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What layer does ifconfig operate at when setting a MTU. Does it set the MTU for the physical device (Network Interface Layer) or the MTU of the packet (Internet Layer)?

By executing

ifconfig eth0 mtu 1300

it leads me to believe this is causing the Network Interface Layer to be restricted because an interface must be specified. I am not sure of this though.

Can anyone confirm this for me? Furthermore, if anyone has a good source that would be great!

Solution was found at this link: debianhelp.co.uk/mtu.htm hidden in joeqwerty's comments.

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Network interfaces operate at layers 1 and 2. The Ethernet protocol operates at layers 1 and 2. Since you're setting the MTU on the interface I'm assuming you're setting it for whatever layer 2 implementation you're using (since layer 1 doesn't deal with the encapsulation of data into frames, packets, segments, etc.). Setting the MTU on the inerface is setting the MTU of the Ethernet frame size (assuming you're using Ethernet).

The network layer is layer 3, which is where TCP\IP operates.

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ethernet itself operates only at layer two I believe, which inherently requires a layer 1 physical device to be present, but ethernet isn't picky about the type of media it uses. – Jonathan Dec 10 '10 at 4:09
Ethernet is a layer 1 and layer 2 standard. It defines both the physical and data link aspect. – joeqwerty Dec 10 '10 at 4:18
I agree with your reasoning but I need to know for sure. – Alex Dec 10 '10 at 6:07
It should be working at level 3 and linking to a level 2 device. MTU changes the size of the packets generated at level 3. MTU may be smaller for particular destinations. – BillThor Dec 10 '10 at 6:15
@BillThor: When setting the MTU on an interface, we're dealing with the physical frame (or packet) size, not the TCP MSS or ip datagram size. As such, we're setting it at layer 2. debianhelp.co.uk/mtu.htm. debianadmin.com/… – joeqwerty Dec 10 '10 at 15:21
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