HowTo: Changing Interface's MTU Size

Wiki defines MTU as

In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The MTU may be fixed by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-to-point serial links). A higher MTU brings higher bandwidth efficiency. However, large packets can block up a slow interface for some time, increasing the lag for further packets. For example, a 1500 byte packet, the largest allowed by Ethernet at the network layer (and hence most of the Internet), would tie up a 14.4k modem for about one second.

Default MTU Size for Different Network Topology
Network MTU(Bytes)
-----------------------------------------------
16 Mbit/Sec Token Ring 17914
4 Mbits/Sec Token Ring 4464
FDDI 4352
Ethernet 1500
IEEE 802.3/802.2 1492
X.25 576
-----------------------------------------------

Now, here's a quick entry on how to change your network interface's MTU size

Changing MTU Size
To check for your current eth0 interface's MTU size, simply

# ifconfig eth0 | grep MTU

The aboves gives you similar lines like this
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To change your interface's MTU size from 1500 to 1492, simply

# ifconfig eth0 mtu 1492

Verify that the above command was executed successfully by checking the interface's MTU size again

# ifconfig eth0 | grep MTU
Result:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
UP BROADCAST RUNNING MULTICAST MTU:1492 Metric:1
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The above changes is temporary.

Making MTU Changes Permanently
To make the current MTU size permanent between reboots, simply append the below lines into your /etc/sysconfig/network-scripts/ifcfg-eth0 file for eth0 interface

MTU=1492

and restart your network service

# service network restart

The above line is for your eth0 config file. Change when needed.

Enjoy.

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