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Adjusting MTU speeds up a problematic VPN connection

Question:

I can connect to the VPN server using Checkpoint SecureRemote but the network is very very slow. A sniffer tool shows many bad packets. Any ideas?

Answer:

I would like to explore one detail with you. Please go to \Program Files\CheckPoint\SecuRemote\bin and find MtuAdjust.exe. Double-click it, choose Yes to continue, choose "Adjust MTU for all adapters", edit 1500 to be 1400, leave "Enable Path MTU Discovery" checked and press OK. This will assure that the MTU for NIC is set to 1400. This will prevent packet fragmentation which could slow or prevent encrypted traffic.

MTU means 'Maximum Transmittable Unit' and specifies the maximum size datagram IP that may pass to a driver. On an Ethernet network, MaxMTU will default to 1500.

Note: This tool window will always come up with a default value of 1500; this does not indicate the actual MTU setting of the NIC. After you set the MTU to 1400, let me know any noticeable changes.

Note 2: If you use a different VPN client software you can still adjust the MTU. E.g. you can find the value directly in your Windows registry here:

HKEY_LOCAL_MACHINE\SYSTEM\CurentControlSet\Services\NdisWan\Parameters\Protocols\0

Change the dword ProtocolMTU to whatever MTU you want to run.

You can also download a tool that does this job for you: MTU-Speed Pro.


Generated 16:00:42 on Oct 19, 2017