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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 |
To change the MTU of an interface on GNU/Linux, you just need to use ifconfig command to do so, like this for example
sudo ifconfig eth0 mtu 1492
To change it permanently on Debian, put it in the /etc/network/interfaces file .where almost all network parameters are found. To do this, just add a line mtu to the definition of your interface and save the file.
sudo gedit /etc/network/interfaces
Example
iface eth0 inet static
address 192.168.0.1
network 192.168.0.0
gateway 192.168.0.254
netmask 255.255.255.0
mtu 1492
Daiup Users
For dialup users: the Maximum Transmission Unit (MTU) value can be changed within the file
/etc/ppp/options
Well ok fine … that tells you how to change the MTU.
But what it does not tell you is how to find out whether you need to change your settings to 1300 or 1600 or whatever….. it cannot be simply trial and error. I hope.
1492 or 1500 seem to be the common defaults but there is never an explanation of what they could / should be …
from what I found on http://www.debianadmin.org after Googling, settings for a static (not necessarily a dynamic [DHCP])address should be:
dialup 576
DSL 1492
cable modem 1500
fiber 4352
Use the command tracepath to know the suitable MTU of your network
The MTU becomes more important for people using Gigabit Ethernet. To get the best throughput (and if your switch supports it), it’s best to use “Jumbo” frames. Jumbo frames are enabled with an MTU of around 9000. Not all Gigabit Ethernet NICs support an MTU of 9000. As an example, my Realtek 8169 based card supports a maximum of 7200. Even at only 7200, transferring files between the computers on my LAN happens much faster than when using the default MTU of 1500. Of course, it’s best to check and see what the manufacturer suggests as the maximum supported MTU for the particular NIC you’re using.
With Intrepid, the network manager has a tab for MTU setting and does it well, whoever thought about this, my hats off to him for incorporating this nifty feature.
Hi all,
It seems tracepath does give what is suitable.
$ tracepath 74.126.24.86
1: Mugglewille-desktop.local (192.168.1.3) 0.355ms pmtu 1500
1: no reply
2: no reply
3: no reply
4: no reply
5: no reply
6: no reply
7: no reply
8: no reply
9: no reply
^C
So seems it is saying the best mtu is 1500
I can’t get ‘mtu size’ to work for eth0 when it has a dynamic IP address. The manual for interfaces describes the ‘mtu size’ option for static IP addresses, as are used above. I’m running Ubuntu 8.04 LTS server.
Is this a bug ?
To find out MTU you can do a ping to any destination, set the DF bit to 1 (Dont Fragment) and play around with the packet size until you find a limit that works.
To do this in ubuntu:
ping -Mdo -s 1500 8.8.8.8
-Mdo sets DF to 1
– s 1500 sets your buffer to 1500 bytes
– 8.8.8.8 is a destination IP
The above command should return that the packet cant be fragmented or the message is too long (or similar). You then reduce the size of the packet (eg. -s 1490) until you get a reply. Then increase and reduce the number until you find the exact MTU you should use.
BUT, what should it be set too?
Evolution mail, youtube-dl, PUBKEY and many more such tools aren’t working on my ubuntu 10.10, however my browser opens all the websites very normally. Can it be a problem with the MTU??
I ran the command sudo ifconfig -a on the terminal and the output is as follows:
eth0 Link encap:Ethernet HWaddr d8:d3:85:30:cf:33
inet addr:192.168.1.4 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::dad3:85ff:fe30:cf33/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:960 errors:0 dropped:0 overruns:0 frame:0
TX packets:783 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1005736 (1.0 MB) TX bytes:90389 (90.3 KB)
Interrupt:45 Base address:0xc000
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:4 errors:0 dropped:0 overruns:0 frame:0
TX packets:4 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:240 (240.0 B) TX bytes:240 (240.0 B)
pimreg Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
UP RUNNING NOARP MTU:1472 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
wlan0 Link encap:Ethernet HWaddr 70:f1:a1:55:b3:d4
inet addr:192.168.1.2 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::72f1:a1ff:fe55:b3d4/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:6 errors:0 dropped:0 overruns:0 frame:0
TX packets:35 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:980 (980.0 B) TX bytes:6398 (6.3 KB)
Interrupt:17 Memory:f87e0000-f87e0100.
Please suggest appropriate changes.
ok so im trying to get on xbox live and its telling me i need to change the mtu i have no idea how to do so can someone help me out with step by step directions
plz
This was helpful. Ever since I switched to WIFI for my Ubuntu computer, yahoo mail wouldn’t work right. Also, other posting & replying functions in websites wouldn’t work right either as well as dropbox. It was driving me nuts. Following the advice here, I figured out the highest I could set MTU on the wireless connection was 1472. Everything now works like a charm!
Thanks!