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The error "Error 51: Unable to communicate with the VPN subsystem" is  caused by an issue with the interprocess/driver communication.


This document will attempt to track known reasons and workaround/fixes for this error.

Issue 1: PPP network interfaces (unlikely to be seen today)

There is a known issue with Mac OS X 10.4 and the earlier versions of 4.9, such as when using PPP based interfaces (e.g. Dial-Up).  This issue is documented in Cisco bug ID CSCsd51157.  A workaround when hitting this issue was to restart the Cisco IPSec VPN Services after the VPN Point-to-Point Protocol (PPP) based connection is up.

The method to restart the VPN Service is:

/System/Library/StartupItems/CiscoVPN/CiscoVPN stop

/System/Library/StartupItems/CiscoVPN/CiscoVPN start


Upgrade to Mac OS X 10.4.9 or later and Cisco IPSec VPN or later.

Issue 2: 64bit Mac OSX Kernel

If one tries to use the Cisco IPSec VPN Client on Mac OS X running the 64-bit kernel one will receive the error 51 failure.


The Cisco IPSec cliet for Mac OS X does not support the 64 bit kernel.  The VPN driver only has i386 and PPC extensions, not x86_64 extensions.


To do a test 32bit boot one can hold the 3 and 2 buttons during Apple System Boot.  If this works then we have proved that your issue is the 64bit kernel.


1. If running Mac OS 10.6 or later, Use the built-in Mac OS X IPSec client.

2. If the built-in Mac OS X client is unavailable, re-configure your Mac to boot into the 32bit kernel.  This issue had not been much of a concern until recently, March 2011, when Apple began to release their Macbook Pro systems configured to boot into 64bit by default

Further Information

Default Architecture

These Macs use the 64-bit kernel by default in Mac OS X v10.6.

    * Mac Pro (Mid 2010)

    * MacBook Pro (Early 2011)

You can see which kernel you are using in System Profiler:

   1. Choose About This Mac from the Apple menu.

   2. Click More Info.

   3. Select Software in the Contents pane.

   4. Look for "64-bit Kernel and Extensions: Yes (or No)" under the System Software Overview heading.

How to Set the Boot Architecture

Issue 3: Mac OS X 10.7 (Lion) or later

If one tries to use the Cisco IPSec VPN Client on Mac OS X 10.7 (Lion) one might encounter this error.


The Cisco IPSec client for Mac OS X does  not support 10.7 (Lion) or later.


Use the built-in Mac OS IPsec client.  Configuration information can be found in the VPN Client for Mac OS X FAQ.

Overall: For IKEv1 IPSec support please use the Apple built-in client.

Using the Apple built-in client will help ensure support as the Mac OS Evolves.  For further information on Mac OS X VPN Support, please read the VPN Client for Mac OS X FAQ.

Community Member

Let me try in Lion tonight.

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