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  1. #1
    Join Date
    Jun 2011
    Posts
    34

    Default Finding the destination MAC address

    Hi,

    Network layer uses ARP to find the local MAC address before handing down the packet to Data link layer. I am wondering,how they can find the remote MAC address for framing in data link layer.

    Can anybody reply?

  2. #2
    Join Date
    Dec 2010
    Posts
    49

    Default

    Each time a packet traverses a router, the framing from the ingress interface is stripped off and the packet is then framed (or encapsulated) as appropriate for the egress interface. The ingress interface might be ethernet and the egress interface might be token ring, ethernet, firewire, or a synchronous serial interface to a T1. ARP is used at each router hop as needed. The actions at each hop need only get you to the next hop. So, there is no need to know the MAC address for anything beyond the next hop.

    See this thread...


    http://www.lammle.com/discussion/showthread.php?t=5678

  3. #3
    Join Date
    Dec 2010
    Location
    Left Coast, California
    Posts
    306

    Default

    Just to add the last step to OMD's excellent answer:

    when the last router (destination router with the destination network attached) gets the frame, strips it off, and sees that the destination network is directly attached to him, he will arp onto that subnet, (ethernet lan), for the last leg of the journey - that is where the ip address will be matched to a hardware address and forwarded to destination device.

    Intervening layer 3 routers don't care about or need the Mac address (src or dest.) ...as a matter of fact, its not even included in the header or any other field of the frame.
    Last edited by ciscodaze; 04-02-2012 at 04:23 PM.
    Kevin NET+SEC+A+CCNA
    'All that is not eternal is eternally out of date' ~ C.S. Lewis

  4. #4
    Join Date
    Jun 2011
    Posts
    34

    Default

    Quote Originally Posted by ciscodaze View Post
    Just to add the last step to OMD's excellent answer:

    when the last router (destination router with the destination network attached) gets the frame, strips it off, and sees that the destination network is directly attached to him, he will arp onto that subnet, (ethernet lan), for the last leg of the journey - that is where the ip address will be matched to a hardware address and forwarded to destination device.

    Intervening layer 3 routers don't care about or need the Mac address (src or dest.) ...as a matter of fact, its not even included in the header or any other field of the frame.
    Hi OMD and ciscodaze,

    Thanks a lot for the nice illustration. So, it basically means that data-link header for the destination MAC address part would be empty (nothing) for the case where the destination is remote. Perfect?

  5. #5
    Join Date
    Apr 2012
    Posts
    1

    Default

    I can't say that both answers above are wrong but I feel that they are hard to understand, at least for me.

    However, yes, ARP is used but there is 2 cases that you can't "really" dismiss and they are the following:

    1. The destination is within the source's LAN: in this case, thr source will send a broadcast message with all 1s turned on or Fs in hex (ff:ff:ff:ff:ff :ff) with its IP and Mac addresses (source info), and it will fill the destination IP and keep the "destinatio n"'s MAC accress null or different words, it says (Hello "All", I've sent you all a message, the one of you holding this IP address [destination IP address] please reply to me. The only device that will reply back to the ARP Request with an ARP Reply is the device holding that Mac address.

    2. When the destination is in a different network (router, internetwork, remember?), it will be forwarded to the defaultn gateway (the directly connected interface's IP), the router will then forward it to the concerned network segment based on the IP address of the destination that is filled in the ARP Requet, the destination in that network segment will reply with an ARP Reply to the source filling his Mac address.

    I hope this was helpful!
    and please, correct me if I am wrong

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