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Home » Forums » Forum Archives » Networking and Internet Sharing » Topic # 2037

MTU in ICS using PPPoe
dalkeith Jul-24-02 11:02 PM
I have followed the instructions below and have determined that the MTU value on my gateway machine is 1492.

However, certain web pages only load when using an MTU value of 1454 on my client machines (rather than 1492 as stated as the MTU value of my gateway machine).

Is this normal? Why wouldn't using the value of 1492 work if this is the actual value of my gateway?

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Can't access some websites while using a shared PPPoE connection
Intended For
Windows XP
Windows 2000
Windows Me
Windows 98

If you're using Windows' built-in support for Internet Connection Sharing, and your Internet connection is facilitated by PPPoE software (such as Enternet 300) or Windows XP's built-in PPPoE, you may experience this problem. Although any web site will be accessible on the "Host" computer, certain web sites will never load successfully from any of the "client" machines. (If you don't know what "Hosts" or "Clients" are with regard to ICS, read Internet Connection Sharing.) The problem is caused by an incompatible MTU networking setting: Windows' default is 1500, but PPPoE uses 1492 or 1454. Here's how to fix it:

Find the IP address of your gateway. If you're using Windows 2000 or XP, run IPCONFIG at a command prompt on the Host computer. If you're using Windows 98 or Me, run WINIPCFG on the Host computer. Either way, you'll get an address that looks like xxx.xxx.xxx.xxx (where the x's represent numbers).
Then, go to one of your Client machines, and type the following:
PING -f -l 1500 xxx.xxx.xxx.xxx
(where xxx.xxx.xxx.xxx is the gateway address you obtained in the first step). You'll probably get an error message indicating that it must be fragmented. If you do, type the following:
PING -f -l 1492 xxx.xxx.xxx.xxx
If that doesn't work, try this:
PING -f -l 1454 xxx.xxx.xxx.xxx
The numbers in each of these examples (1500, 1492, 1454) are the MTU values. Continue issuing this command with lower and lower MTU numbers until you get ping responses instead of an error message. The highest MTU value that works is the one you need to be using. If an MTU of 1500 (the first command, above) does not produce an error, then this solution won't work for you.
The next step is to configure all your Client computers to use the new, lower MTU as the default for all Internet communication.
Windows 2000 and XP:

Run the Registry Editor (REGEDIT.EXE) on one of your "Client" machines.
Navigate to HKEY_LOCAL_MACHINE\ SYSTEM\ CurrentControlSet\ Services\ Tcpip\ Parameters\ Interfaces.
There should be several subkeys under the Interfaces key; most likely, you'll find three. View the contents of each key by clicking, and find the one that corresponds to your primary network adapter; it will be the one with more values than the other two, and will have an IP address value set to something like 192.168.0.x.
Once you've found the correct subkey, create a new DWORD value in it (Edit -> New -> DWORD Value), and name the value MTU.
Double-click the new value, choose the Decimal option, and type the MTU value determined above.
Click Ok when you're done - you'll need to restart Windows for this change take effect.
Repeat this for each Client machine.
Windows 98/Me:

Run the Registry Editor (REGEDIT.EXE) on one of your "Client" machines.
Navigate to HKEY_LOCAL_MACHINE\ System\ CurrentControlSet\ Services\ Class\ Nettrans\.
Under that branch, find a key (numbered, such as 0005) that contains has TCP/IP assigned to the DriverDesc value.
Select New from the Edit menu, then String Value, and type MaxMTU for the name of the new value.
Double-click the new value, choose the Decimal option, and type the MTU value determined above(mine is 1454).
Click Ok when you're done - you'll need to restart Windows for this change take effect.
Repeat this for each Client machine.



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