Q

Reduce MTU for VPN connecting to ADSL

I have a customer with a Checkpoint 650 VPN concentrator and Checkpoint client VPN on a PC. The Client VPN PC is connected to Verizon's ADSL Line that runs PPPoE. We can establish the VPN tunnel, but then we cannot do anything but ping the devices across the tunnel. I read about MTU issues with PPPoE, so I assume the IPSec Header plus the PPPoE header are making the packets larger than the default 1500 and they are getting dropped somewhere in the Verizon ATM cloud. Is there a way I can make the VPN client negotiate a smaller MTU size with the host on the other end of the VPN tunnel (email servers, file servers etc.)? Or is there another solution?
I ran into this problem with Pacific Bell's DSL service. Try reducing the MTU to 1492 bytes or less. The instructions for doing this are on the RASPPPOE site. Here is the link: http://user.cs.tu-berlin.de/~normanb/ (search for MTU on the page).

DSL Reports also has more info on this topic: www.dslresports.com (search for PPPOE and MTU).
This was first published in January 2002

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