In the following figure, what will happen first at the switch when Computer A wants to send Computer B a message?

Computer A will create an ARP (broadcast) request. When that request is received on port Fa0/0, the switch will record Computer A's MAC address on Fa0/0. Then it will forward the message to all ports because the initial ARP packet/frame is a layer 2 broadcast. The switch will not directly communicate with Fa0/1 because the MAC address table is empty and the initial ARP packet/frame is a broadcast. The switch will only record Computer B's MAC address on port Fa0/1 when Computer B responds to the initial ARP packet/frame with a frame containing the source MAC address.