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In another question, Mike Pennington said that IP MTU should not exceed 1500 bytes when you sent packets through the internet.

But then, why there are jumbo frames? It is my understanding that 1500 was designed when ethernet networks share the same coaxial cable. But now, we can expect switches and full duplex everywhere.

Why IP MTU must not exceed 1500 bytes?

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    The specific quote was "rules for MTU sizes on the internet". What you do inside your LAN is one thing, what you do to traverse the internet is something else.
    – Brett Lykins
    Mar 10, 2014 at 18:30
  • well, then clearly, you didn't even read my previous question. I was asking exactly what happen when you have two networks, and one has different MTU than another.
    – Javier Loureiro
    Mar 10, 2014 at 18:35
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    I would recommend reading the wikipedia article on MTU as a good jump off point, then looking at the article on Path MTU Discovery.
    – Brett Lykins
    Mar 10, 2014 at 18:38
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    @JavierLoureiro these Q&A stack exchange sites are easier to use if you keep your thinking compartmentalized. So in THIS question, everyone's comments, and answers should only be about THIS question. Your comment above to Brett muddies the water by trying to mix in the mess happing over in that other question. I've also added another comment in your other question.
    – Craig Constantine
    Mar 10, 2014 at 19:01

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There is a very important distinction between the Internet, and your own LAN, (or even WAN circuits.)

On the Internet, 1500 is the max MTU. (End of discussion.)

On your own LAN, (or in some cases WAN too) you can do whatever you want. As technologies expanded, 1500 MTU was no longer needed. So things like jumbo frames came into use.

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1500 bytes is the standard MTU for Ethernet. Lots of Ethernet hardware now supports larger limits but unfortunately there is no standard for handling segments with mixed MTUs on a single vlan. So increasing the MTU from the default of 1500 bytes means that you have to make sure every device on the vlan is both capable and configured to support the larger size. That is a massive PITA.

For the most part the gain of running jumbo frames doesn't outweigh the pain. Some exceptions are networks that are used to carry tunneled traffic and HPC/SAN networks but those kinds of specialist high performance networks don't tend to cross organization boundaries.

There is no reason two autonomous systems can't make an arrangement to exchange packets larger than 1500 bytes but few do because it's normally not worth the cost/hassle to allow exchange of larger packets with a small number of networks (there is something of a chicken and egg situation here too).

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