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JUMBO MTU

Posted on 2014-01-20
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How does a client and a Server determine what size of MTU to send when connecting over a network.
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Question by:sectel
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by:tmx84
ID: 39794285
It's going to depend on the equipment and is typically preconfigured within the switch.

http://en.wikipedia.org/wiki/Maximum_transmission_unit

http://en.wikipedia.org/wiki/Jumbo_frame
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Cyclops3590 earned 500 total points
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Generally speaking, its configured.   Ethernet based networks have a MTU of 1500.  So when the frames are created, they will contain 1500 bytes of "data".  Now lets say that past your router the MTU of that subnet is 1400.  Well now you get fragmentation.  For IP packets, you can set the do not fragment bit which would then cause the communication to fail.  This is why in cases (especially when having ipsec or other tunnels) the MTU should be determined manually using the ping command to eliminate possible fragmentation.

IPv6 on the other hand has the ability to detect the optimal MTU for the entire path.  IPv4 doesn't really have this though.

By the way, even though I mentioned frames (layer 2) above as to MTU for ethernet, the IP layer tries to match it when passing the "data" down for encapsulation.  So if you have a layer 2 MTU of 1400 and a layer 3 MTU of 1500 (entirely possible), you get fragmentation.  1 frame which will end up equating to a packet with 1400 bytes and one with 100 bytes.  Hence why you want to manipulate layer 3 MTU to play nice with layer 2 MTU.  IPv6's detection tries to figure that out along the entire path so that the layer 3 MTU can be adjusted to minimize that fragmentation.

Hope that makes sense.
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