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Thread: Ethernet Frames

  1. #1
    Join Date
    Sep 2009
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    2

    Question Ethernet Frames

    Really enjoying an outstanding write up.
    I found a little confusing the description of the length field of 802.3 Ethernet frame. It states on page 36 "802.3 uses a length field, but the Ethernet frame uses a type field to identify the upper layer protocols ".
    This is quite different from my research where i found out that
    "For 802.3 frames, Length is the number of bytes in the Data field."
    Could there be any reconcillation?

  2. #2
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    Default

    IEEE defines a 802.3 standard where the Type field is replaced by Length, and an 802.2 LLC header follows with the Type field.

    HTH.
    Maddox Thomas-Clark 14/10/2008
    Bean Thomas-Clark 18/09/2007
    Big Evils Cisco World
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  3. #3
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    Default

    Quote Originally Posted by Big Evil View Post
    IEEE defines a 802.3 standard where the Type field is replaced by Length, and an 802.2 LLC header follows with the Type field.

    HTH.
    Sure. The question is: Is the type field the same as the length field?

  4. #4
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    The 16-bit field after the MAC addresses can be used as a length field or a type field.
    Maddox Thomas-Clark 14/10/2008
    Bean Thomas-Clark 18/09/2007
    Big Evils Cisco World
    Linkedin

  5. #5
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    Default

    The Ethernet frame uses the type field, which specifies the upper layer protocol (IP, IPX etc), but the 802.3 frame uses length, which specifies the length in bytes of the data field.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  6. #6
    Join Date
    Nov 2012
    Location
    Jonesboro Arkansas
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    57

    Default Ethernet Frame Types chap 2

    Okay Questions
    1 . The "first was DIX" Is this Ethernet II what the IEEE used it to create the 802.3 Ethernet from.
    2.) Is there an Ethernet I and do all 802.3 specifications of Ethernet have the length field If so are we expected to identify what each is, also is this reffered to as a control field too?
    3) Which is used more, from the output on pg.52 it looks like the Ethernet II is. I thought the reason 802.3 was created was because some protocols were to long to fit into the length type.
    Any help here

  7. #7
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    Nov 2012
    Location
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    Default Ethernet Frame Types chap 2

    Okay pg. 53 displays Ethernet physical layer specifications. Im using 7th edition. How do I interpit the diagram. Is it that the data link and Physical layers are 802.3 and covers all of the 802.3 standards and that Ethernet Im assuming Ethernet II there on side of data link and physical also covers the same standards as 802.3 ?

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