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Transcript of Draft-francois-segment-routing-ti-lfa-00 Pierre Francois, IMDEA Networks Institute Clarence...
![Page 1: Draft-francois-segment-routing-ti-lfa-00 Pierre Francois, IMDEA Networks Institute Clarence Filsfils, Ahmed Bashandy, Cisco Systems Bruno Decraene, Stephane.](https://reader036.fdocuments.us/reader036/viewer/2022082611/56649ef05503460f94c0110d/html5/thumbnails/1.jpg)
draft-francois-segment-routing-ti-lfa-00
Pierre Francois, IMDEA Networks InstituteClarence Filsfils, Ahmed Bashandy, Cisco Systems
Bruno Decraene, Stephane Litkowski, Orange
IETF 89, SPRING WG
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Topology Independent Fast Reroute using Segment Routing
• Fast Reroute– Local protection of traffic against sudden failures
of links and nodes– IP-FRR behavior when SR comes into play
• Topology Independent coverage– Full coverage for link and node protection
• Segment Routing– Leveraging the SR architecture allows to enforce any
failover path
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Which failover path?
• New in IP-FRR: Post-convergence path from the PLR to the destination
A D
B C
PE1
E
F
100 100PE2
100 100
LFA
TI-LFA
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Why that choice?
• Post-convergence path – Typically in line with capacity planning– Easy to predict
• SR: No need for TLDP sessions – Useless to favor LFA over RLFA over…– Easier to provide the path that the operators
actually want
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Exercise
• draft-ietf-rtgwg-lfa-manageability-01– Cases where default IP-FRR behavior is not ideal– Need for manageability to enforce an ideal path• Turns out to always be the post-convergence one…
• Common sense– The operator configures metrics to obtain
optimum paths for its services (bw, delay, …)– The PLR should respect such objectives upon FRR
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How to do TI-LFA
• Enforcing loop-freeness on post-convergence path– Where can I release the packet?
• At the intersection between the new shortest path and the per-destination Q-Space of the destination
– How do I reach the release point?• By chaining intermediate segments that are assessed to be loop-
free
• Maths: homework• How many segments?
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How many segments?• Link protection, symmetric topology:
– Maximum 2, guaranteed– Most often, 1 is enough– When a post-convergence LFA is available: 0
• Link protection, asymmetric topology– Many asymmetric nets where 2 was the max– A few cases here and there were a bit more are needed for a couple of links
• Node protection– Never more than 4, rarely more than 2
• -01 contains numbers– Eager to increase the number of case studies
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Protecting AdjSID’s
• Packet with 2 top ADJ SIDs [AD, DF, …]– The packet should go to D – Repair: [C, D, DF, …], oif B
A D
B C E
F
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Protecting AdjSID’s
• Packet with 1 top ADJ SID [AD, F, …]• Option– The packet could go to D • Repair: [C, D, F, …], oif B
– The packet could go directly to F• Repair: [C, F, …], oif B
A D
B C E
F
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Summary
• FRR for Segment Routing– Node and Adjacency Segments– No more TLDP session required for FRR purposes
• Full coverage– More than 2 segments are rarely needed
• Post-convergence paths – Better fit with capacity planning– Respect of the ISP policy
• 1 implementation available– At least one more on the way
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Thank you!