Hello !

I have a question regarding the eth. frame size. I was convinced (by my teacher) that the maximum SDU size is 1500bytes or 1518 bytes with the header, and 1522 bytes with a header and a 802.1Q tag. Also I understand that there is a preambule field of 8 bytes which heralds the coming of the frame and a IPG with a variable size of 12 - 20 bytes (size depends on the manufacturer of the NIC).
Are these two components (preambule and IPG) added to the overall data frame size? Wikipedia says they are (max size is 1538-1542) and alot of others say they arent (1518-1522)

Which one is true? Can someone clarify?

Thank you very much.

James

For a first post I'm impressed by the quality of this question.

*tips hat*

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- Pete

Who says they are and aren't? I'm interested to read where each is said.

As far as I know 1500 is the actual data payload, hence any other bits and pieces are added onto that, it's not part of the original 1500 payload.

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www.mellowd.co.uk/ccie/

i believe preamble and SFD (start frame delimiter) is not a part of an actual ethernet frame. NIC's send them so the other end can distinguish when the actual ethernet frame starts. Besides, there is no such thing as max ethernet frame size of 1518bytes (or 1522, if with 802.1q tag)- these are just the defaults, thus, the most commonly used frame sizes. With enabling jumbo frames, the ethernet frame size goes way beyond the default values.

@mellowd : my teacher said that preambule and IPG are not added on the data link layer, but are added on the physical layer. He said that the ethernet header consists of a destination & source MAC, ether type info and finally a hash function result. He also said that preambule and inter packet gap are added on the physical layer and that they belong to that frame but are not added to its size, and that if someone asks me what is the maximum size of a standard ethernet frame I should reply 1522. But now Ive gone to wikipedia's entry on ethernet frame and they say that preambule and IPG are added to the overall size. Im confused

@errtime : So you believe I really shouldnt think about it that much. Well it itches me, I want to understand. I cant help myself..

I would agree with your teacher in that the IPG and the preamble are part of the physical communication (OSI layer 1 if you will) and that they are not part of the L2 frame.

I have seen somewhere else that referred to the IPG and preamble as part of a frame however I can't remember whether it was simply doing a cross-section of what it would look like on the wire (including L1 additions) or whether they were speaking purely hypothetical at L2.

I have always been taught the same as you, that the maximum size is 1518 for a standard frame, then plus any additional tags you may have (Dot1Q etc.)

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David
CCIE R&S #38338, CCIP, CCNP

http://networkbroadcast.co.uk - My Blog
http://twitter.com/davidrothera

Well, if you follow the IEEE there is; jumbo frames are not defined in the standard.

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http://networking.ventrefamily.com

The issue is often confused because the link speed (mbps/gbps/etc) includes the IFG, Preamble, and CRC - but the interface (load-interval) ignores some of it. Some might call those few bytes "noise", but when you're dealing with small payloads it can really add up. Here is an example:

http://networking.ventrefamily.com/2010 ... alyst.html

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http://networking.ventrefamily.com

@javentre : So the link speed includes this minimum 20 bytes, but they are not accordingly incremented in the NOS because they are processed on the physical layer? This varies from implementation to implementation right? So in the other words, every network device will and must first process the preambule and ifg of the frame and afterwards the 18 byte header inside of the frame starting with the hash. The load report of the interface will omit the preambule and ifg, because the statistics that the interface collects accounts only for bytes received on layer 2. So layer 2 devices see the overhead of a maximum of 1500 byte SDU as a value of 18 or a value of 22 bytes. That makes sense.