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We have noticed this happening if you change your network connection (e.g. go from a wired to a wireless connection and soon after start the VPN). Try disconnecting the VPN and reconnecting it. Failing that try rebooting if it is a Windows host. Failing that try re-installing the Cisco AnyConnect client.
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could not ping vpn.slac.stanford.edu
You can't ping vpn (or fwvpn1 or fwvpn2) if you have established a VPN connection.