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Summary
One of the easy and most accurate ways to test for optimum MTU (maximum Transfer Unit) is to do a simple Ping test. You will simply send out ping requests and progressively lower your packet size until the packet no longer needs to be fragmented. Although this simple test is accurate for testing end points, users may find that a lower MTU may be better for their particular circumstances.
Finding the Correct MTU
The command for this ping test is ping 8.8.8.8 -f -l xxxx.
•You can use any well known, pingable domain like ping www.google.com -f -l xxxx in place of 8.8.8.8 for the test.
•There is a single space between each command.
•"-l" is a lower case letter L, not the number one.
•The last four numbers are the test packet size.
Step 1
At the DOS Prompt type in ping 8.8.8.8 -f -l 1500 and hit Enter. Notice that the packet needs to be
fragmented. (Figure 1)
Step 2
Drop the test packet size down (28 bytes in my case) and test again. Notice that the packet still needs to be fragmented. (Figure 2)
Now 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:
1) 1472 Max packet size from Ping Test result,
2) + 28 IP and ICMP headers
3) 1500 is your optimum MTU Setting.
Summary
One of the easy and most accurate ways to test for optimum MTU (maximum Transfer Unit) is to do a simple Ping test. You will simply send out ping requests and progressively lower your packet size until the packet no longer needs to be fragmented. Although this simple test is accurate for testing end points, users may find that a lower MTU may be better for their particular circumstances.
Finding the Correct MTU
The command for this ping test is ping 8.8.8.8 -f -l xxxx.
•You can use any well known, pingable domain like ping www.google.com -f -l xxxx in place of 8.8.8.8 for the test.
•There is a single space between each command.
•"-l" is a lower case letter L, not the number one.
•The last four numbers are the test packet size.
Step 1
At the DOS Prompt type in ping 8.8.8.8 -f -l 1500 and hit Enter. Notice that the packet needs to be
fragmented. (Figure 1)
Step 2
Drop the test packet size down (28 bytes in my case) and test again. Notice that the packet still needs to be fragmented. (Figure 2)
Now 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:
1) 1472 Max packet size from Ping Test result,
2) + 28 IP and ICMP headers
3) 1500 is your optimum MTU Setting.
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