Cisco CCNA CCNP CCIE all you need to study cyber security

ALL you need to study CCNA

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Distance Vector Routing

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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

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•Routers pass periodic copies of routing table to neighbor
routers and accumulate distance vectors.
Distance Vector Routing Protocols

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•Routers discover the best path to
destinations from each neighbor.
Sources of Information and
Discovering Routes

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Selecting the
Best Route with Metrics

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•Updates proceed step-by-step
from router to router.
Maintaining Routing Information

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•Each node maintains the distance from itself to each possible destination network.
Inconsistent Routing Entries

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•Slow convergence produces inconsistent routing.
Inconsistent Routing Entries (Cont.)

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•Router C concludes that the best path to network
10.4.0.0 is through router B.
Inconsistent Routing Entries (Cont.)

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•Router A updates its table to reflect the new but
erroneous hop count.
Inconsistent Routing Entries (Cont.)

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•Hop count for network 10.4.0.0 counts to infinity.
Count to Infinity

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•Define a limit on the number of hops to prevent infinite loops.
Defining a Maximum

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•Packets for network 10.4.0.0 bounce (loop) between
routers B and C.
Routing Loops

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•It is never useful to send information about a route back in the direction from which the
original information came.
Split Horizon

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•Routers advertise the distance of routes that have gone down to infinity.
Route Poisoning

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•Poison reverse overrides split horizon.
Poison Reverse

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•The router keeps an entry for the network’s possible down
state, allowing time for other routers to recompute for this
topology change.
Holddown Timers

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•The router sends updates when a change in its routing table occurs.
Triggered Updates

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Distance Vector Operation

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Distance Vector Operation (Cont.)

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Distance Vector Operation (Cont.)

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Distance Vector Operation (Cont.)

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Distance Vector Operation (Cont.)

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Distance Vector Operation (Cont.)

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Summary
•Distance vector-based routing algorithms
(also known as Bellman-Ford algorithms) pass periodic
copies of a routing table from router to router.
•When the topology in a distance vector protocol
internetwork changes, routing table updates must
occur. As with the network discovery process,
topology change updates proceed step-by-step from
router to router.
•When maintaining the routing information,
inconsistencies can occur if the internetwork’s slow
convergence on a new configuration causes incorrect
routing entries.

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Summary (Cont.)
•The condition called count to infinity arises when
routing table updates continue to increase the metric
to a destination that cannot be reached, rather than
marking the destination as unreachable.
•A routing loop occurs when two or more routers have
incorrect routing information indicating that a valid
path to an unreachable destination exists through the
other routers.
•A number of techniques are available to eliminate
routing loops including: split horizon, route poisoning,
poison reverse, holddown timers, and triggered
updates.