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Avoiding micro-loops using Segment Routing

Stéphane LITKOWSKINetwork Architect, Orange Expert

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A micro-loop ? Does my network have loops ?

Micro-loops are a natural phenomenon in hop by hop routed networks

This applies to MPLS networks !

Caused by a transient disagreement between routers during convergence

Good and bad events may create loops

Duration of micro-loop depends of convergence time

S N1

N2

D

1

101

1

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A micro-loop ? Does my network have loops ?

Micro-loops are a natural phenomenon in hop by hop routed networks

This applies to MPLS networks !

Caused by a transient disagreement between routers during convergence

Good and bad events may create loops

Duration of micro-loop depends of convergence time

S N1

N2

D

1

101

1

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A micro-loop ? Does my network have loops ?

Micro-loops are a natural phenomenon in hop by hop routed networks

This applies to MPLS networks !

Caused by a transient disagreement between routers during convergence

Good and bad events may create loops

Duration of micro-loop depends of convergence time

S N1

N2

D

1

101

1

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A micro-loop ? Does my network have loops ?

Micro-loops are a natural phenomenon in hop by hop routed networks

This applies to MPLS networks !

Caused by a transient disagreement between routers during convergence

Good and bad events may create loops

Duration of micro-loop depends of convergence time

S N1

N2

D

1

101

1

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A micro-loop ? Does my network have loops ?

Micro-loops are a natural phenomenon in hop by hop routed networks

This applies to MPLS networks !

Caused by a transient disagreement between routers during convergence

Good and bad events may create loops

Duration of micro-loop depends of convergence time

S N1

N2

D

1

101

1

! SLOW !

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A micro-loop ? Does my network have loops ?

Micro-loops are a natural phenomenon in hop by hop routed networks

This applies to MPLS networks !

Caused by a transient disagreement between routers during convergence

Good and bad events may create loops

Duration of micro-loop depends of convergence time

S N1

N2

D

1

101

1

! SLOW !

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A micro-loop ? Does my network have loops ?

Micro-loops are a natural phenomenon in hop by hop routed networks

This applies to MPLS networks !

Caused by a transient disagreement between routers during convergence

Good and bad events may create loops

Duration of micro-loop depends of convergence time

S N1

N2

D

1

101

1

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Where do micro-loops happen ?

Anywhere ! They can be local or remote !

R1 R4

R2

DS

10

111

1

R3

Local loop

Remote loop

Remote loop

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1FRR !

Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1FRR !

Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1

! SLOW !

FRR !Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1

! SLOW !

Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1Traffic

Time

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Why do I need to take of micro-loops ?

Micro-loops break Fast-reroute !

Micro-loops affect traffic which is not concerned by the topology change !

Traffic between S and N2 suffers of packet loss !100Mbps of traffic entering a loop on a 5ms RTD

link will become 1Gbps in about 40msec !

S N1

N2

D

1

101

1Traffic

TimeLink congestion !

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Strategies to address micro-loop issues

Not a new topic ! Look at RFC5715 ..

Mitigation• Drop looping packets (not so easy !)• Converge faster (minimize micro-loop duration)

Avoidance• Local delay (draft-ietf-rtgwg-uloop-delay)• Ordered FIB (RFC6976)• PLSN (RFC5715)• Incremental metric (INFOCOM 2007 paper from P. Francois, M. Shand, O. Bonaventure)• …

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Strategies to avoid micro-loops

Local delay = OFIB(1)Upon link down, only local router will delay its convergenceUpon link up, local router will delay flooding

Solves only local micro-loopsMultiple implementations available for link down event

S N R1 R2 D

N2

I will delay by 1200msec

I will delay by 1200msec

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Conclusion on current state of the art

No definitive solution to avoid micro-loopsLocal delay is good but not perfect

Mitigation is not enough and sometimes complex to implement

How can we improve ?

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Why not using Segment Routing ?

Segment Routing can build a loop-free pathSee my talks in 2014 and 2015 on TI-LFA

Why not applying a temporary loop-free path ?Two stages convergenceStage#1 : use loop-free path (timer based)Stage#2 : use standard path

S N R1 D

N2

10

Node_SID_N2

Adj_SID_N2R1

Payload

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Payload

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Payload

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Payload

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Payload

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Payload

Stage#1 : tunnel {Adj-SID_R2-D}

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Stage#1 : tunnel {Adj-SID_R2-D}

PayloadAdj-

SID_R2-D

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Stage#1 : tunnel {Adj-SID_R2-D}

PayloadAdj-

SID_R2-D

Stage#1 : tunnel {Node_SID_R2;Adj-SID_R2-D}

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Stage#1 : tunnel {Adj-SID_R2-D}

PayloadAdj-

SID_R2-D

Stage#1 : tunnel {Node_SID_R2;Adj-SID_R2-D}

Stage#1 : tunnel{forward to D}

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Payload

Stage#1 : tunnel {Node_SID_R2;Adj-SID_R2-D}

Stage#1 : tunnel{forward to D}

Stage#2 : normal FIB update

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Payload

Stage#1 : tunnel{forward to D}

Stage#2 : normal FIB update

Stage#2 : normal FIB update

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Multiple approaches possible to build a loop-free path

One is TI-LFA-like approachUpon link failure, we can compute a loop-free label stack to the destination (as in TI-

LFA)The loop-free path will be kept for a time waiting for the other to convergenceAll the routers applies this two stages convergence

R1S

DR2

10 R4

100

Payload

Stage#2 : normal FIB update

Stage#2 : normal FIB update

Stage#2 : normal FIB update

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Dream or reality ? Technology is a reality !

And we already tested it !

R2R1S

R3 R4

R5 R6

10

10

S,R1,R2 are Cisco XR nodes with early code

R3,R4,R5,R6 are non Cisco

In the setup, SPF delays have been highly increased to make the microloop phenomenon more visible

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Lab evaluation : S-R1 link failure

Without micro-loop avoidance

FRRMicro-loop effect

WITH micro-loop avoidance (LOCAL-DELAY)

FRR

! LOOP AVOIDED !

FRR

! LOOP AVOIDED !

WITH micro-loop avoidance (SR)

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Lab evaluation : R2-R4 link failure

Without micro-loop avoidance

Micro-loop effect

WITH micro-loop avoidance (LOCAL-DELAY)

Micro-loop effect

!!! LOOP NOT AVOIDED !!!

FRR

WITH micro-loop avoidance (SR)

! LOOP AVOIDED !

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Segment routing micro-loop avoidance

Early code testing shown very good results

Works for multiple traffic types (IP, MPLS LDP, SR)

The technology brings a high benefit in avoiding micro-loops

Local mechanism : no protocol extensionno interoperability required (expect support of SR)

Incremental deployment with incremental benefit

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Conclusion

Micro-loops are a real issue

Current solutions are not satisfying (complex or limited)

Segment routing helps in creating temporary loop-free path

Two stages convergence remains the GOOD idea !

SR micro-loop avoidance works ! : see demo from Cisco at their booth

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Thank you