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Routing loops

CCNA 2 Chapter 4

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Routing loops

A problem that can occur with distance vectorrouting protocols

It happens when systems are slow to

converge so that routers have inconsistentrouting tables

Packets can be forwarded in the wrong

direction Packets can be forwarded endlessly round

loops

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Convergence

The system is converged when all routershave consistent information about the network

Suppose a link goes down the adjoiningrouters know about it but the others dont

The system is not converged again until allrouters know about the link going down

This can be slow each router has to updateits routing table and pass it to its neighbors

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Converged

All routers have a route to A

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Link down not converged

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Link down not converged

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Link down not converged

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Link down - converged

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a. Routing loop beginning

To 1 via E1 hop

To 1 via A2 hops

To 1 via A2 hops

To 1 via B3 hopsTo 1 via D

3 hops

No routeto 1

Network 1 goes down.Router E knows.

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b. Routing loop develops

No routeto 1

To 1 via A2 hops

To 1 via A2 hops

To 1 via B3 hopsTo 1 via D

3 hops

No routeto 1

E sends update to A

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c. Routing loop develops

No routeto 1

No routeto 1

No routeto 1

To 1 via B3 hopsTo 1 via D

3 hops

No routeto 1

A sends updates

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d. Routing loop develops

No routeto 1

To 1 via C4 hops

To 1 via C4 hops

No routeto 1

C still thinks it has routes it sends updates

To 1 via B3 hopsTo 1 via D3 hops

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e. Routing loop develops

To 1 via B 5 hops

To 1 via D 5 hops

To 1 via C4 hops

To 1 via C4 hops

To 1 via B3 hopsTo 1 via D

3 hops

No routeto 1

B and D think they havenew routes

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f. Routing loop develops

To 1 via B 5 hops

To 1 via D 5 hops

To 1 via C4 hops

To 1 via C4 hops

To 1 via B3 hopsTo 1 via D

3 hops

A updates E about itsnew route to 1

To 1 via A6 hops

Packets for network 1 arepassed round until they

time out

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Count to infinity

Packet could loop for ever but routing protocolspecifies a limit

RIP regards 16 hops as infinity discard packet

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Split horizon rule

Router B learns about aroute from router C

Router B does not

include this route in itsupdates to router C

This rule reduces thelikelihood of routing

loops

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Routing loop does not develop

No routeto 1

No routeto 1

No routeto 1

To 1 via B3 hopsTo 1 via D

3 hops

No routeto 1

A sends updates

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Routing loop does not develop

No routeto 1

No routeto 1

To 1 via B3 hopsTo 1 via D3 hops

No routeto 1

No routeto 1

Updates do NOTinclude routesto 1

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Routing loop does not develop

No routeto 1

No routeto 1

No routeto 1

No routeto 1

B and D doinform Cabout 1

No routeto 1

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Split horizon rule in action

Router C originally learned the routes tonetwork 1 from B and D

It therefore does not include informationabout network 1 in its updates to B and D

No routing loop develops

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The End