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I see many articles on line stating that the overhead of Ethernet is 18 bytes (example: http://www.learnios.com/viewtopic.php?f=8&t=18887)

Looking at this wiki article (http://en.wikipedia.org/wiki/Ethernet_frame#Structure) 18 bytes is the following fields;

  • Destination MAC
  • Source MAC
  • Ethertype
  • CRC

Why do the preamble, start of frame delimiter and inter-frame gap not count towards this, are they not included in every frame? This would take the Ethernet overhead up to 26 bytes.

The original article I linked was with regards to calculating the required MTU for MPLS networks (encapsulating layer 2 traffic into MPLS frames). I want to understand why 18 bytes is being added on to the standard 1500 bytes and not 26 bytes (plus the MPLS label stacks and any do1q tags).

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Because ethernet has a 1500-byte payload size; but a 1526 packet size. But it's just easier to forget that mac address nonsense, and focus on 1500-byte "packets". –  Ian Boyd Mar 31 '12 at 20:06
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up vote 6 down vote accepted

Why do the preamble, start of frame delimiter and inter-frame gap not count towards this

Because they are not part of the Ethernet frame but of the wire level protocol. Remember that if you run Ethernet over IP for example or via VPLS etc, then those do not exist. MPLS / VPLS does not transmit wire level inforamtion. It is not simulating a cable, physically.

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Ah I see! That makes complete sense :) Thanks. –  jwbensley Mar 29 '12 at 9:25
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