MTU Issues with Windows 7 over VPN Connection

We’ve been in the process of upgrading our office workstations from Windows XP to Windows 7 and so far everything has gone perfectly. However, we’ve been experiencing some major network issues over the Cisco AnyConnect VPN Client, like:

  • Occassional black screen after connecting with remote desktop to a Windows 7 computer (though connecting to an XP machine works perfectly all the time).
  • Can’t remote desktop to Windows 7 computer using host name (only IP address would work).
  • First time connecting to network share it takes a very long time to connect.

I contacted Cisco Support and they suggested changing the MTU value (something also recommend on some other forums I read). Um, wow. That fixed the problem immediately. Here’s what we changed. Hopefully this will help someone else out there.

=====
conf t
group-policy DfltGrpPolicy attributes
webvpn
svc mtu 900
=====

UPDATE: We continued having other odd connectivity and speed issues a couple weeks after making this change. After much hair pulling our ISP, Qwest, finally discovered a faulty interface card in a box outside our building. Once they replaced this our speed returned to normal, our VPN tunnel stopped dropping, we returned the MTU back to the normal value of around 1400 and everything worked perfectly. Apparently it was this dumb interface card all along.

Moral of the story… if you’re experiencing speed issues and flakey VPN, you may have faulty hardware somewhere that is causing inconsistent packet lose, but as a quick fix you can try decreasing the MTU.

2 thoughts on “MTU Issues with Windows 7 over VPN Connection

  1. David Jensen

    You said to fix the problem do this:
    conf t
    group-policy DfltGrpPolicy attributes
    webvpn
    svc mtu 900

    But where do I do that? The Windows “cmd” prompt? I tried that but it didn’t work.
    Thanks for your time.

    Reply

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