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How to change the PPPoE MTU size in Windows XP

Article ID: 283165 - View products that this article applies to.

This article was previously published under Q283165

SUMMARY

This article describes how to change the Point-to-Point Protocol over Ethernet (PPPoE) Maximum Transfer Unit (MTU) size.

The default and maximum PPPoE MTU size is 1,480 bytes. With some Internet service providers (ISPs), you may have to reduce the MTU size of a PPPoE connection to a value between 1,400 and 1,480 (for example, to 1,454). Do not set an MTU size smaller than 1,400.

For additional information about the default MTU size, click the following article number to view the article in the Microsoft Knowledge Base:

314496

(http://support.microsoft.com/kb/314496/ )

Default MTU size for different network topology

For additional information about how to determine if you have to lower the MTU size of a PPPoE connection, click the following article number to view the article in the Microsoft Knowledge Base:

159211

(http://support.microsoft.com/kb/159211/ )

Diagnoses and treatment of black hole routers

Modify the MTU size

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:

322756

(http://support.microsoft.com/kb/322756/ )

How to back up and restore the registry in Windows

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

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Ndiswan\Parameters\Protocols\0

Then add the following registry entries.

Collapse this tableExpand this table

Entry name	Data type	Value data
ProtocolType	REG_DWORD	0x00000800
PPPProtocolType	REG_DWORD	0x00000021
ProtocolMTU	REG_DWORD	`the appropriate MTU size` (in decimal)

To do this:

Click Start, click Run, type regedit, and then click OK.
Locate and then click the following key in the registry:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Ndiswan\Parameters
On the Edit menu, point to New, and then click Key.
Type Protocols, and then press ENTER.
On the Edit menu, point to New, and then click Key.
Type 0, and then press ENTER.
On the Edit menu, point to New, and then click DWORD Value.
Type ProtocolType, and then press ENTER.
On the Edit menu, click Modify.
Type 800, and then click OK.
On the Edit menu, point to New, and then click DWORD Value.
Type PPPProtocolType, and then press ENTER.
On the Edit menu, click Modify.
Type 21, and then click OK.
On the Edit menu, point to New, and then click DWORD Value.
Type ProtocolMTU, and then press ENTER.
On the Edit menu, click Modify.
Type the appropriate MTU size (decimal value), and then click OK.
Quit Registry Editor.

Notes

Therefore, if the MTU size is set to 1460 (decimal), the max value of NUM in the following command line can be 1432:
ping IP_address -f -l NUM
You may have to restart the computer to make the change take effect.