The maximum transmission unit (MTU) is the size (in bytes) of the largest protocol data unit that it 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 greater efficiency because each packet carries more user data while protocol overheads, such as headers or underlying per-packet delays remain fixed, and higher efficiency means a slight improvement in bulk protocol throughput. However, large packets can occupy a slow link for some time, causing greater delays to following packets and increasing lag and minimum latency. 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.

OK, enough for MTU overview,you can read more on Wikipedia pages.

Now here are the step by step to increase the MTU size, on Solaris of course.

by default, if you type “ifconfig -a” you will get the MTU size is 1500:

bash-3.00# ifconfig -a
lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
e1000g0: flags=201000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,CoS> mtu 1500 index 2
inet 10.32.16.1 netmask ffffff00 broadcast 10.32.16.255
ether 8:0:27:73:25:e8
e1000g1: flags=201000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,CoS> mtu 1500 index 3
inet 10.32.16.2 netmask ffffff00 broadcast 10.32.16.255
ether 8:0:27:6a:34:ae
bash-3.00#

you can change MTU size by running “ifconfig <interface> mtu <mtu_size>”, but it doesn’t work for e1000g if the max default is still 1500. try and you will got error like this:

bash-3.00# ifconfig e1000g1 mtu 8000
ifconfig: setifmtu: SIOCSLIFMTU: e1000g1: Invalid argument

{tried on bge interface also on M5000 and the result still same:
root@server # ifconfig bge2 mtu 8000
ifconfig: setifmtu: SIOCSLIFMTU: bge2: Invalid argument

{using dladm command also failed:
root@server # dladm set-linkprop -p mtu=8000 bge2
dladm: warning: invalid link property ‘mtu’

READ from this link and this link, then finally I understand that changing MTU size is different for each interface types. and here are the conclusion:

Change MTU size for E1000g (Intel PRO/1000 Gigabit family device driver) interface:

Scenario:

I have 2 interface; e1000g0 and e1000g1. I need to change the MTU size to 8000  for e1000g1 interface only.

1.  Check current config with “ifconfig -a”

2. edit file “/kernel/drv/e1000g.conf”

go to “MaxFrameSize” line, change the zero values number like this:

MaxFrameSize=0,2,0,0,0,0,0,0,0,0,0,0,0,0,0,0;
# 0 is for normal ethernet frames.
# 1 is for upto 4k size frames.
# 2 is for upto 8k size frames.
# 3 is for upto 16k size frames.
# These are maximum frame limits, not the actual ethernet frame
# size. Your actual ethernet frame size would be determined by
# protocol stack configuration (please refer to ndd command man pages)
# For Jumbo Frame Support (9k ethernet packet)
# use 3 (upto 16k size frames)

Note: Above configuration only affect to e1000g1.

If you want to change MTU size to all interface, just simply change all zero values with 1,2,3 as you need:

Example: MaxFrameSize=2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2;

then, all your interface will support 8K MTU.

3. reboot

4. check the result with “ifconfig -a | grep mtu”

bash-3.00# ifconfig -a | grep mtu
lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> mtu 8232 index 1
e1000g0: flags=201000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,CoS> mtu 1500 index 2
e1000g1: flags=1201000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,CoS> mtu 8106 index 3
bash-3.00#

5. After reboot, the MTU size is 8106. if you want 8000 mtu size, then edit file “/etc/hostname.e1000g1”:

bash-3.00# cat /etc/hostname.e1000g1
solaris10 mtu 8000

bash-3.00# ifconfig -a
lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
e1000g0: flags=201000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,CoS> mtu 1500 index 2
inet 10.32.16.1 netmask ffffff00 broadcast 10.32.16.255
ether 8:0:27:73:25:e8
e1000g1: flags=1201000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,CoS,FIXEDMTU> mtu 8000 index 3
inet 10.32.16.2 netmask ffffff00 broadcast 10.32.16.255
ether 8:0:27:6a:34:ae

Change MTU size for bge (Broadcom Gigabit Ethernet device driver) interface:

bash­3.00# grep bge /etc/path_to_inst
“/pci@0,600000/pci@0/pci@8/pci@0/network@2” 0 “bge”
“/pci@0,600000/pci@0/pci@8/pci@0/network@2,1” 1 “bge”
­

bash­3.00# cat /etc/system
set bge:bge_jumbo_enable = 1
­

bash­3.00# cat /platform/sun4u/kernel/drv/bge.conf
default_mtu=9000;
name=”bge” parent=”/pci@0,600000″ unitaddress=”2″ default_mtu=9000;

bash­3.00# reboot
bash­3.00# ifconfig ­-a

Change MTU size for ce (Cassini Gigabit-Ethernet device driver) interface:

Go to here