One document matched: draft-ietf-l2vpn-vpls-bridge-interop-00.txt
Internet Working Group Ali Sajassi
Internet Draft Cisco Systems
Yetik Serbest
SBC Communications
Frank Brockners
Cisco Systems
Dinesh Mohan
Nortel
Expires: December 2006
VPLS Interoperability with CE Bridges
draft-ietf-l2vpn-vpls-bridge-interop-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006). All Rights Reserved.
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
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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.
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
Table of Contents
1. Overview........................................................3
2. Ethernet Service Instance.......................................3
3. VPLS-Capable PE Model with Bridge Module........................4
4. Mandatory Issues................................................7
4.1. Service Mapping...............................................7
4.2. CE Bridge Protocol Handling...................................9
4.3. Partial-mesh of Pseudowires..................................10
4.4. Multicast Traffic............................................11
5. Optional Issues................................................12
5.1. Customer Network Topology Changes............................12
5.2. Redundancy...................................................14
5.3. MAC Address Learning.........................................15
6. Interoperability with 802.1ad Networks.........................16
7. Acknowledgments................................................16
8. Security Considerations........................................16
9. References.....................................................17
10. Authors' Addresses............................................17
11. Full Copyright Statement......................................18
12. Intellectual Property Statement.....Error! Bookmark not defined.
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1. 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 [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 [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].
This draft 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 four and five discuss the mandatory and optional
issues respectively.
2. Ethernet Service Instance
Before starting the discussion of bridging issues, it is important
to first clarify the Ethernet Service definition. The term VPLS has
different meanings in different contexts. In general, VPLS is used
in the following contexts [Eth-OAM]: a) as an end-to-end bridged-LAN
service over one or more network (one of which being 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 across an MPLS/IP network. For an
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end-to-end service (among different sites of a given customer), we
use the term “Ethernet Service Instance” 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 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 (either for a point-to-point or multipoint connectivity) is
associated with a forwarding path within the service provider’s
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
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/or a
dedicated Service VLAN per ESI in an IEEE 802.1ad network.
3. VPLS-Capable PE Model with Bridge Module
[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.
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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 “Emulated LAN” 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.
+----------------------------------------+
| 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].
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+-------------------------------+
| 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
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 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 customer’s 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
‘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 S-VLAN tag can be, and is, removed at this
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interface by the S-VLAN bridge component. If multiple C-VLANs are
supported by this service instance (e.g., VLAN bundling or port-
based), 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 shall not have been tagged with
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 two (e.g.,
one end of the service be associated with physical Ethernet ports
and the other end be associated with VLANs). In [VPLS-LDP],
unqualified and qualified learning is used to refer to port-based
and VLAN-based operation respectively and it 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. [P802.1ad] allows for each customer AC (either a physical
port, or a VLAN, or a group of VLANs) 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.
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 “port-based w/ 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 w/ tagged and untagged traffic”. 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-based). Finally, in the
fourth scenario, it is assumed that a group of VLANs from the
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Ethernet physical interface is mapped to the corresponding service
instance or ESI.
===================================================================
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 a device 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 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 “port-based” 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 “VLAN-based” 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
“VLAN bundling” 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
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at different sites to a single ESI without coordination among those
sites.
When a port-based or a VLAN-bundling is used, then the may PE 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
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
and 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. [P802.1ad] 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:
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,
ii) Discarding frames, acting as a non-participating barrier to the
operation of the protocol; or,
iii) 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.
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 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-based 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
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example of such protocol is link aggregation protocol [802.3ad];
however, 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 or VLAN-bundling 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.
4.3. 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 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.
[Rosen-Mesh] defines a procedure for detection of partial mesh in
which each PE keeps track of the status of PW Endpoint Entities (EEs
- 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 even 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.
If the PE model, with bridge module as depicted in figure-2, is
used, then [P802.1ag] procedures could be used for detection of
partial-mesh of PWs. [p802.1ag] defines a set of procedures for
fault detection, verification, isolation, and notification per ESI.
Fault detection mechanism of [p8021.ag] can be used to perform
connectivity check among PEs belonging to a given VPLS instance. It
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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). It should be noted that
[P802.1ag] assumes 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 or LAN segment.
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 detected failure in one side is
propagated to the other side.
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 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 “General Query” over that customer UNIs upon
receiving a customer topology change notification as described in
[IGMP-SNOOP]. A “General Query” 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
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 don’t 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
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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 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 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
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 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 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. [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 doesn’t result in 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. 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 “MAC Address Withdrawal” 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
that VPLS instance. The second method is recommended because of ease
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of interoperability between the bridge and the LAN emulation modules
of the PE.
5.2. 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 provider’s 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 4. Bridge Module Model
Figure-4 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 “==” in the figure) to the
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draft-ietf-l2vpn-vpls-bridge-interop.txt October 2004
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 recommended 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 customer’s 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 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.
[P802.1ad] 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 to participate in that customer MAC address
learning, is only three regardless of how many PEs are in the path
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of that service instance. This mechanism can reduce the number of
MAC addressed learned in a 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 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.
Furthermore, [p802.1ah] provides the capability of encapsulating
customers’ MAC addresses within the provider MAC header. A u-PE
capable of this functionality can reduce the number of MAC addressed
learned significantly within the provider network for H-VPLS with
QinQ access as well as H-VPLS with MPLS access.
6. Interoperability with 802.1ad Networks
[VPLS-LDP] discusses H-VPLS provider-network topologies with both
Ethernet [P802.1ad] as well as MPLS access 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 which includes a
[802.1ad] bridge module, then it should operate seamlessly with
Provider Bridges given that the issues discussed in this draft have
been taken into account.
7. Acknowledgments
The authors would like to thank Norm Finn for his comments and
feedbacks.
8. Security Considerations
There are no additional security aspects beyond that of VPLS/H-VPLS
that needs to be discussed here.
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9. Normative 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
[P802.1ad] IEEE Draft P802.1ad/D2.4 “Virtual Bridged Local Area
Networks: Provider Bridges”, Work in progress, September 2004
[P802.1ag] IEEE Draft P802.1ag/D0.1 “Virtual Bridge Local Area
Networks: Connectivity Fault Management”, Work in Progress, October
2004
10. Informative References
[IPLS] Shah, H. and et al, "IP-Only LAN Service (IPLS) ", work in
progress, October 2004
[MFA-Ether] Sajassi, A. and et al, “Ethernet Service Interworking
Over MPLS”, Work in Progress, September 2004
[Rosen-Mesh] “Detecting and Reacting to Failures of the Full Mesh in
IPLS and VPLS”,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 “Media Access Control (MAC)
Bridges”.
[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
[Eth-OAM] Dinesh Mohan, Ali Sajassi, and et al, “L2VPN OAM
Requirements and Framework”, Work in progress, July 2006
11. Authors' Addresses
Ali Sajassi
Sajassi, et al. [Page 17]
draft-ietf-l2vpn-vpls-bridge-interop.txt October 2004
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
Dinesh Mohan
Nortel Networks
3500 Carling Ave
Ottawa, ON K2H8E9
Email: mohand@nortel.com
12. Intellectual Property Considerations
This document is being submitted for use in IETF standards
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Sajassi, et al. [Page 18]
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Sajassi, et al. [Page 19]
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