3Com Switch 7750 Configuration Guide Guide

210 CHAPTER 27: MAC ADDRESS TABLE MANAGEMENT
Figure 52 Packets forwarded by using a MAC address table.
After learning the source address of the packet, 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.
■ If it finds no match, it forwards the packet to all ports, except the receiving
port, within the VLAN to which the receiving port belongs. Normally, this is
referred to as broadcasting the packet.
After broadcasting the packet, the switch will do one of the following based on
whether it receives a response packet:
■ If the network device returns a packet to the switch, this indicates the packet
has been sent to the destination device. The MAC address of the device is
carried in the packet. The switch adds the new MAC address to the MAC
address table through address learning. After that, the switch can directly
forward other packets destined for the same network device by using the
newly added MAC address entry.
■ If the destination device does not respond to the packet, this indicates that the
destination device is unreachable or that the destination device receives the
packet but gives no response. In this case, the switch still cannot learn the
MAC address of the destination device. Therefore, the switch will still broadcast
any other packet with this destination MAC address.
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 removes the MAC
address entries related to a network device if no packet is received from the device
within the aging time. Aging time only applies to dynamic MAC address entries.
You can manually configure (add or modify) a static or dynamic MAC address
entry based on the actual network environment.
n
The switch learns only unicast addresses by using the MAC address learning
mechanism but directly drops any packet with a broadcast source MAC address.
MAC Address Port
MAC A 1
MAC B 1
MAC C 2
MAC D 2
MACD MACA
......
Port 1
MACD MAC A
......
Port 2
MAC address Port
MAC A 1
MAC B 1
MAC C 2
MAC D 2
MAC D MAC A
......
Port 1
MAC D MAC A
......
Port 2
MAC Address Port
MAC A 1
MAC B 1
MAC C 2
MAC D 2
MACD MACA
......
Port 1
MACD MAC A
......
Port 2
MAC address Port
MAC A 1
MAC B 1
MAC C 2
MAC D 2
MAC D MAC A
......
Port 1
MAC D MAC A
......
Port 2