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It is occasionally necessary to reboot the VPN servers in order to install patched software, or to enable new features. This message indicates that the network administrator has disconnected your session so that the VPN server can be rebooted. Click "OK" when you receive this message, and you should be able to connect again after approximately 1 minute. If you want to reconnect immediately, select an alternate server name from the AnyConnect connection window before reconnecting (e.g. change fwvpn1.slac.stanford.edu to fwvpn2.slac.stanford.edu).
If I connect to SLAC's VPN will I lost all my active ssh sessions?
Yes, as all your traffic will be then routed through the VPN tunnel making you appearing from a different IP address to the server.
Only persistent connections are affected (ssh, remote desktop...) all other traffic (web surfing,...) will be transparently rerouted through the VPN.