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User cannot download files across the VPN tunnel because MTU size is too large


Core issue

A VPN session is established successfully. The client can pass most traffic, but cannot download files across the VPN tunnel.


To resolve this issue, perform these steps:

  1. Lower the Maximum Transmission Unit (MTU) by running the setMTU.exe file, found on the VPN Client installation CD, on the PC.

    For more information on lowering the MTU, refer to the Adjusting the MTU Value section of the Usage Notes section of the Release Notes for VPN Client for Windows, Release 3.5.4.

    Note: For VPN Concentrator version 3.6, the MTU can be adjusted through the GUI.

    For more information, refer to VPN 3000 Series Concentrator version 3.6.

  2. If this solution does not solve the issue, the server from which the file is being downloaded could be setting the Do Not Fragment (DF) bit.

    To resolve this issue, reconfigure the server to not set the DF bit or clear the DF bit on Cisco routers and PIX Firewalls.

    For more information on clearing the DF bit, refer to DF Bit Override Functionality with IPSec Tunnels.

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