Question: Choosing the Best Route, Same Subnet ID, Different Mask

certskills
By certskills April 15, 2014 09:05

#CCNA study scenarios often focus on the most common and most obvious combinations of events. However, sometimes it helps to think about some more unusual scenarios just for the purpose of exercising our understanding of how these devices think. This next question does exactly that, using an admittedly bad network design to let us focus on how one router would think about the routes it learns. Details of the question below the fold. Enjoy!

For this question, router R1 runs RIP-2, and OSPF, and EIGRP. Admittedly, that’s a bit unusual for both the ICND1 and ICND2 exams, but to discuss how routers choose routes when learning those routes from multiple sources, at least one router has to run multiple routing protocols. The figure shows the subnet IDs/masks on the right, the routing protocols, and metrics.


If you look at the subnets on the right, its pretty obvious that the subnetting design is busted. Routers R2, R3, and R4 all connect to a subnet with subnet ID 10.1.2.0. Note however that each uses a different subnet mask. That, in a word, is bad.

Now to the question to ponder. Assuming all the links in the diagram work, and R1 indeed learns the routes usig RIP-2, OSPF, and EIGRP, what does R1 put in its routing table? In other words, what will the output of R1’s show ip route command look like? (Select a single answer.)

A. It will list route 10.1.2.0/25, but not routes for the other two subnets

B. It will list route 10.1.2.0/26, but not routes for the other two subnets

C. It will list route 10.1.2.0/27, but not routes for the other two subnets

D. It will list routes for 10.1.2.0/25, 10.1.2.0/26, and 10.1.2.0/27

 

Answer: Router Adding Routes when VLSM Subnets Overlap
Answers to Latest Question: ARP, InARP, EIGRP
certskills
By certskills April 15, 2014 09:05
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9 Comments

  1. jjrinehart April 26, 14:34

    I have used your CCNA certification books for leading study groups and classes and feel they are the best on the market. I find it marvelously fulfilling helping others launch/accelerate their IT careers, having made the journey from CCNA to CCIE myself.

    – Joe

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  2. Efx July 11, 05:05

    Hello. Don’t know why but on packet tracer the answer will be RIPv2. But why he is using that route ? Administrative distance is a higher than OSPF or EIGRP… And Network bits on mask is just 25, not 26 or even 27!

    Even if I disconnect a cable from RIP network, router R1 can’t get updates from OSPF and EIGRP routers

    Reply to this comment
    • Wendell Odom of Certskills Author July 11, 10:19

      Hi EFX,
      Well, I have a couple of guesses. It could be that you don’t have OSPF and EIGRP configured correctly? (No disrespect intended.) I’d say shutdown both the RIP and OSPF link, and work on getting EIGRP alone to work first. (Packet Tracer should do that, right?) Only then add the RIP link, and see what happens.

      What did you config on R1 and R4 for EIGRP?

      And anytime you ask a question about doing something with a Simulator, a problem can always be the Simulator… Also, because the design is admittedly a bad idea, with the overlapped subnets, maybe Packet Tracer is not prepared to show realistic results in that case.

      Thanks,
      Wendell

      Reply to this comment
  3. Efx July 12, 03:10

    Yes , You are right!

    With GNS 3 there was no problem, just copied configs from routers on packet tracer. All three routes is in routing table 🙂

    Reply to this comment
  4. Carlos March 16, 19:25

    I think that R1 learned EIGRP route becouse it has the lower AD. It is correct?

    Reply to this comment
    • certskills Author March 18, 16:06

      Carlos,
      Well, that’s part of it, but not all. Note that the advertisements are for different subnets. R1 actually learns all three routes, because they are not for the same subnet. If the masks has been the same, they would be the same subnet, and indeed, the AD would have been the determining factor.
      Look to the link below, to the April 27 2011 post, and you’ll see a video on it.
      Wendell

      Reply to this comment
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