Group Management Protocol
             (IGMP) and Multicast Listener Discovery (MLD) Snooping
             Switches", RFC 4541, May 2006.
 [RFC4664]   Andersson, L., Ed., and E. Rosen, Ed., "Framework for
             Layer 2 Virtual Private Networks (L2VPNs)", RFC 4664,
             September 2006.
 [RFC4665]   Augustyn, W., Ed., and Y. Serbest, Ed., "Service
             Requirements for Layer 2 Provider-Provisioned Virtual
             Private Networks", RFC 4665, September 2006.
 [RFC6136]   Sajassi, A., Ed., and D. Mohan, Ed., "Layer 2 Virtual
             Private Network (L2VPN) Operations, Administration, and
             Maintenance (OAM) Requirements and Framework", RFC 6136,
             March 2011.
 [802.1D]    IEEE 802.1D-2004, "IEEE Standard for Local and
             Metropolitan Area Networks - Media access control (MAC)
             Bridges (Incorporates IEEE 802.1t-2001 and IEEE 802.1w)".
 [802.1Q]    IEEE Std. 802.1Q-2003 "Virtual Bridged Local Area
             Networks".
 [p802.1Qbe] IEEE Draft Standard P802.1Qbe, "IEEE Draft Standard for
             Local and Metropolitan Area Networks -- Virtual Bridged
             Local Area Networks Amendment: Multiple I-SID
             Registration Protocol".

Sajassi, et al. Informational [Page 19] RFC 6246 VPLS Interop with CE Bridges June 2011

Authors' Addresses

 Ali Sajassi (editor)
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA  95134
 EMail: sajassi@cisco.com
 Frank Brockners
 Cisco Systems, Inc.
 Hansaallee 249
 40549 Duesseldorf
 Germany
 EMail: fbrockne@cisco.com
 Dinesh Mohan (editor)
 Nortel
 Ottawa, ON K2K3E5
 EMail: dinmohan@hotmail.com
 Yetik Serbest
 AT&T Labs
 9505 Arboretum Blvd.
 Austin, TX 78759
 EMail: yetik_serbest@labs.att.com

Sajassi, et al. Informational [Page 20]

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