1

Understanding Route Redistribution

ICNP 2007October 17th, 2007

Franck Le, Geoffrey G. Xie, Hui Zhang

2

Internetwork and Routing

• Common view: – Intra-domain routing using OSPF, RIP– Inter-domain routing using BGP

• In reality, internetworking is much more complex– ISP networks:

• OSPF routes to be redistributed into BGP (and vice versa)

– Enterprise networks: • When BGP is not used, needs mechanism to distribute

routes among OSPF, RIP, EIGRP domains• Also, needs to distribute routes among multiple OSPF

domains

3

What is Route Re-Distribution (RR)?

router ospf 27

redistribute rip metric 200 subnets route-map rip2ospf

distance ospf external 200

!

route-map rip2ospf permit 100

match ip address 100

set tag 22

set metric-type-1

A

B D

E

Office branch 1 Office branch 2

RIP OSPF

RIP OSPF Local

FIB

C

By default, OSPF routers have no visibility of RIP routers

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How Does RR Compare to BGP?

• In many scenarios, RR, not BGP, is used to interconnect network domains,

• Even when BGP is used, RR is required to connect BGP and IGP

• RR can implement policy, like BGP• Unlike BGP, RR is NOT a protocol

– RR is just a configuration mechanism, used separately at each router

RR is more commonly used than BGP, but much less understood, and much more error-prone

5

Problem Statements

• Given an internetwork with RR configurations, what are the loop-free and convergence properties?

• What are the guidelines of using RR if one wants to have loop-free and convergent internetwork?

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Synthesis of the Paper

• Model that reasons about the loop-free and convergence properties

• Sufficient condition to guarantee loop-free and convergence properties

7

Outline

1. Introduction to Route Redistribution (RR)

2. Illustration of routing anomalies

3. A Model for RR

4. Sufficient condition for loop-free and convergent RR

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Route Selection Process

A

B

C

D

E

Office branch 1 Office branch 2RIP OSPF

RIP

FIB

OSPF Local

P

P

P Signaling

Data path

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Route Selection Process

A

B

C

D

E

Office branch 1 Office branch 2RIP OSPF

RIP

FIB

OSPF Local

Selected routing process

P P

PP

P Signaling

Data path

OSPF110120 0/1

10

FIB

Route Redistribution Process

A

B

C

D

E

Office branch 1 Office branch 2RIP OSPF

RIP OSPF Local110120 0/1OSPF

RIP Update

P

P Signaling

Data path

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Outline

1. Introduction to Route Redistribution (RR)

2. Illustration of routing anomalies

3. A Model for RR

4. Sufficient condition for loop-free and convergent RR

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Instabilities

• Wide range of possible routing instabilities

• No general guideline to configure RR

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RIP

OSPF

RIP

Formation of Routing Loops

A

B

C

D

E

RIP(120) OSPF(110)

OSPF Local

FIB

RIP OSPF Local

FIB

P

Next-hop: B

Next-hop: C

Next-hop: E

Next-hop: D

P

P

P Signaling

Data path

14

Outline

1. Introduction to Route Redistribution (RR)

2. Illustration of routing anomalies

3. A Model for RR

4. Sufficient condition for loop-free and convergent RR

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Challenges

• Too many network elements– Hundreds or thousands of routers

• Different router processing order – Routers may process signaling messages in

different order (message delay, router load)– Different order can result in different outcome

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Solutions

• Too many network elements– Abstractions: routing instances– Logics: route selection, RR, network-wide RR

• Different router processing order – Activation sequence1

1 L. Gao and J. Rexford, Stable Internet Routing Without Global Coordination, in Proc. ACM SIGMETRICS, 2000

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A Model for RR

• Abstracts the dynamic exchange of routing information for a prefix P

• Allows to predict paths

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Route Propagation Graph

• Routing instance

• Originating routing instance

• Configured redistribution

• Actual redistribution

• Route vs. no route

• Variables: CL, S

2(110)

1(120)

1(120)

2(110)

80, A, 90

1(120)

2(110)

80, A, 90

1(120)

2(110)

80, A, 90 2(110)

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Illustration of Model

2OSPF1(110)

3RIP

(120)

4OSPF2(110)

F

F

L

L

H

H

E

E

0Local

(0)

1RIP

(120)

A

A

B C D E

F G H I

K L M N

J

RIP RIPOSPF1 OSPF2

P

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Illustration of Model Sequence 1

Signaling

Data path

1RIP

(120)

2OSPF1(110)

3RIP

(120)

4OSPF2(110)

F

F

L

L

H

H

E

E

0Local

(0)

A1

RIP(120)

2OSPF1(110)

3RIP

(120)

CL(t=0) = {A} CL(t=1) = {E, F} CL(t=2) = {E, L} CL(t=3) = {E, H}

CL(t=4) = {E}CL(t=5) = {A, F}CL(t=6) = { }

S(t=1) = {A} S(t=2) = {F} S(t=3) = {L}

S(t=4) = {H}S(t=5) = {E}S(t=6) = {A, F}

4OSPF2(110)

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Route Redistribution Configuration - Cycle Detection (RRC-CD) Problem

• Given a RR configuration, determining whether there is an activation sequence such that the redistributions converge to state including a cycle of active redistributions is NP-hard

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Outline

1. Introduction to Route Redistribution (RR)

2. Illustration of routing anomalies

3. A Model for RR

4. Sufficient condition for loop-free and convergent RR

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Sufficient condition for safety

• Pruning of Route Propagation Graph– For each redistributing router, only conserve

redistributions from the routing processes with lowest administrative distances

• Rationale– Focus on preferred redistributions

1(100)

2(70)

3(120)

4(90)

A A A

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Sufficient condition

If resulting graph satisfies1. Every redistributing router redistributes from a

single routing instance (predictable outcome)

2. For all vertice, there is a redistribution path from a originating vertex (active redistribution)

3. The graph is acyclic (no cycle)

Then, the redistributions converge to an acyclic routing state

No route oscillations No forwarding loops

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Application of Sufficient Condition

1RIP

(120)

2OSPF1(110)

3RIP

(120)

4OSPF2(110)

F

F

L

L

H

H

E

E

0Local

(0)

A

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Application of Sufficient Condition

Modifications

1RIP

(120)

2OSPF1(110)

3RIP

(120)

4OSPF2(110)

80, F

F, 80

L

L

H

H

80, E

E, 80

0Local

(0)

A

27

Application of Sufficient Condition

Pruning

1RIP

(120)

2OSPF1(110)

3RIP

(120)

4OSPF2(110)

80, F

F, 80

L

L

H

H

80, E

E, 80

0Local

(0)

A

28

Application of Sufficient Condition

Pruning

1RIP

(120)

2OSPF1(110)

3RIP

(120)

4OSPF2(110)

80, F L

H

0Local

(0)

A

80, E

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Application of Sufficient Condition

1. Every redistributing router is redistributing from a single routing instance.

2. For all vertice, there is a redistribution path from a originating vertex.

3. The graph is acyclic.

1RIP

(120)

2OSPF1(110)

3RIP

(120)

4OSPF2(110)

80, F L

H

0Local

(0)

A

80, E

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Summary

• Internetwork is far more complex with RR than the conceptual model of BGP/OSPF

• RR serves a fundamental need, but is not well-understood or even well-designed

• First formal study route-free and convergence properties of RR internetwork– Model – Sufficient condition

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Future Work

• If one were to re-design the RR, what should be the solution that supports all the RR applications but avoid the pitfalls?