howto increase MTU size on 100Mbps FE

[Joseph Mack]

I know that jumbo frames increase throughput rate on GigE and was
wondering if a similar thing is possible with regular FE.

according to
 
http://sd.wareonearth.com/~phil/jumbo.html
 
the MTU of 1500 was chosen for 10Mbps ethernet and was kept
for 100Mbps and 1Gbps ethernet for backwards compatibility
on mixed networks. However MTU=1500 is too small
for 100Mbps and 1Gbps ethernet. In Gbps ethernet jumbo frames
(ie bigger MTU) is used to increase throughput.
 
With netpipe I found that throughput on FE was approx linear with
increasing MTU upto the max=1500bytes. I assume that there
is no sharp corner at 1500 and if in principle larger
frames could be sent, then throughput should also increase
for FE. (Let's assume that the larger packets will
never get off the LAN and will never need to be fragmented).
 
I couldn't increase the MTU above 1500 with ifconfig or ip link.
I found that the MTU seemed to be defined in
 
linux/include/if_ether.h
as
ETH_DATA_LEN and ETH_FRAME_LEN
 
and increased these by 1500, recompiled the kernel and net-tools
and rebooted. I still can't install a device with MTU>1500

VLAN sends a packet larger than the standard MTU, having an
extra 4 bytes of out of band data. The VLAN people have 
problems with larger MTUs. Here's their mailing list

http://www.WANfear.com/pipermail/vlan/

where I found the following e-mails

http://www.WANfear.com/pipermail/vlan/2002q2/002385.html
http://www.WANfear.com/pipermail/vlan/2002q2/002399.html
http://www.WANfear.com/pipermail/vlan/2002q2/002401.html

which indicate that the MTU is set in the NIC driver and
that in some cases the MTU=1500 is coded into the hardware
or is at least hard to change.

I don't know whether regular commodity switches (eg Netgear
FS series) care about packet size, but I was going to 
try to send packets over a cross-over cable initially. 

Am I barking up sensible trees here?

Thanks Joe

-- 
Joseph Mack PhD, Senior Systems Engineer, Lockheed Martin
contractor to the National Environmental Supercomputer Center, 
mailto:mack.joseph at epa.gov ph# 919-541-0007, RTP, NC, USA