Cisco's implementation of the DTLS protocol unfortunately does not
comply with the relevant standards, and the OpenSSL in FreeBSD's
base requires a patch to provide full interoperability.  You will
still be able to establish a tunnel, but all traffic will be
transmitted via an HTTPS connection.  This means that any packet
loss on your link will result in a significant detriment to the
speed of your VPN connection.

More information is included in openconnect.html.