Apparently, Redhat 5.2's pppd (2.3pl5) has internal
limits on MTU/MRU that it doesn't check for on the
command line. Specifying an MTU/MRU that is beyond
these undocumented internal limits (e.g. 1524) on the
command line results in pppd starting up, taking these
values, negotiating them with the server, and only after the
line is up and a packet too big is received, generating an
obscure error message and exiting. Here is the error it
Dec 26 14:48:01 blackbird-eth pppd: read: Value too
large for defined data type(75)
BTW, lowering the specified MTU to 1064 works around this pppd bug.
I tried this several times in the test lab and in all cases pppd did
the correct thing. If you specify a number larger that 1500 and both
ends have a max of 1500 then it will drop down to 1500 which is also
the default pppd will use if no mru and mtu are specified. When I
entered a number less that 1500 it used that number specifically. I
was unable to genrate the error that you were reporting.
One possibility is that the server you are dialing into is not using
Linux and their pppd server supports packet sizes larger that 1500
where the linux pppd client does not therefore generating the errors.