IP Fast Reroute Applicability. Pierre Francois Institute IMDEA Networks

Transcription

1 IP Fast Reroute Applicability Pierre Francois Institute IMDEA Networks

2 Agenda IGP (Fast) Convergence IGP Fast Reroute (Hitless maintenance operations)

3 IGP Fast convergence Pushing the IGP to the limits Implementation Configuration Check with your vendor if your requirements for convergence time are really pushy Depends on router performance Depends on network topology

4 Components of the IGP convergence Failure detection Link-State Packet generation / propagation LSDB update, SPF run, RIB update FIB updates

5 Failure detection E C D FAILURE?? A B BFD, L2 alarms,.. Fast detection, no more control plane stress

6 Link-State generation/propagation... B C failed! B C failed! B A D Generation No more fixed wait time 5s, 5s, 5s,... Exponential back off 0ms, 500ms, 1s Propagation Fast-flood vs. fixed Pacing timer No more artificial delaying under normal operation

7 Link-State update propagation 10 E 10 C FAILURE D 1 5 A B 10 Distance to the rerouting nodes msec before updating routers know about the failure Topology dependent Repair vs. Re-optimization

8 Paths recomputation time ispf Full SPF takes a few msec now anyway

9 FIB Updates Bottleneck component of the convergence time x prefixes at y µsec / prefix... Prioritized prefixes

10 IGP Fast Convergence convergence time much below 1s now... but convergence time scaling factors exist number of prefixes in the IGP recovery mechanisms can help

11 Loop-free Alternates C is a loop free alternate? C of A, for the failure of A B, if C would not forward the traffic sent along A B back to A, when A deviates it to C A FAILURE B FIB design to allow direct deviation of traffic when A B is flagged down

12 LFA FIB support per-link C prefix p/p oif --B A FAILURE B FIXED recovery time --B DOWN --C --C UP --B

13 per-link is constraining p x x m A B m FAILURE m C D m

14 LFA FIB support per-prefix prefix oif back. oif C p/p --B --C A FAILURE B FIXED recovery time --B --C DOWN UP

15 per-prefix LFA My favorite Increases coverage Allows for LB ing LFA ed traffic Allows to pick the optimal LFA for each prefix Favors post-convergence paths More complex Not really to compute, but to manage in the FIB LFAs for LDP-established LSP make it necessary

16 Label for p/p LFA FIB support per-link over --C? C prefix p/p oif --B:L1 A FAILURE B FIXED recovery time --B DOWN --C --C UP --B

17 LFA FIB support per-link prefix oif back. oif A C FAILURE B p/p --B:L1 --C:L2 z/z --B:L2 --C:L3 --B DOWN FIXED recovery time --C UP C must have LDP downstream unsolicited turned on or FEC-label must be requested to LFAs

18 LFAs : where does it apply? CR2 meshed part of cores CR1 FAILURE CR3

19 LFAs : where does it apply All the links of a PE P links PE FAILURE P1 P2 non transit node can protect each other These links track many prefixes

20 LFAs : where does it apply P1 Less prefix tracked on these links P PE links PE FAILURE P2

21 LFAs : where it does not apply Ring-ish parts of topologies E C D FAILURE A B

22 Node protecting LFAs An LFA which survives the failure of the entire neighboring node

23 Node protecting LFAs C C D A B FAILURE p A B FAILURE p

24 De facto node protecting LFA C D A B FAILURE p

25 LFA applicability draft-ietf-rtgwg-lfa-applicability-03 cisco, IMDEA, FT, ATT, DT Under IESG review Focus on LFA applicability for PoP Designs

26 Triangle p Fully protected against x x+e link and node failures (intra and inter) C1 c C2 d/u d/u E1 E2

27 Full-mesh p x x+e Fully protected against link and node failures C1 c C2 (intra and inter) d/u d/u A1 a A2 d/u d/u E1 E2

28 Square p C1 has no LFA for destination A1 when c <= a x x+e c C1 C2 Not a big deal if per- d/u prefix LFAs are used a A1 A2 A3 A4 d/u Convergence happens on E1 E2 E3 a small number of prefixes

29 Summary Most typical PoP designs can be covered by LFAs Node protection is usually obtained, sometimes de facto Square has one link not fully covered

30 How to reach 100% coverage? Do you need to? Using more complex techniques U-Turns, NotVia... Tunnels MPLS-FRR

31 Tunnels E p C D FAILURE A B

32 Tunnels In theory, you can get crazy end-to-end paths Within a PoP, less of a concern

33 LFAs in the core? Less obvious Coverage is highly topology dependent Topology design driven by many other factors Should not be a primary design objective

34 Profile 1 Designer relies on fast IGP convergence LFAs come as a bonus when applicable (50msec vs 500msec)

35 Profile 2 Designer seeks for high FRR coverage and cannot (re-)engineer his backbone Forget about LFAs MPLS TE FRR

36 Profile 3 Designer looks for high LFA coverage and can (re-)engineer the topology e.g. for some specific demands in some parts of the networks Tools to perform capacity planning accounting for LFAs

37 Tool

38 IGP Hitless maintenance operations E C D PLANNED FAILURE A B

39 IGP Hitless maintenance operations E C PLANNED FAILURE D 1 Max_Metric-1 5 A B 10

40 ordered FIB updates Modifications to OSPF / IS-IS to enforce a loop-free ordering of the FIB updates upon planned maintenance E C 0 D PLANNED FAILURE A B 1 2

41 Metric-increments Reconfiguration of OSPF / IS-IS to enforce a loop-free transition to post-convergence state E C D 5 A B PLANNED FAILURE 1 10

42 Metric-increments E C D PLANNED FAILURE1,6,16 5 A B 10 Usually, one intermediate metric is sufficient

43 Conclusion Sub-second convergence is conservative now Local FRR mechanisms make it close to Failure Detection time Easy, simple in the PoP Use with care in the Core, consider as bonus (Graceful shutdown is achievable)