M t u

Apr 21, 2008

- Finding the Correct MTU
To find the correct MTU for your configuration you must run a simple DOS Ping test. You will simply send out ping requests and progressively lower your packet size until the packet no longer needs to be fragmented. Please reference the following steps:

The command for this ping test is ping www.urlname.com -f -l xxxx.
You can use any well known, pingable domain like ping www.google.com -f -l xxxx in place of www.urlname.com for the test.
•There is a single space between each command.
•"-l" is a lower case letter L, not the number one.
•The xxxx are the test packet size. (like 1472)
At the DOS Prompt type in ping www.google.com -f -l 1472 and hit Enter. Notice that the packet needs to be fragmented. (you get a message "packet needs to be fragmented but DF set")
Drop the test packet size down (10 or 12 bytes) and test again. If the packet still needs to be fragmented, drop the test packet size down more and test again until your reach a packet size that does not fragment.Once you have a test packet that is not fragmented increase your packet size in small increments and retest until you find the largest possible packet that doesn't fragment. Take the maximum packet size from the ping test and add 28. You add 28 bytes because 20 bytes are reserved for the IP header and 8 bytes must be allocated for the ICMP Echo Request header. Remember: You must add 28 to your results from the ping test!
An example:
1440 Max packet size from Ping Test
+ 28 IP and ICMP headers
1468 your optimum MTU setting