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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