I think I might be getting confused with terminology surrounding MTU.

This definition from Wendell Odom's CCNA book on MTU:

The IEEE 802.3 specification limits the data portion of the 802.3 frame to a minimum of 46 and a maximum of 1500 bytes. The term maximum transmission unit (MTU) defines the maximum layer 3 packet that can be sent over a medium. Because the layer 3 packet rests inside the data portion of an Ethernet frame, 1500 bytes is the largest IP MTU allowed over an Ethernet.

My understanding, is that an Ethernet frame is the last phase of encapsulation before it gets transmitted to the wire. When I look at a diagram of an Ethernet frame, its total size can equal a maximum of 1526 bytes.

Am I right in saying that an Ethernet frame MTU is 1526 while the MTU at the IP layer is 1500? Does the MTU change at each phase of encapsulation, or is the term "MTU" only meant to define the maximum size of a packet at layer 3?

Thanks for any help!

  • 3
    Although this answer addresses your question, the question its answering isn't a duplicate. Perhaps it will assist. – Mike Pennington Nov 19 '13 at 20:00

Am I right in saying that an Ethernet frame MTU is 1526 while the MTU at the IP layer is 1500?

The Ethernet MTU is 1500 bytes, meaning the largest IP packet (or some other payload) an Ethernet frame can contain is 1500 bytes. Adding 26 bytes for the Ethernet header results in a maximum frame (not the same as MTU) of 1526 bytes.

Does the MTU change at each phase of encapsulation, or is the term "MTU" only meant to define the maximum size of a packet at layer 3?

The MTU is often considered a property of a network link, and will generally refer to the layer 2 MTU. The limits at layer 3 are far higher (see below) and cause no issues.

The length of an IP packet (layer 3) is limited by the maximum value of the 16 bit Total Length field in the IP header. For IPv4, this results in a maximum payload size of 65515 (= 2^16 - 1 - 20 bytes header). Because IPv6 has a 40 byte header, it allows for payloads up to 65495. And IIRC using the Jumbo Payload header extension, IPv6 could allow packets up to 4 GB...

When setting up a TCP connection, a Maximum Segment Size (MSS) is agreed upon. This could be considered an MTU at layer 4, but it is not fixed. It is often set to the largest payload that can be sent in a TCP segment without causing fragmentation, thus reflecting the lowest layer 2 MTU on the path. With an ethernet MTU of 1500, this MSS would be 1460 after subtracting 20 bytes for the IPv4 and TCP header.

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    Specifying an Ethernet "header" of 26 bytes seems to be assuming a Q-in-Q encapsulation. The standard Ethernet header is 14 bytes, with an FCS of 4 bytes at the end of the frame. So this leads to an Ethernet frame size of 1518 bytes for a 1500 byte IP packet. Each 802.1Q vlan tag adds another 4 bytes, so a single layer of vlan encapsulation will result in an ethernet overhead of 22 bytes, and it is only when 2 VLAN tags are included that the overhead is 26 bytes (technically only 22 bytes of this is header, and 4 bytes of trailer). – Russell Heilling Feb 21 '14 at 22:23
  • You are correct. Another possible explanation is that the 8 extra bytes are for the Ethernet preamble. Technically, this might not be correct since the question mentions a 'frame', which would not include the preamble. I do not remember which explanation I assumed, the number 1526 was taken from the original question, and is regularly mentioned as the "maximum ethernet frame size". – Gerben Feb 25 '14 at 15:12
  • Ah, that is clearer. Yes, the Ethernet packet includes the preamble, but the MAC Frame does not. I have to admit that I am more familiar with seeing maximum frame sizes of 1518 and 1522 discussed, and have not seen 1526 used that often. – Russell Heilling Feb 25 '14 at 15:17
  • @Gerben, I upvoted your answer long ago, but I looked again and realized that your description of IP MTUs is incorrect. IP MTU does not account for the Total Length field. IP MTU has to deal with the largest IP frame that will fit inside the link it transits; thus 1500 bytes on standard Ethernet. – Mike Pennington Mar 12 '14 at 8:25
  • @MikePennington, I edited the answer to avoid using 'IP MTU' in that way, but I have known the term to be used in this way. So does Google, but it is indeed not very common... – Gerben Mar 12 '14 at 20:39

IP MTU = MSS (Layer 4) MTU = Interface MTU (Layer 2)

Thats the way I interpret it, happy to be corrected.

cheers, Rey

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