This document discusses Topology Independent LFA (TI-LFA), a fast reroute technique that provides 100% node and link protection using Segment Routing. It begins by outlining requirements for fast reroute, then introduces TI-LFA which computes the post-convergence path and encodes it as a loop-free Segment Routing path. The document analyzes applicability on Orange network topologies and presents simulation results showing TI-LFA achieves low stack depth and path compression. It concludes that TI-LFA is a scalable solution that meets requirements by providing optimal fast reroute paths without side effects.
AWS Community Day CPH - Three problems of Terraform
MPLS WC 2014 Segment Routing TI-LFA Fast ReRoute
1. Topology Independent LFA
Orange use case & applicability
Stéphane Litkowski, Orange Expert
Bruno Decraene, Orange Expert
MPLS 2014
2. 2 TI-LFA
Orange Business Service
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One of the largest dedicated network for business
• Worldwide :
• 172 countries, 900+ cities
• France :
• more than 2 million business clients, SMBs and
companies in France
IP VPN
Ethernet
Internet
Cloud
Voice & Telepresence
High value: availability, security,
SLA
5. 5 TI-LFA MPLS 2014
Why using Fast Reroute ?
Applications are more and more sensitive (VoIP, CRM, Sync, transport)
Customers do not want to rewrite or customize their applications code
to handle network failures
Fast convergence is below 1sec BUT:
– hard to have the same performance on all nodes
– hard to maintain convergence time while network is growing
6. 6 TI-LFA
FRR issue Primary path Backup path
MPLS 2014MPLS 2014MPLS 2014
South East
8ms
5ms
8. 8 TI-LFA MPLS 2014
How to improve ?
Requirements
– 100% coverage link and node protection
– No transient congestion
– Optimal routing
– Simple solution to operate and understand
– Scalable solution
9. 9 TI-LFA MPLS 2014
What is the more optimal and natural path upon a failure ?
Post-convergence path from the PLR
Benefits of using Postconvergence path :
– Policy compliant and optimized
– Well sized
– Well known
D
S Potential backup
Path
Postconvergence
PathHow to use Post-convergence path for FRR ?
How to improve ?
11. 11 TI-LFA MPLS 2014
Segment Routing
Segment Routing technology may use any path
without any need of signalling
Allows to use « unlimited » number of paths
Let’s simply compute Post Convergence Path and
enforce it to be loopfree using SR
R1
R6
R5
R2
R3
R4
D
S SR Segments
IP
12. 12 TI-LFA
Topology Independent LFA :
– Segment Routing Fast-Reroute solution
– Providing 100% coverage (node/link/SRLG)
– encoding any FRR path by using Segment Routing blocks :
– any Service Provider policy (LFA policy framework)
– including post-convergence path as new criteria
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Topology Independent LFA
13. 13 TI-LFA MPLS 2014
Topology Independent LFA
Cannot use a strict only Explicit Path due to depth of segment stack
We need to compress the stack
Done by reusing rLFA/dLFA building blocks (P & Q space)
S R1 R2 R3 D
R4 R5 R6
MPLS MPLS MPLS MPLS
2
MPLS
AdjR5
AdjR6
AdjR3
MPLS
AdjR6
AdjR3
MPLS
AdjR3
MPLS
Primary
EPC FRR
14. 14 TI-LFA
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Topology Independent LFA
FRR path is computed as follows :
– Compute postconvergence shortest path (new SPF)
– Enforce loop-freeness by :
– finding a P node on the path
– finding a Q node on the path after P (P and Q may be equal)
– Only P to Q path would be explicit and may be additionnaly
compressed using nodal segments
S R1 R2 R3 D
R4 R5 R6
MPLS MPLS
2
MPLS
NodeR5
MPLS
NodeR5
MPLS
MPLS
PQ
S R1 R2 R3 D
R4 R5 R6
MPLS MPLS
2
MPLS
NodeR5
MPLS MPLS
MPLS
P
50
Q
AdjR6
NodeR5
AdjR6 AdjR6
15. 15 TI-LFA MPLS 2014
Topology Independent LFA
Computation complexity is manageable :
– P-Space comes from old Primary SPT
– Q-Space needs one rSPT per nexthop
– New primary SPT per local failure (link or node)
Expected depth of FRR stack :
– Only 2 segments at max for link protection in symetric networks
– There is always a P adjacent to a Q
– A bit more for some node protection cases but we can add a
second level of compression (by running new fSPFs)
17. 17 TI-LFA
Analysis on topologies : case #1
TI-LFA for path optimality
Paris Paris
Paris Paris STR
STR
Dijon
Lyon
Lyon
Paris
Lyon
Poitiers
Primary
Backup
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Paris
Out of transit
node
18. 18 TI-LFA MPLS 2014
Analysis on topologies : case #1
TI-LFA for path optimality
Paris Paris
Paris Paris STR
STR
Dijon
Lyon
Lyon
Paris
Lyon
Poitiers
LFA
MRT
FRR path
RSVP-TE link
protection (1:n)
Paris
Out of transit
node
19. 19 TI-LFA
Analysis on topologies : case #1
TI-LFA for path optimality
Paris
1
Paris
2
Paris
3
Paris
5
STR
STR
Dijon
Lyon
Lyon
Paris
4
Lyon
Poitiers
EPC FRR
Nodal
EPC stack composed of one segment
Protection stack
Node_Paris4
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Paris
Out of transit
node
25. 25 TI-LFA MPLS 2014
Simulations results on 11 Orange Networks
Depth of protection stack
11 topologies have been
analyzed including multiple
network types and size
26. 26 TI-LFA
Simulations results on 11 Orange Networks
Per node analysis (node protection case)
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92% of nodes from ALL topologies have
99% of its repair_lists with a size <=2
29. 29 TI-LFA
100% FRR link/node protection is a requirement
Current FRR technics may cause some side effects :
– Transient network congestion
– Additionnal management for selection of the backup path
Topology Independent LFA :
– Scalable : no additional state in the network
– Simple to compute
– Provides 100% link/node protection
– Prevents any side effect by using a well sized and optimal path
– Simple to understand : well known path
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Conclusion
