Question 3) Cisco Certified Network Associate

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Objective: Technology
SubObjective: Evaluate the characteristics of routing protocols

Single Answer Multiple Choice

Which technique can be used with distance vector routing protocols to make sure routers get timely notification of changes in network topology?

A. Split horizon
B. Poison reverse
C. Holddown timers
D. Triggered updates

Answer:
D. Triggered updates

Tutorial:
Triggered updates are sent as soon as a change in network topology is discovered. This speeds convergence and helps prevent problems caused by out-of-date information. Triggered updates are combined with holddown timers to make sure new routing information is propagated to the entire network by the triggered updates.

Split horizon stops routing loops by preventing route-update information from being sent back out of the interface on which it arrived. Routing loops occur when there is slow convergence on a network, which causes inconsistent routing entries. These routing loops lead to excessive bandwidth being used and can cripple the network.

Poison reverse “poisons” a failed route by increasing its cost to infinity. This is needed only for very large loops, because small ones can be prevented by split horizon, holddown timers, and triggered updates.

Holddown timers prevent routers from making a change that will affect routes for a specified period of time. The holddown time is set to be slightly greater than the time required to update the entire network.

For more information on triggered updates, see the Interior Gateway Routing Protocol documentation at
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/igrp.htm

Reference:
1. ICND Student Guide v2.1 – Volume 1 – Distance Vector Routing – Solutions to Eliminate Routing Loops

These questions are derived from the Self Test Software Practice Test for Cisco exam 640-801 – Cisco Certified Network Associate (single-exam option)

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