Re: howto increase MTU size?

To: Joseph Mack <mack.joseph@xxxxxxx>
Subject: Re: howto increase MTU size?
Cc: "lvs-users@xxxxxxxxxxxxxxxxxxxxxx" <lvs-users@xxxxxxxxxxxxxxxxxxxxxx>, Julian Anastasov <ja@xxxxxx>, Roberto Nibali <ratz@xxxxxx>, jmack@xxxxxxxx
From: Horms <horms@xxxxxxxxxxxx>
Date: Mon, 29 Apr 2002 04:08:20 +1000
On Thu, Apr 25, 2002 at 03:11:59PM -0400, Joseph Mack wrote:
> according to
> 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.
> On my LVS performance page I found that decreasing the MTU
> decreased max throughput on 100Mbps ethernet. Since the decrease
> was approx linear with MTU, I wondered if the linearity would
> still be linear beyond 1500, and if increasing the MTU
> on 100Mbps would increase max throughput (ignoring
> the problem of fragmenting packets once they got off the LAN).
> 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 
> and increased these by 1500, recompiled the kernel and net-tools
> and rebooted. I still can't install a device with MTU>1500
> Any suggestions?

I have used jumbo packets in conjunction with gigabit ethernet
in the past, but I was not aware that there are any 100Mb/s devices
that suport jumbo frames.


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