DA (destination address)This 40-bit field is a multicast address and is set at 0x01-00-0C-00-00 or 0x03-00-0c-00-00. The receiving host determines that the frame is encapsulated in ISL by reading the 40-bit DA field and matching it to one of the two ISL multicast addresses.接收方只需要知道这两种组播地址，就可以确定收到的是ISL帧附上cisco.com的地
