1-3
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, the switch unicasts the packet instead of broadcasting
it to User A through GigabitEthernet 1/0/1, 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 directly unicasts the communication packets between User A and
User B 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.
Managing MAC Address Table
Aging of MAC address table
To fully utilize a MAC address table, which has a limited capacity, the switch uses an aging mechanism
for updating the table. That is, the switch starts an aging timer for an entry when dynamically creating
the entry. The switch removes the MAC address entry if no more packets with the MAC address
recorded in the entry are received within the aging time.
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