1-3
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
Figure 1-4
. When the response
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
Figure
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.
z
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.
z
The switch learns only unicast addresses by using the MAC address learning mechanism but
directly drops any packet with a broadcast source MAC address.
