I just want to ask a quick , really confusing question. Online on Wikipedia, it states that the MTU for ethernet is 1500. Then as written on the same page, Ethernet Frame types: Ethernet II, it states on line 9 that "EtherType values must be greater than or equal to 1536 (0x0600). That value was chosen because the maximum length of the payload field of an Ethernet 802.3 frame is 1500 octets (0x05DC)". We clearly know that the MTU is 1500. In this case, 1536 bytes clearly contradicts the IEEE standard for MTU. what is going on here...???????
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en.wikipedia.org/wiki/Ethernet_frame#Ethernet_II– user21483Dec 20, 2015 at 13:56
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Please check this Q&A let us know if you still have questions– Mike PenningtonDec 20, 2015 at 14:12
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I'm guessing that 1536 decimal does not equate to 1536 bytes– user21483Dec 20, 2015 at 14:30
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OK there's only 1 possible explanation left. The >= 1536 byte limit applies to the whole Ethernet II frame.– user21483Dec 21, 2015 at 5:34
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Either that or, my understanding from the article tools.ietf.org/html/draft-ietf-isis-ext-eth-01, would be that Ethernet II payloads are not affected by the 1500 byte MTU limits. Or the MTU which is imposed upon Ethernet II is more than 1500 bytes.– user21483Dec 21, 2015 at 5:45
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1 Answer
The short answer is because that is what the spec requires - values between 1500 and 1536 are undefined and should not be used.
The relevant text of the 802.3-2012 spec is as follows:
This two-octet field takes one of two meanings, depending on its numeric value. For numerical evaluation, the first octet is the most significant octet of this field.
- a) If the value of this field is less than or equal to 1500 decimal (05DC hexadecimal), then the Length/ Type field indicates the number of MAC client data octets contained in the subsequent MAC Client Data field of the basic frame (Length interpretation).
b) If the value of this field is greater than or equal to 1536 decimal (0600 hexadecimal), then the Length/Type field indicates the Ethertype of the MAC client protocol (Type interpretation).
The Length and Type interpretations of this field are mutually exclusive.
There are historic (practical) reasons for making the lower bound on type to be substantially clear of the upper bound of length, given the vagaries of vendor implementations available at the time.
