Enabling jumbo frames on your network

Jumbo frames are Ethernet frames with up to 9000 bytes of payload, in contrast to normal frames which have up to 1500 bytes per payload. They are useful on fast (Gigabit Ethernet and faster) networks, because they reduce the overhead. Not only will it result in a higher throughput, it will also reduce CPU usage.

To use jumbo frames, you whole network needs to support it. That means that your switch needs to support jumbo frames (it might need to be enabled by hand), and also all connected hosts need to support jumbo frames. Jumbo frames should also only be used on reliable networks, as the higher payload will make it more costly to resend a frame if packets get lost.

So first you need to make sure your switch has jumbo frame support enabled. I’m using a HP Procurve switch and for this type of switch you can find the instructions here:

$ netcat telnet
ProCurve Switch 2824# config
ProCurve Switch 2824(config)# show vlans
ProCurve Switch 2824(config)# vlan $VLAN_ID jumbo
ProCurve Switch 2824(config)# show vlans
ProCurve Switch 2824(config)# write memory

Now that your switch is configured properly, you can configure the hosts.

For hosts which have a static IP configured in /etc/network/interfaces you need to add the line

    mtu 9000

to the iface stanza of the interface on which you want to enable jumbo frames. This does not work for interfaces getting an IP via DHCP, because they will use the MTU value sent by the DHCP server.

To enable jumbo frames via DHCP, edit the /etc/dhcp/dhcpd.conf file on the DHCP server, and add this to the subnet stanza:

option interface-mtu 9000;

Now bring the network interface offline and online, and jumbo frames should be enabled. You can verify with the command

# ip addr show

which will show the mtu values for all network interfaces.