Web Pages Freeze across VPN

Hi All

I hope you can help me

I've set up a VPN tunnel via a 2 Mbps SDSL link using a cisco 1811 at the remote end and a 2621xm at the central end (3DES SHA). At the remote end there are a number of Dell desktops and my HP Pavillion Laptop (also in the mix is a Cisco 2800 that acts as the default gateway before routing traffic to the 1811).

What is happening is via the VPN tunnel my HP Laptop can access everything with no problems however a number of the Dells are having connectivity problems for example my laptopcan access via http the opening page of a cirix metaframe server whilst a number of the Dells on display a quarter of the page, before it freezes.

Placing Ethereal onto one of the Dells and doing a capture. I see that there are incorrect checksums taking place from the Dell and retransmissions to the server(I disabled Checksum Offload on the Dell but it is still showing) My hunch that it is a MTU problem with the Dells./routers but I need further advice.

Can any body Help

Many Thanks


Reply to
simon watson
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Simon, There are many posts regarding MTU and VPN's so you might be on to something. I'm guessing this is a true IPSec VPN?

You can test your theory of MTU easily... Try pinging accross the VPN with the do-not-fragment bit set and a large packet example: ping -l 1492 -f (that is a dash-letter-l-for-lambda not a dash-number-one)

Reduce the number by 10 or so until you stop getting the message: "Packet needs to be fragmented but DF set." Then increase it one at a time until it breaks again.

Once you find the right size to use, set it perminantly for the NIC in your computer. It will also help to set the MTU size on the inside interface of the routers where the VPN terminates.

Good Luck


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You could try these settings on the tunnel interfaces: ip mtu 1400

ip tcp adjust-mss 1360

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