Re: howto increase MTU size?

To: lvs-users@xxxxxxxxxxxxxxxxxxxxxx
Subject: Re: howto increase MTU size?
From: Roberto Nibali <ratz@xxxxxxxxxxxx>
Date: Fri, 26 Apr 2002 00:25:08 +0200
Hi Joe,

Check the source for your NIC driver (/usr/src/linux/drivers/net).  You
may also want to check the VLAN mailing list

laphish:/usr/src/linux/drivers/net # grep -r jumbo *.c | cut -d: -f1 | sort -u
laphish:/usr/src/linux/drivers/net #

They seem to be 1000Mbit/s NICs and I think the sk98lin driver support jumbo frames too.

as to Michael Brown's posting...
SHould I expect a standard 100Mbps switch (eg Netgear) to drop large MTU packets?

Depends how intelligent the switch tries to be. "An efficient implementation of Gigabit Ethernet with MTU greater than 1500 bytes would probably require switches to resegment Ethernet frames greater than 1500 bytes and recompute the checksums. This would add to the cost of switches." (from: Communication Performance over a Gigabit Ethernet Network by Paul A. Farrell and Hong Ong)

AFAIK 100Mbit/s NICs don't support mtu>1500 per standard definition, but I would need to check my copy of the IEEE-802.3 draft. There was a meeting about things like that [1].

And Mr. Dijkstra is talking merely about Alteon. As everyone knows, Alteon just tries to elude every possible standard below OSI 3 :)

I was going to try it locally at first, and then between two boxes connected by a cross-over cable, to at least see if it was practical.

I doubt you get the NICs to send 1500 mtu without major patching but I stand corrected. And I don't really think you gain that much in performance. You might reduce CPU load but since there would not be a early congestion notification or dynamic socket buffer sizes, you might end up even decreasing performance.

After that I was going to deal with the switch problem.

Take a hub and get 2 Broadcom Tigon3 NICs, compile and 'ifconfig eth0 mtu 9000' :)

For some tests see [2].


Cheers and see you at OLS,
Roberto Nibali, ratz

echo '[q]sa[ln0=aln256%Pln256/snlbx]sb3135071790101768542287578439snlbxq'|dc

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