3Com 4210 9-Port - Managing MAC Address Table

1-4
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.
The MAC address aging timer only takes effect on dynamic MAC address entries.
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 by the network operator and cannot age out by
themselves. Using static MAC address entries can greatly reduce broadcast packets 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. Blackhole entries are configured for filtering out frames with specific source
or destination MAC addresses.
Table 1-1 lists the different types of MAC address entries and their characteristics.
Table 1-1 Characteristics of different types of MAC address entries
MAC address entry
Configuration
method
Aging time
Reserved or not at
reboot (if the
configuration is
saved)
Static MAC address
entry
Manually configured Unavailable Yes
Dynamic MAC address
entry
Manually configured or
generated by MAC
address learning
mechanism
Available No
Blackhole MAC
address entry
Manually configured Unavailable Yes