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A fix for 'Error 51' with the Cisco VPN Client Apps
To connect to Macworld's servers, I use the Cisco VPN Client (version 4.9.01). While this works well most of the time, I occasionally receive this error message when I launch the app:
Error 51: Unable to communicate with the VPN subsystem.
Please make sure that you have at least one network interface that is currently active and has an IP address and start this application again.
When this occurs, I definitely have an active network interface that has an IP address, so it's quite clearly not an issue with my machine's settings. The problem seems to be related to sleeping the Mac after using the Cisco VPN Client; this sometimes (but not always) will lead to the above error message when I next try to use the VPN Client. What makes it extra frustrating is that it's not consistent -- the app will work for days at a time through sleep and wake cycles, then spontaneously decide it's time for error 51 to appear.

I used to think rebooting was the only way to solve this problem -- I hadn't had much luck with the solutions posted in this older hint. A while back, after getting "51'd" again, I went in search of a better solution -- and I found one. In Terminal, use this relatively simple command to stop and restart the kernel extension that the Cisco VPN Client uses:
sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart
In the two or three times I've had this problem occur since finding the above solution, restarting the kernel extension has fixed the problem every time. So thanks to Anders.com for solving this frustrating issue with the Cisco VPN Client. I would hope a future Cisco VPN Client update would solve the issue entirely, but that fix has been published for two years already, so I'm not holding my breath.
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A fix for 'Error 51' with the Cisco VPN Client | 15 comments | Create New Account
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A fix for 'Error 51' with the Cisco VPN Client
Authored by: fltman on Sep 02, '08 08:16:41AM

A cleaner way of doing it is like this:

sudo SystemStarter restart CiscoVPN

And if it is a common problem why not make an alias out of it? Add the following line to your .bash_profile

alias resetcisco='sudo SystemStarter restart CiscoVPN'



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: JaxMyers on Sep 02, '08 10:12:12AM

Great hint, I had the same problem with my Cisco client and this is so much better than rebooting every time. I wish there was a way to use the native Leopard VPN client rather than Cisco though.



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: fltman on Sep 03, '08 01:14:53AM
Try Shimo (http://www.shimoapp.com/) which is a great replacement for many VPN clients such as Ciscos own. Works like a charm and best spent $20 in a long time.

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I've had the same problem
Authored by: porkchop_d_clown on Sep 02, '08 12:37:58PM

but I associated it with running VMware. It didn't occur to me that this might be a common issue!

---
Everyone loves a clown, but no one will lend him money!



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: dr_bob_cms on Sep 02, '08 07:49:07PM

I've seen this a few times, but typically only after a system upgrade or a security patch. This is typically after a reboot, and I've always figured that something got screwed with the kernel extension. I keep a copy of the Cisco installer, and just reinstall. Even though the installer claims that I have to reboot, I've always found that just reinstalling brings the VPN client back to life.



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: chefhomer on Sep 04, '08 03:14:43PM

Yes - this fix does work great if you're an admin user - we've been using it successfully for a few years now. We also see this error occur a lot recently when switching between network locations and/or switching AirPort networks - most of the time if you quit the VPN client, 'Turn AirPort Off' from the AirPort menu - and then turn it back on and rejoin the airport network - the Cisco VPN Client can then be relaunched and will recognize the airport network interface. (which is the only workaround I have found for the non-admin accounts)






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A fix for 'Error 51' with the Cisco VPN Client
Authored by: rfrost on Sep 06, '08 07:45:54AM

I maybe ran into an even simpler "fix," after getting the same Cisco VPN messa ge when connecting to my university's network. I simply opened the Network Pref Pane and clicked on the "details" or "advanced" button for whatever connection method was active, and that enabled Cisco to connect. My guess is that mt trick just forces an additional round of port-polling.



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: ovrdrvn on Oct 26, '08 06:53:45AM

I've found that with Remote Management on (or Back to My Mac for that matter) The Cisco client produces this error. Is there a fix for this that anyone is aware of?



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: jnyfeler on Jun 04, '09 02:37:02AM

Please help.

I am a new mac user, moving from a PC, and this is my first experience. I received this Error 51 when started the Cisco VPN. Since then, I have tried these commands multiple times, rebooted, reinstalled the VPN client, turned off AirPort, and rebooted many more times.

Nothing will get rid of this error message, and it is quite frustrating. I'm on a brand new macbook purchased June 3, 2009.

Can someone please help?



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: jnyfeler on Jun 04, '09 07:03:37AM

just a quick note -- I downloaded the latest version of the dialer (from my company), and it worked.



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: neopod on Sep 04, '09 04:08:45PM
Download r180 of Cisco VP here, 100% works with Snow X.6
<http://www.mcmaster.ca/uts/network/vpn/vpnclient_macosx.html>;

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A fix for 'Error 51' with the Cisco VPN Client
Authored by: jkoetje on Oct 30, '09 11:29:00AM

This fix hasn't worked for me. I had to "dump the Cisco program and reinstall, only to have the error reappear two days later. It doesn't matter if I put the computer to sleep or restart, the error still appears.



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: benc247 on Sep 03, '10 07:35:52AM

This is what I used to fix my problem:
http://forum.maas360.com/go/mobileitexpertise/fix-for-cisco-vpn-error-51-unable-to-communicate-with-the-vpn-subsystem/



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: craneb on Apr 04, '11 09:15:55AM

I could fix my problem with any of the fixes posted here. It turns out my MacBook Pro came set up to boot with the 64 bit kernel. Once I changed it back to book with the 32 bit kernel it was not a problem. Apparently Cisco does not currently support the 64 kernel with their VPN.



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A fix for 'Error 51' with the Cisco VPN Client
Authored by: HommeDeJava on Mar 16, '12 10:28:58AM

Sometimes this is simply not working...

Another workaround is to configure a VPN client using the Mac OS network preferences tool.

A lot less technical but it has worked for me...



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