Finding your Maximum Transfer Unit (MTU)

Before we start: Vista has auto-tuning, so we won’t go there…
also, is a great resource for this stuff.

In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest protocol data unit that a given layer of a communications protocol can pass onwards.
– wikipedia

Find the optimum setting:
In a simple network situation, you can use a ping test. You will send out ping requests and progressively lower your packet size until the packet no longer needs to be fragmented.
The command for this ping test is ping -f -l xxxx
•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.
•Remember to add 28 to this number when setting the optimum MTU

If you are having trouble with a VPN client, it’s a little more difficult. Although tedious, manually adjusting the MTU by trial and error testing is the best method. You will change the MTU and test the VPN for connectivity then adjust the MTU to the largest size possible that works.

Setting the MTU can be done at the registry, or through a third party app like DrTCP


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