Internet Engineering Task Force (IETF) Z. Liu
Request for Comments: 7309 China Telecom
Category: Standards Track L. Jin
ISSN: 2070-1721
R. Chen
ZTE Corporation
D. Cai
S. Salam
Cisco
July 2014
Redundancy Mechanism for Inter-domain VPLS Service
Abstract
In many existing Virtual Private LAN Service (VPLS) inter-domain
deployments (based on RFC 4762), pseudowire (PW) connectivity offers
no Provider Edge (PE) node redundancy, or offers PE node redundancy
with only a single domain. This deployment approach incurs a high
risk of service interruption, since at least one domain will not
offer PE node redundancy. This document describes an inter-domain
VPLS solution that provides PE node redundancy across domains.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/rfc7309.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Network Use Case . . . . . . . . . . . . . . . . . . . . . . 4
5. PW Redundancy Application Procedure for Inter-domain
Redundancy . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. ICCP Switchover Condition . . . . . . . . . . . . . . . . 6
5.1.1. Inter-domain PW Failure . . . . . . . . . . . . . . . 6
5.1.2. PE Node Isolation . . . . . . . . . . . . . . . . . . 6
5.1.3. PE Node Failure . . . . . . . . . . . . . . . . . . . 6
5.2. Inter-domain Redundancy with Two PWs . . . . . . . . . . 6
5.3. Inter-domain Redundancy with Four PWs . . . . . . . . . . 7
6. Management Considerations . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative references . . . . . . . . . . . . . . . . . . 10
10.2. Informative references . . . . . . . . . . . . . . . . . 10
1. Introduction
In many existing Virtual Private LAN Service (VPLS) deployments based
on [RFC4762], pseudowire (PW) connectivity offers no Provider Edge
(PE) node redundancy, or offers PE node redundancy with only a single
domain. This deployment approach incurs a high risk of service
interruption, since at least one domain will not offer PE node
redundancy. This document describes an inter-domain VPLS solution
that provides PE node redundancy across domains. The redundancy
mechanism will provide PE node redundancy and link redundancy in both
domains. The PE throughout the document refers to a routing and
bridging capable PE defined in [RFC4762], Section 10. The domain in
this document refers to an autonomous system (AS), or other
administrative domains.
The solution relies on the use of the Inter-Chassis Communication
Protocol (ICCP) [RFC7275] to coordinate between the two redundant
edge nodes, and use of PW Preferential Forwarding Status Bit
[RFC6870] to negotiate the PW status. There is no change to any
protocol message formats and no new protocol options are introduced.
This solution is a description of reusing existing protocol building
blocks to achieve the desired function, but also defines
implementation behavior necessary for the function to work.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Motivation
Inter-AS VPLS offerings are widely deployed in service provider
networks today. Typically, the Autonomous System Border Router
(ASBR) and associated physical links that connect the domains carry a
multitude of services. As such, it is important to provide PE node
and link redundancy, to ensure high service availability and meet the
end customer service level agreements (SLAs).
Several current deployments of inter-AS VPLS are implemented like
inter-AS option A as described in [RFC4364], Section 10, where the
Virtual Local Area Network (VLAN) is used to hand-off the services
between two domains. In these deployments, PE node/link redundancy
is achieved using Multi-Chassis Link Aggregation (MC-LAG) and ICCP
[RFC7275]. This, however, places two restrictions on the
interconnection: the two domains must be interconnected using
Ethernet links, and the links must be homogeneous, i.e., of the same
speed, in order to be aggregated. These two conditions cannot always
be guaranteed in live deployments. For instance, there are many
scenarios where the interconnection between the domains uses packet
over Synchronous Optical Networking (SONET) / Synchronous Digital
Hierarchy (SDH), thereby ruling out the applicability of MC-LAG as a
redundancy mechanism. As such, from a technical point of view, it is
desirable to use PWs to interconnect the VPLS domains, and to offer
resiliency using PW redundancy mechanisms.
Multiprotocol Border Gateway Protocol (MP-BGP) can be used for VPLS
inter-domain protection, as described in [RFC6074], using either
option B or option C inter-AS models. However, with this solution,
the protection time relies on BGP control-plane convergence. In
certain deployments, with tight SLA requirements on availability,
this mechanism may not provide the desired failover time
characteristics. Furthermore, in certain situations MP-BGP is not
deployed for VPLS. The redundancy solution described in this
document reuses ICCP [RFC7275] and PW redundancy [RFC6718] to provide
fast convergence.
Furthermore, in the case where label switched multicast is not used
for VPLS multicast [RFC7117], the solution described here provides a
better behavior compared to inter-AS option B: with option B, each PE
must perform ingress replication to all other PEs in its local as
well as the remote domain. Whereas, with the ICCP solution, the PE
only replicates to local PEs and to the ASBR. The ASBR then sends
traffic point to point to the remote ASBR, and the remote ASBR
replicates to its local PEs. As a result, the load of replication is
distributed and is more efficient than option B.
Two PW redundancy modes defined in [RFC6718], namely independent mode
and master/slave mode, are applicable in this solution. In order to
maintain control-plane separation between two domains, the
independent mode is preferred by operators. The master/slave mode
provides some enhanced capabilities and, hence, is included in this
document.
4. Network Use Case
There are two network use cases for VPLS inter-domain redundancy:
two-PWs redundancy case, and four-PWs redundancy case.
Figure 1 presents an example use case with two inter-domain PWs.
PE3/PE4/PE5/PE6 may be ASBRs of their respective AS, or VPLS PEs
within its own AS. PE3 and PE4 belong to one redundancy group (RG),
and PE5 and PE6 belong to another RG. A deployment example of this
use case is where there are only two physical links between two
domains and PE3 is physically connected with PE5, and PE4 is
physically connected with PE6.
+---------+ +---------+
+---+ | +-----+ | active PW1 | +-----+| +---+
|PE1|---|-| PE3 |-|-----------------|--| PE5 ||----|PE7|
+---+\ |/+-----+ | | +-----+\ /+---+
| \ / | * | | * | |\ / |
| \| | |ICCP| |ICCP| | | \ |
| / \ | * | | * | |/ \ |
+---+/ |\+-----+ | | +-----+/ \+---+
|PE2|---|-| PE4 |-|-----------------|--| PE6 ||----|PE8|
+---+ | +-----+ | standby PW2 | +-----+| +---+
| | | |
| | | |
| RG1 | | RG2 |
+---------+ +---------+
operator A network operator B network
Figure 1
Figure 2 presents a four-PWs inter-domain VPLS redundancy use case.
PE3/PE4/PE5/PE6 may be ASBRs of their respective AS, or VPLS PEs
within its own AS. A deployment example of this use case is where
there are four physical links between two domains and four PEs are
physically connected with each other with four links.
+---------+ +---------+
+---+ | +-----+ | | +-----+| +---+
|PE1|---|-| PE3 |-|--------PW1------|--| PE5 ||----|PE7|
| | | | |-|-PW3\ /------|--| || | |
+---+\ |/+-----+ | \ / | +-----+\ /+---+
| \ / | * | \ / | * | |\ / |
| \| | |ICCP| X |ICCP| | | \ |
| / \ | * | / \ | * | |/ \ |
+---+/ |\+-----+ | / \ | +-----+/ \+---+
| | | | |-|-PW4/ \------|--| || | |
|PE2|---|-| PE4 |-|----PW2----------|--| PE6 ||----|PE8|
+---+ | +-----+ | | +-----+| +---+
| | | |
| | | |
| RG1 | | RG2 |
+---------+ +---------+
operator A network operator B network
Figure 2
5. PW Redundancy Application Procedure for Inter-domain Redundancy
PW redundancy application procedures are described in Section 9.1 of
[RFC7275]. When a PE node encounters a failure, the other PE takes
over. This document reuses the PW redundancy mechanism defined in
[RFC7275], with new ICCP switchover conditions as specified in
following section.
There are two PW redundancy modes defined in [RFC6870]: Independent
mode and Master/Slave mode. For the inter-domain four-PW scenario,
it is required that PEs ensure that the same mode be supported on the
two ICCP peers in the same RG. This can be achieved using manual
configuration at the ICCP peers. Other methods for ensuring
consistency are out of the scope of this document.
5.1. ICCP Switchover Condition
5.1.1. Inter-domain PW Failure
When a PE receives advertisements from the active PE, in the same RG,
indicating that all the inter-domain PW status has changed to DOWN/
STANDBY, then if it has the highest priority (after the advertising
PE), it SHOULD advertise active state for all of its associated
inter-domain PWs.
5.1.2. PE Node Isolation
When a PE detects failure of all PWs to the local domain, it SHOULD
advertise standby state for all its inter-domain PWs to trigger
remote PE to switchover.
5.1.3. PE Node Failure
When a PE node detects that the active PE, that is a member of the
same RG, has gone down, if the local PE has redundant PWs for the
affected services and has the highest priority (after the failed PE),
it SHOULD advertise the active state for all associated inter-domain
PWs.
5.2. Inter-domain Redundancy with Two PWs
In this use case, it is recommended that the operation be as follows:
o ICCP deployment option: ICCP is deployed on VPLS edge nodes in
both domains;
o PW redundancy mode: independent mode only;
o Protection architectures: 1:1 (1 standby, 1 active).
The switchover rules described in Section 5.1 apply. Before
deploying this inter-domain VPLS, the operators should negotiate to
configure the same PW high/low priority at two PW endpoints. The
inter-domain VPLS relationship normally involves a contractual
process between operators, and the configuration of PW roles forms
part of this process. For example, in Figure 1, PE3 and PE5 must
both have higher/lower priority than PE4 and PE6; otherwise, both PW1
and PW2 will be in standby state.
5.3. Inter-domain Redundancy with Four PWs
In this use case, there are two options to provide protection: 1:1
and 3:1 protection. The inter-domain PWs that connect to the same PE
should have proper PW priority to advertise the same active/standby
state. For example, in Figure 2, both PW1 and PW3 are connected to
PE3 and should advertise active/standby state.
For the 1:1 protection model, the operation would be as follows:
o ICCP deployment option: ICCP is deployed on VPLS edge nodes in
both domains;
o PW redundancy mode: independent mode only;
o Protection architectures: 1:1 (1 standby, 1 active).
The switchover rules described in Section 5.1 apply. In this case,
the operators do not need to do any coordination of the inter-domain
PW priority. The PE detecting one PW DOWN SHOULD set the other PW to
STANDBY if available, and then synchronize the updated state to its
ICCP peer. When a PE detects that the PWs from the ICCP peer PE are
DOWN or STANDBY, it SHOULD switchover as described in Section 5.1.1.
There are two variants of the 3:1 protection model. We will refer to
them as options A and B. The implementation MUST support option A
and MAY support option B. Option B will be useful when the two
legacy PEs in one domain do not support the function in this
document. The two legacy PEs still need to support PW redundancy
defined in [RFC6870] and be configured as slave node.
For option A of the 3:1 protection model, the support of the Request
Switchover status bit [RFC6870] is required. The operation is as
follows:
o ICCP deployment option: ICCP is deployed on VPLS edge nodes in
both domains;
o PW redundancy mode: Independent mode with 'request switchover' bit
support;
o Protection architectures: 3:1 (3 standby, 1 active).
In this case, the procedure on the PE for the PW failure is per
Section 6.3 of [RFC6870] and with the following additions:
o When the PE detects failure of the active inter-domain PW, it
SHOULD switch to the other local standby inter-domain PW if
available, and send an updated LDP PW status message with the
'request switchover' bit set on that local standby inter-domain PW
to the remote PE;
o Local and remote PE SHOULD also update the new PW status to their
ICCP peers, respectively, in Application Data Messages with the
PW-RED Synchronization Request TLV for corresponding service, so
as to synchronize the latest PW status on both PE sides.
o While waiting for the acknowledgement, the PE that sends the
'request switchover' bit may receive a switchover request from its
ICCP peer's PW remote endpoint by virtue of the ICCP
synchronization. The PE MUST compare IP addresses with that PW
remote peer. The PE with a higher IP address SHOULD ignore the
request and continue to wait for the acknowledgement from its peer
in the remote domain. The PE with the lower IP address SHOULD
clear the 'request switchover' bit and set the 'Preferential
Forwarding' local status bit, and update the PW status to ICCP
peer.
o The remote PE receiving the 'request switchover' bit SHOULD
acknowledge the request and activate the PW only when it is ready
to take over as described in Section 5.1, 5.1; otherwise, it SHOULD
ignore the request.
The PE node isolation failure and PE node failure is described in
Section 5.1.
For option B of the 3:1 protection model, master/slave mode support
is required and should be as follows:
o ICCP deployment option: ICCP is deployed on VPLS edge nodes in
only one domain;
o PW redundancy mode: master/slave only;
o Protection architectures: 3:1 (3 standby, 1 active).
When master/slave PW redundancy mode is employed, the network
operators of two domains must agree on which domain PEs will be
master, and configure the devices accordingly. The inter-domain PWs
that connect to one PE should have higher PW priority than the PWs on
the other PE in the same RG. The procedure on the PE for PW failure
is as follows:
o The PE with higher PW priority should only enable one PW active,
and the other PWs standby. should be in the standby state.
o When the PE detects an active PW DOWN, it SHOULD enable the other
local standby PW to be active with preference. Only when two
inter-domain PWs connected to the PE are DOWN, the ICCP peer PE in
the same RG SHOULD switchover as described in Section 5.1.
The PE node isolation failure and PE node failure are described in
Section 5.1.
6. Management Considerations
When deploying the inter-domain redundancy mechanism described in
this document, consistent provisioning is required for proper
operation. The two domains must both use the same use case
(Section 5.2 or Section 5.3). Within each section, all of the
described modes and options must be provisioned identically both
within each RG and between the RGs. Additionally, for the two-PWs
redundancy options defined in Section 5.2, the two operators must
also negotiate to configure same high/low PW priority at the two PW
endpoints. If the provisioning is inconsistent, then the inter-
domain redundancy mechanism may not work properly.
7. Security Considerations
Besides the security properties of [RFC7275] for the ICCP control
plane, and [RFC4762] and [RFC6870] for the PW control plane, this
document has additional security considerations for the ICCP control
plane.
In this document, ICCP is deployed between two PEs or ASBRs. The two
PEs or ASBRs should only be connected by a network that is well
managed and whose service levels and availability are highly
monitored. This should be ensured by the operator.
The state flapping on the inter-domain and intra-domain PW may cause
security threats or be exploited to create denial-of-service attacks.
For example, excessive PW state flapping (e.g., by malicious peer
PE's implementation) may lead to excessive ICCP exchanges.
Implementations SHOULD provide mechanisms to perform control-plane
policing and mitigate such types of attacks.
8. Acknowledgements
The authors would like to thank Sami Boutros, Giles Heron, Adrian
Farrel, Andrew G. Malis, and Stephen Kent for their valuable
comments.
9. Contributors
Daniel Cohn
Email:daniel.cohn.ietf@gmail.com
Yubao Wang
ZTE Corporation
Nanjing, China
Email: wang.yubao@zte.com.cn
10. References
10.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6870] Muley, P. and M. Aissaoui, "Pseudowire Preferential
Forwarding Status Bit", RFC 6870, February 2013.
[RFC7275] Martini, L., Salam, S., Sajassi, A., Bocci, M.,
Matsushima, S., and T. Nadeau, "Inter-Chassis
Communication Protocol for Layer 2 Virtual Private Network
(L2VPN) Provider Edge (PE) Redundancy", RFC 7275, June
2014.
10.2. Informative references
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[RFC6074] Rosen, E., Davie, B., Radoaca, V., and W. Luo,
"Provisioning, Auto-Discovery, and Signaling in Layer 2
Virtual Private Networks (L2VPNs)", RFC 6074, January
2011.
[RFC6718] Muley, P., Aissaoui, M., and M. Bocci, "Pseudowire
Redundancy", RFC 6718, August 2012.
[RFC7117] Aggarwal, R., Kamite, Y., Fang, L., Rekhter, Y., and C.
Kodeboniya, "Multicast in Virtual Private LAN Service
(VPLS)", RFC 7117, February 2014.
Authors' Addresses
Zhihua Liu
China Telecom
109 Zhongshan Ave.
Guangzhou 510630
P.R.China
EMail: zhliu@gsta.com
Lizhong Jin
Shanghai
P.R.China
EMail: lizho.jin@gmail.com
Ran Chen
ZTE Corporation
NO.19 East Huayuan Road
Haidian District Beijing 100191
P.R.China
EMail: chen.ran@zte.com.cn
Dennis Cai
Cisco
3750 Cisco Way,
San Jose, California 95134
USA
EMail: dcai@cisco.com
Samer Salam
Cisco
595 Burrard Street, Suite:2123
Vancouver, BC V7X 1J1
Canada
EMail: ssalam@cisco.com