Question about Distance Vector Routing - Answer confusing

Dean Romanelli · ‎03-01-2012

Hi All,

In my reviewing for my exam, I came across the following practice question, and I find it a bit ambiguous in sections. I am wondering if someone can help?

What does a router do when running distance vector protocols? (ONLY CHOOSE 2):

A: Sends it's entire routing table to all routers in the routing domain

B: Maintain the topology of the entire network in it's database

C: Send periodic updates regardless of topology changes

D. Updates it's routing table based on updates received by neighbors

The answer unanimously is C & D.

This bothers me, however. I agree 100% with option C. But I believe that both options A & D are correct. However, the correct answer is unanimously D, not A. I am pretty darn certain routers running distance vector protocols send their entire routing tables to all routers in the routing domain, so why is this option incorrect? I feel like this should be a "choose 3."

The only thing I can determine is that they want you to recognize that the periodic updates ARE the entire routing table and thus you shouldn't choose A because C "already covered it," but from a black and white, cold hard facts standpoint, I believe routing protocols perform each A,C & D.

If I am mistaken, or if anyone can help, it would be most appreciated.

dslice · ‎03-01-2012

Typical exam question that makes you guess what they are looking for. My responses with #DS#

What does a router do when running distance vector protocols? (ONLY CHOOSE 2):

A: Sends it's entire routing table to all routers in the routing domain

B: Maintain the topology of the entire network in it's database

C: Send periodic updates regardless of topology changes

D. Updates it's routing table based on updates received by neighbors

The answer unanimously is C & D.

#DS#. Unanimous by whom?

This bothers me, however. I agree 100% with option C.

#DS# I don't. It depends on the protocol. EIGRP is DV and doesn't send periodic updates.

But I believe that both options A & D are correct. However, the correct answer is unanimously D, not A. I am pretty darn certain routers running distance vector protocols send their entire routing tables to all routers in the routing domain, so why is this option incorrect? I feel like this should be a "choose 3."

#DS# Distance Vector protocols by definition only exchange info with immediate peers and do not flood beyond this point. Each router sends to its peers based on its own view of the network rather than flooding info like link state protocols. A is absolutely incorrect.

The only thing I can determine is that they want you to recognize that the periodic updates ARE the entire routing table and thus you shouldn't choose A because C "already covered it," but from a black and white, cold hard facts standpoint, I believe routing protocols perform each A,C & D.

#DS# Like I said, DV protocols don't necessarily send periodic updates. Bellman Ford DV protocols do, but DUAL based EIGRP does not. D is actually the only correct answer.

If I am mistaken, or if anyone can help, it would be most appreciated.

Sent from Cisco Technical Support iPad App

Dean Romanelli · ‎03-02-2012

Donald,

Thanks for replying, and I agree with your explanations.

By "unanimous," I mean I've googled this question and read different wordings of it about 15 times in hopes of seeing if the original source just had the proposed answer incorrect, and all sites list C & D as the correct answers. As for option C not being 100% accurate (send periodic updates regardless of topology changes); I guess they want you to rule out EIGRP since it's a hybrid, justifying their selection of answer C.

I agree with your explanation of answer A being incorrect. I knew there was something I was missing. Only exchange info with immediate peers, NOT the whole network. Makes sense.

Thanks for your help.

dslice · ‎03-02-2012

And just to dispel one more myth, there actually isn't anything hybrid about EIGRP. It's an Advanced DV protocol in comparison to traditional Bellman-Ford based protocols like RIP, but it doesn't share any of the primary attributes of a link-state protocol. It maintains more information that it's currently using and uses different methods of determining whether a path is loop-free, but it still only has local knowledge (local links and info from immediate peers) so I don't really see anything link-state-like in it.

The only thing hybrid about EIGRP is that early Cisco docs stated it was a hybrid protocol. Sounds like marketing to me. I've never agreed with this designation, but it pre-dates my involvement with EIGRP.