One document matched: draft-sajassi-l2vpn-vpls-bridge-interop-00.txt
Internet Draft Document Ali Sajassi
Provider Provisioned VPN Working Group Cisco Systems
draft-sajassi-l2vpn-vpls-bridge-interop-
00.txt Yetik Serbest
SBC Communications
Frank Brockners
Cisco Systems
Expires: April 2005 October 2004
VPLS Interoperability with CE Bridges
draft-sajassi-l2vpn-vpls-bridge-interop-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
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2.
Abstract
One of the main motivations behind VPLS is its ability to provide
connectivity not only among customer routers and servers/hosts but
also among customer bridges. If only connectivity among customer IP
routers/hosts was desired, then IPLS solution [IPLS] could have been
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 today or the same level of service that they receive from
their Ethernet Service Providers using IEEE 802.1ad-based networks
[P802.1ad] (or its predecessor ¡ QinQ-based network).
When CE devices are IEEE bridges, then there are certain issues and
challenges that need to be accounted for in a VPLS network. Majority
of these issues have currently been addressed in IEEE 802.1ad
standard for provider bridges and they need to be addressed for VPLS
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networks. This draft discusses these issues and wherever possible,
the recommended solutions to these issues.
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-
02.txt
www.ietf.org/internet-drafts/draft-ietf-ppvpn-l2-framework-05.txt
www.ietf.org/internet-drafts/draft-ietf-l2vpn-vpls-ldp-05.txt
WHERE DOES THIS FIT IN THE PICTURE OF THE ROUTING WORK
L2VPN
WHY IS IT TARGETED AT THIS WG
This draft describes interoperability issues with customer bridges
in a VPLS network.
JUSTIFICATION
Existing Internet drafts specify how to provide multipoint Ethernet
services over MPLS (VPLS). This draft describes some of the issues
associated with the provision of such multipoint services when
customer devices are bridges.
Table of Contents
1. Status of this Memo.............................................1
2. Abstract........................................................1
3. Conventions.....................................................2
4. Overview........................................................3
5. Ethernet Service Instance.......................................3
5.1. Attachment Circuits Mapping to an ESI.........................5
6. VPLS-Capable PE Model...........................................7
7. CE Bridge Protocol Handling.....................................9
7.1. Customer Network Topology Changes............................10
8. Partial-mesh of Pseudowires....................................12
9. Redundancy.....................................................13
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10. MAC Address Learning..........................................14
11. Multicast Traffic.............................................15
12. Interoperability with 802.1ad Networks........................16
13. Acknowledgments...............................................17
14. Security Considerations.......................................17
15. Full Copyright Statement......................................17
16. IPR Notice....................................................17
17. References....................................................18
18. Authors' Addresses............................................19
4.
Overview
Virtual Private LAN Service is a LAN emulation service intended for
providing connectivity between geographically dispersed customer
sites across MAN/WAN (over MPLS/IP) network(s), 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 customer bridges. If only
connectivity among customer IP routers/hosts was desired, then IPLS
solution [IPLS] could have been 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 today or the same level
of service that they receive from their Ethernet Service Providers
using IEEE 802.1ad-based networks [P802.1ad] (or its predecessor ¡
QinQ-based network).
When CE devices are IEEE bridges, then there are certain issues and
challenges that need to be accounted for in a VPLS network. Majority
of these issues have currently been addressed in IEEE 802.1ad
standard for provider bridges and they need to be addressed for VPLS
networks. This draft discusses these issues and wherever possible,
the recommended solutions to these issues. It 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 [VPLS-LDP].
5.
Ethernet Service Instance
Before starting the discussion of bridging issues, it is important
to first clarify the Ethernet Service definition. The term ôVPLSö is
used for both: the network architecture that provides a multipoint-
Ethernet service as well as for the service itself. Also sometimes,
it refers to the end-to-end network or service between different
customer sites as long as a portion of the network is MPLS/IP. For
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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
across an MPLS/IP network. For an end-to-end service (among
different sites of a given customer), we use the term ôEthernet
Service Instanceö or ESI.
[MFA-Ether] defines 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 UNI or a NNI; furthermore, it can be an Ethernet interface or a
VLAN, it can be an ATM or FR VC, or it can be a PPP/HDLC interface.
If an ESI is associated with more than two ACs, then it is a
multipoint ESI. In this document, where ever the keyword ESI is
used, it means multipoint ESI, unless it is 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 [P802.1ad]. Furthermore, an ESI can correspond to
both a VPLS instance and a Service VLAN if its associated ACs are
connected to 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.
[MEF-Ethernet] defines an Ethernet Virtual Connection (EVC) as an
association of two or more UNIs, where the UNI is a standard
Ethernet interface and point of demarcation between Customer
Equipment and service providerÆs network. An EVC is either point-to-
point or multipoint-to-multipoint. It should be noted that an ESI
cannot be directly compare to an EVC per MEF definition since an ESI
is associated with a set of ACs; whereas, an EVC is associated with
a set of UNIs. However, if one limits the ACs associated with a
given ESI to only UNIs, then for a multipoint connection, an ESI
corresponds to a multipoint-to-multipoint EVC.
An ESI (either for a point-to-point or multipoint connectivity) is
associated with a forwarding path within the service providerÆs
network and that is different from an Ethernet Class of Service
(CoS) which is associated with the frame Quality-of-Service
treatment by each node along the path defined by the ESI. An ESI can
have one or more CoS associated with it.
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 MAC address domain and broadcast domain
per ESI. A separate MAC address domain is provided by using a
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separate filtering database per ESI (for both VPLS and IEEE 802.1ad
networks) and separate broadcast domain is provided by using a full-
mesh of PWs per ESI over the IP/MPLS core in a VPLS network and a
dedicated Service VLAN per ESI in an IEEE 802.1ad network.
Different Ethernet AC types can be associated with an ESI. For
example, an ESI can be associated with only physical Ethernet ports,
VLANs, or a combination of two (e.g., one end of the service be
associated with physical Ethernet ports and the other end be
associated with VLANs). In the VPLS terminology, unqualified and
qualified learning is used to refer to port-based and vlan-based
operation respectively. Based on this VPLS definition, it is not
clear how to classify a customer service where traffic from some of
its sites (that is untagged and port-based) needs to be mapped to a
VLAN at another site. 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. [P802.1ad]
allows for each customer AC to be mapped independently to an ESI
which provides better service offering 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.
5.1.
Attachment Circuits Mapping to an ESI
The following table lists possible mappings that can exist between
customer ACs and its associated ESI ¡ this table is extracted from
[MFA-Ether]. As it can be seen, there are several possible ways to
perform such mapping. In the first scenario, it is assumed that an
Ethernet physical port only carries untagged traffic and all the
traffic is mapped to the corresponding service instance or ESI. This
is referred to as ôport-based w/ untagged trafficö. 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 ôport-
based w/ tagged and untagged trafficö. In the third scenario, it is
assumed that only a single VLAN is mapped to the corresponding
service instance or ESI (referred to as VLAN mapping). 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.
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===================================================================
Ethernet I/F & Associated Service Instance(s)
-------------------------------------------------------------------
Port-based port-based VLAN VLAN
Untagged tagged & mapping bundling
untagged
-------------------------------------------------------------------
Port-based Y N Y(Note-1) N
untagged
Port-based N Y Y(Note-2) Y
tagged &
untagged
VLAN Y(Note-1) Y(Note-2) Y Y(Note-3)
Mapping
VLAN N Y Y(Note-3) Y
Bundling
===================================================================
Note-1: In this asymmetric mapping scenario, it is assumed that the
CE device with ôVLAN mappingö AC is a device capable of supporting
[802.1Q] frame format.
Note-2: In this asymmetric mapping scenario, it is assumed that the
CE device with ôVLAN mappingö AC is a device that can support
[P802.1ad] frame format because it will receive Ethernet frames
with two tags; where the outer tag is S-VLAN and the inner tag is C-
VLAN received from ôport-basedö AC. One application example for such
CE device is in feature server for DSL aggregation over Metro
Ethernet network.
Note¡3: In this asymmetric mapping scenario, it is assumed that the
CE device with ôVLAN mappingö AC can support the [P802.1ad] frame
format because it will receive Ethernet frames with two tags; where
the outer tag is S-VLAN and the inner tag is C-VLAN received from
ôVLAN bundlingö AC.
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 [P802.1ad].
As mentioned before, the mapping function between the customer AC
and its associated ESI is a local function and thus when the AC is a
single customer VLAN, it is possible to map different customer VLANs
at different sites to a single ESI ¡ e.g., no coordination is
required among different customer sites for that service instance
and each customer site can independently identify the same service
instance via a different VLAN pertinent to its local site.
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When a port-based or a VLAN-bundling is used, then if the PE uses an
additional S-VLAN tag to mark the customer traffic received over
that AC as belonging to a given ESI, then that 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
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] and the PE shall translate the customer tag (C-VLAN) into
the provider tag (S-VLAN) upon receiving a frame from the customer.
6.
VPLS-Capable PE Model
[L2VPN-FRWK] 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 include both routers/hosts and IEEE
bridges, then the second model is the most suitable and adequate one
and it is consistent with IEEE standards for Provider Bridges
[P802.1ad]. We briefly describe the second model and then elaborate
upon this model to show its sub-components based on [P802.1ad]
Provider Bridge model. Finally, we show how this model can be used
to support all the different services and AC mapping (both symmetric
and asymmetric) described in the previous section.
As described in [L2VPN-FRWK], 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 Service VLAN or S-VLAN). The bridge
module has at least a single ôEmulated LANö interface over which
each VPLS instance is represented by a unique S-VLAN tag. Each VPLS
instance can consist of a set of PWs and its associated forwarder
corresponding to a single Virtual LAN (VLAN) as depicted in the
following figure. Thus, sometimes it is referred to as V-LAN
emulation.
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+----------------------------------------+
| VPLS-capable PE model |
| +---------------+ +------+ |
| | | |VPLS-1|------------
| | |==========|Fwdr |------------ PWs
| | Bridge ------------ |------------
| | | S-VLAN-1 +------+ |
| | Module | o |
| | | o |
| | (802.1ad | o |
| | bridge) | o |
| | | o |
| | | S-VLAN-n +------+ |
| | ------------VPLS-n|-------------
| | |==========| Fwdr |------------- PWs
| | | ^ | |-------------
| +---------------+ | +------+ |
| | |
+-------------------------|--------------+
LAN emulation 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 PWs associated with
that VPLS instance (assuming raw mode PW is used as specified in
[PWE3-Ethernet]).
The bridge module can itself consist of one or two sub-components
depending on the functionality that it needs to perform. The
following figure depicts the model for bridge module based on
[P802.1ad].
+-------------------------------+
| 802.1ad Bridge Module Model |
| |
+---+ | +------+ +-----------+ |
|CE |---------|C-VLAN|------| | |
+---+ | |bridge|------| | |
| +------+ | | |
| o | S-VLAN | |
| o | | |
| o | Bridge | |
+---+ | +------+ | | |
|CE |---------|C-VLAN|------| | |
+---+ | |bridge|------| | |
| +------+ +-----------+ |
+-------------------------------+
Figure 2. The Model of 802.1ad Bridge Module
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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 providerÆs bridge protocol such as RSTP, MSTP, GVRP,
GMRP, etc. among provider bridges within a single administrative
domain.
The 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 customerÆs bridging protocol
such as RSTP and MSTP. The reason that such participation is
required is because a customer VLAN (C-VLAN) at one site can be
mapped into a different C-VLAN at a different site or in case of
asymmetric mapping (as describe in the previous section), a customer
Ethernet port at one site can be mapped into a customer VLAN (or
group of C-VLANs) at a different site.
In scenarios where C-VLAN bridge component is required, then there
will be one such component per customer UNI port in order to avoid
local switching within the C-VLAN bridge component and thus limiting
local switching among different UNIs for the same customer to S-VLAN
bridge component.
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
æLANÆ (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 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., VLAN bundling), 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 mapping),
then the frames shall not have tagged with C-VLAN tag since C-VLAN
can be derived from the S-VLAN. The C-VLAN aware bridge component
applies a port VLAN ID (PVID) to untagged frames received on each
internal æLANÆ, allowing full control over the delivery of frames
for each C-VLAN through the Customer UNI Port.
7.
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. [P802.1ad] states that
when a PE is connected to a CE bridge, then the service offered by
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the PE may appear to specific customer protocols running on the CE
in one of the four ways:
i) Transparent to the operation of the protocol among CEs of
different sites using the service provided, appearing as an
individual LAN without bridges; or,
i
i) Discarding frames, acting as a non-participating barrier to the
operation of the protocol; or,
i
i
i) Peering, with a local protocol entity at the point of provider
ingress and egress, participating in and terminating the
operation of the protocol; or,
iv) Participation in individual instances of customer protocols.
For example, when an Attachment Circuit is port-based, then the
bridge module of the PE can operate transparently with respect to
the CEÆs RSTP/MSTP protocols (and thus no C-VLAN component is
required for that customer UNI). However, when an Attachment Circuit
is VLAN-based (either VLAN mapping or VLAN bundling), then the
bridge module of the PE needs to peer with the RSTP/MSTP protocols
running on the CE (and thus the C-VLAN bridge component is
required). There are also protocols that require peering but are
independent from the type of Attachment Circuit. An example of such
protocol is link aggregation protocol [802.3ad]; however, one can
argue that this is a media-dependent protocol as its name implies.
Therefore, the peering requirement can be generalized such that the
media-independent protocols (RSTP/MSTP, CFM, etc) that require
peering are for VLAN-based Attachment Circuit.
[P802.1ad] reserves a block of 16 MAC addresses for the operation of
C-VLAN and S-VLAN bridge components and it shows which of these
reserved MAC addresses are only for C-VLAN bridge component and
which ones are only for S-VLAN bridge component and which ones apply
to both C-VLAN and S-VLAN components.
7.1.
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. [L2VPN-REQ] provides some examples
of such customer network connectivity that are depicted in the
figure below. Such network topologies are designed to protect
against the failure or removal of network components with 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 to avoid
black holing of customer frames.
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+----------- +---------------
| |
+------+ +------+ +------+ +------+
| 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 spanning tree
protocol to configure and partition their active topology, so that
the provider connectivity doesnÆt result in data loop.
Reconfiguration of a customerÆs active topology can result in the
apparent movement of customer end stations from the point of view of
the PEs. However, the requirement for mutual independence of the
distinct ESIs that can be supported by a single provider spanning
tree active topology does not permit either the direct receipt of
provider topology change notifications from the CEs or the use of
received customer spanning tree protocol topology change
notifications to stimulate topology change signaling on a provider
spanning tree.
To address this issue, [P802.1ad] requires that customer topology
change notification to be detected at the ingress of the S-VLAN
bridge component and the S-VLAN bridge transmits a Customer Change
Notification (CCN) BPDU tagged with the S-VLAN ID associated with
that service instance and a destination MAC address as specified in
the block of 16 reserved multicast MAC addresses. Upon receiving the
CCN, the provider bridge will flush all the customer MAC addresses
associated with that S-VLAN ID on all the provider bridge interfaces
except the one that the CCN message is received from.
Based on the provider bridge model depicted in figure (1), there are
two methods of propagating the CCN message over the VPLS network.
The first method is to translate the in-band CCN message into an
out-of-band ôMAC Address Withdrawalö message as specified in [VPLS-
LDP] and the second method is to treat the CCN message as customer
data and pass it transparently over the set of PWs associated with
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that VPLS instance. The second method is recommended because of ease
of interoperability between the bridge and the LAN emulation modules
of the PE.
8.
Partial-mesh of Pseudowires
The effect of a PW failure (resulting in creation of partial-mesh of
PWs) on the CE devices and their supported services should be well
known. 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 cause ôblack holesö 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.
[P802.1ag] defines a set of procedures for fault detection,
verification, isolation, and notification per ESI. However, these
procedures are not very suitable for detection and isolation of PW
failure for a number of reasons.
First, [P802.1ag] 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 coverage includes bridge module within a PE and
it is not limited to just PWs. Furthermore, [P802.1ag] operates
transparently over the full-mesh of PWs for a given service instance
since it operates at the Ethernet level (and not at PW level).
Second, [P802.1ag] is basically a slow protocol intended to check
the integrity for a given service instance end-to-end. Therefore,
the detection time may be longer than the detection time needed for
loop prevention inside the customer network. Some Enterprise
customers running spanning tree protocols in their network require
loop detection time in order of tens of milliseconds.
Third, [P802.1ag] assume that the Ethernet links or LAN segments
connecting provider bridges are full-duplex and the failure in one
direction results in the failure of the whole link. However, that is
not the case for VPLS instance since a PW consists of two uni-
directional LSPs and one direction can fail independent from the
other causing inconsistent view of the full-mesh by the
participating PEs till the failure is detected and propagated to the
other PEs.
[Rosen-Mesh] defines a procedure for detection of partial mesh in
which each PE keeps track of the status of PW Endpoint Entities (EEs
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- e.g., VPLS forwarders) for itself as wells the ones reported by
other PEs. Therefore, upon a PW failure, the PE that detects the
failure not only takes notice locally but it notifies other PEs
belonging to that service instance of such failure so that all the
participants PEs have a consistent view of the PW mesh. The
procedure defined in [Rosen-Mesh] is for detection of partial mesh
per service instance and in turn it relies on additional procedure
for PW failure detection such as BFD or VCCV. Given that there can
be ten or hundreds of thousands of PWs in a PE, the scalability
aspects of this procedure needs to be worked out. Also [Rosen-Mesh]
acknowledges that many of the details regarding operational aspects
of such procedure are missing and need to be worked out.
9.
Redundancy
[VPLS-LDP] talks about dual-homing of a given u-PE to two n-PEs over
a provider MPLS access network to provide protection against link
and node failure ¡ e.g., in case the primary n-PE fails or the
connection to it fails, then the u-PE uses the backup PWs to reroute
the traffic to the backup n-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 limited
to hub-and-spoke) can be supported using the providerÆs MSTP
protocol and 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 mechanisms (Ethernet versus MPLS access
networks), only one n-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 n-PE are blocked by the MSTP protocol;
whereas, in case of a MPLS access network, the access-facing PW is
blocked at the u-PE for a given VPLS instance.
-------------------------+ Provider +-------------------------
. Core .
+------+ . . +------+
| n-PE |======================| n-PE |
Provider | (P) |---------\ /-------| (P) | Provider
Access +------+ ._ \ / . +------+ Access
Network . \/ . Network
(1) +------+ . /\ . +------+ (2)
| n-PE |----------/ \--------| n-PE |
| (B) |----------------------| (B) |_
+------+ . . +------+
. .
------------------------+ +-------------------------
Figure 3. Bridge Module Model
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draft-sajassi-l2vpn-vpls-bridge-interop.txt October 2004
Figure-3 shows two provider access networks each with two n-PEs
where the n-PEs are connected via a full mesh of PWs for a given
VPLS instance. As shown in the figure, only one n-PE in each access
network is serving as a Primary PE (P) for that VPLS instance and
the other n-PE is serving 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 n-PE from the access network side, the n-PE replicates the
frame over both PWs in the core even though it only needs to send
the frames over a single PW (shown with ô==ö in the figure) to the
primary n-PE on the other side. This is an unnecessary replication
of the customer frames that consumes core-network bandwidth (half of
the frames get discarded at the receiving n-PE). This issue gets
aggravated when there are more than two n-PEs per provider access
network ¡ e.g., if there are three n-PEs or four n-PEs per access
network, then 67% or 75% of core-BW for multicast, broadcast and
unknown unicast are respectively wasted.
Therefore, it is important to have a protocol among n-PEs that can
disseminate the status of PWs (active or blocked) among themselves
and furthermore to have it tied up with the redundancy mechanism
such that per VPLS instance the status of active/backup n-PE gets
reflected on the corresponding PWs emanating from that n-PE.
The above discussion was centered on the lack of efficiency with
regards to packet replication over MPLS core network 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 RSTP/MSTP protocols, RSTP
convergence and detection time is much faster than its predecessor
(IEEE 802.1D STP which is obsolete). Therefore, if the provider
network offers VPLS redundancy mechanism, then it should provide
transparency to the customerÆs network during a failure within its
network ¡ e.g., the failure detection and recovery time within the
service provider network to 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 customerÆs network that is dual homed.
10.
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.
[P802.1ad] has devised a mechanism to alleviate MAC address learning
within provider Ethernet networks that can equally be applied to
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draft-sajassi-l2vpn-vpls-bridge-interop.txt October 2004
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 to participate in that customer MAC address
learning, is only three regardless of how many PEs are in the path
of that service instance.
If the provider access network is of type Ethernet (e.g., IEEE
802.1ad-based network), then the MSTP protocol can be used to
partition 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 (and 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.
11.
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 PtP 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 causing additional burden
on the ingress PE for unnecessary packet replication, causing
additional load on the MPLS core network, and causing additional
processing at the receiving PE where multicast packet is discarded.
One possible approach, to deliver multicast more efficiently over
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 as
described in section 7, then for proper operation of IGMP snooping,
the PE must generate a ôGeneral Queryö over that customer UNIs upon
receiving a customer topology change notification as described in
[IGMP-SNOOP]. A ôGeneral Queryö by the PE results in proper
registration of the customer multicast MAC address(s) at the PE when
there is customer topology change. It should be noted that IGMP
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draft-sajassi-l2vpn-vpls-bridge-interop.txt October 2004
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
sub-set of PWs for packet replication; therefore, avoiding sending
multicast packets to the egress PEs that donÆt need them. However,
the replication is still performed by the ingress PE. In order to
avoid, replication at ingress PE, one may want to use multicast
distribution trees (MDTs) in the provider core network; however,
this comes with its potential pitfalls. If the MDT is used for all
multicast traffic of a given customer, then this results in customer
multicast and unicast traffic to be 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 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 distribution of
its data traffic. Similarly, if multicast connectivity is lost, but
unicast is working, then the customer spanning tree will activate
the blocked port which will result in 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 potential
issue even when it is used for customer data traffic since the MDT
doesnÆt 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 CFM procedures (802.1ag) used for
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 applicability
of the provider MDT to customer multicast traffic is for future
study.
12.
Interoperability with 802.1ad Networks
[VPLS-LDP] discusses H-VPLS provider-network topologies with both
Ethernet [P802.1ad] as well as MPLS access networks. Furthermore,
[VPLS-APP] discusses several examples and scenarios where the end-to-
end Ethernet service spans across both VPLS and Provider Bridged
[P802.1ad] networks. Therefore, it is of paramount importance to
ensure seamless interoperability between these two types of networks.
Provider bridges as specified in [P802.1ad] are intended to operate
seamlessly with customer bridges and provide the required services.
Therefore, if a PE is modeled based on Figures 1&2 that includes a
[802.1ad] bridge module, then it should operate seamlessly with
Provider Bridges. The issues discussed in this draft have been taken
into account.
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draft-sajassi-l2vpn-vpls-bridge-interop.txt October 2004
13.
Acknowledgments
The authors would like to thank Norm Finn for his valuable comments.
14.
Security Considerations
Security aspects of this draft will be discussed at a later point.
15.
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
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.
16.
IPR Notice
The IETF takes no position regarding the validity or scope of any
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pertain to the implementation or use of the technology described in
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claims of rights made available for publication and any assurance of
Sajassi, et al. [Page 17]
draft-sajassi-l2vpn-vpls-bridge-interop.txt October 2004
licenses to be made available, or the result of an attempt made to
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proprietary rights by implementors or users of this specification
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The IETF invites any interested party to bring to its attention any
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this standard. Please address the information to the IETF Executive
Director.
17.
References
[L2VPN-REQ] Agustyn, W. et al, "Service Requirements for Layer-2
Provider Provisioned Virtual Provider Networks", work in progress
[L2VPN-FRWK] Andersson, L. and et al, "Framework for Layer 2 Virtual
Private Networks (L2VPNs)", Work in Progress
[VPLS-LDP] Lasserre, M. and et al, "Virtual Private LAN Services
over MPLS", work in progress
[IPLS] Shah, H. and et al, "IP-Only LAN Service (IPLS) ", work in
progress, October 2004
[MFA-Ether] Sajassi, A. and et al, ôEthernet Service Interworking
Over MPLSö, Work in Progress, September 2004
[P802.1ad] IEEE Draft P802.1ad/D2.4 ôVirtual Bridged Local Area
Networks: Provider Bridgesö, Work in progress, September 2004
[P802.1ag] IEEE Draft P802.1ag/D0.1 ôVirtual Bridge Local Area
Networks: Connectivity Fault Managementö, Work in Progress, October
2004
[Rosen-Mesh] ôDetecting and Reacting to Failures of the Full Mesh in
IPLS and VPLSö,draft-rosen-l2vpn-mesh-failure-01.txt, March 2004
[PWE3-Ethernet] "Encapsulation Methods for Transport of Ethernet
Frames Over IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-
01.txt, Work in progress, November 2002.
[802.1D-REV] IEEE Std. 802.1D-2003 ôMedia Access Control (MAC)
Bridgesö.
[802.1Q] IEEE Std. 802.1Q-2003 "Virtual Bridged Local Area
Networks".
[IGMP-SNOOP] Christensen, M. and et al, "Considerations for IGMP and
MLD Snooping Switches", Work in progress, May 2004
Sajassi, et al. [Page 18]
draft-sajassi-l2vpn-vpls-bridge-interop.txt October 2004
18.
Authors' Addresses
Ali Sajassi
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Email: sajassi@cisco.com
Yetik Serbest
SBC Labs
9505 Arboretum Blvd.
Austin, TX 78759
Email: yetik_serbest@labs.sbc.com
Frank Brockners
Cisco Systems, Inc.
Hansaallee 249
40549 Duesseldorf
Germany
Email: fbrockne@cisco.com
Sajassi, et al. [Page 19]
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