3Com WX3000 operation manual Managing MAC Address Table, Aging of MAC address table

Figure 1-4MAC address learning diagram (3)

User B User C

Geth 1/0/4	Geth 1/0/3
	Geth 1/0/1

User A

4)At this time, the MAC address table of the device includes two forwarding entries shown in Figure 1-5. When forwarding the response packet, the device 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 1-5MAC address table entries of the switch (2)

MAC-address	Port	VLAN ID
MAC-A	GigabitEthernet1/0/1	1
MAC-B	GigabitEthernet1/0/4	1

5)After this interaction, the device directly unicasts the communication packets between User A and User B based on the corresponding MAC address table entries.

z

z

Under some special circumstances, for example, User B is unreachable or User B receives the packet but does not respond to it, the device cannot learn the MAC address of User B. Hence, the device still broadcasts the packets destined for User B.

The device 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 device uses an aging mechanism for updating the table. That is, the device starts an aging timer for an entry when dynamically creating the entry, and removes the MAC address entry if no more packets with the MAC address recorded in the entry are received within the aging time.

1-3