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TIP_Jumbo_Frames

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Situation

You have two or more PC's with gigabit NIC's that support jumbo frames.
You must have a gigabit switch that is jumbo frame clean or have just two machines connected via a crossover cable or a newer NIC with Auto-MDI/MDI-X.

On the Fly Configuration

You may simply do (as root):

 # ifconfig eth0 mtu 9000

Permanent Configuration

This assumes that you have assigned the static IP address 192.168.1.2 to eth0

Edit /etc/conf.d/net and add the desired mtu value:

config_eth0=( "192.168.1.2 netmask 255.255.255.0" )

becomes:

config_eth0=( "192.168.1.2 netmask 255.255.255.0" )
mtu_eth0="9000"

or for CIDR notation:

config_eth0=( "192.168.1.2/24" )

becomes:

config_eth0=( "192.168.1.2/24" )
mtu_eth0="9000"

Restart the Interface (as root):

# /etc/init.d/net.eth0 restart

Notes

Jumbo Frames are ethernet frames larger than the standard MTU of 1500.

AFAIK, you cannot mix computers with jumbo frames and the standard MTU value on the same LAN without VLAN tagging or a router that will fragment the packets.

Use cat5e or better ethernet cable with all eight wires connected.

You may need to tune the network protocol you're using, e.g., for NFS: rsize=32768, wsize=32768 in /etc/fstab

Don't expect a 10x increase in throughput; your disks and PCI bus are only so fast. You should see a big decrease in CPU usage when transferring larger files.

The SMC 8508T seems to only be jumbo frame clean to about 6000. Details here.

Someone added: Since I just purchased my SMC 8508T in combination with my WRT54GS I went out to look how this all is supposed to work. I found this interesting read about Path MTU discovery. Granted they only speak about 1400 mtu's and 1500 mtu's, but to me it seems this is the same as 1500 v.s. 9000.

Rufnut added: On my r8169 Gigabit Cards the maximum mtu was 7200, which when set increased throughput from 13.5 MB/s to 44.5 MB/s. I feel they would have went better in a 66 MHz PCI slot with more optimization.

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Last modified: Mon, 22 Sep 2008 13:47:00 +1000 Hits: 28,679

Created by NickStallman.net, Luxury Homes Australia
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