Chapter 1: Course Introduction
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© 2002, Cisco Systems, Inc. All rights reserved.
Distance Vector Routing
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2
Objectives
Upon completing this lesson, you will be
able to:
• Describe the features offered by distance vector
routing protocols and give examples of each
• Describe the issues associated with distance
vector routing and identify solutions to those
issues
© 2002, Cisco Systems, Inc. All rights reserved.
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Distance Vector Routing Protocols
• Routers pass periodic copies of routing table to neighbor
routers and accumulate distance vectors.
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Sources of Information and
Discovering Routes
• Routers discover the best path to
destinations from each neighbor.
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Selecting the
Best Route with Metrics
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Maintaining Routing Information
ã Updates proceed step-by-step
from router to router.
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Inconsistent Routing Entries
• Each node maintains the distance from itself to each possible destination network.
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Inconsistent Routing Entries (Cont.)
ã Slow convergence produces inconsistent routing.
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Inconsistent Routing Entries (Cont.)
• Router C concludes that the best path to network
10.4.0.0 is through router B.
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Inconsistent Routing Entries (Cont.)
• Router A updates its table to reflect the new but
erroneous hop count.
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Count to Infinity
• Hop count for network 10.4.0.0 counts to infinity.
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Defining a Maximum
• Define a limit on the number of hops to prevent infinite loops.
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Routing Loops
• Packets for network 10.4.0.0 bounce (loop) between
routers B and C.
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Split Horizon
• It is never useful to send information about a route back in the direction from which the
original information came.
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Route Poisoning
• Routers advertise the distance of routes that have gone down to infinity.
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Poison Reverse
ã Poison reverse overrides split horizon.
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Holddown Timers
• The router keeps an entry for the network’s possible down
state, allowing time for other routers to recompute for this
topology change.
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Triggered Updates
• The router sends updates when a change in its routing table occurs.
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Distance Vector Operation
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