1-33) Because the switch broadcasts the packet, both User B and User C can receive the packet.However, User C is not the destination device of the packet, and therefore does not process thepacket. Normally, User B will respond to User A, as shown in Figure 1-4. When the responsepacket from User B comes into the switch on GigabitEthernet 1/0/4, the switch records theassociation between the MAC address of User B and the corresponding port to the MAC addresstable of the switch.Figure 1-4 MAC address learning diagram (3)4) At this time, the MAC address table of the switch includes two forwarding entries shown in Figure1-5. When forwarding the response packet from User B to User A, the switch sends the response toUser A through GigabitEthernet 1/0/1 (technically called unicast), because MAC-A is already in theMAC address table.Figure 1-5 MAC address table entries of the switch (2)5) After this interaction, the switch sends packets destined for User A and User B in unicast modebased on the corresponding MAC address table entries.z Under some special circumstances, for example, User B is unreachable or User B receives thepacket but does not respond to it, the switch cannot learn the MAC address of User B. Hence, theswitch still broadcasts the packets destined for User B.z The switch learns only unicast addresses by using the MAC address learning mechanism butdirectly drops any packet with a broadcast source MAC address.
