Xserve: How to Enable Ethernet Jumbo Frames on the PCI Gigabit Card

Warnings:

1. All devices on a local network must be configured to use Jumbo Frames, or loss of connectivity could occur on that network.

2. All devices on the local network must support use of Jumbo Frames. Few network switches support Jumbo Frames, and sending them to a switch that does not support them may cause the switch to stop operating. Check the specifications of network hardware prior to enabling Jumbo Frames. Even if your devices are Jumbo Frame compatible, Jumbo Frames are not needed under most circumstances. Be sure you have a need for them before using this procedure.

3. This procedure is intended only for computers with Mac OS X Server and the PCI Gigabit Card, or for the Xserve G5. Jumbo Frames can be used on the built-in Ethernet port only on the Xserve G5.

Enabling Jumbo Frames

If you have Mac OS X Server 10.3 or later, you can set up Jumbo Frames via normal administration applications. The steps below would not be needed.

If you have Mac OS X Server 10.2.4 to 10.2.8, follow these steps to create a script that is executed during startup:

Warning: Users who are unfamiliar with command line interface and manual configuration in general should proceed with caution or preferably seek assistance. Assistance in setting up or creating Startup Items is not part of Apple's complimentary technical support for new products. Users interested in fee-based support for setting up and creating Startup Items or shell scripts should contact Apple Professional Services (http://www.apple.com/services/). If you do not follow these steps precisely, the script may not work, which could prevent the computer from starting up normally.

1. Make sure you are logged in with an administrator account.
2. Open Terminal (/Applications/Utilities).
3. Type: mkdir -p /Library/StartupItems/JumboFrame
4. Press Return.
5. Type: cd /Library/StartupItems/JumboFrame
6. Press Return.
7. Type: pico JumboFrame
8. Press Return.
9. In the pico editor, paste in the following text.

Begin copying below this line.

---

#!/bin/sh

##
# Configure kernel parameters for Jumbo Frame support
##

. /etc/rc.common

StartService ()
{
ConsoleMessage "Configuring for Jumbo Frames"

if [ "${JUMBOFRAME:=-NO-}" = "-YES-" ]; then

/usr/sbin/sysctl -w  net.inet.raw.maxdgram=16384
/usr/sbin/sysctl -w  net.inet.raw.recvspace=16384
/usr/sbin/sysctl -w  net.inet.tcp.sendspace=65536
/usr/sbin/sysctl -w  net.inet.tcp.recvspace=65536
/usr/sbin/sysctl -w  net.inet.udp.recvspace=65536

fi

}

StopService ()
{
return 0
}

RestartService ()
{
return 0
}

RunService "$1"

---

End copying above this line.

10. Save the file (Control-O, Return), and exit pico (Control-X).
17. Type: chmod 755 JumboFrame
18. Press Return.
19. Type: pico StartupParameters.plist
20. Press Return.
21. Paste the following text into the pico editor:

Begin copying below this line.

---

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN"
"http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>Description</key>
        <string>Can set Jumbo Ethernet Frames</string>
        <key>OrderPreference</key>
        <string>None</string>
        <key>Provides</key>
        <array>
                <string>JumboFrame</string>
        </array>
        <key>Requires</key>
        <array>
                <string>Network Configuration</string>
        </array>
</dict>
</plist>

---

End copying above this line.

22. Save the file (Control-O, Return) and exit pico (Control-X).
23. Type: chmod 644 StartupParameters.plist
24. Press Return.
25. Type: sudo pico /etc/hostconfig
26. When prompted, enter your account password.
27. Press Return.
28. In the pico editor, add this line at the bottom (use the down arrow key to navigate to the file end):

JUMBOFRAME=-YES-

29. Save it (Control-O, Return) and exit pico (Control-X).


When you restart the computer, the script makes the kernel parameter settings necessary to support Ethernet Jumbo Frames. Once that is done, you can enable Jumbo Frames on the PCI Gigabit port using this command:

sudo networksetup -setMTU en1 9000

If your PCI Gigabit card has a port name different from "en1", you should change the port name in the command to match. Repeat this command if you have multiple PCI Gigabit cards, substituting the other port names.

This setting is retained after a restart and does not need to be entered after each startup.

How to disable Jumbo Frames

If you experience any issues or do not want to use Jumbo Frames, you can change the port back by using this command:

sudo networksetup -setMTU en1 1500

Then change the JUMBOFRAME line in /etc/hostconfig to:

JUMBOFRAME=-NO-

After that, restart the computer.