Greetings:

For some reason, whenever I attempt to increase the MTU on my in kernel
pppoe interface beyond 1460, it fails:

        # ifconfig pppoe0 mtu 1460
        *OKAY*
        # ifconfig pppoe0 mtu 1461
        ifconfig: SIOCSIFMTU: Invalid argument

Note that pppoe0 is tried to physical ethernet interface fxp0, which
itself has a MTU of 1500.

However, other PPPoE implimentations can use MTU values as high as 1492.
Even the NetBSD PPPoE documentation at:

	http://www.netbsd.org/Documentation/network/pppoe/

uses 1492 as an example MTU value for in kernel PPPoE.

Any ideas as to why I can't use the higher MTU value for the NetBSD in
kernel PPPoE?  The reason why this is a problem is that Windows clients
(running WinXP or Win98SE) behind a NetBSD NAT firewall are unable to
use Windows Update or down load large files from some websites.  In my
/etc/ipnat.conf:

map pppoe0 192.168.1.0/24 -> 123.123.123.123/32 portmap tcp/udp 20000:40000 mssclamp 1420
map pppoe0 192.168.1.0/24 -> 123.123.123.123/32 mssclamp 1420
map pppoe0 192.168.1.0/24 -> 123.123.123.123/32

Unfortunately, higher mssclamp values (including 1430 & 1440) do not
work with my pppoe.

Thanks in advance.
Alicia.