Internet DRAFT - draft-ietf-ppvpn-vpls-ldp
draft-ietf-ppvpn-vpls-ldp
Internet Draft Document Marc Lasserre
Provider Provisioned VPN Working Group Riverstone Networks
draft-ietf-ppvpn-vpls-ldp-00.txt Vach Kompella
Nick Tingle
Sunil Khandekar
Timetra Networks
Ali Sajassi
Cisco Systems
Pascal Menezes Loa Andersson
Terabeam Networks Consultant
Andrew Smith Pierre Lin
Consultant Yipes Communication
Juha Heinanen Giles Heron
Song Networks PacketExchange Ltd.
Ron Haberman Tom S.C. Soon
Masergy, Inc. Yetik Serbest
Eric Puetz
Nick Slabakov SBC Communications
Rob Nath
Riverstone Networks
Luca Martini
Vasile Radaoca Level 3
Nortel Networks Communications
Expires: December 2003 June 2003
Virtual Private LAN Services over MPLS
draft-ietf-ppvpn-vpls-ldp-00.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
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The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2. Abstract
This document describes a virtual private LAN service (VPLS)
solution over MPLS, also known as Transparent LAN Services (TLS). A
VPLS creates an emulated LAN segment for a given set of users. It
delivers a layer 2 broadcast domain that is fully capable of
learning and forwarding on Ethernet MAC addresses that is closed to
a given set of users. Many VPLS services can be supported from a
single PE node.
This document describes the control plane functions of signaling
demultiplexor labels, extending [PWE3-CTRL] and rudimentary support
for availability (multi-homing). It is agnostic to discovery
protocols. The data plane functions of forwarding are also
described, focusing, in particular, on the learning of MAC
addresses. The encapsulation of VPLS packets is described by [PWE3-
ETHERNET].
3. 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 RFC 2119
Placement of this Memo in Sub-IP Area
RELATED DOCUMENTS
www.ietf.org/internet-drafts/draft-ietf-ppvpn-l2vpn-requirements-
01.txt
www.ietf.org/internet-drafts/draft-ietf-ppvpn-l2-framework-03.txt
www.ietf.org/internet-drafts/draft-ietf-pwe3-ethernet-encap-02.txt
www.ietf.org/internet-drafts/draft-ietf-pwe3-control-protocol-01.txt
WHERE DOES THIS FIT IN THE PICTURE OF THE SUB-IP WORK
PPVPN
WHY IS IT TARGETED AT THIS WG
The charter of the PPVPN WG includes L2 VPN services and this draft
specifies a model for Ethernet L2 VPN services over MPLS.
JUSTIFICATION
Existing Internet drafts specify how to provide point-to-point
Ethernet L2 VPN services over MPLS. This draft defines how
multipoint Ethernet services can be provided.
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Table of Contents
1. Status of this Memo.............................................1
2. Abstract........................................................2
3. Conventions.....................................................2
4. Overview........................................................4
5. Topological Model for VPLS......................................5
5.1. Flooding and Forwarding.......................................5
5.2. Address Learning..............................................6
5.3. LSP Topology..................................................6
5.4. Loop free L2 VPN..............................................7
6. Discovery.......................................................7
7. Control Plane...................................................7
7.1. LDP Based Signaling of Demultiplexors.........................7
7.2. MAC Address Withdrawal........................................9
7.2.1. MAC TLV.....................................................9
7.2.2. Address Withdraw Message Containing MAC TLV................10
8. Data Forwarding on an Ethernet VC Type.........................11
8.1. VPLS Encapsulation actions...................................11
8.1.1. VPLS Learning actions......................................12
9. Operation of a VPLS............................................12
9.1. MAC Address Aging............................................13
10. A Hierarchical VPLS Model.....................................13
10.1. Hierarchical connectivity...................................14
10.1.1. Spoke connectivity for bridging-capable devices...........14
10.1.2. Advantages of spoke connectivity..........................16
10.1.3. Spoke connectivity for non-bridging devices...............17
10.2. Redundant Spoke Connections.................................18
10.2.1. Dual-homed MTU device.....................................18
10.2.2. Failure detection and recovery............................19
10.3. Multi-domain VPLS service...................................20
11. Hierarchical VPLS model using Ethernet Access Network.........20
11.1. Scalability.................................................21
11.2. Dual Homing and Failure Recovery............................21
12. Significant Modifications.....................................22
13. Acknowledgments...............................................22
14. Security Considerations.......................................22
15. Intellectual Property Considerations..........................22
16. Full Copyright Statement......................................22
17. References....................................................23
18. Authors' Addresses............................................24
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4. Overview
Ethernet has become the predominant technology for Local Area
Networks (LANs) connectivity and is gaining acceptance as an access
technology, specifically in Metropolitan and Wide Area Networks (MAN
and WAN respectively). An Ethernet port is used to connect a
customer to the Provider Edge (PE) router acting as an LER. Customer
traffic is subsequently mapped to a specific MPLS L2 VPN by
configuring L2 FECs based upon the input port ID and/or VLAN tag
depending upon the VPLS service.
Broadcast and multicast services are available over traditional
LANs. MPLS does not support such services currently. Sites that
belong to the same broadcast domain and that are connected via an
MPLS network expect broadcast, multicast and unicast traffic to be
forwarded to the proper location(s). This requires MAC address
learning/aging on a per LSP basis, packet replication across LSPs
for multicast/broadcast traffic and for flooding of unknown unicast
destination traffic.
The primary motivation behind Virtual Private LAN Services (VPLS) is
to provide connectivity between geographically dispersed customer
sites across MAN/WAN network(s), as if they were connected using a
LAN. The intended application for the end-user can be divided into
the following two categories:
- Connectivity between customer routers – LAN routing application
- Connectivity between customer Ethernet switches – LAN switching
application
[PWE3-ETHERNET] defines how to carry L2 PDUs over point-to-point
MPLS LSPs, called pseudowires (PW). Such PWs can be carried over
MPLS or GRE tunnels. This document describes extensions to [PWE3-
CTRL] for transporting Ethernet/802.3 and VLAN [802.1Q] traffic
across multiple sites that belong to the same L2 broadcast domain or
VPLS. Note that the same model can be applied to other 802.1
technologies. It describes a simple and scalable way to offer
Virtual LAN services, including the appropriate flooding of
Broadcast, Multicast and unknown unicast destination traffic over
MPLS, without the need for address resolution servers or other
external servers, as discussed in [L2VPN-REQ].
The following discussion applies to devices that are VPLS capable
and have a means of tunneling labeled packets amongst each other.
While MPLS LSPs may be used to tunnel these labeled packets, other
technologies may be used as well, e.g., GRE [MPLS-GRE]. The
resulting set of interconnected devices forms a private MPLS VPN.
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5. Topological Model for VPLS
An interface participating in a VPLS must be able to flood, forward,
and filter ethernet frames.
+----+ +----+
+ C1 +---+ ........................... +---| C1 |
+----+ | . . | +----+
Site A | +----+ +----+ | Site B
+---| PE |------ Cloud -------| PE |---+
+----+ | +----+
. | .
. +----+ .
..........| PE |...........
+----+ ^
| |
| +-- Emulated LAN
+----+
| C1 |
+----+
Site C
The set of PE devices interconnected via pseudowires appears as a
single emulated LAN to customer C1. Each PE device will learn remote
MAC address to pseudowire associations and will also learn directly
attached MAC addresses on customer facing ports.
We note here again that while this document shows specific examples
using MPLS transport tunnels, other tunnels that can be used by
pseudo-wires, e.g., GRE, L2TP, IPSEC, etc., can also be used, as
long as the originating PE can be identified, since this is used in
the MAC learning process.
The scope of the VPLS lies within the PEs in the service provider
network, highlighting the fact that apart from customer service
delineation, the form of access to a customer site is not relevant
to the VPLS [L2VPN-REQ].
The PE device is typically an edge router capable of running a
signaling protocol and/or routing protocols to set up pseudowires.
In addition, it is capable of setting up transport tunnels to other
PEs and deliver traffic over a pseudowire.
5.1. Flooding and Forwarding
One of attributes of an Ethernet service is that all broadcast and
destination unknown MAC addresses are flooded to all ports. To
achieve flooding within the service provider network, all address
unknown unicast, broadcast and multicast frames are flooded over the
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corresponding pseudowires to all relevant PE nodes participating in
the VPLS.
Note that multicast frames are a special case and do not necessarily
have to be sent to all VPN members. For simplicity, the default
approach of broadcasting multicast frames can be used. Extensions
explaining how to interact with 802.1 GMRP protocol, IGMP snooping
and static MAC multicast filters will be discussed in a future
revision if needed.
To forward a frame, a PE must be able to associate a destination MAC
address with a pseudowire. It is unreasonable and perhaps impossible
to require PEs to statically configure an association of every
possible destination MAC address with a pseudowire. Therefore, VPLS-
capable PEs must have the capability to dynamically learn MAC
addresses on both physical ports and virtual circuits and to forward
and replicate packets across both physical ports and pseudowires.
5.2. Address Learning
Unlike BGP VPNs [BGP-VPN], reachability information does not need to
be advertised and distributed via a control plane. Reachability is
obtained by standard learning bridge functions in the data plane.
As discussed previously, a pseudowire consists of a pair of uni-
directional VC LSPs. When a new MAC address is learned on an
inbound VC LSP, it needs to be associated with the outbound VC LSP
that is part of the same pair. The state of this pseudowire is
considered operationally up when both incoming and outgoing VC LSPs
are established. Similarly, it is considered operationally down
when one of these two VC LSPs is torn down.
Standard learning, filtering and forwarding actions, as defined in
[802.1D-ORIG], [802.1D-REV] and [802.1Q], are required when a
logical link state changes.
5.3. Tunnel Topology
PE routers typically run an IGP between them, and are assumed to
have the capability to establish transport tunnels. Tunnel are set
up between PEs to aggregate traffic. Pseudowires are signaled to
demultiplex the L2 encapsulated packets that traverse the tunnels.
In an Ethernet L2VPN, it becomes the responsibility of the service
provider to create the loop free topology. For the sake of
simplicity, we define that the topology of a VPLS is a full mesh of
tunnels and pseudowires.
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5.4. Loop free L2 VPN
For simplicity, a full mesh of pseudowires is established between
PEs. Ethernet bridges, unlike Frame Relay or ATM where the
termination point becomes the CE node, have to examine the layer 2
fields of the packets to make a switching decision. If the frame is
directed to an unknown destination, or is a broadcast or multicast
frame, the frame must be flooded.
Therefore, if the topology isn't a full mesh, the PE devices may
need to forward these frames to other PEs. However, this would
require the use of spanning tree protocol to form a loop free
topology, that may have characteristics that are undesirable to the
provider. The use of a full mesh and split-horizon forwarding
obviates the need for a spanning tree protocol.
Each PE MUST create a rooted tree to every other PE router that
serve the same VPLS. Each PE MUST support a "split-horizon" scheme
in order to prevent loops, that is, a PE MUST NOT forward traffic
from one pseudowire to another in the same VPLS (since each PE has
direct connectivity to all other PEs in the same VPLS).
Note that customers are allowed to run STP such as when a customer
has "back door" links used to provide redundancy in the case of a
failure within the VPLS. In such a case, STP BPDUs are simply
tunneled through the provider cloud.
6. Discovery
Currently, no discovery mechanism has been prescribed for VPLS.
There are three potential candidates, [BGP-DISC], [RADIUS-DISC],
[LDP-DISC].
7. Control Plane
This document describes the control plane functions of Demultiplexor
Exchange (signaling of VC labels). Some foundational work in the
area of support for multi-homing is laid, although that work is
described in a different document [VPLS-BRIDGING].
7.1. LDP Based Signaling of Demultiplexors
In order to establish a full mesh of pseudowires, all PEs in a VPLS
must have a full mesh of LDP sessions.
Once an LDP session has been formed between two PEs, all pseudowires
are signaled over this session.
In [PWE3-CTRL], the L2 VPN information is carried in a Label Mapping
message sent in downstream unsolicited mode, which contains the
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following VC FEC TLV. VC, C, VC Info Length, Group ID, Interface
parameters are as defined in [PWE3-CTRL].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VC tlv |C| VC Type |VC info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VCID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface parameters |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This document uses the VC type value for Ethernet as defined in
[PWE3-CTRL]:
VC Type Description
0x0001 Frame Relay DLCI
0x0002 ATM AAL5 VCC transport
0x0003 ATM transparent cell transport
0x0004 Ethernet VLAN
0x0005 Ethernet
0x0006 HDLC
0x0007 PPP
0x8008 CEM [8]
0x0009 ATM VCC cell transport
0x000A ATM VPC cell transport
VC types 0x0004 and 0x0005 identify pseudowire types that carry VLAN
tagged and untagged Ethernet traffic respectively, for point-to-
point connectivity.
We use the VC type Ethernet with codepoint 0x0005 to identify
pseudowires that carry Ethernet traffic for multipoint connectivity.
The Ethernet VC Type described below, conforms to the Ethernet VC
Type defined in [PWE3-CTRL].
In a VPLS, we use a VCID (to be substituted with a VPNID TLV later,
to address extending the scope of a VPLS) to identify an emulated
LAN segment. Note that the VCID as specified in [PWE3-CTRL] is a
service identifier, identifying a service emulating a point-to-point
virtual circuit. In a VPLS, the VCID is a single service
identifier.
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7.2. MAC Address Withdrawal
It MAY be desirable to remove or relearn MAC addresses that have
been dynamically learned for faster convergence.
We introduce an optional MAC TLV that is used to specify a list of
MAC addresses that can be removed or relearned using the Address
Withdraw Message.
The Address Withdraw message with MAC TLVs MAY be supported in order
to expedite removal of MAC addresses as the result of a topology
change (e.g., failure of the primary link for a dual-homed MTU-s).
If a notification message is sent on the backup link (blocked link),
which has transitioned into an active state (e.g., similar to
Topology Change Notification message of 802.1w RSTP), with a list of
MAC entries to be relearned, the PE will update the MAC entries in
its FIB for that VPLS instance and send the message to other PEs
over the corresponding directed LDP sessions.
If the notification message contains an empty list, this tells the
receiving PE to remove all the MAC addresses learned for the
specified VPLS instance except the ones it learned from the sending
PE (MAC address removal is required for all VPLS instances that are
affected). Note that the definition of such a notification message
is outside the scope of the document, unless it happens to come from
an MTU connected to the PE as a spoke. In such a scenario, the
message will be just an Address Withdraw message as noted above.
7.2.1. MAC TLV
MAC addresses to be relearned can be signaled using an LDP Address
Withdraw Message that contains a new TLV, the MAC TLV. Its format
is described below. The encoding of a MAC TLV address is the 6-byte
MAC address specified by IEEE 802 documents [g-ORIG] [802.1D-REV].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC address #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC address #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U bit
Unknown bit. This bit MUST be set to 0. If the MAC address
format is not understood, then the TLV is not understood, and MUST
be ignored.
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F bit
Forward bit. This bit MUST be set to 0. Since the LDP
mechanism used here is Targeted, the TLV MUST NOT be forwarded.
Type
Type field. This field MUST be set to 0x0404 (subject to IANA
approval). This identifies the TLV type as MAC TLV.
Length
Length field. This field specifies the total length of the MAC
addresses in the TLV.
MAC Address
The MAC address(es) being removed.
The LDP Address Withdraw Message contains a FEC TLV (to identify the
VPLS in consideration), a MAC Address TLV and optional parameters.
No optional parameters have been defined for the MAC Address
Withdraw signaling.
7.2.2. Address Withdraw Message Containing MAC TLV
When MAC addresses are being removed or relearned explicitly, e.g.,
the primary link of a dual-homed MTU-s has failed, an Address
Withdraw Message can be sent with the list of MAC addresses to be
relearned.
The processing for MAC TLVs received in an Address Withdraw Message
is:
For each MAC address in the TLV:
- Relearn the association between the MAC address and the
interface/pseudowire over which this message is received
- Send the same message to all other PEs over the corresponding
directed LDP sessions.
For an Address Withdraw message with empty list:
- Remove all the MAC addresses associated with the VPLS instance
(specified by the FEC TLV) except the MAC addresses learned
over this link (over the pseudowire associated with the
signaling link over which the message is received)
- Send the same message to all other PEs over the corresponding
directed LDP sessions.
The scope of a MAC TLV is the VPLS specified in the FEC TLV in the
Address Withdraw Message. The number of MAC addresses can be
deduced from the length field in the TLV.
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Further descriptions of how to deal with failures expeditiously with
different configurations will be described in other documents, such
as [VPLS-BRIDGING].
8. Data Forwarding on an Ethernet VC Type
This section describes the dataplane behavior on an Ethernet VPLS
pseudowire. While the encapsulation is similar to that described in
[PWE3-ETHERNET], the NSP functions of stripping the service-
delimiting tag, and using a "normalized" Ethernet packet are
described.
8.1. VPLS Encapsulation actions
In a VPLS, a customer Ethernet packet without preamble is
encapsulated with a header as defined in [PWE3-ETHERNET]. A
customer Ethernet packet is defined as follows:
- If the packet, as it arrives at the PE, has an encapsulation
that is used by the local PE as a service delimiter, i.e., to
identify the customer and/or the particular service of that
customer, then that encapsulation is stripped before the packet
is sent into the VPLS. As the packet exits the VPLS, the
packet may have a service-delimiting encapsulation inserted.
- If the packet, as it arrives at the PE, has an encapsulation
that is not service delimiting, then it is a customer packet
whose encapsulation should not be modified by the VPLS. This
covers, for example, a packet that carries customer specific
VLAN-Ids that the service provider neither knows about nor
wants to modify.
As an application of these rules, a customer packets may arrive at a
customer-facing port with a VLAN tag that identifies the customer's
VPLS instance. That tag would be stripped before it is encapsulated
in the VPLS. At egress, the packet may be tagged again, if a
service-delimiting tag is used, or it may be untagged if none is
used.
Likewise, if a customer packet arrives at a customer-facing port
over an ATM VC that identifies the customer's VPLS instance, then
the ATM encapsulation is removed before the packet is passed into
the VPLS.
Contrariwise, if a customer packet arrives at a customer-facing port
with a VLAN tag that identifies a VLAN domain in the customer L2
network, then the tag is not modified or stripped, as it belongs
with the rest of the customer frame.
By following the above rules, the Ethernet packet that traverses a
VPLS is always a customer Ethernet packet. Note that the two
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actions, at ingress and egress, of dealing with service delimiters
are local actions that neither PE has to signal to the other. They
allow, for example, a mix-and-match of VLAN tagged and untagged
services at either end, and do not carry across a VPLS a VLAN tag
that has local significance only. The service delimiter may be an
MPLS label also, whereby an Ethernet pseudowire given by [PWE3-
ETHERNET] can serve as the access side connection into a PE. An
RFC1483 PVC encapsulation could be another service delimiter. By
limiting the scope of locally significant encapsulations to the
edge, hierarchical VPLS models can be developed that provide the
capability to network-engineer VPLS deployments, as described below.
8.1.1. VPLS Learning actions
Learning is done based on the customer Ethernet packet, as defined
above. The Forwarding Information Base (FIB) keeps track of the
mapping of customer Ethernet packet addressing and the appropriate
pseudowire to use. We define two modes of learning: qualified and
unqualified learning. However, the model followed in this VPLS
document is the unqualified learning model.
In unqualified learning, all the customer VLANs are handled by a
single VPLS, which means they all share a single broadcast domain
and a single MAC address space. This means that MAC addresses need
to be unique and non-overlapping among customer VLANs or else they
cannot be differentiated within the VPLS instance and this can
result in loss of customer frames. An application of unqualified
learning is port-based VPLS service for a given customer (e.g.,
customer with non-multiplexed UNI interface where all the traffic on
a physical port, which may include multiple customer VLANs, is
mapped to a single VPLS instance).
In qualified learning, each customer VLAN is assigned to its own
VPLS instance, which means each customer VLAN has its own broadcast
domain and MAC address space. Therefore, in qualified learning, MAC
addresses among customer VLANs may overlap with each other, but they
will be handled correctly since each customer VLAN has its own FIB ,
i.e., each customer VLAN has its own MAC address space. Since VPLS
broadcasts multicast frames, qualified learning offers the advantage
of limiting the broadcast scope to a given customer VLAN.
9. Operation of a VPLS
We show here an example of how a VPLS works. The following
discussion uses the figure below, where a VPLS has been set up
between PE1, PE2 and PE3.
Initially, the VPLS is set up so that PE1, PE2 and PE3 have a full-
mesh of Ethernet pseudowires. The VPLS instance is assigned a
unique VCID.
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For the above example, say PE1 signals VC Label 102 to PE2 and 103
to PE3, and PE2 signals VC Label 201 to PE1 and 203 to PE3.
Assume a packet from A1 is bound for A2. When it leaves CE1, say it
has a source MAC address of M1 and a destination MAC of M2. If PE1
does not know where M2 is, it will multicast the packet to PE2 and
PE3. When PE2 receives the packet, it will have an inner label of
201. PE2 can conclude that the source MAC address M1 is behind PE1,
since it distributed the label 201 to PE1. It can therefore
associate MAC address M1 with VC Label 102.
-----
/ A1 \
---- ----CE1 |
/ \ -------- ------- / | |
| A2 CE2- / \ / PE1 \ /
\ / \ / \---/ \ -----
---- ---PE2 |
| Service Provider Network |
\ / \ /
----- PE3 / \ /
|Agg|_/ -------- -------
-| |
---- / ----- ----
/ \/ \ / \ CE = Customer Edge Router
| A3 CE3 --C4 A4 | PE = Provider Edge Router
\ / \ / Agg = Layer 2 Aggregation
---- ----
9.1. MAC Address Aging
PEs that learn remote MAC addresses need to have an aging mechanism
to remove unused entries associated with a VC Label. This is
important both for conservation of memory as well as for
administrative purposes. For example, if a customer site A is shut
down, eventually, the other PEs should unlearn A's MAC address.
As packets arrive, MAC addresses are remembered. The aging timer
for MAC address M SHOULD be reset when a packet is received with
source MAC address M.
10. A Hierarchical VPLS Model
The solution described above requires a full mesh of tunnel LSPs
between all the PE routers that participate in the VPLS service.
For each VPLS service, n*(n-1)/2 pseudowires must be setup between
the PE routers. While this creates signaling overhead, the real
detriment to large scale deployment is the packet replication
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requirements for each provisioned VCs on a PE router. Hierarchical
connectivity, described in this document reduces signaling and
replication overhead to allow large scale deployment.
In many cases, service providers place smaller edge devices in
multi-tenant buildings and aggregate them into a PE device in a
large Central Office (CO) facility. In some instances, standard IEEE
802.1q (Dot 1Q) tagging techniques may be used to facilitate mapping
CE interfaces to PE VPLS access points.
It is often beneficial to extend the VPLS service tunneling
techniques into the MTU (multi-tenant unit) domain. This can be
accomplished by treating the MTU device as a PE device and
provisioning pseudowires between it and every other edge, as an
basic VPLS. An alternative is to utilize [PWE3-ETHERNET]
pseudowires or Q-in-Q logical interfaces between the MTU and
selected VPLS enabled PE routers. Q-in-Q encapsulation is another
form of L2 tunneling technique, which can be used in conjunction
with MPLS signaling as will be described later. The following two
sections focus on this alternative approach. The VPLS core
pseudowires (Hub) are augmented with access pseudowires (Spoke) to
form a two tier hierarchical VPLS (H-VPLS).
Spoke pseudowires may be implemented using any L2 tunneling
mechanism, expanding the scope of the first tier to include non-
bridging VPLS PE routers. The non-bridging PE router would extend a
Spoke pseudowire from a Layer-2 switch that connects to it, through
the service core network, to a bridging VPLS PE router supporting
Hub pseudowires. We also describe how VPLS-challenged nodes and
low-end CEs without MPLS capabilities may participate in a
hierarchical VPLS.
10.1. Hierarchical connectivity
This section describes the hub and spoke connectivity model and
describes the requirements of the bridging capable and non-bridging
MTU devices for supporting the spoke connections.
For rest of this discussion we will refer to a bridging capable MTU
device as MTU-s and a non-bridging capable PE device as PE-r. A
routing and bridging capable device will be referred to as PE-rs.
10.1.1. Spoke connectivity for bridging-capable devices
As shown in the figure below, consider the case where an MTU-s
device has a single connection to the PE-rs device placed in the CO.
The PE-rs devices are connected in a basic VPLS full mesh. For each
VPLS service, a single spoke pseudowire is set up between the MTU-s
and the PE-rs based on [PWE3-CTRL]. Unlike traditional pseudowires
that terminate on a physical (or a VLAN-tagged logical) port at each
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end, the spoke pseudowire terminates on a virtual bridge instance on
the MTU-s and the PE-rs devices.
PE2-rs
------
/ \
| -- |
| / \ |
CE-1 | \B / |
\ \ -- /
\ /------
\ MTU-s PE1-rs / |
\ ------ ------ / |
/ \ / \ / |
| \ -- | VC-1 | -- |---/ |
| / \--|- - - - - - - - - - - |--/ \ | |
| \B / | | \B / | |
\ /-- / \ -- / ---\ |
/----- ------ \ |
/ \ |
---- \ ------
|Agg | / \
---- | -- |
/ \ | / \ |
CE-2 CE-3 | \B / |
\ -- /
MTU-s = Bridging capable MTU ------
PE-rs = VPLS capable PE PE3-rs
--
/ \
\B / = Virtual VPLS(Bridge)Instance
--
Agg = Layer-2 Aggregation
The MTU-s device and the PE-rs device treat each spoke connection
like an access port of the VPLS service. On access ports, the
combination of the physical port and/or the VLAN tag is used to
associate the traffic to a VPLS instance while the pseudowire tag
(e.g., VC label) is used to associate the traffic from the virtual
spoke port with a VPLS instance, followed by a standard L2 lookup to
identify which customer port the frame needs to be sent to.
10.1.1.1. MTU-s Operation
MTU-s device is defined as a device that supports layer-2 switching
functionality and does all the normal bridging functions of learning
and replication on all its ports, including the virtual spoke port.
Packets to unknown destination are replicated to all ports in the
service including the virtual spoke port. Once the MAC address is
learned, traffic between CE1 and CE2 will be switched locally by the
MTU-s device saving the link capacity of the connection to the PE-
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rs. Similarly traffic between CE1 or CE2 and any remote destination
is switched directly on to the spoke connection and sent to the PE-
rs over the point-to-point pseudowire.
Since the MTU-s is bridging capable, only a single pseudowire is
required per VPLS instance for any number of access connections in
the same VPLS service. This further reduces the signaling overhead
between the MTU-s and PE-rs.
If the MTU-s is directly connected to the PE-rs, other encapsulation
techniques such as Q-in-Q can be used for the spoke connection
pseudowire. However, to maintain a uniform end-to-end control plane
based on MPLS signaling, [PWE3-CTRL] can be used for distribution of
pseudowire tags (e.g., Q-in-Q tags or pseudowire labels) between
MTU-s and PE-rs.
10.1.1.2. PE-rs Operation
The PE-rs device is a device that supports all the bridging
functions for VPLS service and supports the routing and MPLS
encapsulation, i.e. it supports all the functions described in
[VPLS].
The operation of PE-rs is independent of the type of device at the
other end of the spoke pseudowire. Thus, the spoke pseudowire from
the PE-r is treated as a virtual port and the PE-rs device will
switch traffic between the spoke pseudowire, hub pseudowires, and
access ports once it has learned the MAC addresses.
10.1.2. Advantages of spoke connectivity
Spoke connectivity offers several scaling and operational advantages
for creating large scale VPLS implementations, while retaining the
ability to offer all the functionality of the VPLS service.
- Eliminates the need for a full mesh of tunnels and full mesh of
pseudowires per service between all devices participating in the
VPLS service.
- Minimizes signaling overhead since fewer pseudowires are required
for the VPLS service.
- Segments VPLS nodal discovery. MTU-s needs to be aware of only
the PE-rs node although it is participating in the VPLS service
that spans multiple devices. On the other hand, every VPLS PE-rs
must be aware of every other VPLS PE-rs device and all of it’s
locally connected MTU-s and PE-r.
- Addition of other sites requires configuration of the new MTU-s
device but does not require any provisioning of the existing MTU-s
devices on that service.
- Hierarchical connections can be used to create VPLS service that
spans multiple service provider domains. This is explained in a
later section.
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10.1.3. Spoke connectivity for non-bridging devices
In some cases, a bridging PE-rs device may not be deployed in a CO
or a multi-tenant building while a PE-r might already be deployed.
If there is a need to provide VPLS service from the CO where the PE-
rs device is not available, the service provider may prefer to use
the PE-r device in the interim. In this section, we explain how a
PE-r device that does not support any of the VPLS bridging
functionality can participate in the VPLS service.
As shown in this figure, the PE-r device creates a point-to-point
tunnel LSP to a PE-rs device. Then for every access port that needs
PE2-rs
------
/ \
| -- |
| / \ |
CE-1 | \B / |
\ \ -- /
\ /------
\ PE-r PE1-rs / |
\ ------ ------ / |
/ \ / \ / |
| \ | VC-1 | -- |---/ |
| ------|- - - - - - - - - - - |--/ \ | |
| -----|- - - - - - - - - - - |--\B / | |
\ / / \ -- / ---\ |
------ ------ \ |
/ \ |
---- \------
| Agg| / \
---- | -- |
/ \ | / \ |
CE-2 CE-3 | \B / |
\ -- /
------
PE3-rs
to participate in a VPLS service, the PE-r device creates a point-
to-point [PWE3-ETHERNET] pseudowire that terminates on the physical
port at the PE-r and terminates on the virtual bridge instance of
the VPLS service at the PE-rs.
10.1.3.1. PE-r Operation
The PE-r device is defined as a device that supports routing but
does not support any bridging functions. However, it is capable of
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setting up [PWE3-ETHERNET] pseudowires between itself and the PE-rs.
For every port that is supported in the VPLS service, a [PWE3-
ETHERNET] pseudowire is setup from the PE-r to the PE-rs. Once the
pseudowires are setup, there is no learning or replication function
required on part of the PE-r. All traffic received on any of the
access ports is transmitted on the pseudowire. Similarly all
traffic received on a pseudowire is transmitted to the access port
where the pseudowire terminates. Thus traffic from CE1 destined for
CE2 is switched at PE-rs and not at PE-r.
This approach adds more overhead than the bridging capable (MTU-s)
spoke approach since a pseudowire is required for every access port
that participates in the service versus a single pseudowire required
per service (regardless of access ports) when a MTU-s type device is
used. However, this approach offers the advantage of offering a
VPLS service in conjunction with a routed internet service without
requiring the addition of new MTU device.
10.2. Redundant Spoke Connections
An obvious weakness of the hub and spoke approach described thus far
is that the MTU device has a single connection to the PE-rs device.
In case of failure of the connection or the PE-rs device, the MTU
device suffers total loss of connectivity.
In this section we describe how the redundant connections can be
provided to avoid total loss of connectivity from the MTU device.
The mechanism described is identical for both, MTU-s and PE-r type
of devices
10.2.1. Dual-homed MTU device
To protect from connection failure of the pseudowire or the failure
of the PE-rs device, the MTU-s device or the PE-r is dual-homed into
two PE-rs devices, as shown in figure-3. The PE-rs devices must be
part of the same VPLS service instance.
An MTU-s device will setup two [PWE3-ETHERNET] pseudowires (one each
to PE-rs1 and PE-rs2) for each VPLS instance. One of the two
pseudowires is designated as primary and is the one that is actively
used under normal conditions, while the second pseudowire is
designated as secondary and is held in a standby state. The MTU
device negotiates the pseudowire labels for both the primary and
secondary pseudowires, but does not use the secondary pseudowire
unless the primary pseudowire fails. Since only one link is active
at a given time, a loop does not exist and hence 802.1D spanning
tree is not required.
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PE2-rs
------
/ \
| -- |
| / \ |
CE-1 | \B / |
\ \ -- /
\ /------
\ MTU-s PE1-rs / |
\------ ------ / |
/ \ / \ / |
| -- | Primary PW | -- |---/ |
| / \--|- - - - - - - - - - - |--/ \ | |
| \B / | | \B / | |
\ -- \/ \ -- / ---\ |
------\ ------ \ |
/ \ \ |
/ \ \ ------
/ \ / \
CE-2 \ | -- |
\ Secondary PW | / \ |
- - - - - - - - - - - - - - - - - |-\B / |
\ -- /
------
PE3-rs
10.2.2. Failure detection and recovery
The MTU-s device controls the usage of the pseudowires to the PE-rs
nodes. Since LDP signaling is used to negotiate the pseudowire
labels, the hello messages used for the LDP session can be used to
detect failure of the primary pseudowire.
Upon failure of the primary pseudowire, MTU-s device immediately
switches to the secondary pseudowire. At this point the PE3-rs
device that terminates the secondary pseudowire starts learning MAC
addresses on the spoke pseudowire. All other PE-rs nodes in the
network think that CE-1 and CE-2 are behind PE1-rs and may continue
to send traffic to PE1-rs until they learn that the devices are now
behind PE3-rs. The relearning process can take a long time and may
adversely affect the connectivity of higher level protocols from CE1
and CE2. To enable faster convergence, the PE3-rs device where the
secondary pseudowire got activated may send out a flush message,
using the MAC TLV as defined in Section 6, to PE1-rs, who relays it
to all other PE-rs devices participating in the VPLS service. Upon
receiving the message, all PE-rs nodes flush the MAC addresses
associated with that VPLS instance.
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10.3. Multi-domain VPLS service
Hierarchy can also be used to create a large scale VPLS service
within a single domain or a service that spans multiple domains
without requiring full mesh connectivity between all VPLS capable
devices. Two fully meshed VPLS networks are connected together
using a single LSP tunnel between the VPLS gateway devices. A
single spoke pseudowire is setup per VPLS service to connect the two
domains together. The VPLS gateway device joins two VPLS services
together to form a single multi-domain VPLS service. The
requirements and functionality required from a VPLS gateway device
will be explained in a future version of this document.
11. Hierarchical VPLS model using Ethernet Access Network
In the previous section, a two-tier hierarchical model that consists
of hub-and-spoke topology between MTU-s devices and PE-rs devices and
a full-mesh topology among PE-rs devices was discussed. In this
section the two-tier hierarchical model is expanded to include an
Ethernet access network. This model retains the hierarchical
architecture discussed previously in that it leverages the full-mesh
topology among PE-rs devices; however, no restriction is imposed on
the topology of the Ethernet access network (e.g., the topology
between MTU-s and PE-rs devices are not restricted to hub and spoke).
The motivation for an Ethernet access network is that Ethernet-based
networks are currently deployed by some service providers to offer
VPLS services to their customers. Therefore, it is important to
provide a mechanism that allows these networks to integrate with an
IP or MPLS core to provide scalable VPLS services.
One approach of tunneling a customer's Ethernet traffic via an
Ethernet access network is to add an additional VLAN tag to the
customer's data (which may be either tagged or untagged). The
additional tag is referred to as Provider's VLAN (P-VLAN). Inside the
provider's network each P-VLAN designates a customer or more
specifically a VPLS instance for that customer. Therefore, there is a
one to one correspondence between a P-VLAN and a VPLS instance.
In this model, the MTU-S device needs to have the capability of
adding the additional P-VLAN tag for non-multiplexed customer UNI
port where customer VLANs are not used as service delimiter. If
customer VLANs need to be treated as service delimiter (e.g.,
customer UNI port is a multiplexed port), then the MTU-s needs to
have the additional capability of translating a customer VLAN (C-
VLAN) to a P-VLAN in order to resolve overlapping VLAN-ids used by
different customers. Therefore, the MTU-s device in this model can be
considered as a typical bridge with this additional UNI capability.
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The PE-rs device needs to be able to perform bridging functionality
over the standard Ethernet ports toward the access network as well as
over the pseudowires toward the network core. The set of pseudowires
that corresponds to a VPLS instance would look just like a P-VLAN to
the bridge portion of the PE-rs and that is why sometimes it is
referred to as Emulated VLAN. In this model the PE-rs may need to run
STP protocol in addition to split-horizon. Split horizon is run over
MPLS-core; whereas, STP is run over the access network to accommodate
any arbitrary access topology. In this model, the PE-rs needs to map
a P-VLAN to a VPLS-instance and its associated pseudowires and vise
versa.
The details regarding bridge operation for MTU-s and PE-rs (e.g.,
encapsulation format for QinQ messages, customer’s Ethernet control
protocol handling, etc.) are outside of the scope of this document
and they are covered in [802.1ad]. However, the relevant part is the
interaction between the bridge module and the MPLS/IP pseudowires in
the PE-rs device.
11.1. Scalability
Given that each P-VLAN corresponds to a VPLS instance, one may think
that the total number of VPLS instances supported is limited to 4K.
However, the 4K limit applies only to each Ethernet access network
(Ethernet island) and not to the entire network. The SP network, in
this model, consists of a core MPLS/IP network that connects many
Ethernet islands. Therefore, the number of VPLS instances can scale
accordingly with the number of Ethernet islands (a metro region can
be represented by one or more islands). Each island may consist of
many MTU-s devices, several aggregators, and one or more PE-rs
devices. The PE-rs devices enable a P-VLAN to be extended from one
island to others using a set of pseudowires (associated with that
VPLS instance) and providing a loop free mechanism across the core
network through split-horizon. Since a P-VLAN serves as a service
delimiter within the provider's network, it does not get carried over
the pseudowires and furthermore the mapping between P-VLAN and the
pseudowires is a local matter. This means a VPLS instance can be
represented by different P-VLAN in different Ethernet islands and
furthermore each island can support 4K VPLS instances independent
from one another.
11.2. Dual Homing and Failure Recovery
In this model, an MTU-s can be dual or triple homed to different
devices (aggregators and/or PE-rs devices). The failure protection
for access network nodes and links can be provided through running
MSTP in each island. The MSTP of each island is independent from
other islands and do not interact with each other. If an island has
more than one PE-rs, then a dedicated full-mesh of pseudowires is
used among these PE-rs devices for carrying the SP BPDU packets for
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that island. On a per P-VLAN basis, the MSTP will designate a single
PE-rs to be used for carrying the traffic across the core. The loop-
free protection through the core is performed using split-horizon and
the failure protection in the core is performed through standard
IP/MPLS re-routing.
12. Significant Modifications
Between rev 04 and this one, these are the changes:
o minor revisions of text
o cleanup of use of MPLS LSPs for tunnels
o clearly states qualified learning is out of scope for current
model
o corrected MAC TLV description
13. Acknowledgments
We wish to thank Joe Regan, Kireeti Kompella, Anoop Ghanwani, Joel
Halpern, Rick Wilder, Jim Guichard, Steve Phillips, Norm Finn, Matt
Squire, Muneyoshi Suzuki, Waldemar Augustyn, and Eric Rosen for
their valuable feedback. In addition, we would like to thank Rajiv
Papneja (ISOCORE), Winston Liu (ISOCORE), and Charlie Hundall
(Extreme) for identifying issues with the draft in the course of the
interoperability tests.
14. Security Considerations
Security issues resulting from this draft will be discussed in
greater depth at a later point. It is recommended in [RFC3036] that
LDP security (authentication) methods be applied. This would
prevent unauthorized participation by a PE in a VPLS. Traffic
separation for a VPLS is effected by using VC labels. However, for
additional levels of security, the customer MAY deploy end-to-end
security, which is out of the scope of this draft. In addition, the
L2FRAME] document describes security issues in greater depth.
15. Intellectual Property Considerations
This document is being submitted for use in IETF standards
discussions.
16. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
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document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
17. References
[PWE3-ETHERNET] "Encapsulation Methods for Transport of Ethernet
Frames Over IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-
02.txt, Work in progress, February 2003.
[PWE3-CTRL] "Transport of Layer 2 Frames Over MPLS", draft-ietf-
pwe3-control-protocol-02.txt, Work in progress, February 2003.
[802.1D-ORIG] Original 802.1D - ISO/IEC 10038, ANSI/IEEE Std 802.1D-
1993 "MAC Bridges".
[802.1D-REV] 802.1D - "Information technology - Telecommunications
and information exchange between systems - Local and metropolitan
area networks - Common specifications - Part 3: Media Access Control
(MAC) Bridges: Revision. This is a revision of ISO/IEC 10038: 1993,
802.1j-1992 and 802.6k-1992. It incorporates P802.11c, P802.1p and
P802.12e." ISO/IEC 15802-3: 1998.
[802.1Q] 802.1Q - ANSI/IEEE Draft Standard P802.1Q/D11, "IEEE
Standards for Local and Metropolitan Area Networks: Virtual Bridged
Local Area Networks", July 1998.
[BGP-VPN] Rosen and Rekhter, "BGP/MPLS VPNs". draft-ietf-ppvpn-
rfc2547bis-04.txt, Work in Progress, May 2003.
[RFC3036] "LDP Specification", L. Andersson, et al. RFC 3036.
January 2001.
[RADIUS-DISC] " Using Radius for PE-Based VPN Discovery", Juha
Heinanen, draft-heinanen-radius-pe-discovery-04.txt, Work in
Progress, June 2003.
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[BGP-DISC] "Using BGP as an Auto-Discovery Mechanism for Network-
based VPNs", Ould-Brahim, et. al., draft-ietf-ppvpn-bgpvpn-auto-
05.txt, Work in Progress, May 2003.
[LDP-DISC] "Discovering Nodes and Services in a VPLS Network", O.
Stokes et al, draft-stokes-ppvpn-vpls-discover-00.txt, Work in
Progress, June 2002.
[VPLS-BRIDGING] "Bridging and VPLS", draft-finn-ppvpn-bridging-vpls-
00.txt, Work in Progress, June 2002.
[L2VPN-SIG] "LDP-based Signaling for L2VPNs", draft-rosen-ppvpn-l2-
signaling-03.txt, Work in Progress, May 2003.
[L2FRAME] "L2VPN Framework", draft-ietf-ppvpn-l2-framework-03, Work
in Progress, February 2003.
[L2VPN-REQ] "Service Requirements for Layer 2 Provider Provisioned
Virtual Private Networks", draft-ietf-ppvpn-l2vpn-requirements-
00.txt, Work in Progress, May 2003.
[802.1ad] "IEEE standard for Provider Bridges", Work in Progress,
December 2002.
18. Authors' Addresses
Marc Lasserre
Riverstone Networks
Email: marc@riverstonenet.com
Vach Kompella
TiMetra Networks
274 Ferguson Dr.
Mountain View, CA 94043
Email: vkompella@timetra.com
Sunil Khandekar
TiMetra Networks
274 Ferguson Dr.
Mountain View, CA 94043
Email: sunil@timetra.com
Nick Tingle
TiMetra Networks
274 Ferguson Dr.
Mountain View, CA 94043
Email: nick@timetra.com
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Ali Sajassi
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Email: sajassi@cisco.com
Loa Andersson
Email: loa@pi.se
Pascal Menezes
Email: pascalm1@yahoo.com
Andrew Smith
Consultant
Email: ah_smith@pacbell.net
Giles Heron
PacketExchange Ltd.
Email: giles@packetexchange.net
Juha Heinanen
TutPro
Email: jh@tutpro.com
Tom S. C. Soon
SBC Technology Resources Inc.
Email: sxsoon@tri.sbc.com
Yetik Serbest
SBC Communications
serbest@tri.sbc.com
Eric Puetz
SBC Communications
puetz@tri.sbc.com
Ron Haberman
Masergy Inc.
Email: ronh@masergy.com
Luca Martini
Level 3 Communications, LLC.
Email: luca@level3.net
Rob Nath
Riverstone Networks
Email: rnath@riverstonenet.com
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