Network Working Group Y. Liu
Internet Draft W. Cheng
Intended status: Informational China Mobile
Expires: 10 July 2024 C. Lin
New H3C Technologies
X. Geng
Huawei Technologies
Y. Liu
ZTE
10 January 2024
Considerations for Protection of SR Networks
draft-liu-rtgwg-sr-protection-considerations-02
Abstract
This document describes the considerations for protection of Segment
Routing (SR) networks.
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This Internet-Draft will expire on 10 July 2024.
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Table of Contents
1. Introduction...................................................3
1.1. Requirements Language.....................................3
1.2. Terminology...............................................3
2. Forwarding over SR Network.....................................3
2.1. SR BE Path................................................4
2.2. SR TE Path................................................4
3. Protection Mechanisms..........................................6
3.1. Path Protection...........................................6
3.1.1. Local Proctection Mechanisms.........................6
3.1.2. Liveness Check for Local Protection..................7
3.1.3. Micro-Loop Avoidance.................................7
3.1.4. End-to-End Protection Mechanisms.....................8
3.1.5. Liveness Check for End-to-End Protection.............8
3.2. Egress Protection.........................................9
3.2.1. Local Repair.........................................9
3.2.2. Ingress Node Switchover..............................9
4. Implementation Recommendations................................10
4.1. Single-homed Scenario....................................11
4.2. Multi-homed Scenario.....................................12
4.3. Liveness Check...........................................12
5. Considerations for SRv6 Segment List Compression..............13
5.1. TI-LFA with C-SID........................................13
5.2. Micro-Loop Avoidance with C-SID..........................13
6. Security Considerations.......................................13
7. IANA Considerations...........................................14
8. References....................................................14
8.1. Normative References.....................................14
8.2. Informative References...................................15
Contributors.....................................................15
Authors' Addresses...............................................16
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Appendix A. Examples.............................................17
A.1 Example of SR BE Scenario.................................17
A.2 Example of SR TE Scenario.................................19
1. Introduction
Segment Routing (SR) [RFC8402] leverages the source routing
paradigm. An ingress node steers a packet through an ordered list of
instructions, called "segments".
SR can be instantiated on the MPLS data plane (MPLS-SR) and the IPv6
data plane (SRv6). On the MPLS-SR data plane, a segment is encoded
as an MPLS label, and an ordered list of segments is encoded as a
stack of labels. On the SRv6 data plane, a segment is encoded as an
IPv6 address (SRv6 SID) [RFC8986], and an ordered list of segments
is encoded as an ordered list of SRv6 SIDs in the SR header (SRH)
[RFC8754].
This document describes the common failure scenarios and protection
mechanisms in SR networks. Then implementation recommendations for
protection of SR networks are proposed.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
BE: Best Effort
TE: Traffic Engineering
MPLS-SR: Segment Routing over MPLS
SRv6: Segment Routing over IPv6
G-SRv6: Generalized SRv6 Network Programming
2. Forwarding over SR Network
In an SR network, the ingress node steers a packet through an
ordered list of segments, which instructs the SR network to forward
the packet via a specific path to the egress node. The forwarding
path is either an SR BE path or an SR TE path.
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2.1. SR BE Path
An SR BE path is based on shortest path forwarding.
On the MPLS-SR data plane, the ingress PE encapsulates the payload
in an MPLS payload with the Prefix-SID of the egress PE and the VPN
label. The Prefix-SID steers the packet to the egress PE along
shortest path computed by IGP or BGP.
-----------------------
| MPLS Label Stack |
| 16001 (Prefix-SID) |
| 90001 (VPN Label) |
-----------------------
| Payload |
-----------------------
Ingress PE ---> P nodes ---> Egress PE
Figure 1: Forwarding over MPLS-SR BE
On the SRv6 data plane, the ingress PE encapsulates the payload in
an outer IPv6 header where the destination address is the SRv6
Service SID provided by the egress PE. The underlay P nodes between
the PEs only need to perform plain IPv6 shortest path forwarding.
-----------------------
| IPv6 Header |
| DA = 2001:DB8:1:1:: |
-----------------------
| Payload |
-----------------------
Ingress PE ---> P nodes ---> Egress PE
Figure 2: Forwarding over SRv6 BE
2.2. SR TE Path
In an SR TE path, the ingress PE steers the traffic flow into an SR
Policy [RFC9256] with an ordered list of segments associated with
that SR Policy. The underlay P nodes whose SIDs are part of the
segment list are called endpoint nodes. They will be involved in the
forwarding path and execute the function associated with the SID.
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On the MPLS-SR data plane, the ingress PE encapsulates the payload
packet with a stack of MPLS labels allocated for segments associated
with the SR policy.
------------------------
| MPLS Label Stack |
| 16006 |
| 16005 |
| 16004 |
| 16003 |
| 16002 |
| 16001 |
| 90101 |
------------------------
| Payload |
------------------------
Ingress PE ---> P nodes ---> Egress PE
Figure 3: Forwarding over MPLS-SR TE
On the SRv6 data plane, the ingress PE encapsulates the payload
packet in an outer IPv6 header with the Segment Routing Header (SRH)
carrying the segment list of the SR policy.
------------------------
| IPv6 Header |
| DA = 2001:DB8:6:1:: |
------------------------
| SRH |
| Seg[0]= 2001:DB8:1:1:: |
| Seg[1]= 2001:DB8:2:1:: |
| Seg[2]= 2001:DB8:3:1:: |
| Seg[3]= 2001:DB8:4:1:: |
| Seg[4]= 2001:DB8:5:1:: |
| Seg[5]= 2001:DB8:6:1:: |
------------------------
| Payload |
------------------------
Ingress PE ---> P nodes ---> Egress PE
Figure 4: Forwarding over SRv6 TE
If Compressed Segment List encoding is enabled in the SRv6 network
[I-D.ietf-spring-srv6-srh-compression], the segment list in the SRH
will be encoded in the compressed way. The compressed SRv6 Segment-
List encoding can optimize the packet header length by avoiding the
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repetition of the Locator-Block and trailing bits with each
individual SID.
The G-SRv6 mechanism will be used as an example for the encoding of
SRv6 TE path in this document. Figure 5 shows the encapsulation of
packet using the G-SRv6 mechanism.
------------------------
| IPv6 Header |
| DA = 2001:DB8:6:1:: |
------------------------
| SRH |
|Seg[0]= 2001:DB8:1:1:: |
|Seg[1]= 2:1|3:1|4:1|5:1 |
|Seg[2]= 2001:DB8:6:1:: |
------------------------
| Payload |
------------------------
Ingress PE ---> P nodes ---> Egress PE
Figure 5: Forwarding over G-SRv6 Encoded TE
3. Protection Mechanisms
Two main categories of protection mechanism in SR networks are
described in this section: path protection and egress protection.
Path protection works when the failure occurs along the forwarding
path, including SR BE paths and SR TE paths. Path protection is
further divided into local protection, which is performed by the
node adjacent to the failed component, and end-to-end protection,
which is performed by the ingress PE node.
In multi-homed scenarios, egress protection works instead when the
failure occurs on the egress PE node, and traffics will be forwarded
to another backup Egress PE node. Egress protection can be performed
by either local repair or ingress node switchover.
The corresponding liveness check mechanisms are also described along
with the protection mechanisms.
3.1. Path Protection
3.1.1. Local Proctection Mechanisms
Local protection is performed by the node adjacent to the failed
component using fast-reroute techniques [RFC5286] [RFC5714]. The
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common method of local repair is to provide a repair path for the
destination avoiding the failed component.
[I-D.ietf-rtgwg-segment-routing-ti-lfa] describes the Topology
Independent Loop-free Alternate Fast Re-route technology (TI-LFA)
using Segment Routing, which is able to provide a loop free backup
path irrespective of the topologies used in the network. For each
destination in the network, TI-LFA pre-installs a backup forwarding
entry for each protected destination ready to be activated upon
detection of the failure of a link used to reach the destination.
On the MPLS-SR data plane, the TI-LFA repair path is encoded as a
stack of MPLS labels and pushed onto the packet.
On the SRv6 data plane, the TI-LFA repair path is encoded as an SRv6
SID list, and encapsulated in the SRH along with an outer IPv6
header.
3.1.2. Liveness Check for Local Protection
In order to perceive the failures of links and neighbors, a node
should monitor the liveness of its adjacent components.
[RFC5880] and [RFC7880] provide widely used mechanisms for liveness
check, called Bidirectional Forwarding Detection (BFD) and Seamless
Bidirectional Forwarding Detection (S-BFD).
BFD can be associated with the interface state to detect the failure
of directly-connected links. Two adjacent nodes may establish BFD or
S-BFD sessions between each other, and send BFD control packets to
monitor the liveness of each other. In another way, a node may send
BFD echo packets to all the neighbors, and they will reflect the
packets back, without establishing BFD sessions.
Other OAM methods, such as Ping, TWAMP or STAMP, may also be used
for liveness check for local protection, which will not be
enumerated here in detail.
3.1.3. Micro-Loop Avoidance
When a component fails or comes back up, the topology is changed.
The routing convergence happens in each node at different times and
during a different lapse of time. These transient routing
inconsistencies may cause micro-loops.
[I-D.bashandy-rtgwg-segment-routing-uloop] provides a mechanism
leveraging segment routing to ensure loop-freeness during the IGP
reconvergence process, which relies on the temporary use of SR
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policies ensuring loop-freeness over the post-convergence paths from
the converging node to the destination.
On the MPLS-SR data plane, the loop-free post-convergence path is
encoded as a stack of MPLS labels and pushed onto the packet.
On the SRv6 data plane, the loop-free post-convergence path is
encoded as an SRv6 SID list, and encapsulated in the SRH along with
an outer IPv6 header.
3.1.4. End-to-End Protection Mechanisms
End-to-end protection lets the ingress PE node be in charge of the
failure recovery. The ingress node should steer the flow from the
failed path into another alive path.
In the case of SR TE path, the SR Policy itself allows for multiple
candidate paths, of which at any point in time there is a single
active candidate path that is provisioned in the forwarding plane
and used for traffic steering [RFC9256]. The candidate path with
highest preference is selected as the primary path, and the
candidate path with second highest preference can be selected as the
hot-standby backup. When the primary candidate path fails,
switchover to the backup candidate path can be triggered by fast re-
route mechanism.
If all the candidate paths fail, the ingress node may use SR BE path
for best-effort forwarding as a backup.
3.1.5. Liveness Check for End-to-End Protection
It is essential that the ingress PE node should check the end-to-end
liveness of paths, including primary path and backup path. So that
the ingress PE node can perceive the path failure and then trigger
the switchover.
In the case of SR TE path, BFD or S-BFD can be used to monitor the
liveness of SR Policy at the level of segment list. If all the BFD
sessions associated with segment lists in a candidate path are down,
the candidate path is deemed to be failed. If all the candidate
paths are failed, the SR Policy is deemed to be failed.
Moreover, If the SR TE path is strict (every hop along the path
appearing in the SID list), the reverse path of the BFD packets
should be the same with the forward path. Otherwise, the failure in
the reverse path may cause the misjudgement of the liveness of SR
Policy. To achieve the consistence of forward path and reverse path,
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the egress node should be instructed to use specific path to send
packets back to the ingress node.
Other OAM methods, such as Ping, TWAMP or STAMP, may also be used
for liveness check for end-to-end protection, which will not be
enumerated here in detail.
Local protection and end-to-end protection may both be used in the
same SR network. Since the speed of failure detection for local
protection is faster than end-to-end protection, local protection
usually performs the local repair in advance, which allows the path
to remain alive. In this case, the ingress node will not perceive
the failure and does not need to trigger end-to-end protection.
3.2. Egress Protection
If the failure occurs on the egress PE node, the TI-LFA or the hot-
standby backup candidate path of SR Policy will not work. To provide
protection, the packet should be forwarded to another backup Egress
PE node, if it exists.
3.2.1. Local Repair
In the case of egress PE node failure, the local repair node, which
is usually the penultimate hop on the SR path, should forward packet
to another Egress PE node. If a failure occurs on the link between
PE and CE, that PE should work as the local repair node and forward
packet to another Egress PE node. That mechanism is beyond the scope
of this document.
On the SRv6 data plane, [I-D.ietf-rtgwg-srv6-egress-protection]
provides a method to use Mirror SID for egress protection. The
Mirror SID is configured on the backup egress PE to protect the
primary egress PE, and it will be used by the repair node to encode
the segment list of repair path. But that method does not apply to
the MPLS-SR data plane.
3.2.2. Ingress Node Switchover
If there are multiple egress PE nodes, the ingress PE node receives
all their advertisements of the same service, and builds paths for
each of them respectively. The ingress PE node may use Fast Reroute
(FRR) for these different paths. When the primary egress PE node
fails, the ingress node steers the flow to the path belonging to
another egress PE node for protection.
BFD can be used to monitor the liveness of the service SID, locator
or interface address of the egress PE node. If the BFD session is
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down, the egress PE node is deemed to be unreachable. The ingress PE
node may also use the IGP routes of the locator or interface address
of the egress PE node to evaluate if that egress PE node is alive.
The IGP convergence is slower than BFD, but it can be useful in some
cases. For example, in the BGP-based VPN service network, the
ingress node switchover based on IGP convergence of egress PE routes
is usually faster than BGP convergence of VPN routes.
Egress protection and path protection may both be used in the same
SR network. Among the different paths to the same egress PE node and
the paths to different egress PE nodes, one is selected as the
primary path and others are used as backup. The priorities of
multiple backup paths may be decided by the egress-node-first
strategy or the TE-first strategy.
By the Egress-node-first strategy, paths to the primary egress PE
nodes are prioritized. For example, if a failure occurs on the
primary path, the ingress PE node will select another path still
leading to the primary egress PE nodes. Unless all the paths to the
primary egress PE node are failed, the ingress PE node would use the
path to the backup egress PE node.
By the TE-first strategy, SR TE paths to any egress PE node have
higher priorities than SR BE paths. For example, if a failure occurs
on the primary path and there is no other alive SR TE path to the
primary egress PE node, the ingress node will select an SR TE path
to the backup egress PE node, rather than an SR BE path still
leading to the primary egress PE node.
4. Implementation Recommendations
This section will introduce the implementation recommendations of
protection for SR networks. Section 4.1 describes the single-homed
scenario, and Section 4.2 describes the multi-homed scenario. In the
following scenarios, we assume that both SR BE paths or SR TE paths
are used in the same network to steer traffics with different
requirements.
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4.1. Single-homed Scenario
CE1--PE1-----P1----PE3--CE2
| | |
| | |
| | |
| | |
| | |
PE2-----P2----PE4
In the single-homed scenario, the combination of following
mechanisms can be used for the protection of SR network:
o TI-LFA
o Multiple Candidate Paths
o BE as Backup for TE
For traffics steered by SR BE paths, protection is performed locally
by the node adjacent to the failed component using TI-LFA mechanism.
BFD for links and neighbors are used as triggers of TI-LFA.
For traffics steered by SR TE paths, in some cases, end-to-end
protection (switchover to backup candidate path) is preferred over
local protection (TI-LFA) due to SLA requirements. BFD or S-BFD is
enabled to monitor the liveness of candidate paths. If the main
candidate path is down, the SR Policy will switch to the backup
candidate path. In some other cases, local protection is preferred
over backup candidate path due to the requirements of traffic
restoring time, like less than 200ms.
o In the cases with high SLA requirements: For strict SR TE path,
TI-LFA is not used along the path. For loose SR TE path, local
protection only works for the loose part on the path.
o In the cases with fast traffic restoring requirements: TI-LFA
preforms local protection in advance. The ingress node will
perceive the failure on the main candidate path after routing
convergence, and then switch to backup candidate path.
In addition, SR BE path can be used as a final backup for SR TE path
in case of multi-point faults. When all candidate paths of an SR
Policy are failed, the traffics will be switched to the SR BE path
instead of being dropped. Except for the cases where dropping is
more preferred due to strong SLA requirements or where there is no
requirement of fast traffic restoration for multi-point faults.
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4.2. Multi-homed Scenario
PE1-----P1----PE3
/ | | | \
/ | | | \
CE1 | | | CE2
\ | | | /
\ | | | /
PE2-----P2----PE4
In the multi-homed scenario, egress protection is also taken into
consideration besides path protection. In addition to the mechanisms
mentioned in the previous single-homed scenario, the following ones
are also used for the protection of SR network:
o Ingress Node Switchover to Backup Egress Node
The ingress node monitors the liveness of egress nodes, such as
enabling BFD for egress nodes, or validating IGP routes of egress
nodes. When the failure occurs on the main egress node, the ingress
node performs the switchover from the main egress node to the backup
egress node. This mechanism works for both the traffics steered by
SR TE paths and SR BE paths in the multi-home scenario. Note that,
in the multi-homed scenario, the ingress node switchover works among
the paths towards different egress nodes. Taking the SR TE paths as
an example, the ingress node switches among multiple SR Policies
with different endpoints, while in the single-homed scenario the
ingress node switches among multiple candidate paths within the same
SR Policy.
In the cases with fast traffic restoring requirements, like less
than 200ms, the local repair for egress node failure should be
deployed.
The path protection is the same as the previous single-homed
scenarios.
4.3. Liveness Check
As described in Section 4.1 and 4.2, BFD/S-BFD is used to monitor
the liveness of links, neighbors, SR Policies and egress nodes.
The BFD time interval for links and neighbors is recommended to be
10ms * 3 and thus the local protection provided by TI-LFA would
restore traffics in less than 50ms.
The BFD time interval for main candidate paths of SR Polices is
recommended to be 50ms * 3, while the time interval for backup
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candidate paths can be relaxed to 100ms * 3. Thus, the end-to-end
protection would restore traffics in less than 300ms.
The BFD time interval for egress nodes is recommended to be 50ms *
3.
5. Considerations for SRv6 Segment List Compression
[I-D.ietf-spring-srv6-srh-compression] enables a compressed encoding
of the SRv6 Segment List in the SRH, which can reduce the SRv6
encapsulation size. The SRv6 Segment-List compression may have an
effect on the protection of SRv6 networks, which is discussed in
this section.
5.1. TI-LFA with C-SID
When SRv6 Segment List compression is enabled, the repair node may
check the compression capabilities of nodes along the repair path
and try to use C-SIDS to encode the repair path.
If NEXT-C-SID flavors are preferred, the TI-LFA repair list consist
of the End SID with NEXT-C-SID flavor of the P node and the End.X
SID(s) with NEXT-C-SID flavor of the path from P node to Q node,
except for the last End.X SID which must not have NEXT-C-SID flavor.
In addition, the End SID must be a global C-SID, and the End.X
SID(s) can be local C-SID(s).
If REPLACE-C-SID flavors are preferred, the TI-LFA repair list
consist of the End SID with REPLACE-C-SID flavor of the P node and
the End.X SID(s) with REPLACE-C-SID flavor of the path from P node
to Q node, except for the last End.X SID which must not have
REPLACE-C-SID flavor.
5.2. Micro-Loop Avoidance with C-SID
If SRv6 Segment List compression is enabled, the converging node may
check the compression capabilities of nodes along the post-
convergence path and try to use C-SIDs to encode the path.
The TI-LFA mechanism can be used to compute the loop-free post-
convergence path. If so, the building of TI-LFA repair list with C-
SIDs is similar with the previous section.
6. Security Considerations
TBD.
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7. IANA Considerations
This document has no IANA actions.
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.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
<https://www.rfc-editor.org/info/rfc7880>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
[I-D.ietf-spring-srv6-srh-compression] Cheng, W., Filsfils, C., Li,
Z., Decraene, B., Cai, D., Clad, F., Zadok, S., Guichard,
J., Aihua, L., Raszuk, R. and C. Li, " Compressed SRv6
Segment List Encoding in SRH", draft-ietf-spring-srv6-srh-
compression-05 (work in progress), June 2023.
[I-D.ietf-rtgwg-segment-routing-ti-lfa] Litkowski, S., Bashandy, A.,
Filsfils, C., Francois, P., Decraene, B., and D. Voyer,
"Topology Independent Fast Reroute using Segment Routing",
draft-ietf-rtgwg-segment-routing-ti-lfa-11 (work in
progress), June 2023.
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8.2. Informative References
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286, DOI
10.17487/RFC5286, September 2008, <https://www.rfc-
editor.org/info/rfc5286>.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986, DOI
10.17487/RFC8986, February 2021, <https://www.rfc-
editor.org/info/rfc8986>.
[I-D.ietf-rtgwg-srv6-egress-protection] Hu, Z., Chen, H., Chen, H.,
Wu, P., Toy, M., Cao, C., He, T., Liu, L., and X. Liu,
"SRv6 Path Egress Protection", Work in Progress, Internet-
Draft, draft-ietf-rtgwg-srv6-egress-protection-09, March
2023.
[I-D.bashandy-rtgwg-segment-routing-uloop] Bashandy, A., Filsfils,
C., Litkowski, S., Decraene, B., Francois, P. and P.,
Psenak, "Loop avoidance using Segment Routing", draft-
bashandy-rtgwg-segment-routing-uloop-15 (work in
progress), June 2023.
Contributors
Mengxiao Chen
H3C
Email: chen.mengxiao@h3c.com
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Authors' Addresses
Yisong Liu
China Mobile
China
Email: liuyisong@chinamobile.com
Weiqiang Cheng
China Mobile
China
Email: chengweiqiang@chinamobile.com
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Xuesong Geng
Huawei Technologies
China
Email: gengxuesong@huawei.com
Yao Liu
ZTE Corp.
China
Email: liu.yao71@zte.com.cn
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Appendix A. Examples
Figure 6 is used as a reference topology to illustrate the
deployments of protection in SR networks. PE1 and PE3 are primary PE
nodes for VPN service access. PE2 and PE4 are used as backup. The
prefix of CE2, along with VPN service SID, is advertised by BGP
routes from PE3 and PE4 to PE1 and PE2. The VPN traffic is from CE1
to CE2.
PE1-----P1-----P3-----P5-----P7----PE3
/ | \ / | \ / | \ / | \ / | \ / | \
/ | \/ | \/ | \/ | \/ | \/ | \
CE1 | /\ | /\ | /\ | /\ | /\ | CE2
\ | / \ | / \ | / \ | / \ | / \ | /
\ |/ \|/ \|/ \|/ \|/ \| /
PE2-----P2-----P4-----P6-----P8----PE4
Figure 6: Reference Topology
The link metrics are configured as follows:
o Metrics of PE1-P2, PE2-P1, P1-P4, P2-P3, P3-P6, P4-P5, P5-P8, P6-
P7, P7-PE4, P8-PE3, PE1-PE2 and PE3-PE4 links are 11.
o Metrics of all other links are 5.
o Link metrics are bidirectional.
A.1 Example of SR BE Scenario
BE scenario: SR BE paths are used to steer the VPN service. The
deployments of protection are as follows:
o All nodes enable TI-LFA for local protection.
o All nodes enable BFD for links and neighbors.
o Ingress PE node enables FRR of SR BE path to backup egress PE
node for service protection.
o Ingress PE node enables BFD for egress PE node to monitor the
liveness of SR BE path.
Assume that the data plane is MPLS-SR. The MPLS labels are assigned
using the following rules (just for the convenience of
illustration).
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NodeID: n for PEn, n+10 for Pn
Prefix-SID: 16000 + NodeID
Adj-SID: 24000 + NeigborNodeID
VPN label: 90000 + NodeID
For example, the labels assigned on PE1 and P8 are as follows.
PE1:
Prefix-SID: 16001
VPN label: 90001
For PE1->P1:
Adj-SID: 24011
For PE1->P2:
Adj-SID: 24012
P8:
Prefix-SID: 16018
For P8->P5:
Adj-SID: 24015
For P8->P6:
Adj-SID: 24016
For P8->P7:
Adj-SID: 24017
For P8->PE3:
Adj-SID: 24003
For P8->PE4:
Adj-SID: 24004
PE1 installs the SR BE path to PE3 with the label stack of [16003,
90003] as the primary next-hop for the VPN flow. Meanwhile, PE1 also
installs the SR BE path to PE4 with the label stack of [16004,
90004] as the backup next-hop.
PE1 enables BFD for Prefix-SID 16003 and 16004 to monitor the
liveness of SR BE paths.
TI-LFA is enabled on all nodes. Take P1 for example. The shortest
path from P1 to PE3 is via neighbor P3. In order to provide local
protection for P3 node failure, P1 computes and installs the repair
path P1->P2->P4->P6, using [16014, 24016] as the label stack.
All nodes use BFD to monitor the liveness of links and adjacent
nodes.
Under normal circumstances, PE1 encapsulates the VPN payload in a
label stack of [16003, 90003].
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Assume that a failure occurs on P3. The fail-timer of BFD from P1 to
P3 expires, so P1 perceives the failure. When P1 forwards the VPN
packet, the TI-LFA repair path is used. Then, P1 pushes [16014,
24016] onto the label stack. The packet is forwarded in the repair
path P1->P2->P4->P6 according to the top two labels. So the failure
is repaired by local protection.
Assume that a failure occurs on PE3. TI-LFA does not work and the
packets along the SR BE path are dropped. Then the BFD session from
PE1 to Prefix-SID 16003 is down, so PE1 triggers the switchover to
the SR BE path to PE4 and encapsulates the VPN payload in the label
stack of [16004, 90004]. After that, the VPN traffic from CE1 to CE2
is recovered.
Assume that a failure occurs on link PE3-CE2. Since the BFD session
from PE1 to Prefix-SID 16003 is still alive, PE1 continues to
forward the VPN packets to PE3. When PE3 receives the packet, it
pops all the labels, looks up the VPN table and forwards the packet
to CE2. However, the link PE3-CE2 is failed. So PE3 selects the FRR
alternate next-hop which is the SR BE path to PE4. Then PE3
encapsulates the packet in the label stack of [16004, 90004], and
forwards it through the link PE3-PE4.
A.2 Example of SR TE Scenario
TE scenario: SR TE paths are used to steer the VPN service. The
deployments of protection are as follows:
o In the SR Policy of SR TE strict path, disjoint backup candidate
path is used as hot standby for end-to-end protection.
o Ingress PE node uses SR BE paths as backup for end-to-end
protection of SR TE paths.
o Ingress PE node enables BFD for SR Policy. In the case of SR TE
strict path, the reverse path of BFD packet keeps consistent with
forward path.
o Ingress PE node enables BFD for locator of egress PE node to
monitor the liveness of SR BE path.
o Ingress PE node enables FRR of paths to backup egress PE node for
service protection.
o All nodes enable TI-LFA for local protection. All nodes enable
BFD for links and neighbors.
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In this scenario, the SR TE strict path is used to steer the VPN
traffic flows to the primary egress node PE3, and the SR TE loose
path is used for the backup egress node PE4.
Assume that the data plane is SRv6. The SRv6 SIDs are assigned using
the following rules (just for the convenience of illustration), with
G-SRv6 compression enabled.
NodeID: An for PEn, Bn for Pn
Locator: 2001:DB8:NodeID::/48
End SID: Locator:1::
End SID with COC: Locator:2::
End DT: Locator:100:: (Only for PE nodes)
End.X SID: Locator:NeigborNodeID + F1::
End.X SID with COC: Locator:NeigborNodeID + F2::
For example, the SRv6 SIDs assigned for PE1 and P8 are as follows.
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PE1:
Locator: 2001:DB8:A1::/48
End SID: 2001:DB8:A1:1::
End SID with COC: 2001:DB8:A1:2::
End DT: 2001:DB8:A1:100::
For PE1->P1:
End.X SID: 2001:DB8:A1:B1F1::
End.X SID with COC: 2001:DB8:A1:B1F2::
For PE1->P2:
End.X SID: 2001:DB8:A1:B2F1::
End.X SID with COC: 2001:DB8:A1:B2F2::
P8:
Locator: 2001:DB8:B8::/48
End SID: 2001:DB8:B8:1::
End SID with COC: 2001:DB8:B8:2::
For P8->P5:
End.X SID: 2001:DB8:B8:B5F1::
End.X SID with COC: 2001:DB8:B8:B5F2::
For P8->P6:
End.X SID: 2001:DB8:B8:B6F1::
End.X SID with COC: 2001:DB8:B8:B6F2::
For P8->P7:
End.X SID: 2001:DB8:B8:B7F1::
End.X SID with COC: 2001:DB8:B8:B7F2::
For P8->PE3:
End.X SID: 2001:DB8:B8:A3F1::
End.X SID with COC: 2001:DB8:B8:A3F2::
For P8->PE4:
End.X SID: 2001:DB8:B8:A4F1::
End.X SID with COC: 2001:DB8:B8:A4F2::
The SR Policies on PE1 are configured as follows:
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SR Policy 1 (Strict Path to PE3)
Candidate Path 1
Preference: 20
Segment List: 2001:DB8:A1:B1F2::, 2001:DB8:B1:B3F2::,
2001:DB8:B3:B5F2::, 2001:DB8:B5:B7F2::, 2001:DB8:B7:A3F1::
Candidate Path 2
Preference: 10
Segment List: 2001:DB8:A1:B2F2::, 2001:DB8:B2:B4F2::,
2001:DB8:B4:B6F2::, 2001:DB8:B6:B8F2::,2001:DB8:B8:A3F1::
SR Policy 2 (Loose Path to PE4)
Candidate Path 1
Preference: 20
Segment List: 2001:DB8:B4:2::, 2001:DB8:B8:2::,2001:DB8:A4:1::
PE1 installs SR Policy 1, which is the SR TE strict path to PE3, as
the primary next-hop for the VPN flow. SR Policy 1 has two disjoint
candidate paths. The candidate path with higher preference is
selected as the primary candidate path, and the candidate path with
lower preference is selected as hot standby backup.
Meanwhile, the SR BE path to PE3, the SR TE loose path to PE4 (SR
Policy 2), and the SR BE path to PE4 are also installed as backup
next-hops. The priorities of multiple backup paths may be decided by
either of the egress-node-first strategy or the TE-first strategy.
Egress-node-first strategy:
o primary: SR TE path to primary egress node PE3 (SR Policy 1)
o backup(1st priority): SR BE path to primary egress node PE3
o backup(2nd priority): SR TE path to backup egress node PE4 (SR
Policy 2)
o backup(3rd priority): SR BE path to backup egress node PE4
TE-first strategy:
o primary: SR TE path to primary egress node PE3 (SR Policy 1)
o backup(1st priority): SR TE path to backup egress node PE4 (SR
Policy 2)
o backup(2nd priority): SR BE path to primary egress node PE3
o backup(3rd priority): SR BE path to backup egress node PE4
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Egress-node-first strategy is used as an example below.
PE1 enables BFD for SR Policy 1 and SR Policy 2 to monitor the
liveness of SR TE paths. For SR Policy 1 which is the strict path,
the forward and reverse paths of BFD packet should be the same. For
example, the primary path of SR Policy 1 is PE1->P1->P3->P5->P7-
>PE3, so the reverse path should be PE3->P7->P5->P3->P1->PE1. A
segment list of such reverse path is installed on PE3. PE1 may send
BFD packet with the segment list of SR Policy 1 along with the BSID
of reverse path. When the BFD packet is forwarded along the strict
path to PE3, PE3 will add an outer IPv6 header with SRH carrying the
segment list of [2001:DB8:A3:B7F2::, B7:B5F2, B5:B3F2, B3:B1F2,
B1:A1F1], which instructs the packet to be forwarded along the same
strict path back to PE1.
PE1 enables BFD for locator 2001:DB8:A3::/48 and 2001:DB8:A4::/48 to
monitor the liveness of SR BE paths.
TI-LFA is enabled on all nodes. BFD are used to monitor the liveness
of links and adjacent nodes.
Under normal circumstances, PE1 encapsulates the VPN payload in an
outer IPv6 header with SRH carrying the segment list of primary
candidate path of SR Policy 1 along with the VPN SID advertised by
PE3. Using G-SRv6 compression, the segment list will be encoded as
[2001:DB8:A1:B1F2::, B1:B3F2, B3:B5F2, B5:B7F2, B7:A3F1,
2001:DB8:A3:100::].
Assume that a failure occurs on P3. The packets are dropped since
the failed P3 is on the path. The BFD session of the segment list in
the primary candidate path of SR Policy 1 is down, so PE1 triggers
the switchover to the backup candidate path of SR Policy 1. Then PE1
encapsulates the VPN payload in an outer IPv6 header with SRH
carrying the segment list of [2001:DB8:A1:B2F2::, B2:B4F2, B4:B6F2,
B6:B8F2, B8:A3F1, 2001:DB8:A3:100::].
Before the recovery of P3, assume that P8 also fails. The BFD
session of the segment list in the backup candidate path of SR
Policy 1 is also down. Then PE1 triggers the switchover to the 1st
priority backup next-hop which is the SR BE path to PE3. PE1
encapsulates the VPN payload in an outer IPv6 header where the
destination address is 2001:DB8:A3:100::.
Assume that a failure occurs on PE3. Both the BFD sessions of SR
Policy 1 and locator 2001:DB8:A3::/48 are down, which means the
primary next-hop and the 1st priority backup next-hop are down. So
PE1 triggers the switchover to the 2nd priority backup next-hop,
which is the SR TE loose path to PE4. Then PE1 encapsulates the VPN
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payload in an outer IPv6 header with SRH carrying the segment list
of [2001:DB8:B4:2::, B8:2, A4:1, 2001:DB8:A4:100::].
Before the recovery of PE3, assume that a failure occurs on P6. The
fail-timer of BFD from P4 to P6 expires, so P4 perceives the
failure. When P4 forwards the VPN packet, the TI-LFA repair path is
used. Then, P4 encapsulates the packet in an outer IPv6 Header with
SRH carrying a compressed segment-list of [2001:DB8:B5:2::,
B5:B7F1]. The packet is forwarded in the repair path P4->P3->P5->P7
according to the outer IPv6 Header and SRH. So the failure is
repaired by local protection.
Before the recovery of PE3, assume that a failure occurs on P8. When
P6 forwards the VPN packet to destination address 2001:DB8:B8:2::
which is one of the segments in the segment list of SRH, the TI-LFA
on P6 does not work, since the failed node P8 is the destination. So
the packets are dropped. The BFD session of SR Policy 2 is down, and
PE1 triggers the switchover to the 3rd priority backup next-hop
which is the SR BE path to PE4. Then PE1 encapsulates the VPN
payload in an outer IPv6 header where the destination address is
2001:DB8:A4:100::. If the routing convergence is not completed at
the moment, P6 will use TI-LFA repair path P6->P5->P7->PE4 to
forward the packet. After the routing convergence is done, P nodes
will forward the packet along new shortest path excluding P8.
Assume that a failure occurs on link PE3-CE2. This is similar with
the same failure in section 4.1. The BFD session is still alive, PE1
continues to forward the VPN packets to PE3. PE3 will select the FRR
alternate next-hop for CE1 and forward the packet to PE4 with SR BE
path.
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