Network Working Group Praveen Muley, Ed.
Internet Draft Mustapha Aissaoui, Ed.
Intended Status: Standards Track Alcatel-Lucent
Expires: August 27, 2012
February 27, 2012
Pseudowire Preferential Forwarding Status Bit
draft-ietf-pwe3-redundancy-bit-06.txt
Abstract
This document describes a mechanism for standby status signaling of
redundant pseudowires (PWs) between their termination points. A set
of redundant PWs is configured between provider edge (PE) nodes in
single-segment pseudowire (SS-PW) applications, or between
terminating provider edge (T-PE) nodes in multi-segment pseudowire
(MS-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 status bit is needed
to indicate a preferential forwarding status of Active or Standby for
each PW in a redundant set.
In addition, a second status bit is defined to allow peer PE nodes to
coordinate a switchover operation of the PW.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 27, 2012.
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Copyright Notice
Copyright (c) 2011 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 [1].
Table of Contents
1. Introduction...................................................3
2. Motivation and Scope...........................................5
3. Terminology....................................................7
4. PE Architecture................................................8
5. Modes of Operation.............................................9
5.1. Independent Mode:.........................................9
5.2. Master/Slave Mode:.......................................11
6. PW State Transition Signaling Procedures......................13
6.1. PW Standby Notification Procedures in Independent mode...13
6.2. PW Standby notification procedures in Master/Slave mode..14
6.2.1. PW State Machine....................................15
6.3. Coordination of PW Switchover............................16
6.3.1. Procedures at the requesting endpoint...............18
6.3.2. Procedures at the receiving endpoint................19
7. Status Mapping................................................20
7.1. AC Defect State Entry/Exit...............................20
7.2. PW Defect State Entry/Exit...............................20
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8. Applicability and Backward Compatibility......................21
9. Security Considerations.......................................21
10. MIB Considerations...........................................21
11. IANA Considerations..........................................22
11.1. Status Code for PW Preferential Forwarding Status.......22
11.2. Status Code for PW Request Switchover Status............23
12. Major Contributing Authors...................................23
13. Acknowledgments..............................................24
14. References...................................................24
14.1. Normative References....................................24
14.2. Informative References..................................24
15. Appendix A - Applications of PW Redundancy Procedures........25
15.1. One Multi-homed CE with single SS-PW redundancy.........25
15.2. Multiple Multi-homed CEs with single SS-PW redundancy...27
15.3. Multi-homed CE with MS-PW redundancy....................29
15.4. Multi-homed CE with MS-PW redundancy and S-PE protection30
15.5. Single Homed CE with MS-PW redundancy...................32
15.6. PW redundancy between H-VPLS MTU-s and PE-rs............33
Author's Addresses...............................................35
1. Introduction
In Virtual Private Wire Services (VPWS) or Virtual Private Local Area
network Services (VPLS) that use SS-PWs, protection for the PW is
provided by the packet switched network (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. There are, however, applications where PSN
protection is insufficient to fully protect the PW-based service.
These include the following:
In a VPWS service where the Customer Edge (CE) node is dual homed to
a pair of PE nodes, PW redundancy mechanisms are required to ensure
that the correct PW is used for forwarding when attachment circuit
(AC) redundancy is used. PW redundancy mechanisms are also required
when multiple redundant MS-PWs are used between T-PEs, to ensure that
both T-PEs use the same MS-PW to forward to one another.
In a hierarchical VPLS (H-VPLS) service, PW redundancy mechanisms are
required to enable a multi-tenant unit switch (MTU-s) to be dual-
homed to two PE-rs devices.
In these cases, pseudowire redundancy mechanisms are required. These
scenarios are described in the PW redundancy and framework document
[5].
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Scenarios, such as those above, therefore rely on a set of two or
more pseudowires to protect a given VPWS or VPLS . Only one of these
pseudowires is used by the PEs to forward user traffic on at any
given time. This is the active PW. The other PWs in the set are
considered standby and are not used for forwarding unless they become
active. This provides a 1:1 or N:1 PW protection with the possibility
of multi-homing between the CE and the PEs.
In order to support AC or spoke PW redundancy, at least one of the
PEs on which a PW terminates must be different from that on which the
primary PW terminates, as described in [5]. Figure 1-1 illustrates an
application of active and standby PWs.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudowire ------->| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| | | +-----+
| |----------|....|...PW1.(active)...|....|----------| |
| | | |==================| | | CE2 |
| CE1 | +----+ |PE2 | | |
| | +----+ | | +-----+
| | | |==================| |
| |----------|....|...PW2.(standby)..| |
+-----+ | | PE3|==================| |
AC +----+ +----+
Figure 1-1: Reference Model for PW Redundancy
In MS-PW applications, PW redundancy is also required to protect the
service against failures of the switching PEs, which cannot be
protected by PSN mechanisms. In addition, PW redundancy is also
required if CEs are dual-homed to the PEs, as described above. In
this case, multiple MS-PWs are configured between a pair of T-PE
nodes, as described in Figure 2 of [5]. The paths of these MS-PWs are
diverse in that they are switched at different S-PE nodes. Only one
of these MS-PWs is active at any given time, while the others are
standby.
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This document specifies a new PW status bit to indicate the
preferential forwarding status of the PW for the purpose of notifying
the remote PE of the preferential forwarding state of each PW in the
redundancy set i.e. Active or Standby. This status bit is different
from the PW status bits already defined in the PWE3 control protocol
[2]. In addition, a second status bit is defined to allow peer PE
nodes to coordinate a switchover operation of the PW from Active to
standby, or vice versa.
2. Motivation and Scope
The PWE3 control protocol [2] defines the following status codes in
the PW status TLV to indicate the state for an AC and a PW:
0x00000000 - Pseudowire forwarding (clear all failures)
0x00000001 - Pseudowire Not Forwarding
0x00000002 - Local Attachment Circuit (ingress) Receive Fault
0x00000004 - Local Attachment Circuit (egress) Transmit Fault
0x00000008 - Local PSN-facing PW (ingress) Receive Fault
0x00000010 - Local PSN-facing PW (egress) Transmit Fault
The scenarios defined in [5] allow the provisioning of a primary PW
and one or many secondary PWs in the same VPWS or VPLS service.
A PE node makes a selection of which PW to activate at any given time
for the purpose of forwarding user packets. This selection takes into
account the local state of the PW and AC, as well as the remote state
of the PW and AC as indicated in the PW status bits it received from
the peer PE node.
In the absence of faults, all PWs are UP both locally and remotely
and a PE node needs to select a single PW to forward user packets to.
This is referred to as the active PW. All other PWs will be in
standby and must not be used to forward user packets.
In order for both ends of the service to select the same PW for
forwarding user packets, this document defines a new status bit, the
'Preferential Forwarding' status bit, to allow a PE node to indicate
the preferential forwarding state of a PW to its peer PE node.
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In addition, a second status bit is defined to allow peer PE nodes to
coordinate a switchover operation of the PW if required by the
application. This is known as the 'request switchover' status bit.
Together, the mechanisms described in this document achieve the
following PW protection capabilities:
a. A MANDATORY 1:1 PW protection with a single active PW and one
standby PW. An active PW can forward data traffic and control
plane traffic, such as Operations, Administration, and
Maintenance (OAM) packets. A standby PW does not carry data
traffic.
b. An OPTIONAL N:1 PW protection scheme with a single active PW
and N standby PWs.
c. An OPTIONAL mechanism to allow PW endpoints to coordinate the
switchover to a given PW by using an explicit
request/acknowledgment switchover procedure. This mechanism is
complementary to the Independent mode of operation and is
described in Section 6.3. . This mechanism can be invoked
manually by the user, effectively providing a manual
switchover capability. It can also be invoked automatically to
resolve a situation where the PW endpoints could not match the
two directions of the PW.
d. An OPTIONAL, locally configured precedence to govern the
selection of a PW when more than one PW qualify for the active
state, as defined in sections 5.1. and 5.2. The PW with the
lowest precedence value has the highest priority. Precedence
may be configured via, for example, a local configuration
parameter at the PW endpoint.
e. OPTIONALLY, implementations can designate by configuration one
PW in the 1:1 or N:1 set as a primary PW and the remaining as
secondary PWs. If more than one PW qualify for the active
state, as defined in sections 5.1. and 5.2. , a PE node
selects the primary PW in preference to a secondary PW. In
other words, the primary PW has implicitly the lowest
precedence value. Furthermore, a PE node reverts to the
primary PW immediately after it comes back up or after the
expiration of a delay. The PE node can use the PW precedence
to select a secondary PW among many that qualify for active
state.
These protection schemes are provided using the following operational
modes:
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1. An independent mode of operation in which each PW endpoint
node uses its own local rule to select which PW it intends
to activate at any given time and advertises it to the
remote endpoints. Only a PW which is UP and which indicated
Active status bit locally and remotely is in the active
state and can be used to forward data packets. This is
described in Section 5.1.
2. A Master/Slave mode in which one PW endpoint, the Master
endpoint, selects and dictates to the other endpoint(s),
the Slave endpoint(s), which PW to activate. This is
described in Section 5.2.
The above mechanisms and operational modes allow the following:
a.Multi-homing of a CE device to two or more PE nodes.
b.Multi-homing of a PE node to two or more PE nodes.
More details of how these schemes are used can be found in
Informative Appendix A.
Note that this document specifies the mechanisms to support PW
redundancy where a set of redundant PWs terminate on either a PE (for
SS-PW) or a T-PE (for MS-PW). PW redundancy scenarios where the
redundant set of PW segments terminate on an S-PE are for further
study.
3. Terminology
UP PW: A PW which has been configured (label mapping exchanged
between PEs) and is not in any of the PW or AC defect
states specified in [2]. Such a PW is available for
forwarding traffic.
DOWN PW: A PW that has either not been fully configured, or has been
configured and is in any of the PW or AC defect states
specified in [2], such a PW is not available for forwarding
traffic.
Active PW: An UP PW used for forwarding user, OAM and control plane
traffic.
Standby PW: An UP PW that is not used for forwarding user traffic,
but may forward OAM and specific control plane traffic.
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Primary PW: The PW which a PW endpoint activates in preference to any
other PW when more than one PW qualifies for active state.
When the primary PW comes back up after a failure and
qualifies for active state, the PW endpoint always reverts
to it. The designation of Primary is performed by local
configuration for the PW at the PE.
Secondary PW: When it qualifies for 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.
PW Precedence: This is a configuration local to the PE that dictates
the order in which a forwarder chooses to use a PW when
multiple PWs all qualify for the active state. Note that a
PW which has been configured as Primary has implicitly the
lowest precedence value.
PW Endpoint: A PE where a PW terminates on a point where Native
Service Processing is performed, e.g., A SS-PW PE, an MS-PW
T-PE, or an H-VPLS MTU-s or PE-rs.
OAM: Operations, Administration, and Maintenance.
VCCV: Virtual Connection Connectivity Verification.
This document uses the term 'PE' to be synonymous with both PEs as
per RFC3985 and T-PEs as per RFC5659.
This document uses the term 'PW' to be synonymous with both PWs as
per RFC3985 and SS-PWs and MS-PWs as per RFC5659.
4. PE Architecture
Figure 4-1 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 RFC3985 [6]. The
forwarder selects which of the redundant PWs to using the criteria
described in this document.
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+----------------------------------------+
| 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 4-1 PE Architecure for PW redundancy
5. Modes of Operation
There are two modes of operation for the use of the PW Preferential
Forwarding status bits:
o Independent mode
o Master/Slave mode.
5.1. Independent Mode:
PW endpoint nodes independently select which PW they intend to make
active and which PWs they intend to make standby. They advertise the
corresponding Active/Standby preferential forwarding status for each
PW. Each PW endpoint compares local and remote status bits and uses
the PW that is UP at both endpoints and that advertised Active
preferential forwarding status at both the local and remote
endpoints.
In this mode of operation, the preferential forwarding status
indicates the preferred forwarding state of each endpoint but the
actual forwarding state of the PW is the result of the comparison of
the local and remote forwarding status bits.
If more than one PW qualifies for the Active state, each PW endpoint
MUST implement a common mechanism to choose the PW for forwarding.
The default mechanism MUST be supported by all implementations and
operates as follows:
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1. For FEC128 PW, the PW with the lowest pw-id value is selected.
2. For FEC129 PW, each PW in a redundant set is uniquely identified
at each PE using the following triplet: AGI::SAII::TAII. The
unsigned integer form of the concatenated word can be used in the
comparison. However, the SAII and TAII values as seen on a PE
node are the mirror values of what the peer PE node sees. To have
both PE nodes compare the same value we propose that the PE with
the lowest system IP address use the unsigned integer form of
AGI::SAII::TAII while the PE with the highest system IP address
use the unsigned integer form of AGI::TAII::SAII. This way, both
PEs will compare the same values. The PW which corresponds to the
minimum of the compared values across all PWs in the redundant is
selected.
Note 1: in the case where the system IP address is not known, it
is recommended to implement the optional tie-breaking mechanism
described next.
Note 2: in the case of segmented PW, the operator needs to make
sure that the pw-id or AGI::SAII::TAII of the redundant PWs within
the first and last segment are ordered consistently such that the
same end-to-end MS-PW gets selected. Otherwise, it is recommended
to implement the optional tie-breaking mechanism described next.
The PW endpoints MAY also implement the following optional tie-
breaking mechanism.
1. If the PW endpoint is configured with the precedence parameter on
each PW in the redundant set, it must select the PW with the
lowest configured precedence value.
2. If the PW endpoint is configured with one PW as primary and one
or more PWs as secondary, it must select the primary PW in
preference to all secondary PWs. If a primary PW is not
available, it must use the secondary PW with the lowest
precedence value. If the primary PW becomes available, a PW
endpoint must revert to it immediately or after the expiration of
a configurable delay.
In steady state with consistent configuration, a PE will always find
an active PW. However, it is possible that such a PW is not found due
to a mis-configuration. In the event that an active PW is not found,
a management indication SHOULD be generated. If a management
indication for failure to find an active PW was generated and an
active PW is subsequently found, a management indication should be
generated, so clearing the previous failure indication. Additionally,
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a PE may use the optional request switchover procedures described in
Section 6.3. to have both PE nodes switch to a common PW.
There may also be transient conditions where endpoints do not share a
common view of the Active/Standby state of the PWs. This could be
caused by propagation delay of the T-LDP status messages between
endpoints. In this case, the behavior of the receiving endpoint is
outside the scope of this document.
Thus, in this mode of operation, the following definition of Active
and Standby PW states apply:
o Active State
A PW is considered to be in Active state when the PW labels are
exchanged between its two endpoints and the status bits exchanged
between the endpoints indicate the PW is UP and its preferential
forwarding status is Active at both endpoints. In this state user
traffic can flow over the PW in both directions. As described in
Section 5.1. , the PE nodes must implement a common mechanism to
select one PW for forwarding in case multiple PWs qualify for the
Active state.
o Standby State
A PW is considered to be in Standby state when the PW labels are
exchanged between its two endpoints, but the Preferential Forwarding
status bits exchanged indicate the PW preferential forwarding status
is Standby at one or both endpoints. In this state the endpoints MUST
NOT forward data traffic over the PW but MAY allow PW OAM packets,
e.g., Virtual Connection Connectivity Verification (VCCV) packets
[10], to be sent and received in order to test the liveliness of
standby PWs. The endpoints of the PW may also allow the forwarding of
specific control plane packets of applications using the PW. The
specification of applications and the allowed control plane packets
is outside the scope of this document. If the PW is a spoke in H-
VPLS, any MAC addresses learned via the PW SHOULD be flushed when it
transitions to Standby state according to the procedures in RFC4762
[3] and [9].
5.2. Master/Slave Mode:
One endpoint node of the redundant set of PWs is designated the
Master and is responsible for selecting which PW both endpoints must
use to forward user traffic.
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The Master indicates the forwarding state in the PW Preferential
Forwarding status bit. The other endpoint node, the Slave, MUST
follow the decision of the Master node based on the received status
bits. In other words, the Preferential Forwarding status bit sent by
the Master node indicates the actual forwarding state of the PW at
the Master node.
There is a single PE Master PW endpoint node and one or many PE PW
endpoint Slave nodes. The assignment of Master/Slave roles to the PW
endpoints is performed by local configuration. Note that the behavior
described in this section assumes correct configuration of the Master
and Slave endpoints. This document does not define a mechanism to
detect errors in the configuration.
One endpoint of the PW, the Master, actively selects which PW to
activate and uses it for forwarding user traffic. This status is
indicated to the Slave node by setting the Preferential Forwarding
status bit in the status bit TLV to Active. It does not forward user
traffic to any other of the PW's in the redundancy set to the slave
node and indicates this by setting the Preferential Forwarding status
bit in the status bit TLV to Standby for those PWs. The master node
MUST ignore any PW Preferential Forwarding status bits received from
the Slave nodes.
If more than one PW qualify for the Active state, and the PW endpoint
is configured with one PW as primary, the Master endpoint must use
the primary PW in preference to all secondary PWs. If a primary PW is
not available, it must use the secondary PW with the lowest
precedence value. If the primary PW becomes available, a PW endpoint
must revert to it immediately or after the expiration of a
configurable delay. These primary/secondary procedures are optional.
The Slave endpoint(s) are required to act on the status bits received
from the Master. When the received status bit transitions from Active
to Standby, a Slave node MUST stop forwarding over the previously
active PW. When the received status bit transitions from Standby to
Active for a given PW, the Slave node MUST start forwarding user
traffic over this PW.
In this mode of operation, the following definition of Active and
Standby PW states apply:
o Active State
A PW is considered to be in Active state when the PW labels are
exchanged between its two endpoints, and the status bits exchanged
between the endpoints indicate the PW is UP at both endpoints, and
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the preferential forwarding status at the Master endpoint is Active.
In this state user traffic can flow over the PW in both directions.
o Standby State
A PW is considered to be in Standby state when the PW labels are
exchanged between its two endpoints, and the status bits exchanged
between the endpoints indicate the preferential forwarding status at
the Master endpoint is Standby. In this state the endpoints MUST NOT
forward data traffic over the PW but MAY allow PW OAM packets, e.g.,
VCCV, to be sent and received. The endpoints of the PW may also allow
the forwarding of specific control plane packets of applications
using the PW. The specification of applications and the allowed
control plane packets is outside the scope of this document. If the
PW is a spoke in H-VPLS, any MAC addresses learned via the PW SHOULD
be flushed when it transitions to standby state according to the
procedures in RFC4762 [3] and [9].
6. PW State Transition Signaling Procedures
This section describes the extensions to PW status signaling and the
processing rules for these extensions. It defines a new "PW
Preferential Forwarding" bit Status Code that is to be used with the
PW Status TLV specified in RFC 4447 [2].
The PW Preferential Forwarding bit, when set, is used to signal
either the Preferred or Actual Active/Standby forwarding state of the
PW by one PE to the far end PE. The actual semantics of the value
being signaled vary according to whether the PW is acting in a
Master/Slave or Independent mode.
6.1. PW Standby Notification Procedures in Independent mode
PEs that contain PW endpoints independently select which PW they
intend to use for forwarding, depending on the specific application
(example applications are described in [5]). They advertise the
corresponding preferred Active/Standby forwarding state for each PW.
An Active Preferential Forwarding state is indicated by clearing the
PW Preferential Forwarding status bit in the PW status TLV. A Standby
Preferential Forwarding State is indicated by setting the PW
Preferential Forwarding status bit in the PW status TLV. This
advertisement occurs in both the initial label mapping message and in
a subsequent notification message when the forwarding state
transitions as a result of a state change in the specific
application.
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Each PW endpoint compares the updated local and remote status and
effectively activates the PW which is UP at both endpoints and which
shows both local Active and remote Active Preferential Forwarding
states. The PE nodes must implement a common mechanism to select one
PW for forwarding in case multiple PWs qualify for the Active state.
When a PW is in Active state, the PEs can forward user packets, OAM
packets, and other control plane packets over the PW.
When a PW is in Standby state, the PEs MUST NOT forward user packets
over the PW but MAY forward PW OAM packets and specific control plane
packets.
For MS-PWs, S-PEs MUST relay the PW status notification containing
both the existing status bits and the new Preferential Forwarding
status bits between ingress and egress PWs as per the procedures
defined in [4].
6.2. PW Standby notification procedures in Master/Slave mode
Whenever the Master PW endpoint selects or deselects a PW for
forwarding user traffic at its end, it explicitly notifies the event
to the remote Slave endpoint. The slave endpoint carries out the
corresponding action on receiving the PW state change notification.
If the PW Preferential Forwarding bit in PW Status TLV received by
the slave is set, it indicates that the PW at the Master end is not
used for forwarding and is thus kept in the Standby state, the PW
MUST also not be used for forwarding at Slave endpoint. Clearing the
PW Preferential Forwarding bit in PW Status TLV indicates that the PW
at the Master endpoint is used for forwarding and is in Active state,
and the receiving Slave endpoint MUST activate the PW if it was
previously not used for forwarding.
When this mechanism is used, a common Group ID in the PWid FEC
element or a PW Grouping TLV in the Generalized PWid FEC Element as
defined in [2] MAY be used to signal PWs in groups in order to
minimize the number of LDP status messages that must be sent. When
PWs are provisioned with such grouping a termination point sends a
single "wildcard" Notification message to denote this change in
status for all affected PWs. This status message contains either the
PW FEC TLV with only the Group ID, or else it contains the PW
Generalized FEC TLV with only the PW Grouping ID TLV. As mentioned in
[2], the Group ID field of the PWid FEC Element, or the PW Grouping
TLV in the Generalized PWid FEC Element, can be used to send status
notification for an arbitrary set of PWs.
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For MS-PWs, S-PEs MUST relay the PW status notification containing
both the existing and the new Preferential Forwarding status bits
between ingress and egress PW segments as per the procedures defined
in [4].
6.2.1. PW State Machine
It is convenient to describe the PW state change behavior in terms of
a state machine (Table 1). The PW state machine is explained in
detail in the two defined states and the behavior is presented as a
state transition table. The same state machine is applicable to PW
Groups.
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STATE EVENT NEW STATE
ACTIVE PW put in Standby (master) STANDBY
Action: Transmit PW preferential
forwarding bit set
Receive PW Preferential Forwarding STANDBY
bit set (slave)
Action: Stop forwarding over PW
Receive PW Preferential Forwarding ACTIVE
bit set but bit not supported
Action: None
Receive PW Preferential Forwarding ACTIVE
bit clear
Action: None.
STANDBY PW activated (master) ACTIVE
Action: Transmit PW preferential
forwarding bit clear
Receive PW Preferential Forwarding ACTIVE
bit clear (slave)
Action: Activate PW
Receive PW Preferential Forwarding STANDBY
bit clear but bit not supported
Action: None
Receive PW Preferential Forwarding STANDBY
bit set
Action: No action
Table 1 PW State Transition Table in Master/Slave Mode
6.3. Coordination of PW Switchover
There are PW redundancy applications which require that PE nodes
coordinate the switchover to a PW such that both endpoints will
forward over the same PW at any given time. One such application for
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redundant MS-PW is identified in [5]. Multiple MS-PWs are configured
between a pair of T-PE nodes. The paths of these MS-PWs are diverse
and are switched at different S-PE nodes. Only one of these MS-PWs is
active at any given time. The others are put in standby. The
endpoints follow the Independent Mode procedures to use the PW which
is both UP and for which both endpoints advertise an Active
'Preferential Forwarding' status bit.
The trigger for sending a request to switchover of the MS-PW by one
endpoint can be an operational event, for example a failure, which
causes the endpoints to be unable to find a common PW for which both
endpoints advertise an Active 'Preferential Forwarding' status bit.
The other trigger is the execution of an administrative maintenance
operation by the network operator in order to move the traffic away
from the nodes or links currently used by the active PW.
Unlike the case of a Master/Slave mode of operation, the endpoint
requesting the switchover requires explicit acknowledgement from the
peer endpoint that the request can be honored before it switches to
another PW. Furthermore, any of the endpoints can make the request to
switchover.
This document specifies a second status bit that is used by a PE to
request that its peer PE switchover to use a different active PW.
This bit is referred to as the 'request PW switchover' status bit.
The 'Preferential Forwarding' status bit continues to be used by each
endpoint to indicate its current local settings of the Active/Standby
state of each PW in the redundancy set. In other words, as in the
Independent mode, it indicates to the far-end which of the PWs is
being used to forward packets and which is being put in standby. It
can thus be used as a way for the far-end to acknowledge the
requested switchover operation.
The request switchover bit is OPTIONAL and, if received by a PE, is
ignored if not understood.
If the request switchover bit is supported by both sending and
receiving PEs, the following procedures MUST be followed by both
endpoints of a PW to coordinate the switchover of the PW.
S-PEs nodes MUST relay the PW status notification containing the
existing status bits, as well as the new 'Preferential Forwarding'
and 'request switchover' status bits between ingress and egress PW
segments as per the procedures defined in [4].
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6.3.1. Procedures at the requesting endpoint
a. The requesting endpoint sends a Status TLV in the LDP
notification message with the 'request switchover' bit set on the
PW it desires to switch to.
b. The endpoint does not activate forwarding on that PW at this
point in time. It MAY, however, enable receiving on that PW. Thus
the 'Preferential Forwarding' status bit still reflects the
currently-used PW.
c. The requesting endpoint starts a timer while waiting the remote
endpoint to acknowledge the request.
d. If while waiting for the acknowledgment, the requesting endpoint
receives a request from its peer to switchover to the same or a
different PW, it must perform the following:
i. If its address is higher than that of the peer, this
endpoint ignores the request and continues to wait for
the acknowledgement from its peer.
ii. If its system IP address is lower than that of its peer,
it aborts the timer and immediately starts the
procedures of the receiving endpoint in Section 6.3.2.
e. If while waiting for the acknowledgment, the requesting
endpoint receives a status notification message from its peer
with the 'Preferential Forwarding' status bit cleared in the
requested PW, it must treat this as an explicit acknowledgment of
the request and must perform the following:
i. Abort the timer.
ii. Activate the PW.
iii. Send an update status notification message with the
'Preferential Forwarding' status bit and the 'request
switchover' bit clear on the newly active PW and send an
update status notification message with the
'Preferential Forwarding' status bit set in the
previously active PW.
f. If while waiting for the acknowledgment, the requesting endpoint
detects that the requested PW went into DOWN state locally, and
could use an alternate PW which is UP, it must perform the
following:
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i. Abort the timer.
ii. Issue a new request to switchover to the alternate PW.
iii. Re-start the timer.
g. If, while waiting for the acknowledgment, the requesting endpoint
detects that the requested PW went into the DOWN state locally,
and could not use an alternate PW which is UP, it must perform
the following:
i. Abort the timer.
ii. Send an update status notification message with the
'Preferential Forwarding' status bit unchanged and the
'request switchover' bit reset for the requested PW.
h. If, while waiting for the acknowledgment, the timer expires, the
requesting endpoint MUST assume that the request was rejected and
MAY issue a new request.
i. If the requesting node receives the acknowledgment after the
request expired, it will treat it as if the remote endpoint
unilaterally switched between the PWs without issuing a request.
In that case, it may issue a new request and follow the
requesting endpoint procedures to synchronize which PW to use for
the transmit and receive directions of the emulated service.
6.3.2. Procedures at the receiving endpoint
a. Upon receiving a status notification message with the 'request
switchover' bit set on a PW different from the currently active
one, and the requested PW is UP, the receiving endpoint must
perform the following:
i. Activate the PW.
ii. Send an update status notification message with the
'Preferential Forwarding' status bit clear and the
'request switchover' bit reset on the newly active PW ,
and send an update status notification message with the
'Preferential Forwarding' status bit set in the
previously active PW.
iii. Upon receiving a status notification message with the
'request switchover' bit set on a PW different from the
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currently active one, and the requested PW is DOWN, the
receiving endpoint MUST ignore the request.
7. Status Mapping
The generation and processing of the PW Status TLV must follow the
procedures in RFC 4447 [2]. The PW status TLV is sent on the active
PW and standby PWs to make sure the remote AC and PW states are
always known to the local PE node.
The generation and processing of PW Status TLV by an S-PE node in a
MS-PW must follow the procedures in [4].
The procedures for determining and mapping PW and AC states must
follow the rules in [7] with the following modifications.
7.1. AC Defect State Entry/Exit
A PE enters the AC receive (or transmit) defect state for a PW when
one or more of the conditions specified for this PW type in [7] are
met.
When a PE enters the AC receive (or transmit) defect state for a PW,
it must send a forward (reverse) defect indication to the remote
peers over all PWs in the redundancy set when required by the PW type
in [7].
When a PE exits the AC receive (or transmit) defect state for a PW
service, it must clear the forward (or reverse) defect indication to
the remote peers over all PWs in the redundancy set when required by
the PW type in [7].
7.2. PW Defect State Entry/Exit
A PE enters the PW receive (or transmit) defect state for a PW
service when one or more of the conditions specified in Section 8.2.1
(Section 8.2.2) in [7] are met for all PWs in the redundancy set.
When a PE enters the PW receive (or transmit) defect state for a PW
service, it must send a reverse (or forward) defect indication over
one or more of the PWs in the redundancy set if the PW failure was
detected by this PE without receiving a forward defect indication
from the remote PE [7].
When a PE exits the PW receive (or transmit) defect state for a PW,
it must clear the reverse (or forward) defect indication over any PW
in the redundancy set if applicable.
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8. Applicability and Backward Compatibility
The mechanisms defined in this document are applicable to
applications where standby state signaling of a PW or PW group is
required. Both PW FEC 128 and 129 are supported. All PWs which are
part of a redundant set must use the same FEC type. When the set uses
FEC 128 PWs, each PW is uniquely identified by its PW-ID. When the
redundant set uses FEC 129 PWs, each PW must have a unique identifier
which consists of the triplet AGI::SAII::TAII.
A PE implementation that uses the mechanisms described in this
document MUST negotiate the use of PW status TLV between its T-LDP
peers as per RFC 4447 [2]. If PW Status TLV is found to be not
supported by either of its endpoint after status negotiation
procedures, then the mechanisms specified in this document cannot be
used.
A PE implementation compliant to RFC 4447 [2], and which does not
support the generation or processing of the 'Preferential Forwarding'
status bit or of the 'request switchover' status bit, will ignore
these status bits if they are received from a peer PE.
9. Security Considerations
This document uses the LDP extensions that are needed for protecting
pseudowires. It will have the same security properties as in the PWE3
control protocol [2].
10. MIB Considerations
This document makes the following update to the PwOperStatusTC
textual convention in RFC5542 [8]:
PwOperStatusTC ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Indicates the operational status of the PW.
- up(1): Ready to pass packets.
- down(2): PW signaling is not yet finished, or
indications available at the service
level indicate that the PW is not
passing packets.
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- testing(3): AdminStatus at the PW level is set to
test.
- dormant(4): The PW is not in a condition to pass
packets but is in a 'pending' state,
waiting for some external event. For example, the
PW Preferential Forwarding status state machine as defined in
[RFCXXXX (this document)] is in state "STANDBY".
- notPresent(5): Some component is missing to accomplish
the setup of the PW. It can be
configuration error, incomplete
configuration, or a missing H/W component.
- lowerLayerDown(6): One or more of the lower-layer interfaces
responsible for running the underlying PSN
is not in OperStatus 'up' state."
SYNTAX INTEGER {
up(1),
down(2),
testing(3),
dormant(4),
notPresent(5),
lowerLayerDown(6)
}
11. IANA Considerations
This document defines the following PW status codes for the PW
redundancy application. IANA is requested to allocate these from the
PW Status Codes registry.
11.1. Status Code for PW Preferential Forwarding Status
0x00000020 When the bit is set, it indicates "PW forwarding
standby".
When the bit is cleared, it indicates "PW forwarding
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active".
11.2. Status Code for PW Request Switchover Status
0x00000040 When the bit is set, it represents "Request switchover to
this PW".
When the bit is cleared, it represents no specific
action.
12. Major Contributing Authors
The editors would like to thank Matthew Bocci, Pranjal Kumar Dutta,
Giles Heron, Marc Lasserre, Luca Martini, Thomas Nadeau, Jonathan
Newton, Hamid Ould-Brahim, and Olen Stokes, who made a major
contribution to the development of this document.
Matthew Bocci
Alcatel-Lucent
Email: matthew.bocci@alcatel-lucent.com
Pranjal Kumar Dutta
Alcatel-Lucent
Email: pranjal.dutta@alcatel-lucent.com
Giles Heron
Cisco Systems, Inc.
giles.heron@gmail.com
Marc Lasserre
Alcatel-Lucent
Email: marc.lasserre@alcatel-lucent.com
Luca Martini
Cisco Systems, Inc.
Email: lmartini@cisco.com
Thomas Nadeau
CA Technologies
thomas.nadeau@ca.com
Jonathan Newton
Cable & Wireless Worldwide
Email: Jonathan.Newton@cw.com
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Hamid Ould-Brahim
Nortel
Email: hbrahim@nortel.com
Olen Stokes
Extreme Networks
Email: ostokes@extremenetworks.com
13. Acknowledgments
The authors would like to thank the following individuals for their
valuable comments and suggestions which improved the document both
technically and editorially:
Vach Kompella, Kendall Harvey, Tiberiu Grigoriu, John Rigby,
Prashanth Ishwar, Neil Hart, Kajal Saha, Florin Balus, Philippe
Niger, Dave McDysan, Roman Krzanowski, Italo Busi, Robert Rennison,
Nicolai Leymann and Daniel Cohn.
14. References
14.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Martini, L., et al., "Pseudowire Setup and Maintenance using
LDP", RFC 4447, April 2006.
[3] Kompella,V., Lasserrre, M. , et al., "Virtual Private LAN
Service (VPLS) Using LDP Signalling", RFC 4762, January 2007.
14.2. Informative References
[4] Martini, L., et al., "Segmented Pseudo Wire", RFC 6073, January
2011.
[5] Muley, P., et al., "Pseudowire (PW) Redundancy", draft-ietf-
pwe3-redundancy-06.txt", February 2012.
[6] Bryant, S., et al., "Pseudo Wire Emulation Edge-to-Edge (PWE3)
Architecture", RFC 3985, March 2005
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[7] Aissaoui, M., et al., "Pseudo Wire (PW) OAM Message Mapping",
RFC 6310, July 2011.
[8] Nadeau, T., Zelig, D., Nicklass, O., "Definitions of Textual
Conventions for Pseudowire (PW) Management", RFC5542, May 2009
[9] Dutta, P., Lasserre, M., Stokes, O., "LDP Extensions for
Optimized MAC Address Withdrawal in H-VPLS", draft-ietf-l2vpn-
vpls-ldp-mac-opt-05.txt, October 2011
[10] [RFC5085] Nadeau, T. and C. Pignataro, "Pseudowire Virtual
Circuit Connectivity Verification (VCCV): A Control Channel for
Pseudowires", RFC 5085, December 2007.
15. Appendix A - Applications of PW Redundancy Procedures
This section shows how the mechanisms described in this document are
used to achieve the desired protection behavior for the scenarios
described in the PW redundancy requirements and framework document
[5].
15.1. One Multi-homed CE with single SS-PW redundancy
The following figure illustrates an application of single segment
pseudowire redundancy.
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|<-------------- 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 15-1 Multi-homed CE with single SS-PW redundancy
The application in Figure 15-1 makes use of the Independent mode of
operation.
CE1 is dual homed to PE1 and to PE3 by attachment circuits. The
method for dual-homing of CE1 to PE1 and to PE3 nodes, and the
protocols used, are outside the scope of this document (see [5]).
In this example, the AC from CE1 to PE1 is active, while the AC from
CE1 to PE3 is standby, as determined by the redundancy protocol
running on the ACs. Thus, in normal operation, PE1 and PE3 will
advertise "Active" and "Standby" 'Preferential Forwarding' status bit
respectively to PE2, reflecting the forwarding state of the two ACs
to CE1 as determined by the AC dual-homing protocol. PE2 advertises
'Preferential Forwarding' status bit of "Active" on both PW1 and PW2
since the AC to CE2 is single homed. As both the local and remote
UP/DOWN status and preferential forwarding status for PW1 are UP and
Active, traffic is forwarded over PW1 in both directions.
On failure of the AC between CE1 and PE1, the forwarding state of the
AC on PE3 transitions to Active. PE3 then announces the newly changed
'Preferential Forwarding' status bit of "Active" to PE2. PE1 will
advertise a PW status notification message indicating that the AC
between CE1 and PE1 is DOWN. PE2 matches the local and remote
preferential forwarding status of "Active" and status of "Pseudowire
forwarding" and select PW2 as the new active pseudowire to send
traffic to.
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On failure of PE1 node, PE3 will detect it and will transition the
forwarding state of its AC to Active. The method by which PE3 detects
that PE1 is down is outside the scope of this document. PE3 then
announces the newly changed 'Preferential Forwarding' status bit of
"Active" to PE2. PE3 and PE2 match the local and remote preferential
forwarding status of "Active" and UP/DOWN status "Pseudowire
forwarding" and select PW2 as the new active pseudowire to send
traffic to. Note that PE2 may have detected that the PW to PE1 went
down via T-LDP Hello timeout or via other means. However, it will not
be able to forward user traffic until it receives the updated status
bit from PE3.
Note in this example, the receipt of the AC status on the CE1-PE1
link is normally sufficient for PE2 to switch to PW2. However, the
operator may want to trigger the switchover of the PW for
administrative reasons, e.g., maintenance, and thus the use of the
'Preferential Forwarding' status bit is required to notify PE2 to
trigger the switchover.
Note that the primary/secondary procedures do not apply in this case
as the PW PW Preferential Forwarding status is driven by the AC
forwarding state as determined by the AC dual-homing protocol used.
15.2. Multiple Multi-homed CEs with single SS-PW redundancy
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V (not shown) V V |
V AC +----+ +----+ AC V
+-----+ | |....|.......PW1........|....| | +-----+
| |----------| PE1|...... .........| PE3|----------| |
| CE1 | +----+ \ / PW3 +----+ | CE2 |
| | +----+ X +----+ | |
| | | |....../ \..PW4....| | | |
| |----------| PE2| | PE4|--------- | |
+-----+ | |....|.....PW2..........|....| | +-----+
AC +----+ +----+ AC
Figure 15-2 Multiple Multi-homed CEs with single SS-PW redundancy
The application in Figure 15-2 makes use of the Independent mode of
operation.
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CE1 is dual-homed to PE1 and PE2. CE2 is dual-homed PE3 and PE4. The
method for dual-homing and the used protocols are outside the scope
of this document. 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.
Assume that the AC from CE1 to PE1 is Active, from CE1 to PE2 is
Standby; furthermore, assume that the AC from CE2 to PE3 is Standby
and from CE2 to PE4 is Active. The method of deriving Active/Standby
status of the AC is outside the scope of this document.
PE1 advertises the preferential status "Active" and UP/DOWN status
"Pseudowire forwarding" for pseudowires PW1 and PW4 connected to PE3
and PE4. This status reflects the forwarding state of the AC
attached to PE1. PE2 advertises preferential status "Standby" and
UP/DOWN status "Pseudowire forwarding" for pseudowires PW2 and PW3
to PE3 and PE4. PE3 advertises preferential status "Standby" and
UP/DOWN status "Pseudowire forwarding" for pseudowires PW1 and PW3
to PE1 and PE2. PE4 advertise the preferential status "Active" and
UP/DOWN status "Pseudowire forwarding" for pseudowires PW2 and PW4
to PE2 and PE1 respectively. Thus by matching the local and remote
preferential forwarding status of "Active" and UP/DOWN status of
"Pseudowire forwarding" of pseudowires, the PE nodes determine which
PW should be in the Active state. In this case it is PW4 that will
be selected.
On failure of the AC between CE1 and PE1, the forwarding state of
the AC on PE2 is changed to Active. PE2 then announces the newly
changed 'Preferential Forwarding' status bit of "active" to PE3 and
PE4. PE1 will advertise a PW status notification message indicating
that the AC between CE1 and PE1 is down. PE2 and PE4 match the local
and remote preferential forwarding status of "Active" and UP/DOWN
status "Pseudowire forwarding" and select PW2 as the new active
pseudowire to send traffic to.
On failure of PE1 node, PE2 will detect it and will transition the
forwarding state of its AC to Active. The method by which PE2
detects that PE1 is down is outside the scope of this document. PE2
then announces the newly changed 'Preferential Forwarding' status
bit of "Active" to PE3 and PE4. PE2 and PE4 match the local and
remote preferential forwarding status of "Active" and UP/DOWN status
"Pseudowire forwarding" and select PW2 as the new active pseudowire
to send traffic to. Note that PE3 and PE4 may have detected that the
PW to PE1 went down via T-LDP Hello timeout or via other means.
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However, they will not be able to forward user traffic until they
received the updated status bit from PE2.
Because each dual-homing algorithm running on the two node sets,
i.e., {CE1, PE1, PE2} and {CE2, PE3, PE4}, selects the active AC
independently, there is a need to signal the active status of the AC
such that the PE nodes can select a common active PW for end-to-end
forwarding between CE1 and CE2 as per the procedures in the
independent mode.
Note that any primary/secondary procedures, as defined in sections
5.1. and 5.2. , do not apply in this use case as the Active/Standby
status is driven by the AC forwarding state as determined by the AC
dual-homing protocol used.
15.3. Multi-homed CE with MS-PW redundancy
The following figure illustrates an application of multi-segment
pseudowire redundancy.
Native |<-----------Pseudo Wire----------->| 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|=========|T-PE4| | +----+
+-----+ +-----+ AC
Figure 15-3 Multi-homed CE with MS-PW redundancy
The application in Figure 15-3 makes use of the Independent mode of
operation.
CE2 is dual-homed to T-PE2 and T-PE4. PW1 and PW2 are used to extend
the resilient connectivity all the way to T-PE1. PW1 has two segments
and is active pseudowire while PW2 has two segments and is a standby
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pseudowire. This application requires support for MS-PW with segments
of the same type as described in [4].
The operation in this case is the same as in the case of SS-PW as
described in Section 15.1. . The only difference is that the S-PE
nodes need to relay the PW status notification containing both the
UP/DOWN and forwarding status to the T-PE nodes.
15.4. Multi-homed CE with MS-PW redundancy and S-PE protection
The following figure illustrates an application of multi-segment
pseudowire redundancy with 1:1 PW protection.
Native |<-----------Pseudo Wire------------->| Native
Service | | Service
(AC) | |<-PSN1-->| |<-PSN2-->| | (AC)
| V V V V V V |
| +-----+ |
| |=============| |=============| |
| |.....PW3-Seg1......|.PW3-Seg2....| |
| |.|===========|S-PE3|===========|.| |
| |.| +-----+ |.| |
| +-----+ +-----+ +-----+ |
+----+ | |T-PE1|=========|S-PE1|=========|T-PE2| | +----+
| |-------|......PW1-Seg1.......|PW1-Seg2.......|-------| |
| | | |=========| |=========| | | |
| CE1| +-----+ +-----+ +-----+ | |
| | |.| |.| +-----+ +-----+ | CE2|
| | |.| |.|=========| |=========| | | |
| | |.| |...PW2-Seg1......|.PW2-Seg2......|-------| |
+----+ |.| |===========|S-PE2|=========|T-PE4| | +----+
|.| +-----+ +-----+ AC
|.| +-----+ |.|
|.|=============| |===========|.|
|.......PW4-Seg1......|.PW4-Seg2....|
|===============|S-PE4|=============|
+-----+
Figure 15-4 Multi-homed CE with MS-PW redundancy and protection
The application in Figure Figure 15-4 makes use of the
Independent mode of operation.
CE2 is dual-homed to T-PE2 and T-PE4. The PW pairs {PW1,PW3} and
{PW2, PW4} are used to extend the resilient connectivity all the
way to T-PE1, like in the case in Section 15.3. , with the
addition that this setup provides for S-PE node protection.
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CE1 is connected to T-PE1 while CE2 is dual-homed to T-PE2 and
T-PE4. There are four segmented PWs. PW1 and PW2 are primary PWs
and are used to support CE2 multi-homing. PW3 and PW4 are
secondary PWs and are used to support 1:1 PW protection. PW1,
PW2, PW3 and PW4 have two segments and they are switched at S-
PE1, S-PE2, S-PE3 and S-PE4 respectively.
It is possible that S-PE1 coincides with S-PE4 and/or SP-2
coincides with S-PE3, in particular where the two PSN domains
are interconnected via two nodes. However Figure 15-4 shows four
separate S-PEs for clarity.
The behavior of this setup is exactly the same as in the setup
in Section 15.3. except that T-PE1 will always see a pair of
PWs eligible for the active state, for example the pair
{PW1,PW3} when the AC between CE2 and T-PE2 is in active state.
Thus, it is important that both T-PE1 and T-PE2 implement a
common mechanism to choose one the two PWs for forwarding as
explained in Section 5.1. Similarly, T-PE1 and T-PE4 must use
the same mechanism to select among the pair {PW2, PW4} when the
AC between CE2 and T-PE4 is in active state.
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15.5. Single Homed CE with MS-PW redundancy
The following is an application of the independent mode of operation
along with the optional request switchover procedures in order to
provide N:1 PW protection. A revertive behavior to a primary PW is
shown as an example of configuring and using the primary/secondary
procedures described in sections 5.1. and 5.2. .
Native |<------------Pseudo Wire------------>| 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 15-5 Single homed CE with multi-segment pseudowire redundancy
CE1 is connected to PE1 in provider Edge 1 and CE2 to PE2 in provider
edge 2 respectively. There are three segmented PWs. A primary PW,
PW1, is switched at S-PE1. A primary PW, PW1 has the lowest
precedence value of zero. A secondary PW, PW2, which is switched at
S-PE2 and has a precedence of 1. Finally, another secondary PW, PW3,
is switched at S-PE3 and has a precedence of 2. The precedence is
locally configured at the endpoints of the PW, i.e., T-PE1 and T-PE2.
Lower the precedence value, higher the priority.
T-PE1 and T-PE2 will select the PW they intend to activate based on
their local and remote UP/DOWN state as well as the local precedence
configuration. In this case, they will both advertise Preferential
Forwarding' status bit of "Active" on PW1 and of "Standby" on PW2 and
PW3 using priority derived from local precedence configuration.
Assuming all PWs are UP, T-PE1 and T-PE2 will use PW1 to forward user
packets.
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If PW1 fails, then the T-PE detecting the failure will send a status
notification to the remote T-PE with a "Local PSN-facing PW (ingress)
Receive Fault" bit set, or a "Local PSN-facing PW (egress) Transmit
Fault" bit set, or a "Pseudowire Not Forwarding" bit set. In
addition, it will set the 'Preferential Forwarding' status bit on PW1
to "Standby". It will also advertise the 'Preferential Forwarding'
status bit on PW2 as "Active" as it has the next lowest precedence
value. T-PE2 will also perform the same steps as soon as it is
informed of the failure of PW1. Both T-PE nodes will perform a match
on the 'preferential forwarding' status of "Active" and UP/DOWN
status of "Pseudowire forwarding" and will use PW2 to forward user
packets.
However this does not guarantee that the T-PEs will choose the same
PW from the redundant set to forward on, for a given emulated
service, at all times. This may be due to a mismatch of the
configuration of the PW precedence in each T-PE. This may also be due
to a failure which caused the endpoints to not be able to match the
Active 'Preferential Forwarding' status bit and UP/DOWN status bits.
In this case, T-PE1 and/or T-PE2 can invoke the optional request
switchover/acknowledgement procedures to synchronize the choice of PW
to forward on in both directions.
The trigger for sending a request to switchover can also be the
execution of an administrative maintenance operation by the network
operator in order to move the traffic away from the T-PE/S-PE nodes
/links to be serviced.
In case the request switchover is sent by both endpoints
simultaneously, both T-PEs send status notification with the newly
selected PW with 'request switchover' bit set, waiting for response
from the other endpoint. In such situation, the T-PE with greater
system address request is given precedence. This helps in
synchronizing PWs in event of mismatch of precedence configuration as
well.
On recovery of primary PW1, PW1 is selected to forward traffic
and the secondary PW, PW2, is set to standby.
15.6. PW redundancy between H-VPLS MTU-s and PE-rs
Following figure illustrates the application of use of PW redundancy
in H-VPLS for the purpose of dual-homing an MTU-s node to PE nodes
using PW spokes. This application makes use of the Master/Slave mode
of operation.
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|<-PSN1-->| |<-PSN2-->|
V V V V
+-----+ +--------+
|MTU-s|=========|PE1-rs |========
|..Active PW group.... | H-VPLS-core
| |=========| |=========
+-----+ +--------+
|.|
|.| +--------+
|.|===========| |==========
|...Standby PW group |.H-VPLS-core
=============| PE2-rs|==========
+--------+
Figure 15-6 Multi-homed MTU-s in H-VPLS core
MTU-s is dual homed to PE1-rs and PE2-rs. The primary spoke PWs from
MTU-s are connected to PE1-rs while the secondary PWs are connected
to PE2. PE1-rs and PE2-rs are connected to H-VPLS core on the other
side of network. MTU-s communicates to PE1-rs and PE2-rs the
forwarding status of its member PWs for a set of VSIs having common
status Active/Standby. It may be signaled using PW grouping with
common group-id in PWid FEC Element or Grouping TLV in Generalized
PWid FEC Element as defined in [2] to scale better. MTU-s derives
the status of the PWs based on local policy configuration. In this
example, the primary/secondary procedures, as defined in Section 5.2.
, are used but this can be based on any other policy.
Whenever MTU-s performs a switchover, it sends a wildcard
Notification Message to PE2-rs for the previously standby PW group
containing PW Status TLV with PW Preferential Forwarding bit cleared.
On receiving the notification PE-2rs unblocks all member PWs
identified by the PW group and state of PW group changes from Standby
to Active. All procedures described in Section 6.2. are applicable.
The use of the 'Preferential Forwarding' status bit in Master/Slave
mode is similar to Topology Change Notification in RSTP controlled
IEEE Ethernet Bridges but is restricted over a single hop. When these
procedures are implemented, PE-rs devices are aware of switchovers at
MTU-s and could generate MAC Withdraw Messages to trigger MAC
flushing within the H-VPLS full mesh. By default, MTU-s devices
should still trigger MAC Withdraw messages as currently defined in
[6] to prevent two copies of MAC withdraws to be sent, one by MTU-s
and another one by PE-rs nodes. Mechanisms to disable MAC Withdraw
trigger in certain devices is out of the scope of this document
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Author's Addresses
Praveen Muley
Alcatel-lucent
701 E. Middlefiled Road
Mountain View, CA, USA
Email: Praveen.muley@alcatel-lucent.com
Mustapha Aissaoui
Alcatel-lucent
600 March Rd
Kanata, ON, Canada K2K 2E6
Email: mustapha.aissaoui@alcatel-lucent.com
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