The IP MASQ MTU problem

[Mark Copper <m.copper@wayne.edu>]

I've just had the pleasure of hunting this one down.  I've included the
option mtu 1492 in /etc/network/interfaces for my NIC and I seem to be
back in business (knock on woody!).  This per the IP-MASQ HOWTO, section
7.15 (I connect via PPPoE).

But I wish I understood this better.  Why would an MTU of 1500 suddenly
become a problem?  I had been doing fine for weeks.  Why would it be a
problem for one machine and not another?  My 2.2 kernel machine has no
problem.  The HOWTO says MTU for PPPoE is 1490 but ifconfig shows 1492; if
a difference of 10=1500-1490 is a problem, might 2 be?

Anyone out there willing to shed light/share experience? (I can't bring up
the MTU thread from the linux kernel list; is the link outdated?)

Thanks.

Mark