Question ID: #299

Nevins

I took the TechExams online test and got this question "wrong".

Question 29 (Question ID: #299)
Which of the following are true concerning Split Horizon?


A. Is used only by link-state routing protocols.
B. Routing updates are sent immediately when new routes are learned rather than waiting on the update timer to expire.
C. Is used only by distance vector routing protocols. (Correct answer)
D. Routing updates received on an interface are not sent out the same interface. (answer I selected)


I feel like both answers are correct but the question only allowed you pick one. On the Cisco exams you are told to pick the "Most Correct" when only given one option but I feel like both of these are equally correct as one is not "more correct" or "less correct" given both points are facts.


Split Horizon is as defined on Wikipedia:

In computer networking, split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned. Thus when a device that participates in such route advertisements receives an update from an interface, it (the device) does not forward updates through the same interface out. By doing so, routing loops are prevented.

(Split horizon route advertisement - Wikipedia, the free encyclopedia)


I know Wikipedia isn't the best source but I'm also basing this on everything I've ever been told about split horizon. I'm sorry if this was brought up in the past and I didn't see the notes somewhere but I'm hoping this:

A.)Gets fixed
or
B.)Someone points out the flaw in my logic.

Vask3n

I also recently retook this practice test and got this one wrong as well. I feel both are correct, right?

vinbuck

BGP is not a distance vector protocol but uses a form of split horizon, so I might question that one also. D seems to be the only clear answer as C is technically incorrect

Nevins

You're correct about BGP being a path-vector protocol, but honestly path-vector and distance vector both use hop count and bgp isn't really taught at the CCNA level so I'm guessing this is just an oversight.

vinbuck

Nevins wrote: »

You're correct about BGP being a path-vector protocol, but honestly path-vector and distance vector both use hop count and bgp isn't really taught at the CCNA level so I'm guessing this is just an oversight.

Are you sure about that? Might want to do some reading on hop count

BGP doesn't use it...

Nevins

You're right I should have said "Without tuning bgp behaves like it uses hop count." I know it doesn't actually use hop count.

vinbuck

I think you're referring to the as-path attribute and while shorter as paths are preferred, this is a behavior of EBGP whereas BGP split horizon is a feature of iBGP where as path isn't used.

Long story short - split horizon is used by distance vector and path vector protocols which makes C incorrect.

Webmaster

Nevins wrote: »

I feel like both answers are correct but the question only allowed you pick one.

Took me a while to figure this one out because both answers are indeed set to correct, which matches the explanation "Split horizon is a mechanism to prevent loops 'and' is used only by distance vector protocols.", but it allowed only one answer to be picked. I fixed that, but:

Nevins wrote: »

You're correct about BGP being a path-vector protocol, but honestly path-vector and distance vector both use hop count and bgp isn't really taught at the CCNA level so I'm guessing this is just an oversight.

Especially back when I wrote that particular question it was just OSPF, IS-IS, RIP, EIGRP, i.e. simply link-state vs distance vector, but Vinbuck makes a valid point. I'm going to disable the question for now and will replace/update it shortly.

Thanks again for the feedback and I apologize for any confusion it may have caused.

-Johan

vinbuck

Webmaster wrote: »

Thanks again for the feedback and I apologize for any confusion it may have caused. -Johan

Thanks for checking on it so quickly. And thanks for running such a great site...we are very thankful to have it