Internet Engineering Task Force (IETF) F. Balus, Ed.

Internet Engineering Task Force (IETF) F. Balus, Ed.
Request for Comments: 7041 Alcatel-Lucent
Category: Informational A. Sajassi, Ed.
ISSN: 2070-1721 Cisco
N. Bitar, Ed.
Verizon
November 2013

Extensions to the Virtual Private LAN Service (VPLS)
Provider Edge (PE) Model for Provider Backbone Bridging

Abstract

The IEEE 802.1 Provider Backbone Bridges (PBBs) specification defines
an architecture and bridge protocols for interconnection of multiple
Provider Bridged Networks (PBNs). Provider backbone bridging was
defined by IEEE as a connectionless technology based on multipoint
VLAN tunnels. PBB can be used to attain better scalability than
Provider Bridges (PBs) in terms of the number of customer Media
Access Control addresses and the number of service instances that can
be supported.

The Virtual Private LAN Service (VPLS) provides a framework for
extending Ethernet LAN services, using MPLS tunneling capabilities,
through a routed MPLS backbone without running the Rapid Spanning
Tree Protocol (RSTP) or the Multiple Spanning Tree Protocol (MSTP)
across the backbone. As a result, VPLS has been deployed on a large
scale in service provider networks.

This document discusses extensions to the VPLS Provider Edge (PE)
model required to incorporate desirable PBB components while
maintaining the service provider fit of the initial model.

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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/rfc7041.

Copyright Notice

Copyright (c) 2013 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
2. General Terminology .............................................4
3. PE Reference Model ..............................................6
4. Packet Walkthrough ..............................................9
5. Control Plane ..................................................11
6. Efficient Packet Replication in PBB VPLS .......................12
7. PBB VPLS OAM ...................................................12
8. Security Considerations ........................................12
9. References .....................................................13
9.1. Normative References ......................................13
9.2. Informative References ....................................13
10. Contributors ..................................................14
11. Acknowledgments ...............................................15

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1. Introduction

The IEEE 802.1 Provider Backbone Bridges specification [PBB] defines
an architecture and bridge protocols for interconnection of multiple
Provider Bridged Networks (PBNs). PBB can be used to attain better
scalability than Provider Bridges [PB] in terms of the number of
customer Media Access Control (MAC) addresses and the number of
service instances that can be supported. PBB provides a data-plane
hierarchy and new addressing designed to achieve such better
scalability in Provider Backbone Networks. A number of Ethernet
control-plane protocols, such as the Rapid Spanning Tree Protocol
(RSTP), the Multiple Spanning Tree Protocol (MSTP), and Shortest Path
Bridging (SPB), could be deployed as the core control plane for loop
avoidance and load balancing for PBB. The applicability of these
control protocols is out of scope for this document.

The Virtual Private LAN Service (VPLS) provides a solution for
extending Ethernet LAN services, using MPLS tunneling capabilities,
through a routed MPLS backbone without requiring the use of a native
Ethernet control-plane protocol across the backbone. VPLS use of
the structured FEC 129 [RFC4762] also allows for inter-domain,
inter-provider connectivity and enables auto-discovery options across
the network, improving the service delivery options.

A hierarchical solution for VPLS was introduced in [RFC4761] and
[RFC4762] to provide improved scalability and efficient handling of
packet replication. These improvements are achieved by reducing the
number of Provider Edge (PE) devices connected in a full-mesh
topology through the creation of two-tier PEs. A User-facing PE
(U-PE) aggregates all the Customer Edge (CE) devices in a lower-tier
access network and then connects to the Network-facing PE (N-PE)
device(s) deployed around the core domain. In VPLS, Media Access
Control (MAC) address learning and forwarding are done based on
Customer MAC addresses (C-MACs); this poses scalability issues on the
N-PE devices as the number of VPLS instances (and thus C-MACs)
increases. Furthermore, since a set of pseudowires (PWs) is
maintained on a "per customer service instance" basis, the number of
PWs required at N-PE devices is proportional to the number of
customer service instances multiplied by the number of N-PE devices
in the full-mesh set. This can result in scalability issues (in
terms of PW manageability and troubleshooting) as the number of
customer service instances grows.

This document describes how PBB can be integrated with VPLS to allow
for useful PBB capabilities while continuing to avoid the use of MSTP
in the backbone. The combined solution referred to in this document

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as PBB-VPLS results in better scalability in terms of the number of
service instances, PWs, and C-MACs that need to be handled in the
VPLS PEs.

Section 2 provides a quick terminology reference. Section 3 covers
the reference model for PBB VPLS PEs. Section 4 describes the packet
walkthrough. Sections 5 through 7 discuss the PBB-VPLS usage of
existing VPLS mechanisms -- the control plane; efficient packet
replication; and Operations, Administration, and Maintenance (OAM).

2. General Terminology

Some general terminology is defined here; most of the terminology
used is from [PBB], [PB], [RFC4664], and [RFC4026]. Terminology
specific to this memo is introduced as needed in later sections.

B-BEB: A backbone edge bridge positioned at the edge of a provider
backbone bridged network. It contains a B-component that supports
bridging in the provider backbone based on Backbone MAC (B-MAC)
and B-tag information.

B-component: A bridging component contained in backbone edge and core
bridges that bridges in the backbone space (B-MAC addresses,
B-VLAN).

B-MAC: The backbone source or destination MAC address fields defined
in the PBB provider MAC encapsulation header.

B-tag: Field defined in the PBB provider MAC encapsulation header
that conveys the backbone VLAN identifier information. The format
of the B-tag field is the same as that of an 802.1ad S-tag field.

B-Tagged Service Interface: The interface between a BEB and a
Backbone Core Bridge (BCB) in a provider backbone bridged network.
Frames passed through this interface contain a B-tag field.

B-VID: The specific VLAN identifier carried inside a B-tag.

B-VLAN: The backbone VLAN associated with a B-component.

B-PW: The pseudowire used to interconnect B-component instances.

BEB: A backbone edge bridge positioned at the edge of a provider
backbone bridged network. It can contain an I-component, a
B-component, or both I-components and B-components.

C-VID: The VLAN identifier in a customer VLAN.

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DA: Destination Address.

I-BEB: A backbone edge bridge positioned at the edge of a provider
backbone bridged network. It contains an I-component for bridging
in the customer space (customer MAC addresses, service VLAN IDs).

I-component: A bridging component contained in a backbone edge bridge
that bridges in the customer space (customer MAC addresses,
service VLAN identifier information (S-VLAN)).

I-SID: The 24-bit service instance field carried inside the I-tag.
I-SID defines the service instance that the frame should be
"mapped to".

I-tag: A field defined in the PBB provider MAC encapsulation header
that conveys the service instance information (I-SID) associated
with the frame.

I-Tagged Service Interface: The interface defined between the
I-components and B-components inside an IB-BEB or between two
B-BEBs. Frames passed through this interface contain an I-tag
field.

IB-BEB: A backbone edge bridge positioned at the edge of a provider
backbone bridged network. It contains an I-component for bridging
in the customer space (customer MAC addresses, service VLAN IDs)
and a B-component for bridging the provider's backbone space
(B-MAC, B-tag).

PBs: Provider Bridges (IEEE amendment (802.1ad) to 802.1Q for "QinQ"
encapsulation and bridging of Ethernet frames [PB]).

PBBs: Provider Backbone Bridges (IEEE amendment (802.1ah) to 802.1Q
for "MAC tunneling" encapsulation and bridging of frames across a
provider network [PBB]).

PBBN: Provider Backbone Bridged Network.

PBN: Provider Bridged Network. A network that employs 802.1ad (QinQ)
technology.

PSN: Packet-Switched Network.

S-tag: A field defined in the 802.1ad QinQ encapsulation header that
conveys the service VLAN identifier information (S-VLAN).

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S-Tagged Service Interface: The interface defined between the
customer (CE) and the I-BEB or IB-BEB components. Frames passed
through this interface contain an S-tag field.

S-VLAN: The specific service VLAN identifier carried inside an S-tag.

SA: Source Address.

S-VID: The VLAN identifier in a service VLAN.

Tag: In Ethernet, a header immediately following the Source MAC
Address field of the frame.

3. PE Reference Model

The following gives a short primer on the Provider Backbone Bridge
(PBB) before describing the PE reference model for PBB-VPLS. The
internal components of a PBB bridge module are depicted in Figure 1.

+-------------------------------+
| PBB Bridge Model |
| |
+---+ | +------+ +-----------+ |
|CE |---------|I-Comp|------| | |
+---+ | | | | |--------
| +------+ | | |
| o | B-Comp | |
| o | |--------
| o | | |
+---+ | +------+ | | |
|CE |---------|I-Comp|------| |--------
+---+ ^ | | | ^ | | | ^
| | +------+ | +-----------+ | |
| +------------|------------------+ |
| | |
| | |
S-tagged I-tagged B-tagged
Service Interface Service I/F Service I/F
(I/F)

Figure 1: PBB Bridge Model

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Provider Backbone Bridges (PBBs) [PBB] offer a scalable solution for
service providers to build large bridged networks. The focus of PBB
is primarily on improving two main areas with provider Ethernet
bridged networks:

- MAC-address table scalability
- Service instance scalability

To obviate the above two limitations, PBB introduces a hierarchical
network architecture with associated new frame formats that extend
the work completed by Provider Bridges (PBs). In the PBBN
architecture, customer networks (using PBs) are aggregated into
PBBNs, which utilize the IEEE PBB frame format. The frame format
employs a MAC tunneling encapsulation scheme for tunneling customer
Ethernet frames within provider Ethernet frames across the PBBN. A
VLAN identifier (B-VID) is used to segregate the backbone into
broadcast domains, and a new 24-bit service identifier (I-SID) is
defined and used to associate a given customer MAC frame with a
provider service instance (also called the service delimiter). It
should be noted that in [PBB] there is a clear segregation between
provider service instances (represented by I-SIDs) and provider VLANs
(represented by B-VIDs), which was not the case for PBs.

As shown in Figure 1, a PBB bridge may consist of a single
B-component and one or more I-components. In simple terms, the
B-component provides bridging in the provider space (B-MAC, B-VLAN),
and the I-component provides bridging in the customer space (C-MAC,
S-VLAN). The customer frame is first encapsulated with the provider
backbone header (B-MAC, B-tag, I-tag); then, the bridging is
performed in the provider backbone space (B-MAC, B-VLAN) through the
network till the frame arrives at the destination BEB, where it gets
decapsulated and passed to the CE. If a PBB bridge consists of both
I-components and B-components, then it is called an IB-BEB, and if it
only consists of either B-components or I-components, then it is
called a B-BEB or an I-BEB, respectively. The interface between an
I-BEB or IB-BEB and a CE is called an S-tagged service interface, and
the interface between an I-BEB and a B-BEB (or between two B-BEBs) is
called an I-tagged service interface. The interface between a B-BEB
or IB-BEB and a Backbone Core Bridge (BCB) is called a B-tagged
service interface.

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To accommodate the PBB components, the VPLS model defined in
[RFC4664] is extended as depicted in Figure 2.

+----------------------------------------+
| PBB-VPLS-Capable PE Model |
| +---------------+ +------+ |
| | | |VPLS-1|------------
| | |==========|Fwdr |------------ PWs
+--+ | | Bridge ------------ |------------
|CE|-|-- | | +------+ |
+--+ | | Module | o |
| | | o |
| | (PBB | o |
| | bridge) | o |
| | | o |
+--+ | | | +------+ |
|CE|-|-- | ------------VPLS-n|-------------
+--+ | | |==========| Fwdr |------------- PWs
| | | ^ | |-------------
| +---------------+ | +------+ |
| | |
+-------------------------|--------------+
LAN Emulation Interface

Figure 2: PBB-VPLS-Capable PE Model

The PBB module as defined in the [PBB] specification is expanded to
interact with VPLS Forwarders. The VPLS Forwarders are used in
[RFC4762] to build a PW mesh or a set of spoke PWs (Hierarchical VPLS
(H-VPLS) topologies). The VPLS instances are represented externally
in the MPLS context by a Layer 2 Forwarding Equivalence Class (L2FEC)
that binds related VPLS instances together. VPLS Signaling
advertises the mapping between the L2FEC and the PW labels and
implicitly associates the VPLS bridging instance to the VPLS
Forwarders [RFC4762].

In the PBB-VPLS case, the backbone service instance in the
B-component space (B-VID) is represented in the backbone MPLS network
using a VPLS instance. In the same way as for the regular VPLS case,
existing signaling procedures are used to generate through PW labels
the linkage between VPLS Forwarders and the backbone service
instance.

Similarly, with the regular H-VPLS, another L2FEC may be used to
identify the customer service instance in the I-component space.
This will be useful, for example, to address the PBB-VPLS N-PE case
where H-VPLS spokes are connecting the PBB-VPLS N-PE to a VPLS U-PE.

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It is important to note that the PBB-VPLS solution inherits the PBB
service aggregation capability where multiple customer service
instances may be mapped to a backbone service instance. In the
PBB-VPLS case, this means multiple customer VPNs can be transported
using a single VPLS instance corresponding to the backbone service
instance, thus substantially reducing resource consumption in the
VPLS core.

4. Packet Walkthrough

Since the PBB bridge module inherently performs forwarding, the PE
reference model of Figure 2 can be expanded as shown in Figure 3.

Furthermore, the B-component is connected via several virtual
interfaces to the PW Forwarder module. The function of the PW
Forwarder is defined in [RFC3985]. In this context, the PW Forwarder
simply performs the mapping of the PWs to the virtual interface on
the B-component, without the need for any MAC lookup.

This simplified model takes full advantage of the PBB module -- where
all the [PBB] procedures, including C-MAC/B-MAC forwarding and PBB
encapsulation/decapsulation, take place -- and thus avoids the need
to specify any of these functions in this document.

Because of text-based graphics, Figure 3 only shows PWs on the
core-facing side; however, in the case of MPLS access with spoke PWs,
the PE reference model is simply extended to include the same PW
Forwarder function on the access-facing side. To avoid cluttering
the figure, but without losing any generality, the access-side PW
Forwarder (Fwdr) is not depicted.

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+------------------------------------------------+
| PBB-VPLS-Capable PE Model |
| +---------------+ +------+ |
| | | | | |
| +------+ | ======== ---------
+--+ | | | | | | --------- PWs
|CE|-|-- | I- ==== ======== PW ---------
+--+ | | Comp | | | | Fwdr |
| +------+ | | | --------- PWs
| | B-Comp ======== ---------
| | | ^ | | |
| +------+ | | | +------+ |
+--+ | | I- | | OOOOOOOOOOOOOOOOOOOOOOOO B-tag
|CE|-|-- | Comp ==== | | | I/Fs
+--+ | | |^ | OOOOOOOOOOOOOOOOOOOOOOOO
| +------+| | | | |
| | +---------------+ | |
| | | |
+-----------|--------------------|---------------+
| |
Internal I-tag I/Fs Virtual Interfaces (I/Fs)

+---------------+ +--------------+
| C-MAC DA,SA | | PSN Header |
|---------------| |--------------|
| S-VID, C-VID | | PW Label |
|---------------| |--------------|
| Payload | | B-MAC DA,SA |
+---------------+ |--------------|
| PBB I-tag |
|--------------|
| C-MAC DA,SA |
|--------------|
| S-VID, C-VID |
|--------------|
| Payload |
+--------------+

Figure 3: Packet Walkthrough for PBB VPLS PE

In order to better understand the data-plane walkthrough, let us
consider the example of a PBB packet arriving over a Backbone
pseudowire (B-PW). The PSN header is used to carry the PBB
encapsulated frame over the backbone while the PW label will point to
the related Backbone Service Instance (B-SI), in the same way as for
regular VPLS. The PW label has in this case an equivalent role with
the backbone VLAN identifier on the PBB B-tagged interface.

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An example of the PBB packet for the regular Ethernet PW is depicted
on the right-hand side of Figure 3. The MPLS packet from the MPLS
core network is received by the PBB-VPLS PE. The PW Forwarder
function of the PE uses the PW label to derive the virtual
interface-id on the B-component, and then, after removing the PSN and
PW encapsulation, it passes the packet to the B-component. From
there on, the processing and forwarding are performed according to
[PBB], where bridging based on the Backbone MAC (B-MAC) Destination
Address (DA) is performed. This scenario results in one of the
following outcomes:

1. The packet is forwarded to a physical interface on the
B-component. In this case, the PBB Ethernet frame is forwarded
as is.

2. The packet is forwarded to a virtual interface on the B-component.
This is not typically the case, because of a single split-horizon
group within a VPLS instance; however, if there is more than one
split-horizon group, then such forwarding takes place. In this
case, the PW Forwarder module adds the PSN and PW labels before
sending the packet out.

3. The packet is forwarded toward the access side via one of the
I-tagged service interfaces connected to the corresponding
I-components. In this case, the I-component removes the B-MAC
header according to [PBB] and bridges the packet using the
C-MAC DA.

If the destination B-MAC is an unknown MAC address or a Group MAC
address (multicast or broadcast), then the B-component floods the
packet to one or more of the three destinations described above.

5. Control Plane

The control-plane procedures described in [RFC6074], [RFC4761], and
[RFC4762] can be reused in a PBB-VPLS to set up the PW infrastructure
in the service provider and/or customer bridging space. This allows
porting the existing control-plane procedures (e.g., BGP
Auto-Discovery (BGP-AD), PW setup, VPLS MAC flushing, PW OAM) for
each domain.

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6. Efficient Packet Replication in PBB VPLS

The PBB VPLS architecture takes advantage of the existing VPLS
features addressing packet replication efficiency. The H-VPLS
hierarchy may be used in both customer and backbone service instances
to reduce the redundant distribution of packets over the core. IGMP
and PIM snooping may be applied on a "per customer service instance"
basis to control the distribution of the multicast traffic to
non-member sites.

[IEEE-802.1Q] specifies the use of the Multiple MAC Registration
Protocol (MMRP) for flood containment in the backbone instances. The
same solution can be ported in the PBB-VPLS solution.

Further optimizations of the packet replication in PBB-VPLS are out
of the scope of this document.

7. PBB VPLS OAM

The existing VPLS, PW, and MPLS OAM procedures may be used in each
customer service instance or backbone service instance to verify the
status of the related connectivity components.

PBB OAM procedures make use of the IEEE Ethernet Connectivity Fault
Management [CFM] and ITU-T Y.1731 [Y.1731] tools in both I-components
and B-components.

Both sets of tools (PBB and VPLS) may be used for the combined
PBB-VPLS solution.

8. Security Considerations

No new security issues are introduced beyond those described in
[RFC4761] and [RFC4762].

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9. References

9.1. Normative References

[RFC4761] Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, January 2007.

[RFC4762] Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol (LDP)
Signaling", RFC 4762, January 2007.

[RFC6074] Rosen, E., Davie, B., Radoaca, V., and W. Luo,
"Provisioning, Auto-Discovery, and Signaling in Layer 2
Virtual Private Networks (L2VPNs)", RFC 6074, January 2011.

9.2. Informative References

[RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.

[RFC4664] Andersson, L., Ed., and E. Rosen, Ed., "Framework for
Layer 2 Virtual Private Networks (L2VPNs)", RFC 4664,
September 2006.

[PBB] Clauses 25 and 26 of "IEEE Standard for Local and
metropolitan area networks - Media Access Control (MAC)
Bridges and Virtual Bridged Local Area Networks", IEEE
Std 802.1Q-REV, 2013.

[PB] Clauses 15 and 16 of "IEEE Standard for Local and
metropolitan area networks - Media Access Control (MAC)
Bridges and Virtual Bridged Local Area Networks", IEEE
Std 802.1Q-REV, 2013.

[CFM] CFM clauses of "IEEE Standard for Local and metropolitan
area networks - Media Access Control (MAC) Bridges and
Virtual Bridged Local Area Networks", IEEE Std 802.1Q-REV,
2013.

[IEEE-802.1Q]
"IEEE Standard for Local and metropolitan area networks -
Media Access Control (MAC) Bridges and Virtual Bridged
Local Area Networks", IEEE Std 802.1Q-REV, 2013.

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[Y.1731] ITU-T Recommendation Y.1731, "OAM functions and mechanisms
for Ethernet based networks", July 2011.

[RFC4026] Andersson, L. and T. Madsen, "Provider Provisioned Virtual
Private Network (VPN) Terminology", RFC 4026, March 2005.

10. Contributors

The following people made significant contributions to this document:

Matthew Bocci
Alcatel-Lucent
Voyager Place
Shoppenhangers Road
Maidenhead
Berks, UK

EMail: [email protected]

Raymond Zhang
Alcatel-Lucent

EMail: [email protected]

Geraldine Calvignac
Orange
2, avenue Pierre-Marzin
22307 Lannion Cedex
France

EMail: [email protected]

John Hoffmans
KPN
Regulusweg 1
2516 AC Den Haag
The Netherlands

EMail: [email protected]

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Olen Stokes
Extreme Networks
PO Box 14129
RTP, NC 27709
USA

EMail: [email protected]

11. Acknowledgments

The authors would like to thank Wim Henderickx, Mustapha Aissaoui,
Dimitri Papadimitriou, Pranjal Dutta, Jorge Rabadan, Maarten Vissers,
and Don Fedyk for their insightful comments and probing questions.

Authors' Addresses

Florin Balus (editor)
Alcatel-Lucent
701 E. Middlefield Road
Mountain View, CA 94043
USA

EMail: [email protected]

Ali Sajassi (editor)
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA

EMail: [email protected]

Nabil Bitar (editor)
Verizon
60 Sylvan Road
Waltham, MA 02145
USA

EMail: [email protected]

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