Internet-Draft Ali Sajassi
L2VPN Working Group Samer Salam
Category: Informational Chris Metz
Expires: January 11, 2012 Cisco
Nabil Bitar
Verizon
Dinesh Mohan
Florin Balus
Alcatel-Lucent
July 11, 2011
VPLS Interoperability with Provider Backbone Bridges
draft-ietf-l2vpn-pbb-vpls-interop-02.txt
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Abstract
The scalability of H-VPLS with Ethernet access network can be
improved by incorporating Provider Backbone Bridge functionality in
VPLS access. Provider Backbone Bridging has been standardized as
IEEE 802.1ah-2008, and aims to improve the scalability of MAC
addresses and service instances in Provider Ethernet networks. This
document describes different interoperability scenarios where
Provider Backbone Bridge functionality is used in H-VPLS with
Ethernet or MPLS access network to attain better scalability in
terms of number of customer MAC addresses and number of service
instances. The document also describes the scenarios and the
mechanisms for incorporating Provider Backbone Bridge functionality
within H-VPLS with existing Ethernet access and interoperability
among them. Furthermore, the document discusses the migration
mechanisms and scenarios by which Provider Backbone Bridge
functionality can be incorporated into H-VPLS with existing MPLS
access.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Table of Contents
Conventions used in this document.................................. 2
1. Introduction.................................................... 3
2. Applicability................................................... 4
3. Terminology..................................................... 5
4. H-VPLS with Homogeneous PBBN Access............................. 6
4.1 Service Interfaces and Interworking Options.................... 8
4.2 H-VPLS with PBBN Access: Type I Service Interface.............. 9
4.3 H-VPLS with PBBN Access: Type II Service Interface............ 11
5. H-VPLS with Mixed PBBN Access and PBN Access................... 12
5.1 H-VPLS with Mixed PBBN & PBN Access: Modified PBN PE.......... 13
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5.2 H-VPLS with Mixed PBBN & PBN Access: Regular PBN PE........... 14
6. H-VPLS with MPLS Access........................................ 15
6.1 H-VPLS with MPLS Access: PBB U-PE............................. 15
6.1.1 PBB U-PEs in Single I-SID Domain............................ 17
6.2 H-VPLS with MPLS Access: PBB N-PE............................. 17
6.2.1 PBB N-PEs in Single I-SID Domain............................ 18
7. H-VPLS with MPLS Access: PBB Migration Scenarios............... 18
7.1 802.1ad Service Frames over VPLS Core......................... 19
7.2 PBB Service Frames over VPLS Core ........................ 20
7.3 Mixed 802.1ad and PBB over VPLS Core ..................... 20
8. Acknowledgments................................................ 22
9. IANA Considerations............................................ 22
10. Security Considerations....................................... 22
11. Intellectual Property Considerations.......................... 22
12. References.................................................... 22
12.1 Normative References......................................... 22
12.2 Informative References....................................... 22
Authors' Addresses................................................ 23
1. Introduction
The scalability of H-VPLS with Ethernet access network can be
improved by incorporating Provider Backbone Bridge functionality in
the VPLS access. Provider Backbone Bridging has been standardized as
IEEE 802.1ah-2008, which is an amendment to IEEE 802.1Q to improve
the scalability of MAC addresses and service instances in Provider
Ethernet networks. This document describes interoperability
scenarios where IEEE 802.1ah functionality is used in H-VPLS with
Ethernet or MPLS access network to attain better scalability in
terms of the number of customer MAC addresses and the number of
services. This document also describes the scenarios and the
mechanisms for incorporating Provider Backbone Bridging
functionality within H-VPLS with existing IEEE 802.1ad Ethernet
access and interoperability among them. Furthermore, this document
discusses the migration mechanisms and scenarios by which Provider
Backbone Bridging functionality can be incorporated into H-VPLS with
existing MPLS access.
This document also covers the interoperability scenarios for
deploying H-VPLS with Provider Backbone Bridging Ethernet access
when other types of access networks are deployed, including existing
802.1ad Ethernet access in either single or multiple service
domains. Furthermore, the document explores the scenarios by which
an operator can gradually migrate an existing H-VPLS network to
Provider Backbone Bridging over VPLS.
Section 2 highlights the applicability of Provider Backbone Bridging
interoperation with VPLS. Section 3 gives a quick terminology
reference. Section 4 describes H-VPLS with homogeneous Provider
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Backbone Bridge Access Network. Section 5 discusses H-VPLS with
mixed 802.1ah/802.1ad access. Section 6 focuses on Provider Backbone
Bridging in H-VPLS with MPLS Access Network including Provider
Backbone Bridge function on U-PE and on N-PE variants. Finally,
section 7 describes gradual migration scenarios from existing H-VPLS
to Provider Backbone Bridging over H-VPLS.
2. Applicability
[RFC4762] describes a two-tier hierarchical solution for VPLS for
the purpose of improved pseudowire (PW) scalability. This
improvement is achieved by reducing the number of PE devices
connected in a full-mesh topology through connecting CE devices via
the lower-tier access network, which in turn is connected to the
top-tier core network. [RFC4762] describes two types of H-VPLS
network topologies - one with MPLS access network and another with
IEEE 802.1ad (QinQ) Ethernet access network. In both types of H-
VPLS, MAC address learning and forwarding are done based on customer
MAC addresses (C-MACs), which poses scalability issues as the number
of VPLS instances (and thus customer MAC addresses) increases.
Furthermore, since a set of 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.
In addition to the above, H-VPLS with 802.1ad Ethernet access
network has another scalability issue in terms of the maximum number
of service instances that can be supported in the access network as
described in [RFC4762]. Since the number of provider VLANs (S-VLANs)
is limited to 4K and each S-VLAN represents a service instance in an
802.1ad network, then the maximum number of service instances that
can be supported is 4K. These issues are highlighted in [VPLS-
Bridge].
This document describes how IEEE 802.1ah (aka Provider Backbone
Bridges) can be integrated with H-VPLS to address these scalability
issues. In case of H-VPLS with 802.1ah Ethernet access, the solution
results in better scalability in terms of both number of service
instances and number of C-MACs in the Ethernet access network and
the VPLS core network, as well as number of PWs in VPLS core
network. And in case of H-VPLS with MPLS access, Provider Backbone
Bridging functionality can be used at the U-PE or N-PE which results
in reduction of customer MAC addresses and number of PWs in the VPLS
core network.
The interoperability scenarios depicted in this document fall into
the following two categories:
- Scenarios where Provider Backbone Bridging seamlessly works
with current VPLS implementations (e.g. section 4.2).
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- Scenarios where VPLS PE implementations need to be upgraded in
order to work with Provider Backbone Bridging (e.g. sections
4.3, 5.1).
3. Terminology
802.1ad: IEEE specification for "QinQ" encapsulation and bridging of
Ethernet frames
802.1ah: IEEE specification for "MAC tunneling" encapsulation and
bridging of frames across a provider backbone bridged network.
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 B-MAC and B-TAG
information
B-MAC: The backbone source or destination MAC address fields defined
in the 802.1ah provider MAC encapsulation header.
BCB: A backbone core bridge running in the core of a provider
backbone bridged network. It bridges frames based on B-TAG
information just as an 802.1ad provider bridge will bridge frames
based on a VLAN identifier (S-VLAN)
BEB: A backbone edge bridge positioned at the edge of a provider
backbone bridged network. It can contain an I-component, B-component
or both I and B components.
B-TAG: field defined in the 802.1ah 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: This is the interface between a BEB and
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
I-component: A bridging component contained in a backbone edge
bridge that bridges in the customer space (customer MAC addresses,
S-VLAN)
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).
I-BEB: A backbone edge bridged positioned at the edge of a provider
backbone bridged network. It contains an I-component for bridging in
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the customer space (customer MAC addresses, service VLAN IDs).
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 802.1ah provider MAC encapsulation
header that conveys the service instance information (I-SID)
associated with the frame.
I-Tagged Service Interface: This the interface defined between the I
and B components inside an IB-BEB or between two B-BEB. Frames
passed through this interface contain an I-TAG field
PBB: Provider Backbone Bridge
PBBN: Provider Backbone Bridged Network
PBN: Provider Bridged Network. A network that employs 802.1ad (QinQ)
technology.
S-TAG: A field defined in the 802.1ad QinQ encapsulation header that
conveys the service VLAN identifier information (S-VLAN).
S-Tagged Service Interface: This 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
4. H-VPLS with Homogeneous PBBN Access
PBBN access offers MAC-address table scalability for H-VPLS PE
nodes. This is due to the MAC tunneling encapsulation scheme of PBB
which only exposes the provider's own MAC addresses to PE nodes (B-
MACs of Provider's PBB-capable devices in the access network), as
opposed to customers' MAC addresses in conventional H-VPLS with MPLS
or 802.1ad access.
PBBN access also offers service instance scalability when compared
to H-VPLS with 802.1Q/802.1ad access networks. This is due to the
new 24-bit service identifier (I-SID) used in PBB encapsulation,
which allows up to 16M services per PBB access network, compared to
4K services per 802.1Q/802.1ad access network.
Another important advantage of PBBN access is that it offers clear
separation between the service layer (represented by I-SID) and the
network layer (represented by B-VLAN). B-VLANs segregate a PBB
access network into different broadcast domains and possibly unique
spanning-tree topologies, with each domain being able to carry
multiple services (i.e. I-SIDs). In 802.1ad access networks, the
network and service layers are the same (represented by S-VLAN).
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This separation allows the Provider to manage and optimize the PBB
access network topology independent of the number of service
instances that are supported.
In this and the following sections we look into different flavors of
H-VPLS with PBBN access. This section discusses the case where H-
VPLS is deployed with homogenous PBBN access networks. Section 5
describes the case where at least one of the access networks is PBN
access (QinQ/802.1ad) while others are PBBN access.
At a macro scale, a network that employs H-VPLS with PBBN access can
be represented as shown in figure 1 below.
+--------------+
| |
+---------+ | IP/MPLS | +---------+
+----+ | | +----+ +----+ | | +----+
| CE |--| | |VPLS| |VPLS| | |--| CE |
+----+ | PBBN |---| PE | | PE |--| PBBN | +----+
+----+ | 802.1ah | +----+ +----+ | 802.1ah | +----+
| CE |--| | | Backbone | | |--| CE |
+----+ +---------+ +--------------+ +---------+ +----+
Figure 1: H-VPLS with PBBN Access
In the context of PBBN and H-VPLS interoperability, "I-SID Domain"
and "B-VID Domain" can be defined as follows:
- "I-SID Domain" refers to a network administrative boundary under
which all the PBB BEBs and VPLS PE devices use the same I-SID
space, i.e. the I-SID assignment is carried out by the same
administration. This effectively means that a given service
instance has the same I-SID designation on all devices within an
I-SID Domain.
- "B-VID Domain" refers to a network administrative boundary under
which all the PBB BEBs and VPLS PE devices employ consistent I-SID
to B-VLAN bundling - e.g., grouping of I-SIDs to B-VLANs are the
same in that domain. Although the two B-VLANs in two PBBNs that
represent the same group of I-SIDs do not need to use the same B-
VID value, in practice they often use the same value because once
the I-SID grouping is made identical in two PBBNs, it is rather
very easy to make the values of the corresponding B-VIDs also
identical.
Consequently, three different kinds of "Service Domains" are defined
in the following manner:
- Tightly Coupled Service Domain - Different PBBN access networks
belonging to the same I-SID Domain and B-VID Domain. However, the
network control protocols (e.g. xSTP) run independently in each
PBB access network.
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- Loosely Coupled Service Domain - Different PBB access networks
belonging to the same I-SID Domain. However, each PBBN access
maintains its own independent B-VID Domain. Again, the network
control protocols (e.g. xSTP) run independently in each PBBN
access.
- Different Service Domain - In this case, each PBBN access
maintains its own independent I-SID Domain and B-VID Domain, with
independent network control protocols (e.g. xSTP) in each PBB
access.
In general, correct service connectivity spanning networks in a
Tightly Coupled Service Domain can be achieved via B-VID mapping
between the networks (often even without B-VID translation).
However, correct service connectivity spanning networks in a Loosely
Coupled Service Domain requires I-SID to B-VID re-mapping (i.e
unbundling and re-bundling of I-SIDs into B-VIDs). Furthermore,
service connectivity spanning networks in Different Service Domains
requires both I-SID translation and I-SID to B-VID re-mapping.
4.1 Service Interfaces and Interworking Options
Customer devices will interface with PBBN edge bridges using
existing Ethernet interfaces including IEEE 802.1Q and IEEE 802.1ad.
At the PBBN edge, customer MAC frames are encapsulated in a PBB
header that includes a service provider source and destination MAC
addresses (B-MAC) and are bridged up to the VPLS PE. The PBB
encapsulated customer MAC frame is then injected into the VPLS
backbone network, delivered to the remote VPLS PE node(s), and
switched onto the remote PBBN access. From there, the PBBN bridges
the encapsulated frame to a PBBN edge bridge where the PBB header is
removed and the customer frame is sent to customer domain.
Interoperating between PBBN devices and VPLS PE nodes will certainly
leverage work already completed. When I-SID visibility is required
at the VPLS PE nodes, a new service interface based on I-SID tag
will need to be defined.
Moreover, by mapping a bridge domain (e.g. B-VLAN) to a VPLS
instance, and bundling multiple end-customer service instances,
represented by I-SIDs, over the same bridge domain, service
providers will be able to significantly reduce the number of full-
mesh PWs required in the core. In this case, I-SID visibility is not
required on the VPLS-PE and the I-SID will serve as the means of
multiplexing/de-multiplexing individual service instances in the
PBBN over a bundle (e.g. B-VLAN).
When I-SID visibility is expected across the service interface at
the VPLS PE, VPLS PE can be considered to offer service-level
interworking between PBBN access and IP/MPLS core. Similarly, when
PE is not expected to have visibility of I-SID at the service
interface, VPLS PE can be considered to offer network-level
interworking between PBBN access and MPLS core.
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A VPLS PE is always part of the IP/MPLS core, and may optionally
participate in the control protocols (e.g. xSTP) of the access
network. When connecting to a PBBN access, the VPLS PE needs to
support one of the following two types of service interfaces:
- Type I: B-Tagged Service Interface with B-VID as Service Delimiter
- The PE connects to a Backbone Core Bridge (BCB) in PBBN access.
The handoff between the BCB and the PE is B-Tagged PBB
encapsulated frame. The PE is transparent to PBB encapsulations
and treats these frames as 802.1ad frames since B-VID EtherType is
the same as S-VID EtherType. The PE does not need to support PBB
functionality. This corresponds to conventional VPLS PE's tagged
service interface. When using Type I service interface, the PE
needs to support either raw-mode or tagged-mode Ethernet PW. Type
I Service Interface is described in detail in Section 4.2.
- Type II: I-Tagged Service Interface with I-SID as Service
Delimiter - The PE connects to a B-BEB (Backbone Edge Bridge with
B-Component) in PBBN access. The PE itself also supports the B-BEB
functionality of [802.1ah]. The handoff between the B-BEB in PBBN
access and the PE is an I-Tagged PBB encapsulated frame. With Type
II service interface, the PE supports the existing raw-mode and
tagged-mode PW types. Type II Service Interface is described in
detail in Section 4.3.
4.2 H-VPLS with PBBN Access: Type I Service Interface
This is a B-Tagged service interface with B-VID as service delimiter
on the VPLS-PE. It does not require any new functionality on the
VPLS-PE. As shown in Figure 2, the PE is always part of the IP/MPLS
core. The PE may also be part of the PBBN Access (e.g. VPLS-PE on
right side of Figure 2) by participating in network control
protocols (e.g. xSTP) of the PBBN access.
PBBN Access IP/MPLS Core PBBN Access
+--------------+
+---------+ | | +---------------+
| | +----+ | | |
| +---+ |VPLS| +-+ | | +---+ |
| |BCB|---| PE |---|P| | | |BCB| |
| +---+ /+----+ /+-+\ | | /+---+ |
|+---+ | / +----+ / \+----+ / +---+|
+--+ ||IB-| +---+/ |VPLS|/ +-+ |VPLS|/ +---+ |IB-|| +--+
|CE|-||BEB|-|BCB|---| PE |---|P|--| PE |---|BCB|-|BEB|--|CE|
+--+ |+---+ +---+ ^ +----+ +-+ +----+ ^ +---+ +---+| +--+
| | | | | | | |
+---------+ | | | +--|------------+
| +--------------+ |
| |
Type I Type I
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Figure 2: H-VPLS with PBBN Access & Type I Service Interface
Type I service interface is applicable to networks with Tightly
Coupled Service Domains, where both I-SID Domains and B-VID Domains
are the same across all PBBN access networks.
The BCB and VPLS PE will exchange PBB encapsulated frames that
include source and destination B-MAC addresses, a B-VID and I-SID.
The service delimiter, from the perspective of the VPLS PE, is the
B-VID; in fact, this interface operates exactly as a current
802.1Q/ad interface into a VPLS PE does today. With Type I service
interface, VPLS PE can be considered as providing network-level
interworking between PBBN and MPLS domains, since VPLS PE does not
have visibility of I-SIDs.
The main advantage of this service interface, when compared to other
types, is that it allows the service provider to save on the number
of full-mesh PWs required in the core. This is primarily because
multiple service instances (I-SIDs) are bundled over a single full-
mesh corresponding to a bridge domain (e.g. B-VID), instead of
requiring a dedicated full-mesh per service instance. Another
advantage is the MAC address scalability in the core since the core
is not exposed to C-MACs.
The disadvantage of this interface is the comparably excessive
replication required in the core: since a group of service instances
share the same full-mesh of PWs, an unknown unicast, multicast or
broadcast on a single service instance will result in a flood over
the core. This, however, can be mitigated via the use of per I-SID
flood containment (B-MAC multicast pruning) as described in [PBB-
VPLS-MCAST].
Three different modes of operation are supported by Type I Service
Interface:
- Port Mode: All traffic over an interface in this mode is mapped to
a single VPLS instance. Existing PW signaling and Ethernet raw
mode (0x0005) PW type, defined in [RFC4447] [RFC4448], are
supported.
- VLAN Mode: all traffic associated with a particular VLAN
identified by the B-VID is mapped to a single VPLS instance.
Existing PW signaling and Ethernet raw mode (0x0005) PW type,
defined in [RFC4447] [RFC4448], are supported.
- VLAN Bundling Mode: all traffic associated with a group or range
of VLANs or B-VIDs is mapped to a single VPLS instance. Existing
PW signaling and Ethernet raw mode (0x0005) PW type, defined in
[RFC4447] [RFC4448], are supported.
For the above three modes, it is also possible to use Ethernet
tagged mode (0x0004) PW, as defined in [RFC4447] [RFC4448], for
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interoperability with equipment that does not support raw mode. The
use of raw mode is recommended to be the default though.
4.3 H-VPLS with PBBN Access: Type II Service Interface
This is an I-Tagged service interface with I-SID as service
delimiter on VPLS-PE. It requires the VPLS-PE to include B-Component
of PBB BEB for I-SID processing in addition to the capability to map
I-SID Bundle to VPLS instance. As shown in Figure 3, the PE is
always part of IP/MPLS core and connects to one or more B-BEB in
PBBN access.
PBBN Access IP/MPLS Core PBBN Access
+--------------+
+---------+ | | +---------+
| | | | | |
| +---+ +-----+ | | +---+ |
| |B- | |PE w/| +-+ | | |BCB| |
| |BEB|--|B-BEB|-|P| | | +---+ |
| +---+ /+-----+ +-+ | | / | |
|+---+ +---+/ +-----+/ \+-----+ +---+ +---+|
+--+ ||IB-| |B- | |PE w/| +-+ |PE w/| |B- | |IB-|| +--+
|CE|-||BEB|-|BEB|--|B-BEB|-|P|-|B-BEB|-|BEB| |BEB|--|CE|
+--+ |+---+ +---+ ^+-----+ +-+ +-----+^+---+ +---+| +--+
| | | | | | | |
+---------+ | | | | +---------+
| +-------------+ |
| |
Type II Type II
Figure 3: H-VPLS with PBBN Access & Type II Service Interface
Type II service interface is applicable to Loosely Coupled Service
Domains and Different Service Domains. B-VID Domains can be
independent and the B-VID is always locally significant in each PBBN
access and does not need to be transported over the IP/MPLS core.
Given the above, it should be apparent that Type II service
interface is applicable to Tightly Coupled Service Domains as well.
By definition the B-BEB connecting to the VPLS PE will remove any B-
VLAN tags for frames exiting the PBB access network. The B-BEB and
VPLS PE will exchange PBB encapsulated frames that include source
and destination B-MAC addresses, and I-SID. The service delimiter,
from the perspective of the VPLS PE, is the I-SID. Since PE has
visibility to I-SIDs, the PE provides service-level interworking
between PBBN access and IP/MPLS core.
The advantage that Type II service interface has compared to Type I
is the potentially less replication in the core without the need for
a per I-SID flood containment (B-MAC multicast pruning) mechanism.
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This is mainly due to the increased segregation of service instances
over disjoint full-meshes of PWs.
The disadvantage of this service interface, compared to Type I, is
that it may require a larger number of full-mesh PWs in the core.
However, the number of full-mesh PWs can still be less than those
required by H-VPLS without PBBN access.
It is expected that this interface type will be used for customers
with significant multicast traffic (but without multicast pruning
capability in VPLS PE) so that a separate VPLS instance is set up
per group of customers with similar geographic locality (per I-SID
group).
Type II Service Interface may operate in I-SID Bundling Mode: all
traffic associated with a group or range of I-SIDs is mapped to a
single VPLS instance. The PE maintains a mapping of I-SIDs to a PE
local bridge domain (e.g. B-VID). The VPLS instance is then
associated with this bridge domain. With Tightly and Loosely Coupled
Service Domains, no I-SID translation needs to be performed. Type II
Service Interface also supports Different Service Domains in this
mode, since the B-BEB link in the PE connecting to the local PBBN
can perform the translation of PBBN-specific I-SID to a local I-SID
within the IP/MPLS core, which may then be translated to the other
PBBN specific I-SID on the egress PE. Such translation can also
occur in the B-BEB of PBBN access. Existing PW signaling and
Ethernet raw mode (0x0005), defined in [RFC4447] [RFC4448], is
supported. It is also possible to use tagged mode (0x0004) PW for
purpose of interoperability with equipment that does not support raw
mode.
Note 1: Port mode is not called out in Type II Service Interface
since it requires the mapping of I-SIDs to be identical on different
I-Tagged interfaces across VPLS network. If this is indeed the case,
Port mode defined in Type I Service Interface (Section 4.2) can be
used.
Note 2: In a degenerate scenario, it is possible to define an I-SID
bundle that comprises a single I-SID. This allows the Type II
service interface to effectively operate as if in I-SID Mode, at the
added expense of consuming more bridge-domains on the PE and
increased number of PW full-mesh in the core.
5. H-VPLS with Mixed PBBN Access and PBN Access
It is foreseeable that service providers will want to interoperate
their existing PBN (QinQ) access networks with PBBN access networks
over H-VPLS. Figure 4 below shows the high-level network topology.
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+--------------+
| |
+---------+ | IP/MPLS | +---------+
+----+ | | +----+ +----+ | | +----+
| CE |--| PBN | |VPLS| |VPLS| | |--| CE |
+----+ | (QinQ) |---| PE1| | PE2|--| PBBN | +----+
+----+ | 802.1ad | +----+ +----+ | 802.1ah | +----+
| CE |--| | | Backbone | | |--| CE |
+----+ +---------+ +--------------+ +---------+ +----+
Figure 4: H-VPLS with Mixed PBN and PBBN Access Networks
Referring to Figure 4 above, two possibilities come into play
depending on whether the interworking is carried out at PE1 or PE2.
These are described in the following sub-Sections.
5.1 H-VPLS with Mixed PBBN & PBN Access: Modified PBN PE
As shown in Figure 5, the operation of VPLS PE2 (connecting to the
PBBN access on the right) is no different from what was discussed in
Section 4. Type II service interface, as discussed in the above
section, is applicable. It is the behavior of VPLS PE1 (connecting
to the PBN access on the left) that is the focus of this section.
PBN Access IP/MPLS Core PBBN Access
(802.1ad) +--------------+ (802.1ah)
| | +---------+
+---------+ | | | |
| | +-----+ | | +---+ |
| +---+ |PE w/| +-+ | | |BCB| |
| |PCB|--|IBBEB|-|P| | | +---+ |
| +---+ /+-----+ +-+ | | / | |
| | / +-----+/ \+-----+ +---+ +---+|
+--+ |+---+ +---+ |PE w/| +-+ |PE w/| |B- | |IB-|| +--+
|CE|-||PEB|-|PCB|--|IBBEB|-|P|-|B-BEB|-|BEB| |BEB|--|CE|
+--+ |+---+ +---+ ^+-----+ +-+ +-----+^+---+ +---+| +--+
| | | |PE1 PE2| | | |
+---------+ | | | | +---------+
| +-------------+ |
| |
S-Tagged Type II (I-Tagged)
Figure 5: H-VPLS with Mixed PBN and PBBN Access: Modified PBN PE
Some assumptions made for this topology include:
- CE is directly connected to PBBN via S-Tagged or port-based
Interface
- I-SID in PBBN access represents the same customer as S-VID in PBN
access
- At S-Tagged Service Interface of PE with IB-BEB functionality
(e.g. PE1 in Figure 5), the only viable service is 1:1 mapping of
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S-VID to I-SID. However, towards the core network side, the same
PE can support I-SID bundling into a VPLS instance.
- PE1 participates in the local ISID domain of the IP/MPLS Core so
the model accommodates for the rest of the PBB network any of the
three domain types described in section 4 - Tightly, Loosely
Coupled and Different Service Domains.
- For ease of provisioning in these disparate access networks, it is
recommended to use the same I-SID Domain among the PBBN access and
PEs with IB-BEB functionality (those connecting to PBN).
This topology operates in I-SID Bundling Mode: at PE connecting to
PBN access, each S-VID is mapped to an I-SID and subsequently a
group of I-SIDs is mapped to a VPLS instance. Similarly, at PE
connecting to PBBN access, each group of I-SIDs is mapped to a VPLS
instance. Similar to Type II interface, no I-SID translation is
performed for I-SID bundling case. Existing PW signaling and
Ethernet raw mode (0x0005) PW type, defined in [RFC4447] [RFC4448],
are supported. It is possible to use tagged mode (0x0004) PW for
backward compatibility as well.
5.2 H-VPLS with Mixed PBBN & PBN Access: Regular PBN PE
As shown in Figure 6, the operation of VPLS PE1 (connecting to the
PBN access on the left) is no different from existing VPLS PEs. It
is the behavior of VPLS PE2 (connecting to the PBBN access on the
right) that is the focus of this section.
PBN Access IP/MPLS Core PBBN Access
(802.1ad) +--------------+ (802.1ah)
| | +---------+
+---------+ | | | |
| | +-----+ | | +---+ |
| +---+ | PE | +-+ | | |BCB| |
| |PCB|--| |-|P| | | +---+ |
| +---+ /+-----+ +-+ | | / | |
| | / +-----+/ \+-----+ +---+ +---+|
+--+ |+---+ +---+ | PE | +-+ |PE w/| |B- | |IB-|| +--+
|CE|-||PEB|-|PCB|--| |-|P|-|IBBEB|-|BEB| |BEB|--|CE|
+--+ |+---+ +---+ ^+-----+ +-+ +-----+^+---+ +---+| +--+
| | | |PE1 PE2| | | |
+---------+ | | | | +---------+
| +-------------+ |
| |
S-Tagged Type II (I-Tagged)
Figure 6: H-VPLS with Mixed PBN and PBBN Access: Regular PBN PE
Some assumptions made for this topology include:
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- CE is directly connected to PBBN via S-Tagged or port-based
Interface
- I-SID in PBBN access represents the same customer as S-VID in PBN
access
- There is 1:1 mapping between the I-SID and VPLS instance
- At S-Tagged Service Interface of PE connecting to PBN (e.g. PE1 in
Figure 6), the PE only provides 1:1 mapping of S-VID to VPLS
instance. S-VID bundling is not a viable option since it does not
correspond to anything in PBBN access.
- The PE connecting to PBBN (e.g. PE2 in Figure 6), supports IB-BEB
functionality and the I-Component is connected to the VPLS
Forwarder (i.e. the I-Component faces the MPLS core whereas the B-
Component faces the PBBN access network). One or more I-SIDs can
be grouped into a B-VID in the PBBN access.
- Since C-VID grouping in different PBBN access networks must be
consistent, it is assumed that same I-SID Domain is used across
these PBBN access networks.
Unlike the other topology, I-SID bundling mode is not supported in
this case. This is primarily because the VPLS core operates in the
same manner as today. The PE with IB-BEB functionality connecting to
PBBN access performs the mapping of each VPLS instance to an I-SID
and one or more of these I-SIDs may be mapped onto a B-VID within
the PBBN access network.
6. H-VPLS with MPLS Access
In this section, the case of H-VPLS with MPLS access network is
discussed. The integration of PBB functionality into VPLS-PE for
such access networks is described to improve the scalability of the
network in terms of the number of MAC addresses and service
instances that are supported.
For this topology, it is possible to embed PBB functionality in
either the U-PE or the N-PE. Both of these cases are described in
the following sub-sections.
6.1 H-VPLS with MPLS Access: PBB U-PE
As stated earlier, the objective for incorporating PBB function at
the U-PE is to improve the scalability of H-VPLS networks in terms
of the number of MAC addresses and service instances that are
supported.
In current H-VPLS, the N-PE must learn customer MAC addresses (C-
MACs) of all VPLS instances that it participates in. This can easily
add-up to hundreds of thousands or even millions of C-MACs at the N-
PE. When the U-PE performs PBB encapsulation, the N-PE only needs to
learn the MAC addresses of the U-PEs, which is a significant
reduction. Furthermore, when PBB encapsulation is used, many I-SIDs
are multiplexed within a single bridge domain (e.g., B-VLAN). If the
VPLS instance is set up per B-VLAN, then one can also achieve a
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significant reduction in the number of full-mesh PWs. It should be
noted that this reduction in full-mesh PWs comes at the cost of
potentially increased replication over the full-mesh of PWs: A given
customer multicast and/or broadcast frames are effectively
broadcasted within the B-VLAN. This may result in additional frame
replication because the full-mesh PWs corresponding to a B-VLAN is
most likely bigger than the full-mesh PWs corresponding to a single
I-SID. However, per I-SID flood containment (B-MAC multicast
pruning) as described in [PBB-VPLS-MCAST] can be used to remedy this
drawback and have multicast traffic replicated efficiently for each
customer (i.e. for each I-SID).
Figure 7 below illustrates the scenario for H-VPLS with MPLS access.
As it can be seen, customer networks or hosts (CE) connect into the
U-PE nodes using standard Ethernet interfaces [802.1D], [802.1Q], or
[802.1ad]. The U-PE is connected upstream to one or more VPLS N-PE
nodes by MPLS PWs (per VPLS instance). These, in turn, are connected
via a full-mesh of PWs (per VPLS instance) traversing the IP/MPLS
core. The U-PE is outfitted with PBB Backbone Edge Bridge (BEB)
functions where it can encapsulate/de-encapsulate customer MAC
frames in provider B-MAC addresses and perform I-SID translation if
needed.
PBB PBB
BEB +----------+ BEB
| | | |
| +-----------+ | IP | +-----------+ |
| | MPLS | | MPLS | | MPLS | |
V | Access +----+ | Core | +----+ Access | V
+--+ +----+ |VPLS|-| |-|VPLS| +----+ +--+
|CE|--|U-PE| |N-PE| | | | PE | |U-PE|--|CE|
+--+ +----+ +----+ | | +----+ +----+ +--+
| | | | | |
+-----------+ +----------+ +-----------+
Figure 7: H-VPLS with MPLS Access Network and PBB U-PE
The U-PE still provides the same type of services toward its
customers as before and they are:
- Port mode (either 802.1D, 802.1Q, or 802.1ad)
- VLAN mode (either 802.1Q or 802.1ad)
- VLAN-bundling mode (either 802.1Q or 802.1ad)
By incorporating PBB function, the U-PE maps each of these services
(for a given customer) onto a single I-SID based on the
configuration at the U-PE. Many I-SIDs are multiplexed within a
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single bridge domain (e.g. B-VLAN). The U-PE can then map a bridge-
domain onto a VPLS instance and the encapsulated frames are sent
over the PW associated with that VPLS instance. Furthermore the
entire Ethernet bridging operation over VPLS network is performed as
defined in [RFC4762]. In other words, MAC forwarding is based on the
B-MAC address space and service delimiter is based on VLAN ID, which
is B-VID in this case. There is no need to inspect or deal with I-
SID values in the VPLS N-PEs.
6.1.1 PBB U-PEs in Single I-SID Domain
In this scenario, I-SID assignment is performed globally across all
MPLS access networks and therefore there is no need for I-SID
translation. This scenario support I-SID bundling mode and it is
assumed that the mapping of the I-SIDs to the bridge domain (e.g.,
B-VLAN) is consistent across all the participating PE devices. In
case of the I-SID bundling mode, a bridge domain (e.g., B-VLAN) is
mapped to a VPLS instance and existing Ethernet raw mode (0x0005) or
tagged mode (0x0004) PW type as defined in [RFC4447] [RFC4448].
I-SID mode can be considered as a degenerate case of I-SID bundling
where a single bridge domain is used per I-SID. However, that
results in increase number of bridge domains and PWs in the PEs. PBB
flood containment (B-MAC multicast pruning) per I-SID as described
in [PBB-VPLS-MCAST] can be used in conjunction with I-SID bundling
mode to limit the scope of flooding per I-SID thus removing the need
for I-SID mode.
6.2 H-VPLS with MPLS Access: PBB N-PE
In this case, the PBB function is incorporated at the N-PE to
improve the scalability of H-VPLS networks in terms of the numbers
of MAC addresses and service instances that are supported.
Customer networks or hosts (CE) connect into the U-PE nodes using
standard Ethernet interfaces [802.1D], [802.1Q], or [802.1ad]. The
U-PE is connected upstream to one or more VPLS N-PE nodes by MPLS
PWs (per customer). These, in turn, are connected via a full-mesh of
PWs (per customer or group of customers) traversing the IP/MPLS
core.
The U-PE still provides the same type of services toward its
customers as before and they are:
- Port mode (either 802.1D, 802.1Q, or 802.1ad)
- VLAN mode (either 802.1Q or 802.1ad)
- VLAN-bundling mode (either 802.1Q or 802.1ad)
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Spoke PW from U-PE to N-PE is not service multiplexed because there
is no PBB functionality on u-PE - i.e. one service per PW.
PBB PBB
BEB +----------+ BEB
| | | |
+-----------+ | | IP | | +-----------+
| MPLS | V | MPLS | V | MPLS |
| Access +----+ | Core | +----+ Access |
+--+ +----+ |VPLS|-| |-|VPLS| +----+ +--+
|CE|--|U-PE| |N-PE| | | | PE | |U-PE|--|CE|
+--+ +----+ +----+ | | +----+ +----+ +--+
| | | | | |
+-----------+ +----------+ +-----------+
Figure 8: H-VPLS with MPLS Access Network and PBB N-PE
By incorporating PBB function, the N-PE maps each of these services
(for a given customer) onto a single I-SID based on the
configuration at the N-PE. Many I-SIDs can be multiplexed within a
single bridge domain (e.g. B-VLAN). The N-PE can, then, either map a
single I-SID into a VPLS instance or it can map a bridge domain
(e.g. B-VLAN) onto a VPLS instance, according to its configuration.
Next, the encapsulated frames are sent over the set of PWs
associated with that VPLS instance.
6.2.1 PBB N-PEs in Single I-SID Domain
In this scenario, I-SID assignment is performed globally across all
MPLS access networks and therefore there is no need for I-SID
translation. This scenario support I-SID bundling mode and it is
assumed that the mapping of the I-SIDs to the bridge domain (e.g.,
B-VLAN) is consistent across all the participating PE devices. In
case of the I-SID bundling mode, a bridge domain (e.g., B-VLAN) is
mapped to a VPLS instance and existing Ethernet raw mode (0x0005) or
tagged mode (0x0004) PW type as defined in [RFC4447] [RFC4448], can
be used.
I-SID mode can be considered as a degenerate case of I-SID bundling
where a single bridge domain is used per I-SID. However, that
results in increase number of bridge domains and PWs in the PE. PBB
flood containment (B-MAC multicast pruning) per I-SID as described
in [PBB-VPLS-MCAST] can be used in conjunction with I-SID bundling
mode to limit the scope of flooding per I-SID thus removing the need
for I-SID mode.
7. H-VPLS with MPLS Access: PBB Migration Scenarios
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Operators and service providers that have deployed H-VPLS with
either MPLS or Ethernet are unlikely to migrate to PBB technology
overnight because of obvious cost implications. Thus, it is
imperative to outline migration strategies that will allow operators
to protect investments in their installed base while still taking
advantage of the scalability benefits of PBB technology.
In the following sub-sections, we explore three different migration
scenarios which allow a mix of existing H-VPLS access networks to
co-exist with newer PBB-based access networks. The scenarios differ
in whether the Ethernet service frames passing over the VPLS core
are PBB-encapsulated or not. The first scenario in section 7.1
involves passing only non PBB-encapsulated frames over the core. The
second scenario in section 7.2 stipulates passing only PBB-
encapsulated frames over the core. Whereas, the final scenario in
section 7.3 depicts a core that supports a mix of PBB-encapsulated
and non PBB-encapsulated frames. The advantages and disadvantages of
each scenario will be discussed in its respective section.
7.1 802.1ad Service Frames over VPLS Core
In this scenario, existing access networks are left unchanged. All
N-PEs would forward frames based on C-MAC addresses. In other words,
Ethernet frames which are traversing the VPLS core (within PWs)
would use the 802.1ad frame format, as in current VPLS. Hence, the
N-PEs in existing access networks do not require any modification.
For new MPLS access networks that have PBB functions on the U-PE,
the corresponding N-PE must incorporate built-in IB-BEB functions in
order to terminate the PBB encapsulation before the frames enter the
core. A key point here is that while both the U-PE and N-PE nodes
implement PBB IB-BEB functionality, the former has the I-Component
facing the customer (CE) and the B-Component facing the core;
whereas the latter has the I-Component facing the core and the B-
Component facing the customer (i.e. access network).
PBB PBB
+----------+ IB-BEB IB-BEB
| | | |
+-----------+ | IP | | +-----------+ |
| MPLS | | MPLS | V | MPLS | |
| Access +----+ | Core | +----+ Access | V
+--+ +----+ |VPLS|-| |-|VPLS| +----+ +--+
|CE|--|U-PE| |N-PE| | | | PE | |U-PE|--|CE|
+--+ +----+ +----+ | | +----+ +----+ +--+
| (Existing)| | | | (New) |
+-----------+ +----------+ +-----------+
Figure 9: Migration with 802.1ad Service Frames over VPLS Core
The main advantage of this approach is that it requires no change to
existing access networks or existing VPLS N-PEs. The main
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disadvantage is that these N-PEs will not leverage the advantages of
PBB in terms of MAC address and PW scalability.
It is worth noting that this migration scenario is an optimal option
for an H-VPLS deployment with a single PBB-capable access network.
When multiple PBB-capable access networks are required, then the
scenario in Section 7.3 is preferred, as it provides a more scalable
and optimal interconnect amongst the PBB-capable networks.
7.2 PBB Service Frames over VPLS Core
This scenario requires that the VPLS N-PE connecting to existing
MPLS access networks be upgraded to incorporate IB-BEB functions.
All Ethernet service frames passing over the VPLS core would be PBB-
encapsulated. The PBB over MPLS access networks would require no
special requirements beyond what is captured in section 6 of this
document.
In this case, both the U-PE and N-PE which implement IB-BEB
functionality have the I-Component facing the customer and the B-
Component facing the core.
PBB PBB
IB-BEB +----------+ IB-BEB
| | | |
+-----------+ | | IP | +-----------+ |
| MPLS | V | MPLS | | MPLS | |
| Access +----+ | Core | +----+ Access | V
+--+ +----+ |VPLS|-| |-|VPLS| +----+ +--+
|CE|--|U-PE| |N-PE| | | | PE | |U-PE|--|CE|
+--+ +----+ +----+ | | +----+ +----+ +--+
| (Existing)| | | | (New) |
+-----------+ +----------+ +-----------+
Figure 10: Migration with PBB Service Frames over VPLS Core
The main advantage of this approach is that it allows better
scalability of the VPLS N-PEs in terms of MAC address and pseudowire
counts. The disadvantage is that it requires upgrading the VPLS N-
PEs of all existing MPLS access networks.
7.3 Mixed 802.1ad and PBB over VPLS Core
In this scenario, existing access networks are left unchanged, and
exchange Ethernet frames with 802.1ad format over the PWs in the
core. The newly added access networks, which incorporate PBB
functionality exchange Ethernet frames that are PBB-encapsulated
amongst each other over core PWs. For service connectivity between
existing access network (non PBB capable) and new access network
(PBB based), the VPLS N-PE of the latter network employs IB-BEB
functionality to de-capsulate the PBB header from frames outbound to
the core, and encapsulate the PBB header for frames inbound from the
core. As a result, a mix of PBB-encapsulated and 802.1ad Ethernet
service frames are exchanged over the VPLS core.
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This mode of operation requires new functionality on the VPLS N-PE
of the PBB-capable access network, so that the PE can send frames in
802.1ad format or PBB format, on a per PW basis, depending on the
capability of the destination access network. Effectively, the PE
would have to incorporate B-BEB as well as IB-BEB functions.
A given PE needs to be aware of the capability of its remote peer in
order to determine whether it connects to the right PW Forwarder.
This can be achieved either via static configuration, or by
extending the VPLS control plane (BGP-based auto-discovery and LDP
Signaling) discussed in [L2VPN-Sig]. The latter approach and the
details of the extensions required are out of scope for this
document and will be covered in a separate document.
PBB
B-BEB PBB
+----------+ IB-BEB IB-BEB
| | | |
+-----------+ | IP | | +-----------+ |
| MPLS | | MPLS | V | MPLS | |
| Access +----+ | Core | +----+ Access | V
+--+ +----+ |VPLS|-| |-|VPLS| +----+ +--+
|CE|--|U-PE| |N-PE| | | |N-PE| |U-PE|--|CE|
+--+ +----+ +----+ | | +----+ +----+ +--+
| (Existing)| | | | (New) |
+-----------+ +----------+ +-----------+
Figure 11: Migration with Mixed 802.1ad &PBB Service Frames over VPLS
Core
The U-PE and N-PE of the PBB-capable access network both employ BEB
functionality: The U-PE implements IB-BEB function where the I-
Component faces the customer (CE) and the B-Component faces the
core. The N-PE, on the other hand, implements IB-BEB functionality
with the I-Component facing the core and the B-Component facing the
customer (access network). In addition, the N-PE implements stand-
alone B-BEB functionality.
This scenario combines the advantages of both previous scenarios
without any of their shortcomings, namely: it does not require any
changes to existing access networks and it allows the N-PE to
leverage the scalability benefits of 802.1ah for PBB to PBB access
network connectivity. The disadvantage of this option is that it
requires new functionality on the N-PE of the PBB-capable access
network.
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8. Acknowledgments
TBD.
9. IANA Considerations
This document has no actions for IANA.
10. Security Considerations
This document does not introduce any additional security aspects
beyond those applicable to VPLS/H-VPLS. VPLS/H-VPLS security
considerations are already covered in [RFC4762].
11. Intellectual Property Considerations
This document is being submitted for use in IETF standards
Discussions.
12. References
12.1 Normative References
[802.1ad] "Virtual Bridged Local Area Networks, Amendment 4:
Provider Bridges", IEEE Std. 802.1ad-2005, May 2006
[802.1ah] "Virtual Bridged Local Area Networks Amendment 7: Provider
Backbone Bridges", IEEE Std. 802.1ah-2008, August 2008
[RFC4447] "Pseudowire Setup and Maintenance using LDP", RFC4447,
April 2006
[RFC4448] "Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC4448, April 2006
[RFC4762] "Virtual Private LAN Service (VPLS) Using Label
Distribution Protocol (LDP) Signaling", RFC4762, January 2007
[L2VPN-Sig] E. Rosen, et Al. "Provisioning, Autodiscovery and
Signaling in L2VPNs", draft-ietf-l2vpn-signaling-08.txt, May 2006
( work in progress )
12.2 Informative References
[802.1Q] "Virtual Bridged Local Area Networks", IEEE Std. 802.1Q-
2005
[802.1D-REV] "Media Access Control (MAC) Bridges", IEEE Std. 802.1D-
2003
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[VPLS-Bridge] "VPLS Interoperability with CE Bridges", draft-ietf-
l2vpn-vpls-bridge-interop-02.txt, Work in progress, November 2007
[VPLS-MCAST] "Multicast in VPLS", draft-ietf-l2vpn-vpls-mcast-
03.txt, Work in progress, November 2007
[PBB-VPLS-MCAST] "Multicast Pruning in Provider Backbone Bridged
VPLS", draft-sajassi-l2vpn-pbb-vpls-mcast-pruning-00.txt, Work in
progress, July 2008
Authors' Addresses
Ali Sajassi
Cisco
170 West Tasman Drive
San Jose, CA 95134, US
Email: sajassi@cisco.com
Samer Salam
Cisco
595 Burrard Street, Suite 2123
Vancouver, BC V7X 1J1, Canada
Email: ssalam@cisco.com
Chris Metz
Cisco
170 West Tasman Drive
San Jose, CA 95134, US
Email: metz@cisco.com
Nabil Bitar
Verizon Communications
Email : nabil.n.bitar@verizon.com
Dinesh Mohan
Nortel
3500 Carling Ave
Ottawa, ON K2H8E9, Canada
Email: dinmohan@hotmail.com
Florin Balus
Alcatel-Lucent
701 E. Middlefield Road
Mountain View, CA, USA 94043
Email: florin.balus@alcatel-lucent.com
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