Pseudowire Redundancy
RFC 6718
| Document | Type | RFC - Informational (August 2012; No errata) | |
|---|---|---|---|
| Authors | Praveen Muley , Mustapha Aissaoui , Matthew Bocci | ||
| Last updated | 2015-10-14 | ||
| Replaces | draft-muley-pwe3-redundancy | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Andy Malis | ||
| IESG | IESG state | RFC 6718 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Stewart Bryant | ||
| IESG note | Andy Malis (amalis@gmail.com) is the document shepherd. | ||
| Send notices to | (None) | ||
Internet Engineering Task Force (IETF) P. Muley
Request for Comments: 6718 M. Aissaoui
Category: Informational M. Bocci
ISSN: 2070-1721 Alcatel-Lucent
August 2012
Pseudowire Redundancy
Abstract
This document describes a framework comprised of a number of
scenarios and associated requirements for pseudowire (PW) redundancy.
A set of redundant PWs is configured between provider edge (PE) nodes
in single-segment PW applications or between terminating PE (T-PE)
nodes in multi-segment PW applications. In order for the PE/T-PE
nodes to indicate the preferred PW to use for forwarding PW packets
to one another, a new PW status is required to indicate the
preferential forwarding status of active or standby for each PW in
the redundant set.
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/rfc6718.
Muley, et al. Informational [Page 1]
RFC 6718 PW Redundancy August 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
2.1. Requirements Language ......................................6
3. Reference Models ................................................6
3.1. PE Architecture ............................................6
3.2. PW Redundancy Network Reference Scenarios ..................7
3.2.1. PW Redundancy for AC and PE Protection: One
Dual-Homed CE with Redundant SS-PWs .................7
3.2.2. PW Redundancy for AC and PE Protection: Two
Dual-Homed CEs with Redundant SS-PWs ................8
3.2.3. PW Redundancy for S-PE Protection:
Single-Homed CEs with Redundant MS-PWs .............10
3.2.4. PW Redundancy for PE-rs Protection in
H-VPLS Using SS-PWs ................................11
3.2.5. PW Redundancy for PE Protection in a VPLS
Ring Using SS-PWs ..................................13
3.2.6. PW Redundancy for VPLS n-PE Protection
Using SS-PWs .......................................14
4. Generic PW Redundancy Requirements .............................15
4.1. Protection Switching Requirements .........................15
4.2. Operational Requirements ..................................15
5. Security Considerations ........................................16
6. Contributors ...................................................16
7. Acknowledgements ...............................................17
8. References .....................................................17
8.1. Normative References ......................................17
8.2. Informative Reference .....................................18
Muley, et al. Informational [Page 2]
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1. Introduction
The objective of pseudowire (PW) redundancy is to maintain
connectivity across the packet switched network (PSN) used by the
emulated service if a component in the path of the emulated service
fails or a backup component is activated. For example, PW redundancy
will enable the correct PW to be used for forwarding emulated service
packets when the connectivity of an attachment circuit (AC) changes
due to the failure of an AC or when a pseudowire (PW) or packet
switched network (PSN) tunnel fails due to the failure of a provider
edge (PE) node.
PW redundancy uses redundant ACs, PEs, and PWs to eliminate single
points of failure in the path of an emulated service. This is
achieved while ensuring that only one path between a pair of customer
edge (CE) nodes is active at any given time. Mechanisms that rely on
more than one active path between the CEs, e.g., 1+1 protection
switching, are out of the scope of this document because they may
require a permanent bridge to provide traffic replication as well as
support for a 1+1 protection switching protocol in the CEs.
Protection for a PW segment can be provided by the PSN layer. This
may be a Resource Reservation Protocol with Traffic Engineering
(RSVP-TE) label switched path (LSP) with a fast-reroute (FRR) backup
or an end-to-end backup LSP. These mechanisms can restore PSN
connectivity rapidly enough to avoid triggering protection by PW
redundancy. PSN protection mechanisms cannot protect against the
failure of a PE node or the failure of the remote AC. Typically,
this is supported by dual-homing a CE node to different PE nodes that
provide a pseudowire emulated service across the PSN. A set of PW
mechanisms that enables a primary and one or more backup PWs to
terminate on different PE nodes is therefore required. An important
requirement is that changes occurring on the dual-homed side of the
network due to the failure of an AC or PE are not propagated to the
ACs on the other side of the network. Furthermore, failures in the
PSN are not propagated to the attached CEs.
In cases where PSN protection mechanisms are not able to recover from
a PSN failure or where a failure of a switching PE (S-PE) may occur,
a set of mechanisms that supports the operation of a primary and one
or more backup PWs via a different set of S-PEs or diverse PSN
tunnels is therefore required. For multi-segment PWs (MS-PWs), the
paths of these PWs are diverse in that they are switched at different
S-PE nodes.
Muley, et al. Informational [Page 3]
RFC 6718 PW Redundancy August 2012
In both of these cases, PW redundancy is important to maximize the
resiliency of the emulated service. It supplements PSN protection
techniques and can operate in addition to or instead of those
techniques when they are not available.
This document describes a framework for these applications and
associated operational requirements. The framework utilizes a new PW
status, called the 'Preferential Forwarding Status' of the PW. This
is separate from the operational states defined in RFC 5601
[RFC5601]. The mechanisms for PW redundancy are modeled on general
protection switching principles.
2. Terminology
o Up PW: A PW that has been configured (label mapping exchanged
between PEs) and is not in any of the PW or AC defect states
represented by the status codes specified in [RFC4446]. Such a PW
is available for forwarding traffic.
o Down PW: A PW that either has not been fully configured or has
been configured and is in any one of the PW or AC defect states
specified in [RFC4446]. Such a PW is not available for forwarding
traffic.
o Active PW: An up PW used for forwarding Operations,
Administration, and Maintenance (OAM) as well as user-plane and
control-plane traffic.
o Standby PW: An up PW that is not used for forwarding user traffic
but may forward OAM and specific control-plane traffic.
o PW Endpoint: A PE where a PW terminates on a point where native
service processing is performed, e.g., a single-segment PW (SS-PW)
PE, a multi-segment pseudowire (MS-PW) terminating PE (T-PE), or a
hierarchical Virtual Private LAN Service (VPLS) MTU-s or PE-rs.
o Primary PW: The PW that a PW endpoint activates (i.e., uses for
forwarding) in preference to any other PW when more than one PW
qualifies for the active state. When the primary PW comes back up
after a failure and qualifies for the active state, the PW
endpoint always reverts to it. The designation of primary is
performed by local configuration for the PW at the PE and is only
required when revertive behavior is used and is not applicable
when non-revertive protection switching is used.
Muley, et al. Informational [Page 4]
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o Secondary PW: When it qualifies for the active state, a secondary
PW is only selected if no primary PW is configured or if the
configured primary PW does not qualify for active state (e.g., is
down). By default, a PW in a redundancy PW set is considered
secondary. There is no revertive mechanism among secondary PWs.
o Revertive protection switching: Traffic will be carried by the
primary PW if all of the following is true: it is up, a wait-to-
restore timer expires, and the primary PW is made the active PW.
o Non-revertive protection switching: Traffic will be carried by the
last PW selected as a result of a previous active PW entering the
operationally down state.
o Manual selection of a PW: The ability to manually select the
primary/secondary PWs.
o MTU-s: A hierarchical virtual private LAN service multi-tenant
unit switch, as defined in RFC 4762 [RFC4762].
o PE-rs: A hierarchical virtual private LAN service switch, as
defined in RFC 4762.
o n-PE: A network-facing provider edge node, as defined in RFC 4026
[RFC4026].
o 1:1 protection: One specific subset of a path for an emulated
service, consisting of a standby PW and/or AC, protects another
specific subset of a path for the emulated service. User traffic
is transmitted over only one specific subset of the path at a
time.
o N:1 protection: N specific subsets of paths for an emulated
service, consisting of standby PWs and/or ACs, protect another
specific subset of the path for the emulated service. User
traffic is transmitted over only one specific subset of the path
at a time.
o 1+1 protection: One specific subset of a path for an emulated
service, consisting of a standby PW and/or AC, protects another
specific subset of a path for the emulated service. Traffic is
permanently duplicated at the ingress node on both the currently
active and standby subsets of the paths.
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This document uses the term 'PE' to be synonymous with both PEs as
per RFC 3985 [RFC3985] and T-PEs as per RFC 5659 [RFC5659].
This document uses the term 'PW' to be synonymous with both PWs as
per RFC 3985 and SS-PWs, MS-PWs, and PW segments as per RFC 5659.
2.1. Requirements Language
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 [RFC2119].
3. Reference Models
The following sections show the reference architecture of the PE for
PW redundancy and the usage of the architecture in different
topologies and applications.
3.1. PE Architecture
Figure 1 shows the PE architecture for PW redundancy when more than
one PW in a redundant set is associated with a single AC. This is
based on the architecture in Figure 4b of RFC 3985 [RFC3985]. The
forwarder selects which of the redundant PWs to use based on the
criteria described in this document.
+----------------------------------------+
| PE Device |
+----------------------------------------+
Single | | Single | PW Instance
AC | + PW Instance X<===========>
| | |
| |----------------------|
<------>o | Single | PW Instance
| Forwarder + PW Instance X<===========>
| | |
| |----------------------|
| | Single | PW Instance
| + PW Instance X<===========>
| | |
+----------------------------------------+
Figure 1: PE Architecture for PW Redundancy
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3.2. PW Redundancy Network Reference Scenarios
This section presents a set of reference scenarios for PW redundancy.
These reference scenarios represent example network topologies that
illustrate the use of PW redundancy. They can be combined together
to create more complex or comprehensive topologies, as required by a
particular application or deployment.
3.2.1. PW Redundancy for AC and PE Protection: One Dual-Homed CE with
Redundant SS-PWs
Figure 2 illustrates an application of single-segment pseudowire
redundancy where one of the CEs is dual-homed. This scenario is
designed to protect the emulated service against a failure of one of
the PEs or ACs attached to the multi-homed CE. Protection against
failures of the PSN tunnels is provided using PSN mechanisms such as
MPLS fast reroute, so that these failures do not impact the PW.
CE1 is dual-homed to PE1 and PE3. A dual-homing control protocol,
the details of which are outside the scope of this document, enables
the PEs and CEs to determine which PE (PE1 or PE3) should forward
towards CE1 and therefore which AC CE1 should use to forward towards
the PSN.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| | | +-----+
| |----------|....|...PW1.(active)...|....|----------| |
| | | |==================| | | CE2 |
| CE1 | +----+ |PE2 | | |
| | +----+ | | +-----+
| | | |==================| |
| |----------|....|...PW2.(standby)..| |
+-----+ | | PE3|==================| |
AC +----+ +----+
Figure 2: One Dual-Homed CE and Redundant SS-PWs
In this scenario, only one of the PWs should be used for forwarding
between PE1/PE3 and PE2. PW redundancy determines which PW to make
active based on the forwarding state of the ACs so that only one path
is available from CE1 to CE2. This requires an additional PW state
Muley, et al. Informational [Page 7]
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that reflects this forwarding state, which is separate from the
operational status of the PW. This is the 'Preferential Forwarding
Status'.
Consider the example where the AC from CE1 to PE1 is initially active
and the AC from CE1 to PE3 is initially standby. PW1 is made active
and PW2 is made standby in order to complete the path to CE2.
On failure of the AC between CE1 and PE1, the forwarding state of the
AC on PE3 transitions to active. The preferential forwarding state
of PW2 therefore needs to become active, and PW1 standby, in order to
re-establish connectivity between CE1 and CE2. PE3 therefore uses
PW2 to forward towards CE2, and PE2 uses PW2 instead of PW1 to
forward towards CE1. PW redundancy in this scenario requires that
the forwarding status of the ACs at PE1 and PE3 be signaled to PE2 so
that PE2 can choose which PW to make active.
Changes occurring on the dual-homed side of the network due to a
failure of the AC or PE are not propagated to the ACs on the other
side of the network. Furthermore, failures in the PSN are not
propagated to the attached CEs.
3.2.2. PW Redundancy for AC and PE Protection: Two Dual-Homed CEs with
Redundant SS-PWs
Figure 3 illustrates an application of single-segment pseudowire
redundancy where both of the CEs are dual-homed. This scenario is
also designed to protect the emulated service against failures of the
ACs and failures of the PEs. Both CE1 and CE2 are dual-homed to
their respective PEs, CE1 to PE1 and PE2, and CE2 to PE3 and PE4. A
dual-homing control protocol, the details of which are outside the
scope of this document, enables the PEs and CEs to determine which
PEs should forward towards the CEs and therefore which ACs the CEs
should use to forward towards the PSN.
Note that the PSN tunnels are not shown in this figure for clarity.
However, it can be assumed that each of the PWs shown is encapsulated
in a separate PSN tunnel. Protection against failures of the PSN
tunnels is provided using PSN mechanisms such as MPLS fast reroute,
so that these failures do not impact the PW.
Muley, et al. Informational [Page 8]
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|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudowire ------->| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | |....|.......PW1........|....| | +-----+
| |----------| PE1|...... .........| PE3|----------| |
| CE1 | +----+ \ / PW3 +----+ | CE2 |
| | +----+ X +----+ | |
| | | |....../ \..PW4....| | | |
| |----------| PE2| | PE4|--------- | |
+-----+ | |....|.....PW2..........|....| | +-----+
AC +----+ +----+ AC
Figure 3: Two Dual-Homed CEs and Redundant SS-PWs
PW1 and PW4 connect PE1 to PE3 and PE4, respectively. Similarly, PW2
and PW3 connect PE2 to PE4 and PE3. PW1, PW2, PW3, and PW4 are all
up. In order to support protection for the emulated service, only
one PW MUST be selected to forward traffic.
If a PW has a preferential forwarding status of 'active', it can be
used for forwarding traffic. The actual up PW chosen by the combined
set of PEs connected to the CEs is determined by considering the
preferential forwarding status of each PW at each PE. The mechanisms
for communicating the preferential forwarding status are outside the
scope of this document. Only one PW is used for forwarding.
The following failure scenario illustrates the operation of PW
redundancy in Figure 3. In the initial steady state, when there are
no failures of the ACs, one of the PWs is chosen as the active PW,
and all others are chosen as standby. The dual-homing protocol
between CE1 and PE1/PE2 chooses to use the AC to PE2, while the
protocol between CE2 and PE3/PE4 chooses to use the AC to PE4.
Therefore, the PW between PE2 and PE4 is chosen as the active PW to
complete the path between CE1 and CE2.
On failure of the AC between the dual-homed CE1 and PE2, the
preferential forwarding status of the PWs at PE1, PE2, PE3 and PE4
needs to change so as to re-establish a path from CE1 to CE2.
Different mechanisms can be used to achieve this and these are beyond
the scope of this document. After the change in status, the
algorithm needs to evaluate and select which PW to forward traffic
on. In this application, each dual-homing algorithm, i.e., {CE1,
PE1, PE2} and {CE2, PE3, PE4}, selects the active AC independently.
Muley, et al. Informational [Page 9]
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There is therefore a need to signal the active status of each AC such
that the PEs can select a common active PW for forwarding between CE1
and CE2.
Changes occurring on one side of network due to a failure of the AC
or PE are not propagated to the ACs on the other side of the network.
Furthermore, failures in the PSN are not propagated to the attached
CEs. Note that end-to-end native service protection switching can
also be used to protect the emulated service in this scenario. In
this case, PW3 and PW4 are not necessary.
If the CEs do not perform native service protection switching, they
may instead use load balancing across the paths between the CEs.
3.2.3. PW Redundancy for S-PE Protection: Single-Homed CEs with
Redundant MS-PWs
Figure 4 shows a scenario where both CEs are single-homed, and MS-PW
redundancy is used. The main objective is to protect the emulated
service against failures of the S-PEs.
Native |<----------- Pseudowires ----------->| Native
Service | | Service
(AC) | |<-PSN1-->| |<-PSN2-->| | (AC)
| V V V V V V |
| +-----+ +-----+ +-----+ |
+----+ | |T-PE1|=========|S-PE1|=========|T-PE2| | +----+
| |-------|......PW1-Seg1.......|.PW1-Seg2......|-------| |
| CE1| | |=========| |=========| | | CE2|
| | +-----+ +-----+ +-----+ | |
+----+ |.||.| |.||.| +----+
|.||.| +-----+ |.||.|
|.||.|=========| |========== .||.|
|.||...PW2-Seg1......|.PW2-Seg2...||.|
|.| ===========|S-PE2|============ |.|
|.| +-----+ |.|
|.|============+-----+============= .|
|.....PW3-Seg1.| | PW3-Seg2......|
==============|S-PE3|===============
| |
+-----+
Figure 4: Single-Homed CE with Redundant MS-PWs
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CE1 is connected to T-PE1, and CE2 is connected to T-PE2. There are
three multi-segment PWs. PW1 is switched at S-PE1, PW2 is switched
at S-PE2, and PW3 is switched at S-PE3. This scenario provides N:1
protection for the subset of the path of the emulated service from
T-PE1 to T-PE2.
Since there is no multi-homing running on the ACs, the T-PE nodes
advertise 'active' for the preferential forwarding status based on a
priority for the PW. The priority associates a meaning of 'primary
PW' and 'secondary PW' to a PW. These priorities MUST be used if
revertive mode is used and the active PW to use for forwarding is
determined accordingly. The priority can be derived via
configuration or based on the value of the PW forwarding equivalence
class (FEC). For example, a lower value of PWid FEC can be taken as
a higher priority. However, this does not guarantee selection of
same PW by the T-PEs because of, for example, a mismatch in the
configuration of the PW priority at each T-PE. The intent of this
application is for T-PE1 and T-PE2 to synchronize the transmit and
receive paths of the PW over the network. In other words, both T-PE
nodes are required to transmit over the PW segment that is switched
by the same S-PE. This is desirable for ease of operation and
troubleshooting.
3.2.4. PW Redundancy for PE-rs Protection in H-VPLS Using SS-PWs
The following figure (based on the architecture shown in Figure 3 of
[RFC4762]) illustrates the application of PW redundancy to
hierarchical VPLS (H-VPLS). Note that the PSN tunnels are not shown
for clarity, and only one PW of a PW group is shown. A multi-tenant
unit switch (MTU-s) is dual-homed to two PE router switches. The
example here uses SS-PWs, and the objective is to protect the
emulated service against failures of a PE-rs.
Muley, et al. Informational [Page 11]
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PE1-rs
+--------+
| VSI |
Active PW | -- |
Group..........|../ \..|.
CE-1 . | \ / | .
\ . | -- | .
\ . +--------+ .
\ MTU-s . . . PE3-rs
+--------+ . . . +--------+
| VSI | . . H-VPlS .| VSI |
| -- ..|.. . Core |.. -- |
| / \ | . PWs | / \ |
| \ /..|.. . | \ / |
| -- | . . .|.. -- |
+--------+ . . . +--------+
/ . . .
/ . +--------+ .
/ . | VSI | .
CE-2 . | -- | .
..........|../ \..|.
Standby PW | \ / |
Group | -- |
+--------+
PE2-rs
Figure 5: MTU-s Dual-Homing in H-VPLS Core
In Figure 5, the MTU-s is dual-homed to PE1-rs and PE2-rs and has
spoke PWs to each of them. The MTU-s needs to choose only one of the
spoke PWs (the active PW) to forward traffic to one of the PEs and
sets the other PW to standby. The MTU-s can derive the status of the
PWs based on local policy configuration. PE1-rs and PE2-rs are
connected to the H-VPLS core on the other side of network. The MTU-s
communicates the status of its member PWs for a set of virtual
switching instances (VSIs) that share a common status of active or
standby. Here, the MTU-s controls the selection of PWs used to
forward traffic. Signaling using PW grouping with a common group-id
in the PWid FEC Element, or a Grouping TLV in Generalized PWid FEC
Element as defined in [RFC4447], to PE1-rs and PE2-rs, is recommended
for improved scaling.
Whenever an MTU-s performs a switchover of the active PW group, it
needs to communicate this status change to the PE2-rs. That is, it
informs PE2-rs that the status of the standby PW group has changed to
active.
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In this scenario, PE devices are aware of switchovers at the MTU-s
and could generate Media Access Control (MAC) Address Withdraw
messages to trigger MAC flushing within the H-VPLS full mesh. By
default, MTU-s devices should still trigger MAC Address Withdraw
messages as defined in [RFC4762] to prevent two copies of MAC Address
Withdraw messages to be sent (one by the MTU-s and another one by the
PE-rs). Mechanisms to disable the MAC withdraw trigger in certain
devices are out of the scope of this document.
3.2.5. PW Redundancy for PE Protection in a VPLS Ring Using SS-PWs
The following figure illustrates the use of PW redundancy for dual-
homed connectivity between PEs in a VPLS ring topology. As above,
PSN tunnels are not shown, and only one PW of a PW group is shown for
clarity. The example here uses SS-PWs, and the objective is to
protect the emulated service against failures of a PE on the ring.
PE1 PE2
+--------+ +--------+
| VSI | | VSI |
| -- | | -- |
......|../ \..|.....................|../ \..|.......
| \ / | PW Group 1 | \ / |
| -- | | -- |
+--------+ +--------+
. .
. .
VPLS Domain A . . VPLS Domain B
. .
. .
. .
+--------+ +--------+
| VSI | | VSI |
| -- | | -- |
......|../ \..|.....................|../ \..|........
| \ / | PW Group 2 | \ / |
| -- | | -- |
+--------+ +--------+
PE3 PE4
Figure 6: Redundancy in a VPLS Ring Topology
In Figure 6, PE1 and PE3 from VPLS domain A are connected to PE2 and
PE4 in VPLS domain B via PW group 1 and PW group 2. The PEs are
connected to each other in such a way as to form a ring topology.
Such scenarios may arise in inter-domain H-VPLS deployments where the
Rapid Spanning Tree Protocol (RSTP) or other mechanisms may be used
to maintain loop-free connectivity of the PW groups.
Muley, et al. Informational [Page 13]
RFC 6718 PW Redundancy August 2012
[RFC4762] outlines multi-domain VPLS services without specifying how
multiple redundant border PEs per domain and per VPLS instance can be
supported. In the example above, PW group 1 may be blocked at PE1 by
RSTP, and it is desirable to block the group at PE2 by exchanging the
PW preferential forwarding status of standby. The details of how PW
grouping is achieved and used is deployment specific and is outside
the scope of this document.
3.2.6. PW Redundancy for VPLS n-PE Protection Using SS-PWs
|<----- Provider ----->|
Core
+------+ +------+
| n-PE |::::::::::::::::::::::| n-PE |
Provider | (P) |.......... .........| (P) | Provider
Access +------+ . . +------+ Access
Network X Network
(1) +------+ . . +------+ (2)
| n-PE |.......... .........| n-PE |
| (B) |......................| (B) |
+------+ +------+
Figure 7: Bridge Module Model
Figure 7 shows a scenario with two provider access networks. The
example here uses SS-PWs, and the objective is to protect the
emulated service against failures of a network-facing PE (n-PE).
Each network has two n-Pes. These 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 serves as the primary PE (P) for that
VPLS instance, and the other n-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, or 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 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 three or more n-PEs
per provider access network. For example, if there are three n-PEs
or four n-PEs per access network, then 67% or 75% of core bandwidth
for multicast, broadcast, and unknown unicast are wasted,
respectively.
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RFC 6718 PW Redundancy August 2012
In this scenario, the n-PEs can communicate the active or standby
status of the PWs among them. This status can be derived from the
active or backup state of an n-PE for a given VPLS.
4. Generic PW Redundancy Requirements
4.1. Protection Switching Requirements
o Protection architectures such as N:1,1:1 or 1+1 are possible. 1:1
protection MUST be supported. The N:1 protection case is less
efficient in terms of the resources that must be allocated; hence,
this SHOULD be supported. 1+1 protection MAY be used in the
scenarios described in the document. However, the details of its
usage are outside the scope of this document, as it MAY require a
1+1 protection switching protocol between the CEs.
o Non-revertive behavior MUST be supported, while revertive behavior
is OPTIONAL. This avoids the need to designate one PW as primary
unless revertive behavior is explicitly required.
o Protection switchover can be initiated from a PE, e.g., using a
manual switchover or a forced switchover, or it may be triggered
by a signal failure, i.e., a defect in the PW or PSN. Manual
switchover may be necessary if it is required to disable one PW in
a redundant set. Both methods MUST be supported, and signal
failure triggers MUST be treated with a lower priority than any
local or far-end forced switch or manual trigger.
o A PE MAY be able to forward packets received from a PW with a
standby status in order to avoid black holing of in-flight packets
during switchover. However, in cases where VPLS is used, all VPLS
application packets received from standby PWs MUST be dropped,
except for OAM and control-plane packets.
4.2. Operational Requirements
o (T-)PEs involved in protecting a PW SHOULD automatically discover
and attempt to resolve inconsistencies in the configuration of
primary/secondary PWs.
o (T-)PEs involved in protecting a PW SHOULD automatically discover
and attempt to resolve inconsistencies in the configuration of
revertive/non-revertive protection switching mode.
o (T-)PEs that do not automatically discover or resolve
inconsistencies in the configuration of primary/secondary,
revertive/non-revertive, or other parameters MUST generate an
alarm upon detection of an inconsistent configuration.
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RFC 6718 PW Redundancy August 2012
o (T-)PEs participating in PW redundancy MUST support the
configuration of revertive or non-revertive protection switching
modes if both modes are supported.
o The MIB(s) MUST support inter-PSN monitoring of the PW redundancy
configuration, including the protection switching mode.
o (T-)PEs participating in PW redundancy SHOULD support the local
invocation of protection switching.
o (T-)PEs participating in PW redundancy SHOULD support the local
invocation of a lockout of protection switching.
5. Security Considerations
The PW redundancy method described in this RFC will require an
extension to the PW setup and maintenance protocol [RFC4447], which
in turn is carried over the Label Distribution Protocol (LDP)
[RFC5036]. This PW redundancy method will therefore inherit the
security mechanisms of the version of LDP implemented in the PEs.
6. Contributors
The editors would like to thank Pranjal Kumar Dutta, Marc Lasserre,
Jonathan Newton, Hamid Ould-Brahim, Olen Stokes, Dave Mcdysan, Giles
Heron, and Thomas Nadeau, all of whom made a major contribution to
the development of this document.
Pranjal Dutta
Alcatel-Lucent
EMail: pranjal.dutta@alcatel-lucent.com
Marc Lasserre
Alcatel-Lucent
EMail: marc.lasserre@alcatel-lucent.com
Jonathan Newton
Cable & Wireless
EMail: Jonathan.Newton@cw.com
Hamid Ould-Brahim
EMail: ouldh@yahoo.com
Olen Stokes
Extreme Networks
EMail: ostokes@extremenetworks.com
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RFC 6718 PW Redundancy August 2012
Dave McDysan
Verizon
EMail: dave.mcdysan@verizon.com
Giles Heron
Cisco Systems
EMail: giles.heron@gmail.com
Thomas Nadeau
Juniper Networks
EMail: tnadeau@lucidvision.com
7. Acknowledgements
The authors would like to thank Vach Kompella, Kendall Harvey,
Tiberiu Grigoriu, Neil Hart, Kajal Saha, Florin Balus, and Philippe
Niger for their valuable comments and suggestions.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4026] Andersson, L. and T. Madsen, "Provider Provisioned Virtual
Private Network (VPN) Terminology", RFC 4026, March 2005.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi-
Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
October 2009.
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RFC 6718 PW Redundancy August 2012
8.2. Informative Reference
[RFC5601] Nadeau, T. and D. Zelig, "Pseudowire (PW) Management
Information Base (MIB)", RFC 5601, July 2009.
Authors' Addresses
Praveen Muley
Alcatel-Lucent
EMail: praveen.muley@alcatel-lucent.com
Mustapha Aissaoui
Alcatel-Lucent
EMail: mustapha.aissaoui@alcatel-lucent.com
Matthew Bocci
Alcatel-Lucent
EMail: matthew.bocci@alcatel-lucent.com
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