H3C Technologies H3C S3600 Series Switches - Manual (Page 236)

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5) After this interaction, the switch directly unicasts the packets destined for 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.
z The MAC address aging timer only takes effect on dynamic MAC address entries.
z With the destination MAC address triggered update function enabled, when a switch finds a packet
with a destination address matching one MAC address entry within the aging time, it updates the
entry and restarts the aging timer.
Entries in a MAC address table
Entries in a MAC address table fall into the following categories according to their characteristics and
configuration methods:
z Static MAC address entry: Also known as permanent MAC address entry. This type of MAC
address entries are added/removed manually and can not age out by themselves. Using static
MAC address entries can reduce broadcast packets remarkably and are suitable for networks
where network devices seldom change.
z Dynamic MAC address entry: This type of MAC address entries age out after the configured aging
time. They are generated by the MAC address learning mechanism or configured manually.
z Blackhole MAC address entry: This type of MAC address entries are configured manually. A switch
discards the packets destined for or originated from the MAC addresses contained in blackhole
MAC address entries.
Table 1-1 lists the different types of MAC address entries and their characteristics.