3Com Switch 4200G Configuration Guide

94 CHAPTER 18: MAC ADDRESS TABLE MANAGEMENT
MAC Address Learning
Mechanism
The MAC address learning mechanism enables a switch to acquire the MAC
addresses of the network devices on the segments connected to the ports of the
switch. A packet can be directly forwarded if its destination MAC address is already
learnt by the switch.
The MAC address learning mechanism is implemented as follows:
■ When a switch receives a packet from one of its ports (referred to as Port A), the
switch extracts the source MAC address (referred to as MAC-S) of the packet and
considers that the packets destined for MAC-S can be forwarded through Port A.
■ If the MAC address table already contains MAC-S, the switch refreshes the aging
time of the corresponding MAC address entry. Otherwise, the switch adds MAC-S
and Port A as a new MAC address entry to the MAC address table.
■ The switch searches the MAC address table for the destination MAC address of
the received packet. If it finds a match, it directly forwards the packet, or else it
broadcasts the packet in the corresponding VLAN.
■ When a broadcast packet reaches the network device whose MAC address is the
destination MAC address of the packet, the network device returns a packet to
the switch, with its MAC address contained in the packet.
■ The switch extracts the MAC address of the network device from the returned
packet and adds a MAC address entry accordingly in its MAC address table. After
that, the switch can directly forward other packets destined for the same network
device by the newly added MAC address entry.
Among the three types of packets (unicast packets, multicast packets, and broadcast
packets), the MAC address learning mechanism enables a switch to learn MAC
addresses from only unicast packets.
Aging Time of MAC
Address Entries
As mentioned previously, an Ethernet switch can acquire MAC addresses of network
devices from its ports and add MAC address entries accordingly in its MAC address
table.
The MAC address table is updated regularly. That is, the switch updates the aging
time of an existing MAC address entry if it learns the same MAC address again before
the specified aging time expires, and removes an existing MAC address entry if it does
not learn the same MAC address again when the specified aging time expires.
Note the following when setting the aging time:
■ If the aging time is too long, the number of the invalid MAC address entries
maintained by the switch may be too many to make room for the MAC address
table. In this case, the MAC address table cannot vary with network changes in
time.
■ If the aging time is too short, MAC address entries that are still valid may be
removed. This results in large amount of broadcast packets wandering across the
network and decreases the performance of the switches.
Aging time only applies to dynamic MAC address entries.
Limit of the Number of
MAC Addresses Learnt
The MAC address learning mechanism enables an Ethernet switch to acquire the
MAC addresses of the network devices on the segment connected to the ports of the
switch.