Internet Engineering Task Force (IETF) A. Sajassi, Ed.
Request for Comments: 6246 F. Brockners
Category: Informational Cisco Systems
ISSN: 2070-1721 D. Mohan, Ed.
Nortel
Y. Serbest
AT&T
June 2011
Virtual Private LAN Service (VPLS) Interoperability
with Customer Edge (CE) Bridges
Abstract
One of the main motivations behind Virtual Private LAN Service (VPLS)
is its ability to provide connectivity not only among customer
routers and servers/hosts but also among customer IEEE bridges. VPLS
is expected to deliver the same level of service that current
enterprise users are accustomed to from their own enterprise bridged
networks or their Ethernet Service Providers.
When customer edge (CE) devices are IEEE bridges, then there are
certain issues and challenges that need to be accounted for in a VPLS
network. The majority of these issues have been addressed in the
IEEE 802.1ad standard for provider bridges and they can be leveraged
for VPLS networks. This document extends the provider edge (PE)
model described in RFC 4664 based on IEEE 802.1ad bridge module, and
it illustrates a clear demarcation between the IEEE bridge module and
IETF LAN emulation module. By doing so, it shows that the majority
of interoperability issues with CE bridges can be delegated to the
802.1ad bridge module, thus removing the burden on the IETF LAN
emulation module within a VPLS PE.
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Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6246.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Contributions published or made publicly available before November
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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Table of Contents
1. Introduction ....................................................3
1.1. Conventions ................................................4
2. Ethernet Service Instance .......................................4
3. VPLS-Capable PE Model with Bridge Module ........................5
4. Mandatory Issues ................................................8
4.1. Service Mapping ............................................8
4.2. CE Bridge Protocol Handling ...............................10
4.3. Partial Mesh of Pseudowires ...............................11
4.4. Multicast Traffic .........................................12
5. Optional Issues ................................................13
5.1. Customer Network Topology Changes .........................13
5.2. Redundancy ................................................15
5.3. MAC Address Learning ......................................16
6. Interoperability with 802.1ad Networks .........................17
7. Acknowledgments ................................................17
8. Security Considerations ........................................17
9. Normative References ...........................................18
10. Informative References ........................................19
1. Introduction
Virtual Private LAN Service (VPLS) is a LAN emulation service
intended for providing connectivity between geographically dispersed
customer sites across MANs/WANs (over MPLS/IP), as if they were
connected using a LAN. One of the main motivations behind VPLS is
its ability to provide connectivity not only among customer routers
and servers/hosts but also among IEEE customer bridges. If only
connectivity among customer IP routers/hosts is desired, then an IP-
only LAN Service [IPLS] solution could be used. The strength of the
VPLS solution is that it can provide connectivity to both bridge and
non-bridge types of CE devices. VPLS is expected to deliver the same
level of service that current enterprise users are accustomed to from
their own enterprise bridged networks [802.1D] [802.1Q] today or the
same level of service that they receive from their Ethernet Service
Providers using IEEE 802.1ad-based networks [802.1ad] (or its
predecessor, QinQ-based networks).
When CE devices are IEEE bridges, then there are certain issues and
challenges that need to be accounted for in a VPLS network. The
majority of these issues have been addressed in the IEEE 802.1ad
standard for provider bridges and they can be leveraged for VPLS
networks. This document extends the PE model described in [RFC4664]
based on the IEEE 802.1ad bridge module and illustrates a clear
demarcation between IEEE bridge module and IETF LAN emulation module.
By doing so, it describes that the majority of interoperability
issues with CE bridges can be delegated to the 802.1ad bridge module,
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thus removing the burden on the IETF LAN emulation module within a
VPLS PE. This document discusses these issues and, wherever
possible, suggests areas to be explored in rectifying these issues.
The detailed solution specification for these issues is outside of
the scope of this document.
This document also discusses interoperability issues between VPLS and
IEEE 802.1ad networks when the end-to-end service spans across both
types of networks, as outlined in [RFC4762].
This document categorizes the CE-bridge issues into two groups: 1)
mandatory and 2) optional. The issues in group (1) need to be
addressed in order to ensure the proper operation of CE bridges. The
issues in group (2) would provide additional operational improvement
and efficiency and may not be required for interoperability with CE
bridges. Sections 5 and 6 discuss these mandatory and optional
issues, respectively.
1.1. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Ethernet Service Instance
Before starting the discussion of bridging issues, it is important to
clarify the Ethernet Service definition. The term VPLS has different
meanings in different contexts. In general, VPLS is used in the
following contexts [RFC6136]: a) as an end-to-end bridged LAN service
over one or more networks (one of which is an MPLS/IP network), b) as
an MPLS/IP network supporting these bridged LAN services, and c) as
(V)LAN emulation. For better clarity, we differentiate between its
usage as network versus service by using the terms VPLS network and
VPLS instance, respectively. Furthermore, we confine VPLS (both
network and service) to only the portion of the end-to-end network
that spans an MPLS/IP network. For an end-to-end service (among
different sites of a given customer), we use the term "Ethernet
Service Instance" or ESI.
We define the Ethernet Service Instance (ESI) as an association of
two or more Attachment Circuits (ACs) over which an Ethernet service
is offered to a given customer. An AC can be either a User-Network
Interface (UNI) or a Network-Network Interface (NNI); furthermore, it
can be an Ethernet interface or a VLAN, it can be an ATM or Frame
Relay Virtual Circuit, or it can be a PPP/HDLC (PPP/High-Level Data
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Link Control) interface. If an ESI is associated with more than two
ACs, then it is a multipoint ESI. In this document, wherever the
keyword ESI is used, it means multipoint ESI unless stated otherwise.
An ESI can correspond to a VPLS instance if its associated ACs are
only connected to a VPLS network, or an ESI can correspond to a
Service VLAN if its associated ACs are only connected to a Provider-
Bridged network [802.1ad]. Furthermore, an ESI can be associated
with both a VPLS instance and a Service VLAN when considering an end-
to-end service that spans across both VPLS and Provider-Bridged
networks. An ESI can span across different networks (e.g., IEEE
802.1ad and VPLS) belonging to the same or different administrative
domains.
An ESI most often represents a customer or a specific service
requested by a customer. Since traffic isolation among different
customers (or their associated services) is of paramount importance
in service provider networks, its realization shall be done such that
it provides a separate Media Access Control (MAC) address domain and
broadcast domain per ESI. A separate MAC address domain is provided
by using a separate MAC forwarding table (e.g., Forwarding
Information Base (FIB), also known as filtering database [802.1D])
per ESI (for both VPLS and IEEE 802.1ad networks). A separate
broadcast domain is provided by using a full mesh of pseudowires per
ESI over the IP/MPLS core in a VPLS network and/or a dedicated
Service VLAN per ESI in an IEEE 802.1ad network.
3. VPLS-Capable PE Model with Bridge Module
[RFC4664] defines three models for VPLS-capable PE (VPLS-PE), based
on the bridging functionality that needs to be supported by the PE.
If the CE devices can be routers/hosts or IEEE bridges, the second
model from [RFC4664] is the most suitable, and it is both adequate to
provide the VPLS level of service and consistent with the IEEE
standards for Provider Bridges [802.1ad]. We briefly describe the
second model and then expand upon this model to show its sub-
components based on the [802.1ad] Provider Bridge model.
As described in [RFC4664], the second model for VPLS-PE contains a
single bridge module supporting all the VPLS instances on that PE ,
where each VPLS instance is represented by a unique VLAN inside that
bridge module (also known as a Service VLAN or S-VLAN). The bridge
module has a single "Emulated LAN" interface over which it
communicates with all VPLS forwarders, and each VPLS instance is
represented by a unique S-VLAN tag. Each VPLS instance can consist
of a set of pseudowires, and its associated forwarder can correspond
to a single VLAN as depicted in Figure 1 below. Thus, sometimes it
is referred to as VLAN emulation.
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+----------------------------------------+
| VPLS-Capable PE Model |
| +---------------+ +------+ |
| | | |VPLS-1|------------
| | |=======+ |Fwdr |------------ PWs
| | Bridge --------|--- |------------
| | | SVID-1| +------+ |
| | Module | | o |
| | | | o |
| | (802.1ad | | o |
| | bridge) | | o |
| | | | o |
| | | SVID-n| +------+ |
| | --------|---VPLS-n|-------------
| | |=======+ | Fwdr |------------- PWs
| | | ^ | |-------------
| +---------------+ | +------+ |
| | |
+-----------------------|----------------+
|
LAN emulation (multi-access) interface
Figure 1. VPLS-Capable PE Model
Customer frames associated with a given ESI carry the S-VLAN ID for
that ESI over the LAN emulation interface. The S-VLAN ID is stripped
before transmitting the frames over the set of pseudowires (PWs)
associated with that VPLS instance (assuming raw mode PWs are used as
specified in [RFC4448]).
The bridge module can itself consist of one or two sub-components,
depending on the functionality that it needs to perform. Figure 2
depicts the model for the bridge module based on [802.1ad].
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+-------------------------------+
| 802.1ad Bridge Module Model |
| |
+---+ AC | +------+ +-----------+ |
|CE |---------|C-VLAN|------| | |
+---+ | |bridge|------| | |
| +------+ | | |
| o | S-VLAN | |
| o | | | ---> to VPLS Fwdr
| o | Bridge | |
+---+ AC | +------+ | | |
|CE |---------|C-VLAN|------| | |
+---+ | |bridge|------| | |
| +------+ | | |
+---+ AC | | | |
|CE |-----------------------| | |
+---+ | +-----------+ |
+-------------------------------+
Figure 2. Model of the 802.1ad Bridge Module
The S-VLAN bridge component is always required and it is responsible
for tagging customer frames with S-VLAN tags in the ingress direction
(from customer UNIs) and removing S-VLAN tags in the egress direction
(toward customer UNIs). It is also responsible for running the
provider's bridge protocol -- such as Rapid Spanning Tree Protocol
(RSTP), Multiple Spanning Tree Protocol (MSTP), Generic VLAN
Registration Protocol (GVRP), GARP Multicast Registration Protocol
(GMRP), etc. -- among provider bridges within a single administrative
domain.
The customer VLAN (C-VLAN) bridge component is required when the
customer Attachment Circuits are VLANs (aka C-VLANs). In such cases,
the VPLS-capable PE needs to participate in some of the customer's
bridging protocol such as RSTP and MSTP. Such participation is
required because a C-VLAN at one site can be mapped into a different
C-VLAN at a different site or, in case of asymmetric mapping, a
customer Ethernet port at one site can be mapped into a C-VLAN (or
group of C-VLANs) at a different site.
The C-VLAN bridge component does service selection and identification
based on C-VLAN tags. Each frame from the customer device is
assigned to a C-VLAN and presented at one or more internal port-based
interfaces, each supporting a single service instance that the
customer desires to carry that C-VLAN. Similarly, frames from the
provider network are assigned to an internal interface or 'LAN' (e.g,
between C-VLAN and S-VLAN components) on the basis of the S-VLAN tag.
Since each internal interface supports a single service instance, the
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S-VLAN tag can be, and is, removed at this interface by the S-VLAN
bridge component. If multiple C-VLANs are supported by this service
instance (e.g., via VLAN bundling or port-based service), then the
frames will have already been tagged with C-VLAN tags. If a single
C-VLAN is supported by this service instance (e.g., VLAN-based), then
the frames will not have been tagged with a C-VLAN tag since C-VLAN
can be derived from the S-VLAN (e.g., one-to-one mapping). The
C-VLAN-aware bridge component applies a port VLAN ID (PVID) to
untagged frames received on each internal 'LAN', allowing full
control over the delivery of frames for each C-VLAN through the
Customer UNI Port.
4. Mandatory Issues
4.1. Service Mapping
Different Ethernet AC types can be associated with a single Ethernet
Service Instance (ESI). For example, an ESI can be associated with
only physical Ethernet ports, VLANs, or a combination of the two
(e.g., one end of the service could be associated with physical
Ethernet ports and the other end could be associated with VLANs). In
[RFC4762], unqualified and qualified learning are used to refer to
port-based and VLAN-based operation, respectively. [RFC4762] does
not describe the possible mappings between different types of
Ethernet ACs (e.g., 802.1D, 802.1Q, or 802.1ad frames). In general,
the mapping of a customer port or VLAN to a given service instance is
a local function performed by the local PE, and the service
provisioning shall accommodate it. In other words, there is no
reason to restrict and limit an ESI to have only port-based ACs or to
have only VLAN-based ACs. [802.1ad] allows for each customer AC
(either a physical port, a VLAN, or a group of VLANs) to be mapped
independently to an ESI that provides better service offerings to
enterprise customers. For better and more flexible service offerings
and for interoperability purposes between VPLS and 802.1ad networks,
it is imperative that both networks offer the same capabilities in
terms of customer ACs mapping to the customer service instance.
The following table lists possible mappings that can exist between
customer ACs and their associated ESIs. As can be seen, there are
several possible ways to perform such mappings. In the first
scenario, it is assumed that an Ethernet physical port only carries
untagged traffic and all traffic is mapped to the corresponding
service instance or ESI. This is referred to as "port-based with
untagged traffic". In the second scenario, it is assumed that an
Ethernet physical port carries both tagged and untagged traffic and
all that traffic is mapped to the corresponding service instance or
ESI. This is referred to as "port-based with tagged and untagged
traffic". In the third scenario, it is assumed that only a single
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VLAN is mapped to the corresponding service instance or ESI. This is
referred to as "VLAN-based". Finally, in the fourth scenario, it is
assumed that a group of VLANs from the Ethernet physical interface is
mapped to the corresponding service instance or ESI. This is
referred to as "VLAN bundling".
===================================================================
Ethernet I/F & Associated Service Instance(s)
-------------------------------------------------------------------
Port-based Port-based VLAN-based VLAN
untagged tagged & bundling
untagged
-------------------------------------------------------------------
Port-based Y N Y(Note-1) N
untagged
Port-based N Y Y(Note-2) Y
tagged &
untagged
VLAN-based Y(Note-1) Y(Note-2) Y Y(Note-3)
VLAN N Y Y(Note-3) Y
Bundling
===================================================================
Note-1: In this asymmetric mapping scenario, it is assumed that the
CE device with "VLAN-based" AC is capable of supporting [802.1Q]
frame format.
Note-2: In this asymmetric mapping scenario, it is assumed that the
CE device with "VLAN-based" AC can support [802.1ad] frame format
because it will receive Ethernet frames with two tags, where the
outer tag is an S-VLAN and the inner tag is a C-VLAN received from
"port-based" AC. One application example for such CE device is in a
Broadband Remote Access Server (BRAS) for DSL aggregation over a
Metro Ethernet network.
Note-3: In this asymmetric mapping scenario, it is assumed that the
CE device with "VLAN-based" AC can support the [802.1ad] frame format
because it will receive Ethernet frames with two tags, where the
outer tag is an S-VLAN and the inner tag is a C-VLAN received from
"VLAN bundling" AC.
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If a PE uses an S-VLAN tag for a given ESI (either by adding an
S-VLAN tag to customer traffic or by replacing a C-VLAN tag with a
S-VLAN tag), then the frame format and EtherType for S-VLAN SHALL
adhere to [802.1ad].
As mentioned before, the mapping function between the customer AC and
its associated ESI is a local function; thus, when the AC is a single
customer VLAN, it is possible to map different customer VLANs at
different sites to a single ESI without coordination among those
sites.
When a port-based mapping or a VLAN-bundling mapping is used, then
the PE may use an additional S-VLAN tag to mark the customer traffic
received over that AC as belonging to a given ESI. If the PE uses
the additional S-VLAN tag, then in the opposite direction the PE
SHALL strip the S-VLAN tag before sending the customer frames over
the same AC. However, when VLAN-mapping mode is used at an AC and if
the PE uses the S-VLAN tag locally, then if the Ethernet interface is
a UNI, the tagged frames over this interface SHALL have a frame
format based on [802.1Q]. In such a case, the PE SHALL translate the
customer tag (C-VLAN) into the provider tag (S-VLAN) upon receiving a
frame from the customer. In the opposite direction, the PE SHALL
translate from provider frame format (802.1ad) back to customer frame
format (802.1Q).
All the above asymmetric services can be supported via the PE model
with the bridge module depicted in Figure 2 (based on [802.1ad]).
4.2. CE Bridge Protocol Handling
When a VPLS-capable PE is connected to a CE bridge, then -- depending
on the type of Attachment Circuit -- different protocol handling may
be required by the bridge module of the PE. [802.1ad] states that
when a PE is connected to a CE bridge, then the service offered by
the PE may appear to specific customer protocols running on the CE in
one of the four ways:
a) Transparent to the operation of the protocol among CEs of
different sites using the service provided, appearing as an
individual LAN without bridges;
b) Discarding frames, acting as a non-participating barrier to the
operation of the protocol;
c) Peering, with a local protocol entity at the point of provider
ingress and egress, participating in and terminating the
operation of the protocol; or
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d) Participation in individual instances of customer protocols.
All the above CE bridge protocol handling can be supported via the PE
model with the bridge module depicted in Figure 2 (based on
[802.1ad]). For example, when an Attachment Circuit is port-based,
then the bridge module of the PE can operate transparently with
respect to the CE's RSTPs or MSTPs (and thus no C-VLAN component is
required for that customer UNI). However, when an Attachment Circuit
is VLAN-based (either VLAN-based or VLAN bundling), then the bridge
module of the PE needs to peer with the RSTPs or MSTPs running on the
CE (and thus the C-VLAN bridge component is required). In other
words, when the AC is VLAN-based, then protocol peering between CE
and PE devices may be needed. There are also protocols that require
peering but are independent from the type of Attachment Circuit. An
example of such protocol is the link aggregation protocol [802.1AX];
however, this is a media-dependent protocol as its name implies.
[802.1ad] reserves a block of 16 MAC addresses for the operation of
C-VLAN and S-VLAN bridge components. Also, it shows which of these
reserved MAC addresses are only for C-VLAN bridge components, which
are only for S-VLAN bridge components, and which apply to both C-VLAN
and S-VLAN components.
4.3. Partial Mesh of Pseudowires
A VPLS service depends on a full mesh of pseudowires, so a pseudowire
failure reduces the underlying connectivity to a partial mesh, which
can have adverse effects on the VPLS service. If the CE devices
belonging to an ESI are routers running link state routing protocols
that use LAN procedures over that ESI, then a partial mesh of PWs can
result in "black holing" traffic among the selected set of routers.
And if the CE devices belonging to an ESI are IEEE bridges, then a
partial mesh of PWs can cause broadcast storms in the customer and
provider networks. Furthermore, it can cause multiple copies of a
single frame to be received by the CE and/or PE devices. Therefore,
it is of paramount importance to be able to detect PW failure and to
take corrective action to prevent creation of partial mesh of PWs.
When the PE model depicted in Figure 2 is used, then [802.1ag]
procedures could be used for detection of partial mesh of PWs.
[802.1ag] defines a set of procedures for fault detection,
verification, isolation, and notification per ESI.
The fault detection mechanism of [802.1ag] can be used to perform
connectivity check among PEs belonging to a given VPLS instance. It
checks the integrity of a service instance end-to-end within an
administrative domain, e.g., from one AC at one end of the network to
another AC at the other end of the network. Therefore, its path
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coverage includes the bridge module within a PE and it is not limited
to just PWs. Furthermore, [802.1ag] operates transparently over the
full mesh of PWs for a given service instance since it operates at
the Ethernet level (and not at the PW level). It should be noted
that since a PW consists of two unidirectional Label Switched Paths
(LSPs), then one direction can fail independently of the other. Even
in this case, the procedures of [802.1ag] can provide a consistent
view of the full mesh to the participating PEs by relying on remote
defect indication (RDI).
Another, less preferred, option is to define a procedure for
detection of partial mesh; in this procedure, each PE keeps track of
the status of its PW Endpoint Entities (EEs, e.g., VPLS forwarders)
as well as the EEs reported by other PEs. Therefore, upon a PW
failure, the PE that detects the failure not only takes notice
locally but also notifies other PEs belonging to that service
instance so that all the participant PEs have a consistent view of
the PW mesh. Such a procedure is for the detection of partial mesh
per service instance, and in turn it relies on additional procedure
for PW failure detection such as Bidirectional Forward Detection
(BFD) or Virtual Circuit Connectivity Verification (VCCV). Given
that there can be tens (or even hundreds) of thousands of PWs in a
PE, there can be scalability issues with such fault
detection/notification procedures.
4.4. Multicast Traffic
VPLS follows a centralized model for multicast replication within an
ESI. VPLS relies on ingress replication. The ingress PE replicates
the multicast packet for each egress PE and sends it to the egress PE
using point-to-point PW over a unicast tunnel. VPLS operates on an
overlay topology formed by the full mesh of pseudo-wires. Thus,
depending on the underlying topology, the same datagram can be sent
multiple times down the same physical link. VPLS currently does not
offer any mechanisms to restrict the distribution of multicast or
broadcast traffic of an ESI throughout the network, which causes an
additional burden on the ingress PE through unnecessary packet
replication. This in turn causes additional load on the MPLS core
network and additional processing at the receiving PE where
extraneous multicast packets are discarded.
One possible approach to delivering multicast more efficiently over a
VPLS network is to include the use of IGMP snooping in order to send
the packet only to the PEs that have receivers for that traffic,
rather than to all the PEs in the VPLS instance. If the customer
bridge or its network has dual-home connectivity, then -- for proper
operation of IGMP snooping -- the PE must generate a "General Query"
over that customer's UNIs upon receiving a customer topology change
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notification as described in [RFC4541]. A "General Query" by the PE
results the customer multicast MAC address(es) being properly
registered at the PE when there are customer topology changes. It
should be noted that IGMP snooping provides a solution for IP
multicast packets and is not applicable to general multicast data.
Using the IGMP snooping as described, the ingress PE can select a
subset of PWs for packet replication, thus avoiding sending multicast
packets to the egress PEs that don't need them. However, the
replication is still performed by the ingress PE. In order to avoid
replication at the ingress PE, one may want to use multicast
distribution trees (MDTs) in the provider core network; however, this
brings some potential pitfalls. If the MDT is used for all multicast
traffic of a given customer, then this results in customer multicast
and unicast traffic being forwarded on different PWs and even on a
different physical topology within the provider network. This is a
serious issue for customer bridges because customer Bridge Protocol
Data Units (BPDUs), which are multicast data, can take a different
path through the network than the unicast data. Situations might
arise where either unicast OR multicast connectivity is lost. If
unicast connectivity is lost but multicast forwarding continues to
work, the customer spanning tree would not take notice which results
in loss of its unicast traffic. Similarly, if multicast connectivity
is lost, but unicast is working, then the customer spanning tree will
activate the blocked port, which may result in a loop within the
customer network. Therefore, the MDT cannot be used for both
customer multicast control and data traffic. If it is used, it
should only be limited to customer data traffic. However, there can
be a potential issue even when it is used for customer data traffic
since the MDT doesn't fit the PE model described in Figure 1 (it
operates independently from the full mesh of PWs that correspond to
an S-VLAN). It is also not clear how connectivity fault management
(CFM) procedures (802.1ag) used for the ESI integrity check (e.g.,
per service instance) can be applied to check the integrity of the
customer multicast traffic over the provider MDT. Because of these
potential issues, the specific applications of the provider MDT to
customer multicast traffic shall be documented and its limitations be
clearly specified.
5. Optional Issues
5.1. Customer Network Topology Changes
A single CE or a customer network can be connected to a provider
network using more than one User-Network Interface (UNI).
Furthermore, a single CE or a customer network can be connected to
more than one provider network. [RFC4665] provides some examples of
such customer network connectivity; they are depicted in Figure 3
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below. Such network topologies are designed to protect against the
failure or removal of network components from the customer network,
and it is assumed that the customer leverages the spanning tree
protocol to protect against these cases. Therefore, in such
scenarios, it is important to flush customer MAC addresses in the
provider network upon the customer topology change in order to avoid
black-holing of customer frames.
+----------- +---------------
| |
+------+ +------+ +------+ +------+
| CE |-----| PE | | CE |-----| PE |
|device| |device| |device| |device| SP network
+------+\ +------+ +------+\ +------+
| \ | | \ |
|Back \ | |Back \ +---------------
|door \ | SP network |door \ +---------------
|link \ | |link \ |
+------+ +------+ +------+ +------+
| CE | | PE | | CE | | PE |
|device|-----|device| |device|-----|device| SP network
+------+ +------+ +------+ +------+
| |
+------------ +---------------
(a) (b)
Figure 3. Combination of Dual-Homing and Backdoor Links for
CE Devices
The customer networks use their own instances of the spanning tree
protocol to configure and partition their active topology so that the
provider connectivity doesn't result in a data loop. Reconfiguration
of a customer's active topology can result in the apparent movement
of customer end stations from the point of view of the PEs. There
are two methods for addressing this issue based on the provider
bridge model depicted in Figure 1. In the first method, the Topology
Change Notification (TCN) message received from the CE device is
translated into one or more out-of-band "MAC Address Withdrawal"
messages as specified in [RFC4762]. In the second method, the TCN
message received from the CE device is translated into one or more
in-band "Flush" messages per [p802.1Qbe]. The second method is
recommended because of ease of interoperability between the bridge
and LAN emulation modules of the PE.
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5.2. Redundancy
[RFC4762] talks about dual-homing of a given Multi-Tenant Unit switch
(MTU-s) to two PEs over a provider MPLS access network to provide
protection against link and node failure. For example, in case the
primary PE fails or the connection to it fails, then the MTU-s uses
the backup PWs to reroute the traffic to the backup PE. Furthermore,
it discusses the provision of redundancy when a provider Ethernet
access network is used and how any arbitrary access network topology
(not just hub-and-spoke) can be supported using the provider's MSTP
protocol. It also discusses how the provider MSTP for a given access
network can be confined to that access network and operate
independently from MSTP protocols running in other access networks.
In both types of redundancy mechanism (Ethernet and MPLS access
networks), only one PE is active for a given VPLS instance at any
time. In case of an Ethernet access network, core-facing PWs (for a
VPLS instance) at the PE are blocked by the MSTP; whereas, in case of
a MPLS access network, the access-facing PW is blocked at the MTU-s
for a given VPLS instance.
------------------------+ Provider +-----------------------
. Core .
+------+ . . +------+
| PE |======================| PE |
Provider | (P) |---------\ /-------| (P) | Provider
Access +------+ . \ / . +------+ Access
Network . \/ . Network
(1) +------+ . /\ . +------+ (2)
| PE |----------/ \--------| PE |
| (B) |----------------------| (B) |
+------+ . . +------+
. .
------------------------+ +-----------------------
Figure 4. Bridge Module Model
Figure 4 shows two provider access networks each with two PEs that
are connected via a full mesh of PWs for a given VPLS instance. As
shown in the figure, only one PE in each access network serves as a
Primary PE (P) for that VPLS instance and the other PE serves as the
backup PE (B). In this figure, each primary PE has two active PWs
originating from it. Therefore, when a multicast, broadcast, and
unknown unicast frame arrives at the primary PE from the access
network side, the PE replicates the frame over both PWs in the core
even though it only needs to send the frame over a single PW (shown
with "==" in Figure 4) to the primary PE on the other side. This is
an unnecessary replication of the customer frames and consumes core-
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network bandwidth (half of the frames get discarded at the receiving
PE). This issue is aggravated when there are more than two PEs per
provider access network -- e.g., if there are three PEs or four PEs
per access network, then 67% or 75%, respectively, of core-network
bandwidth for multicast, broadcast, and unknown unicast are
respectively wasted.
Therefore, it is recommended to have a protocol among PEs that can
disseminate the status of PWs (active or blocked) among themselves.
Furthermore, it is recommended to have the protocol tied up with the
redundancy mechanism such that (per VPLS instance) the status of
active/backup PE gets reflected on the corresponding PWs emanating
from that PE.
The above discussion was centered on the inefficiency regarding
packet replication over MPLS core networks for current VPLS
redundancy mechanism. Another important issue to consider is the
interaction between customer and service provider redundancy
mechanisms, especially when customer devices are IEEE bridges. If
CEs are IEEE bridges, then they can run RSTPs or MSTPs. RSTP
convergence and detection time is much faster than its predecessor
(IEEE 802.1D STP, which is obsolete). Therefore, if the provider
network offers a VPLS redundancy mechanism, then it should provide
transparency to the customer's network during a failure within its
network, e.g., the failure detection and recovery time within the
service provider network should be less than the one in the customer
network. If this is not the case, then a failure within the provider
network can result in unnecessary switch-over and temporary
flooding/loop within the customer's network that is dual-homed.
5.3. MAC Address Learning
When customer devices are routers, servers, or hosts, then the number
of MAC addresses per customer sites is very limited (most often one
MAC address per CE). However, when CEs are bridges, then there can
be many customer MAC addresses (e.g., hundreds of MAC addresses)
associated with each CE.
[802.1ad] has devised a mechanism to alleviate MAC address learning
within provider Ethernet networks that can equally be applied to VPLS
networks. This mechanism calls for disabling MAC address learning
for an S-VLAN (or a service instance) within a provider bridge (or
PE) when there is only one ingress and one egress port associated
with that service instance on that PE. In such cases, there is no
need to learn customer MAC addresses on that PE since the path
through that PE for that service instance is fixed. For example, if
a service instance is associated with four CEs at four different
sites, then the maximum number of provider bridges (or PEs) that need
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to participate in that customer MAC address learning is only three,
regardless of how many PEs are in the path of that service instance.
This mechanism can reduce the number of MAC addresses learned in a
hierarchical VPLS (H-VPLS) with QinQ access configuration.
If the provider access network is of type Ethernet (e.g., IEEE
802.1ad-based network), then the MSTP can be used to partition the
access network into several loop-free spanning tree topologies where
Ethernet service instances (S-VLANs) are distributed among these tree
topologies. Furthermore, GVRP can be used to limit the scope of each
service instance to a subset of its associated tree topology (thus
limiting the scope of customer MAC address learning to that sub-
tree). Finally, the MAC address disabling mechanism (described
above) can be applied to that sub-tree to further limit the number of
nodes (PEs) on that sub-tree that need to learn customer MAC
addresses for that service instance.
Furthermore, [802.1ah] provides the capability of encapsulating
customers' MAC addresses within the provider MAC header. A MTU-s
capable of this functionality can significantly reduce the number of
MAC addresses learned within the provider network for H-VPLS with
QinQ access, as well as H-VPLS with MPLS access.
6. Interoperability with 802.1ad Networks
[RFC4762] discusses H-VPLS provider-network topologies with both
Ethernet [802.1ad] and MPLS access networks. Therefore, it is
important to ensure seamless interoperability between these two types
of networks.
Provider bridges as specified in [802.1ad] are intended to operate
seamlessly with customer bridges and provide the required services.
Therefore, if a PE is modeled based on Figures 1 and 2, which include
a [802.1ad] bridge module, then it should operate seamlessly with
Provider Bridges given that the issues discussed in this document
have been taken into account.
7. Acknowledgments
The authors would like to thank Norm Finn and Samer Salam for their
comments and valuable feedback.
8. Security Considerations
In addition to the security issues described in [RFC4762], the
following considerations apply:
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- When a CE that is a customer bridge is connected to the VPLS
network, it may be desirable to secure the end-to-end communication
between the customer bridge nodes across the VPLS network. This
can be accomplished by running [802.1AE] MAC security between the
C-VLAN components of the customer bridges. In this case, the VPLS
PEs must ensure transparent delivery of the encryption/security
protocol datagrams using the Bridge Group Address [802.1ad].
- When a CE that is a customer bridge is connected to the VPLS
network, it may be desirable to secure the communication between
the customer bridge and its directly connected PE. If the PE is
modeled to include a [802.1ad] bridge module, then this can be
achieved by running MAC security between the customer bridge and
the S-VLAN component of the VPLS PE as described in Section 7.7.2
of [802.1AX].
- When an 802.1ad network is connected to a VPLS network, it is
possible to secure the NNI between the two networks using the
procedures of [802.1AE] and [802.1AX] between the S-VLAN components
of the Provider Edge Bridge and the attached VPLS PE, as long as
the PE is modeled to include an [802.1ad] bridge module.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4762] Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol
(LDP) Signaling", RFC 4762, January 2007.
[802.1ad] IEEE 802.1ad-2005, "Amendment to IEEE 802.1Q-2005. IEEE
Standard for Local and Metropolitan Area Networks -
Virtual Bridged Local Area Networks Revision-Amendment 4:
Provider Bridges".
[802.1AE] IEEE 802.1AE-2006, "IEEE Standard for Local and
Metropolitan Area Networks - Media Access Control (MAC)
Security".
[802.1ag] IEEE 802.1ag-2007, "IEEE Standard for Local and
Metropolitan Area Networks - Virtual Bridged Local Area
Networks Amendment 5: Connectivity Fault Management".
[802.1ah] IEEE 802.1ah-2008, "IEEE Standard for Local and
Metropolitan Area Networks - Virtual Bridged Local Area
Networks Amendment 7: Provider Backbone Bridges".
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RFC 6246 VPLS Interop with CE Bridges June 2011
[802.1AX] IEEE 802.1AX-2008 "IEEE Standard for Local and
Metropolitan Area Networks - Link Aggregation".
10. Informative References
[IPLS] Shah, H., Rosen, E., Le Faucheur, F., and G. Heron, "IP-
Only LAN Service (IPLS)", Work in Progress, February
2010.
[RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over
MPLS Networks", RFC 4448, April 2006.
[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping
Switches", RFC 4541, May 2006.
[RFC4664] Andersson, L., Ed., and E. Rosen, Ed., "Framework for
Layer 2 Virtual Private Networks (L2VPNs)", RFC 4664,
September 2006.
[RFC4665] Augustyn, W., Ed., and Y. Serbest, Ed., "Service
Requirements for Layer 2 Provider-Provisioned Virtual
Private Networks", RFC 4665, September 2006.
[RFC6136] Sajassi, A., Ed., and D. Mohan, Ed., "Layer 2 Virtual
Private Network (L2VPN) Operations, Administration, and
Maintenance (OAM) Requirements and Framework", RFC 6136,
March 2011.
[802.1D] IEEE 802.1D-2004, "IEEE Standard for Local and
Metropolitan Area Networks - Media access control (MAC)
Bridges (Incorporates IEEE 802.1t-2001 and IEEE 802.1w)".
[802.1Q] IEEE Std. 802.1Q-2003 "Virtual Bridged Local Area
Networks".
[p802.1Qbe] IEEE Draft Standard P802.1Qbe, "IEEE Draft Standard for
Local and Metropolitan Area Networks -- Virtual Bridged
Local Area Networks Amendment: Multiple I-SID
Registration Protocol".
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Authors' Addresses
Ali Sajassi (editor)
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: sajassi@cisco.com
Frank Brockners
Cisco Systems, Inc.
Hansaallee 249
40549 Duesseldorf
Germany
EMail: fbrockne@cisco.com
Dinesh Mohan (editor)
Nortel
Ottawa, ON K2K3E5
EMail: dinmohan@hotmail.com
Yetik Serbest
AT&T Labs
9505 Arboretum Blvd.
Austin, TX 78759
EMail: yetik_serbest@labs.att.com
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RFC, FYI, BCP