Distance Vector routing error?

[Dogma]

In doing the practice Bonus exams that came along with the 6th edition book, I noticed two contradictions. Maybe I'm interpretting them wrong?

Bonus exam 3
What does RIPv2 use to prevent routing loops?

answers were: Split Horizon & Holddown timers

Bonus exam 4
Which of the following are used to reduce chance of possible routing loops in a network with Distance Vector routing protocol?

answers were: Split Horizon, Route Poisining.
(Holddown Timer was given as option)

In the exam 4 question, how come Holddown Timers was not included as a correct answer as it was in exam 3?

[sukarabi]

I'm not so agree with the 3rd question's answer.
For me, I didn't think that Holddown timers would stop looping problems.
Their role is more for avoiding the problems of non-convergence for a network, or for preventing inoperatives routes to be prematurely restored on other router's routing tables ( as I see on page 382 ).

So the 4th question's answers are looking good for me.

Am I wrong ?

[pconlan]

Hey Guys,

sukarabi, while your explanation of what a Holddown timer is, is very good. You have to understand that all of these things are typically considered loop avoidance mechanisms. Yes, they are called "loop advoidance mechanisms" when we describe them, but possibly a better name to describe them all would be distance vector "efficiency mechanisms". As they all assist in making the protocols operate better.
There are actually more than just the three that you saw in those answers. The full list as they are typically taught is:

Setting a maximum infinity count
Split Horizon
Route Poisoning
Poison Reverse
Triggered updates
Holddown Timers
Again, like I said, I think they are better named efficiency mechanisms; rather than loop avoidance mechanism. However, you will read in most places that they are known as the latter.

Hope this helps
Pat

[sukarabi]

Thanks pconlan for this answer who will help me to give the best answer if the question would come on ccna.