What are jumbo frames? Jumbo frames are ethernet frames with more than 1500 bytes of payload. Conventionally, jumbo frames can carry up to 9000 bytes of payload, but variations exist and some care must be taken when using the term.
Why use jumbo frames? Enabling them on your network equipment and on your NICs you will experiment a performance boost specially with iSCSI protocol that works over a standard ethernet network.
Implementation of Jumbo frames must be done with some rules:
- Same MTU for all server present in the network
- Network card must support a MTU over 1500
- Switch must support a MTU over 1500
- Switch must support a MTU over 1500 on a VLAN
How to enable jumbo frames on RHEL/CentOS? Enabling jumbo frames on linux is really simple: edit the NIC configuration and append MTU=9000.
Don’t forget to enable them also on your switch/router!
# vi /etc/sysconfig/network-script/ifcfg-<your_nic> # ex. eth0
MTU=9000
Then restart the single interface…
ifdown eth0; ifup eth0
…or the entire network service
service network restart
After all verify that the new configuration has been correctly applied:
# ifconfig eth0
If the configuration is ok you will see a response like this:
eth0     Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
inet addr:x.x.x.x Bcast:x.x.x.x Mask:x.x.x.x
UP BROADCAST RUNNING MULTICASTÂ MTU:9000 Metric:1
If you’re using bonding, you need to enable jumbo frames only on bond device configuration:
# vi /etc/sysconfig/network-script/ifcfg-bond0
MTU=9000
To avoid MTU hinting you may check your dmesg:
# grep jumbo /var/log/dmesg
r8169 0000:03:00.0: eth2: jumbo features [frames: 7152 bytes, tx checksumming: ok]
r8169 0000:08:00.0: eth0: jumbo features [frames: 9200 bytes, tx checksumming: ko]
The “jumbo frame†is link-dependant. You may need to set lower values:
– wi-fi supports about 7935
You can try some good values on your interfaces and check for errors, then set the value on the bonded interface.
#ping supports MTU hinting. Reading the number in parentheses
$ ping 10.0.0.1 -M do
PING 10.0.0.1 (10.0.0.1) 56(84) bytes of data
^^^^^^
The ICMP payload was 56, and the total payload 84 bytes. So to hit an MTU=1500 we have to send a pkt with a payload of:
1500-84+56=1472
The highest unfragmentable payload is tested with
$ ping 10.0.0.1 -M do -s 1472
PING 10.0.0.1 (10.0.0.1) 1472(1500) bytes of data
Higher payloads cause a nice error:
Frag needed and DF set (mtu = 1500)