HOW TO: Change the Default Maximum Transmission Unit (MTU) Size Settings for PPP Connections or for VPN Connections

This step-by-step article describes how to edit the registry to change the default maximum transmission unit (MTU) size settings for Point-to-Point Protocol (PPP) connections or for virtual private network (VPN) connections.

Microsoft Windows Server 2003, Microsoft Windows 2000, and Microsoft Windows XP use a fixed MTU size of 1500 bytes for all PPP connections and use a fixed MTU size of 1400 bytes for all VPN connections. This is the default setting for PPP clients, for VPN clients, for PPP servers, or for VPN servers that are running Routing and Remote Access.

PPP connections are connections such as modem connections, Integrated Services Digital Network (ISDN) connections, or direct cable connections over null serial cable or over parallel cable. VPN connections are Point-to-Point Tunneling Protocol (PPTP) connections or Layer 2 Tunneling Protocol (L2TP) connections.

Note Use the methods in this article to edit the registry to modify the MTU size settings. If you experience any problems or any performance-related issues after you modify the MTU size settings, remove the registry keys that you added.

Change the MTU Settings for PPP Connections

To change the MTU settings for PPP connections, add the ProtocolType DWORD value, the PPPProtocolType DWORD value, and the ProtocolMTU DWORD value to the following registry key: To do so, follow these steps.

Important 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:

1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate and then click the following subkey in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NdisWan\Parameters
3. Add a Protocols subkey (if it does not already exist). To do so:
   1. On the Edit menu, point to New, and then click Key.
   2. Type Protocols, and then press ENTER.
4. Add a 0 (zero) subkey to the Protocols subkey. To do so:
   1. Click the Protocols subkey that you created step 3.
   2. On the Edit menu, point to New, and then click Key.
   3. Type 0 (zero), and then press ENTER.
5. Click the 0 subkey that you created in step 4.
6. On the Edit menu, point to New, and then click DWORD Value.
7. In the Value data box, type ProtocolType, and then click OK.
8. On the Edit menu, click Modify.
9. In the Value data box, type 800, make sure Hexadecimal is selected under Base, and then click OK.
10. On the Edit menu, point to New, and then click DWORD Value.
11. Type PPPProtocolType, and then press ENTER.
12. On the Edit menu, click Modify.
13. In the Value data box, type 21, make sure Hexadecimal is selected under Base, and then click OK.
14. On the Edit menu, point to New, and then click DWORD Value.
15. Type ProtocolMTU, and then press ENTER.
16. On the Edit menu, click Modify.
17. Under Base, click Decimal, type the MTU size that you want in the Value data box, and then click OK.
18. Quit Registry Editor.
19. Restart your computer.

Change the MTU Settings for VPN Connections

To change the MTU settings for VPN connections, add the ProtocolType DWORD value, the PPPProtocolType DWORD value, and the TunnelMTU DWORD value to the following registry key: To do so, follow these steps.

Important 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:

1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate and then click the following subkey in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NdisWan\Parameters
3. Add a Protocols subkey (if it does not already exist). To do so:
   1. On the Edit menu, point to New, and then click Key.
   2. Type Protocols, and then press ENTER.
4. Add a 0 (zero) subkey to the Protocols subkey. To do so:
   1. Click the Protocols sub key that you created in step 3.
   2. On the Edit menu, point to New, and then click Key.
   3. Type 0 (zero), and then press ENTER.
5. Click the 0 subkey that you created in step 4.
6. On the Edit menu, point to New, and then click DWORD Value.
7. In the Value data box, type ProtocolType, and then click OK.
8. On the Edit menu, click Modify.
9. In the Value data box, type 800, make sure Hexadecimal is selected under Base, and then click OK.
10. On the Edit menu, point to New, and then click DWORD Value.
11. Type PPPProtocolType, and then press ENTER.
12. On the Edit menu, click Modify.
13. In the Value data box, type 21, make sure Hexadecimal is selected under Base, and then click OK.
14. On the Edit menu, point to New, and then click DWORD Value.
15. Type TunnelMTU, and then press ENTER.
16. On the Edit menu, click Modify.
17. Under Base, click Decimal, type the MTU size that you want in the Value data box, and then click OK.
18. Quit Registry Editor.
19. Restart your computer.

For additional information about MTU size in Microsoft Windows NT 4.0, click the following article number to view the article in the Microsoft Knowledge Base: For more information about PPP, see Request for Comments (RFC) 1548. To do so, visit the following Internet Engineering Task Force (IETF) Web site: