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doubt in ccna question answer

Discussion in 'Routing & Switching' started by p1xels, Jul 7, 2013.

  1. p1xels

    p1xels Bit Poster

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    Hi all,
    I faced this question in my ccna class test and the teacher confused me with the answers! I had different answers in my mind.
    Please look at the problem and tell me what is the correct answer?
    Here is the problem-
    ex2.JPG
    I don't understand why option B,D and F should be the answers?
    Why only the 3 switches and not the routers require the destination MAC addresses of the packet for determining a forwarding path?

    Thanks in advance.
     
  2. Theprof

    Theprof Petabyte Poster Forum Leader

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    From my understanding, the question here asks about MAC addresses and not IP addresses... Switches are layer two devices which means they use MAC address tables to route packets. Routers operate at layer 3 which means they rely on IP address tables to route packets. Hubs simply forward the packets on to the switch, they don't make any intelligent decisions.

    In this question, each switch (Switch 1, 2, and 3) will route packets based on MAC addresses. Since the packet is traveling from Mary to Robert, the packet will be hitting each of the switches relying on those switches for the correct destination address, in this case the MAC address.

    It's a trick question because the packet does go through the router, however here they're asking about MAC address so switches will use MAC address tables to route the packet to the correct destination.

    To the network guys, correct me if I am wrong :)
     
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  3. p1xels

    p1xels Bit Poster

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    Hi TheProf,
    Really great answer professor!
    But then , what do the routers use for forwarding addresses? They just use the IP addresses? I mean say switch 1 only requires the mac address,but what about the Router 1? How does it forward the packets here? Does it use only the IP address of the next device as the destination address? In this it the Switch 2,so the Router1 use the ip address of the Switch 2 as the destination address? But I have heard that when packet passes from one device to another ,only the MAC address changes and there is not change in the source or destination IP address in the packet?

    Thanks
     
    Last edited: Jul 9, 2013
  4. Theprof

    Theprof Petabyte Poster Forum Leader

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    That's a good question, I can see where you're coming from...

    What I would recommend you do, is look at how ARP and ARP requests work. You'll understand right away how the switch forwards the data to the router or how the packet gets routed outside your network.
     
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  5. Cunningfox

    Cunningfox Byte Poster

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    Hi,

    First off lab it up and see, packet tracer is great for that kind of stuff :).

    Routers make their decisions on where to forward on IP address, not mac address, it uses the routing table to look up where to send it. You have to distinguish L2 and L3 differently. A switch will simply check the mac address and send it to the correct port - Layer 2. That's why only the switches are the correct answer.

    A router will check where to send the packet by looking the destination IP address up in its routing table and then arp for the mac address of the destination (assuming it doesn't know it) and send it there. The source mac address will be that of the router and destination will be whatever the arp response was. The source and destination IP addresses are not changed.
     
    Last edited: Jul 8, 2013
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  6. Telephasic

    Telephasic New Member

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    I do think it's a little ambiguous, because although the router is stripping the incoming frame from the switch and looking at the destination IP; it still needs to use ARP to get a MAC for the router on the other side of the switch. (probably already in its table but whatever) It still needs to encapsulate that IP packet in an ethernet frame with the appropriate destination MAC. At least that's how it could be seen in my opinion.

    Whether that counts as needing MAC addresses to forward the packet is debatable, which makes this a bit of a poor question in my estimation.
     

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