Starts a packet capture.

        :param port_number: allocated port number
        :param output_file: PCAP destination file for the capture
        :param data_link_type: PCAP data link type (DLT_*), default is DLT_EN10MB

def start_capture(self, port_number, output_file, data_link_type="DLT_EN10MB"):
        """
        Starts a packet capture.

        :param port_number: allocated port number
        :param output_file: PCAP destination file for the capture
        :param data_link_type: PCAP data link type (DLT_*), default is DLT_EN10MB
        """

        if port_number not in self._mappings:
            raise DynamipsError("Port {} is not allocated".format(port_number))

        nio = self._mappings[port_number]

        data_link_type = data_link_type.lower()
        if data_link_type.startswith("dlt_"):
            data_link_type = data_link_type[4:]

        if nio.input_filter[0] is not None and nio.output_filter[0] is not None:
            raise DynamipsError("Port {} has already a filter applied".format(port_number))

        yield from nio.bind_filter("both", "capture")
        yield from nio.setup_filter("both", '{} "{}"'.format(data_link_type, output_file))

        log.info('Ethernet hub "{name}" [{id}]: starting packet capture on port {port}'.format(name=self._name,
                                                                                               id=self._id,
                                                                                               port=port_number))