Frame Relay Question

I was perplexed by the answer the book gave for this question, and don't understand why at this point...

ICND2-DIKTA (page 459)

6. FredsCo has five sites, with routers connected to the same Frame Relay network. Virtual Circuits (VC) have been defined between each pair of routers. What is the fewest subnets that FredsCo could use on the Frame Relay network?

a. 1
b. 2
c. 3
d. 4
e. 5
f. 10

I felt the answer was either (5) or (4), because the book suggest one subnet per VC. The answer turns out to be (10), and I don't understand why?

Below is what I am picturing in my head, as the setup for the question, not sure if I have it setup right in PT, never configured Frame Relay til now...

Time for some more goose-n-juice. Maybe that will help me understand!

attachment.php?attachmentid=367&stc=1&d=1269671759

attachment.php?attachmentid=367&stc=1&d=1269671759

fr6.jpg

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Comments

mikem2te

fieldmonkey wrote: »

FredsCo has five sites, with routers connected to the same Frame Relay network. Virtual Circuits (VC) have been defined between each pair of routers.

The diagram you made in PT is the physical toplogy, this question is asking about the logical topology. Ignore the physical connections to the frame relay switch, the number of virtual circuits is the key here.

The statement above states there is a virtual circuit between each pair of routers implying a full mesh of virtual circuits, every router has a virtual circuit to all other routers, so there are the following VCs

Between Router 0 & Router 1
Between Router 0 & Router 2
Between Router 0 & Router 3
Between Router 0 & Router 4
Between Router 1 & Router 2
Between Router 1 & Router 3
Between Router 1 & Router 4
Between Router 2 & Router 3
Between Router 2 & Router 4
Between Router 3 & Router 4

So 10 virtual circuits == 10 subnets

Make sense?

johnwest43

If thats the case then its a full mesh right? and if its a full mesh then we need only 1 subnet. Am I way off?

notgoing2fail

johnwest43 wrote: »

If thats the case then its a full mesh right? and if its a full mesh then we need only 1 subnet. Am I way off?

That's correct. The answer is choice A.

With a full mesh, you can use just one subnet.

fieldmonkey

That was the other confusing part because it doesnt state whether it was a full or partial mesh, so naturally I assumed it was NOT a full mesh configuration. Therefore A would have been my selection, but since it doesn't state that I began trying to determine what configuration they might use and ruled out a full mesh.

In the end if you have 10 VCs, it makes more sense to use 1 subnet, or am I wrong?

Bad question IMHO

fieldmonkey

Thanks for clarifying, it makes sense now. The question could be worded better

notgoing2fail

fieldmonkey wrote: »

Thanks for clarifying, it makes sense now. The question could be worded better

He didn't want to tell you it was a full mesh because then the answer would have been too easy. So he tried to word it in a way for you to think of the connection points logically as Mike referred to in his post above...

fieldmonkey

The answer would have been totally different if he supplied the info that you are using a full mesh.

mikej412

The question in my book is for BobCo and a full mesh configuration -- and President Bob fires anyone who doesn't use one subnet per VC. That rules out the one subnet option and leaves the 10 subnet option as the answer.

mikem2te

notgoing2fail wrote: »

He didn't want to tell you it was a full mesh because then the answer would have been too easy. So he tried to word it in a way for you to think of the connection points logically as Mike referred to in his post above...

Yeah, bad question. As I see it there is still not enough info, are the interfaces configured point to point, multi point. Both 1 and 10 subnets could be correct depending on the config. 1 subnet may be best practice, 10 subnets nay be better for routing protocols (split horizon etc).

The original question did say fewest so that rules out 10, looks like the author got his own wrong!!

fieldmonkey

Ahhh! Must have had too much goose n juice...lmao....

I was looking at the ANSWER for question #7, for Question 6... The correct answer was A, as we have discussed.

My problem was with question 7, but you all have helped me understand even more now...it was right there all along!!! "Point to point subinterfaces"