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The maximum transmission unit (MTU) of a network interface is the size of the largest block of data that can be transmitted as a single unit. Anything larger than the MTU must be broken into smaller units prior to transmission.

MTUs can be measured either at the network layer or at the link layer. For example, the standard Ethernet MTU is 1500 bytes at the network layer but 1518 bytes at the link layer. The values given here are network-layer MTUs unless stated otherwise, but be aware that equipment manufacturers often specify link-layer MTUs instead.

A common reason for wanting to increase the MTU of an interface is to improve the throughput of high-speed Ethernet. The standard Ethernet MTU has remained fixed at 1500 bytes for backward compatibility with 10 and 100 megabit networks, however this is far from optimal at speeds of 1 gigabit and above. Most modern networking equipment is capable of handling larger frames but must be explicitly configured to do so. Frames which take advantage of this ability are known as ‘jumbo frames’, and 9000 bytes is a popular choice for the MTU.

Possible reasons for wanting to reduce the MTU include:

• Matching the MTU to that of another network (in order to avoid fragmentation when carrying UDP, and to streamline PMTU discovery when carrying TCP).
• Matching the MTU to a whole number of ATM cells (when you know that the traffic will at some point need to be encoded using AAL5).
• Improving throughput on lines with very high error rates.

MTUs should not normally be mixed on any given Internet Protocol subnet, however it is possible (and common) for subnets with different MTUs to be connected to each other by means of a router.

Scenario

Suppose that you wish to configure the interface eth0 for connection to an Ethernet that uses jumbo frames with an MTU of 9000. The assigned IP address of the interface is 192.168.0.2/24.

Method (non-persistent)

The MTU of an interface can be changed temporarily using the mtu option of the ifconfig command:

ifconfig eth0 mtu 9000

The new MTU will not persist beyond a reboot.

Persistently change the MTU of an network interface (Debian)

If the interface has (or can be given) a static configuration in /etc/network/interfaces then its MTU can be persistently altered by adding an mtu option to the relevant iface stanza, for example:

auto eth0
iface eth0 inet static
address 192.168.0.2
netmask 255.255.255.0
mtu 9000

This method does not work when using other configuration methods such as dhcp.

You can activate the new MTU can by taking the interface down using ifdown then bringing it back up using ifup:

ifdown eth0
ifup eth0

Alternatively you could reboot the machine.

Persistently change the MTU of an network interface (Red Hat)

Edit the configuration script for the relevant interface in /etc/sysconfig/network-scripts/. If named directly after the interface eth0 then this would be called ifcfg-eth0, however other names are possible (including ifcfg-Auto_eth0 if using the Gnome Network Manager). If no configuration script exists then you will need to create one.

Add a line to specify the MTU, for example:

DEVICE=eth0
BOOTPROTO=static
ONBOOT=yes
IPADDR=192.168.0.2
NETMASK=255.255.255.0
MTU=9000

Unlike the corresponding method for Debian-based systems, it is possible to set the MTU of interfaces that use DHCP:

DEVICE=eth0
BOOTPROTO=dhcp
HWADDR=02:00:00:00:00:00
ONBOOT=yes
MTU=9000

It is permissible for the values given in this file to be single- or double-quoted.

The new MTU can be activated by taking the interface down using ifdown then bringing it back up using ifup:

ifdown eth0
ifup eth0

Alternatively you could reboot the machine.

Check the configured MTU

You can verify that an interface has the intended MTU using the -i option of the netstat command:

netstat -i

The MTU of each interface is listed in the second column of the output:

Iface     MTU   Met   RX-OK   RX-ERR   RX-DRP   RX-OVR   TX-OK   TX-ERR   TX-DRP   TX-OVR   Flg
eth0      1500   0    9658      0        0         0      308      0         0        0     BMRU
lo    16436 0     12952      0      0      0    12952      0      0      0 LRU

Alternatively you can obtain essentially the same information using the ifconfig command:

ifconfig eth0

The MTU is then listed after the interface address (or addresses) but before the packet counters:

eth0      Link encap:Ethernet  HWaddr 02:00:00:00:00:00
          inet addr:192.168.0.2  Bcast:192.168.0.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:9659 errors:0 dropped:0 overruns:0 frame:0
          TX packets:309 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:2783810 (2.6 MiB)  TX bytes:25318 (24.7 KiB)

Remember that the value listed is not necessarily the one that would apply following a reboot.

Check connectivity using ping

You can verify that the path between two machines has at least the expected MTU using the ping command:

ping -M do -c 4 -s 8972 192.168.0.1

This transmits an ICMP echo request to the specified destination then waits for an ICMP echo reply. As the intention here is to check the behaviour of eth0, it would be best to choose a destination on the same subnet in the first instance. Both machines must have a sufficiently large MTU if the test is to succeed.

The -M do option causes the DF (don't fragment) flag to be set, meaning that the packet should be dropped if it cannot remain in one piece at any point in its journey. The -c option sets the number of pings.

The -s option specifies the number of bytes of padding that should be added to the echo request. In addition to this number there will be 20 bytes for the internet protocol header, and 8 bytes for the ICMP header and timestamp. The amount of padding should therefore be 28 bytes less than the network-layer MTU that you are trying to test (9000 − 28 = 8972).

If the test is successful then you should see a list of echo replies that were received:

PING 192.168.0.1 (192.168.0.1) 8972(9000) bytes of data.
1480 bytes from 192.168.0.1: icmp_seq=1 ttl=64 time=0.241 ms
1480 bytes from 192.168.0.1: icmp_seq=2 ttl=64 time=0.583 ms
1480 bytes from 192.168.0.1: icmp_seq=3 ttl=64 time=0.140 ms
1480 bytes from 192.168.0.1: icmp_seq=4 ttl=64 time=0.123 ms

Note that the IP packet size (9000 bytes) is listed on the first line. You can use this to check that you requested the correct amount of padding.

If the test is unsuccessful then you should see an error in response to each echo request:

PING 192.168.0.1 (192.168.0.1) 8972(9000) bytes of data.
From 192.168.0.2 icmp_seq=1 Frag needed and DF set (mtu = 1500)
From 192.168.0.2 icmp_seq=1 Frag needed and DF set (mtu = 1500)
From 192.168.0.2 icmp_seq=1 Frag needed and DF set (mtu = 1500)
From 192.168.0.2 icmp_seq=1 Frag needed and DF set (mtu = 1500)

Note that the MTU for the hop that failed is listed in the error message. This is not necessarily the MTU for the entire path, only the first MTU that was found to be too small.

Path MTUs are recorded in the routing cache. This can interfere with testing, and in particular, can make a remote MTU restriction appear to be a local one. You can clear the cache using the ip route command:

ip route flush cache

SIOCSIFMTU: Invalid argument

An error of the form:

SIOCSIFMTU: Invalid argument

Indicates that the requested MTU was rejected by the kernel. Typically this would be due to it exceeding the maximum value supported by the interface hardware. In that case you must either reduce the MTU to a value that is supported or obtain more capable hardware.