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  1. Hi Rene,

    Great :slight_smile:

    What about the FCS when changed MAC on every L3 segment ? Its recalculate or not ? ? Header Checksum indicates value of IP header only , no upper layer (TCP/UDP Header+Data) where as FCS indicates total value including Ethernet Heaer + IP Header+ TCP/UDP Hearder+ Data , right ? Please correct me if i am wrong .Many Thanks


  2. Hello Mohammad.

    You are correct, the FCS is calculated on every hop of the journey of a datagram. It is recalculated because the header of the frame in each hop changes. The header checksum of the IP header remains the same for the whole journey since the IP packet does not change.

    I hope this has been helpful!


  3. Hi Laz,
    But as Rene said, Header checksum will get recalculated at every hop since TTL gets decremented. Please correct me if I am wrong?

  4. Hi. Rene,
    Thx for your nice Tutorial.
    I have one question. Is Routing table execute sequentially (Top to bottom) & take action immediately like ACL do with his per ACE?? Appreciate your quick response in this regards .Thx


  5. Hi Laz,
    Many Thanks for your reply as always :slight_smile:
    So , if you have 6+lac (BGP Prefix + Infrastructure prefix ) prefix in a routing table then Router will check with all 6+lac prefix from top to down and find out more specific entry and send the packet to exit interface , right ??

    I cant understand that you have wrote “Keep in mind that a routing table usually has the destinations in some sort of numerical order, so it does save time in finding the most accurate route”. could you please clear to me .Thx


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