3Com 4210 9-Port Configuration Manual page 194

3) 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 the
packet. Normally, User B will respond to User A, as shown in
packet from User B comes into the switch on Ethernet 1/0/4, the switch records the association
between the MAC address of User B and the corresponding port to the MAC address table 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
1-5. When forwarding the response packet from User B to User A, the switch sends the response to
User A through Ethernet 1/0/1 (technically called unicast), because MAC-A is already in the MAC
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 mode
based on the corresponding MAC address table entries.
Under some special circumstances, for example, User B is unreachable or User B receives the
packet but does not respond to it, the switch cannot learn the MAC address of User B. Hence, the
switch still broadcasts the packets destined for User B.
The switch learns only unicast addresses by using the MAC address learning mechanism but
directly drops any packet with a broadcast source MAC address.
1-3
Figure 1-4. When the response
Figure