Re: loopback interface MTU size
Even with regular ethernet devices the mtu may be higher than set, if the specific connection is capable.
For example when I was playing with a mail server on DSL, I could send it small test e-mails from the internet, but for larger e-mail with attachments, sendmail kept timing out awaiting data transfer because packets larger than the mtu of PPPoE would get fragmented, and therefore, rejected. So the sender kept trying to resend every 60 seconds when sendmail timed out.
The server was behind an old broadband router unaware that traffic had to fit through a 1492 mtu hole (PPPoE has an 8-byte header which shrinks its mtu). Once I set the mtu of the sendmail nic to mtu 1492 the stalled mail came through. That only seems to be a potential issue for servers receiving uninitiated connections, not client applications
However, from ping testing with data payload, while internet traffic was limited to the set mtu 1492, local LAN traffic used the full 1500.
i5 650 3.2 GHz, 8 GB, nvidia GTX 550 Ti 32" 1080p | i7-4700, 8 GB, Intel HD 4600/nvidia GTX 765M 15.6" 1080p | etc.