enable: specifies that L2TP is
supported. This parameter is optional.

radius-vpn-instance: specifies the name of the VPN instance
bound to the L2TP tunnel when the RADIUS server returns the attribute of the L2TP
tunnel. This parameter is optional. By default, radius-vpn-instance is set to none,
which indicates that no VPN instance is bound to the L2TP tunnel.

disable: specifies that L2TP is not supported. This parameter is
optional