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.

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
MPLS 2014
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

3. 3 TI-LFA MPLS 2014
Agenda
 Requirements
 Topology Independent LFA
 Applicability on Orange topologies
 Simulation results

4. 4 TI-LFA MPLS 2014
Agenda
 Requirements
 Topology Independent LFA
 Applicability on Orange topologies
 Simulation results

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

7. 7 TI-LFA
Backup
8ms
MPLS 2014
FRR issue
MPLS 2014MPLS 2014
FRR
33ms
Primary path
5ms
FRR

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 ?

10. 10 TI-LFA
Agenda
 Requirements
 Topology Independent LFA
 Applicability on Orange topologies
 Simulation results

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
MPLS 2014
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
MPLS 2014
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)

16. 16 TI-LFA MPLS 2014
Agenda
 Requirements
 Topology Independent LFA
 Applicability on Orange topologies
 Simulation results

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
MPLS 2014
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
MPLS 2014
Paris
Out of transit
node

20. 20 TI-LFA
Analysis on topologies : case #2
TI-LFA vs LFA/rLFA
MPLS 2014
PE1
P1
PE2
P2
P5 P4
P3
PE3
40
15
10k10k
20
20
20
30
39
Primary
Backup
10k
LFA :
• PE2 is defacto node
protection (not guaranteed)
rLFA :
• cannot guarantee node
protection

21. 21 TI-LFA MPLS 2014
Analysis on topologies : case #2
TI-LFA vs LFA/rLFA
SR segment
EPC provides node
protection with 1
segment
Protection stack
Node_P3
Backup
PQ
PE1
P1
PE2
P2
P5 P4
P3
PE3
40
15
10k10k
20
20
20
30
39
10k

22. 22 TI-LFA MPLS 2014
Analysis on topologies : case #3
TI-LFA using multiple segments
PE1
R1
PE2
R2
R3
R4 R5
R6
R7
PE3
1
7
3
3
3
1
1 1
11
2
100 100
Primary
Backup
SR
Segments
Nodal to PE3
Protection stack
compressed
Node_R3
Node_PE3
P
QProtection stack (PQ)
Adj_R3
Adj_R4
Adj_R6
Adj_R7
Adj_PE3
Top
Bottom
Top
Bottom
Nodal to R3
Nodal to PE3

23. 23 TI-LFA MPLS 2014
Analysis on topologies : case #4
Maximum observed stack depth
PE1
PE2
R1
R2
R3
R4
R5
R6
100
100
2
1
2
3
3
1
3
1
3
1000
P
Q
Protection stack (PQ)
Node_R3
Adj_R4
Adj_R5
Adj_R6
Adj_PE2
Protection stack
compressed
Node_R5
Node_R6
Node_R3
Adj_PE2
Primary
Backup
SR
Segments
Top
Bottom Top
Bottom

24. 24 TI-LFA MPLS 2014
Agenda
 Requirements
 Topology Independent LFA
 Applicability on Orange topologies
 Simulation results

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)
MPLS 2014
92% of nodes from ALL topologies have
99% of its repair_lists with a size <=2

27. 27 TI-LFA
Simulations results on 11 Orange Networks
MPLS 2014
80% of nodes have 12 or less distinct repair_lists

28. 28 TI-LFA
Simulations results on 11 Orange Networks
Path compression : computation complexity
MPLS 2014

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
MPLS 2014
Conclusion

30. Thank you !