how to change mtu in windows 7

Modify the MTU size

This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

To modify the PPPoE MTU size, create the following registry key:

Then add the following registry entries.

To do this:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following key in the registry:
   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Ndiswan\Parameters
3. On the Edit menu, point to New, and then click Key.
4. Type Protocols, and then press ENTER.
5. On the Edit menu, point to New, and then click Key.
6. Type 0, and then press ENTER.
7. On the Edit menu, point to New, and then click DWORD Value.
8. Type ProtocolType, and then press ENTER.
9. On the Edit menu, click Modify.
10. Type 800, and then click OK.
11. On the Edit menu, point to New, and then click DWORD Value.
12. Type PPPProtocolType, and then press ENTER.
13. On the Edit menu, click Modify.
14. Type 21, and then click OK.
15. On the Edit menu, point to New, and then click DWORD Value.
16. Type ProtocolMTU, and then press ENTER.
17. On the Edit menu, click Modify.
18. Type the appropriate MTU size (decimal value), and then click OK.
19. Quit Registry Editor.

This is for windows xp

it is similar in win7