Aug 13

To find the proper MTU Size, you’ll have to do a special ping of the destination you’re trying to go to. A destination could be another computer, or a URL.

Step 1 Click on Start and then click Run.

Step 2 Windows NT, 2000, and XP users type in cmd and press Enter (or click OK).

Step 3 Once the window opens, you’ll need to do a special ping. Use the following syntax:

ping [url] [-f] [-l] [MTU value]

Example: ping -f -l 1472


You should start at 1472 and work your way down by 10 each time. Once you get a reply, go up by 2 until you get a fragmented packet. Take that value and add 28 to the value to account for the various TCP/IP headers. For example, lets say that 1452 was the proper value, the actual MTU size would be 1480, which is the optimum for the network we’re working with (1452+28=1480).

Once you find your MTU, you can now configure your router with the proper MTU size.

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Aug 13

A maximum transmission unit (MTU) is the largest size packet or frame, specified in octets (eight-bit bytes), that can be sent in a packet or frame-based network such as the Internet. The Internet´s Transmission Control Protocol uses the MTU to determine the maximum size of each packet in any transmission. Too large an MTU size may mean retransmissions if the packet encounters a router that can´t handle that large a packet. Too small an MTU size means relatively more header overhead and more acknowledgements that have to be sent and handled. Most computer operating systems provide a default MTU value that is suitable for most users. In general, Internet users should follow the advice of their Internet service provider (ISP) about whether to change the default value and what to change it to.

In Windows 95, the default MTU was 1500 octets (eight-bit bytes), partly because this is the Ethernet standard MTU. The Internet de facto standard MTU is 576, but ISPs often suggest using 1500. If you frequently access Web sites that encounter routers with an MTU size of 576, you may want to change to that size. (Apparently some users find that changing the setting to 576 improves performance and others do not find any improvement.) The minimum value that an MTU can be set to is 68.

For more recent Windows systems, the operating system is able to sense whether your connection should use 1500 or 576 and select the appropriate MTU for the connection.

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