Regarding ARP request on remote host

DranizzDranizz Registered Users Posts: 3 ■□□□□□□□□□
Hi everyone, I'm a bit confuse on something.

On one of my books for the CCNA, there is a question regarding the ARP request of a remote host.


If a host request an ARP for a host not on the same subnet, the host will know from the IP address that it's not on the same subnet, so it will send the request to the default gateway. Then the router, will send back is own MAC address to the requesting host. So the host will have the IP and MAC of the router in it's ARP table, right?

How come the answer in the practice exam question is: The Router A will foward the request to router B.

Wheter there is 1 or 2 or even 3 routers between the 2 host, no router will foward any ARP request to any other router. Am I right?

Thanks for your help!

Comments

  • JaCkNiFeJaCkNiFe Member Posts: 96 ■■□□□□□□□□
    Dranizz wrote: »
    Hi everyone, I'm a bit confuse on something.

    On one of my books for the CCNA, there is a question regarding the ARP request of a remote host.


    If a host request an ARP for a host not on the same subnet, the host will know from the IP address that it's not on the same subnet, so it will send the request to the default gateway. Then the router, will send back is own MAC address to the requesting host. So the host will have the IP and MAC of the router in it's ARP table, right?

    How come the answer in the practice exam question is: The Router A will foward the request to router B.

    Wheter there is 1 or 2 or even 3 routers between the 2 host, no router will foward any ARP request to any other router. Am I right?

    Thanks for your help!

    Here is a little explanation of ARP that I put together from memory, correct me If I missed anything (encapsulation details left out intentionally) I hope it helps!

    47448_1607351586504_1317615135_31643564_1173475_n.jpg

    For this example assume that all ARP tables are empty. When host A wants to send something to host B, perhaps an ICMP echo request for example, it will check its ARP table for its default gateway because host B is on another subnet.

    -Host A will then send out an ARP broadcast with its default gateway's IP address.

    -R1 will learn host A' s MAC address from receiving the ARP and will send an ARP reply with its layer 2 address back to host A.

    -Host A then forward the ICMP traffic to its default gateway, when the ICMP traffic reaches R2 it has a destination IP of host B but R2's ARP table is empty.

    -R2 then sends an ARP request using Host B's IP address.

    -Host B learns R2's MAC address from receiving the ARP request and sends an ARP reply to R2 with its layer 2 address.

    -R2 can then forward the ICMP echo request to Host B who will generate an ICMP reply destined to R2's MAC address and Host A's IP address.
    Lab on!
  • DranizzDranizz Registered Users Posts: 3 ■□□□□□□□□□
    So no ARP Request is foward from R1 to R2 coming from Host A, right?
  • fly351fly351 Member Posts: 360
    Dranizz wrote: »
    So no ARP Request is foward from R1 to R2 coming from Host A, right?

    Correct. ARP = IP to MAC mapping. MAC addresses are not send outside of a network since they are Layer 2 addressing. So... the encapsulating frame's destination / source MAC addresses are changed between the point to point connections.


    Example I threw into Visio real fast.
    fly351-albums-examples-picture150-drawing1.png
    CCNP :study:
  • DranizzDranizz Registered Users Posts: 3 ■□□□□□□□□□
    Thanks guys! I knew I was not crazy! :P
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