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An Ethernet switch is mainly used to forward packets at the data link layer, that is, 
transmit the packets to the corresponding ports according to the destination MAC 
address of the packets. To forward packets quickly, a switch maintains a MAC 
address table, which is a Layer 2 address table recording the MAC 
address-to-forwarding port association. Each entry in a MAC address table 
contains the following fields:

■

Destination MAC address

■

ID of the VLAN which a port belongs to

■

Forwarding egress port numbers on the local switch

When forwarding a packet, an Ethernet switch adopts one of the two forwarding 
methods based upon the MAC address table entries.

■

Unicast forwarding: If the destination MAC address carried in the packet is 
included in a MAC address table entry, the switch forwards the packet through 
the forwarding egress port in the entry.

■

Broadcast forwarding: If the destination MAC address carried in the packet is 
not included in the MAC address table, the switch broadcasts the packet to all 
ports except the one receiving the packet.

MAC address table entries can be updated and maintained through the following 
two ways:

■

Manual configuration

■

MAC address learning

Generally, the majority of MAC address entries are created and maintained 
through MAC address learning. The following describes the MAC address learning 
process of a switch:

1 As shown in Figure 41, User A and User B are both in VLAN 1. When User A 

communicates with User B, the packet from User A needs to be transmitted to 
Ethernet 1/0/1. At this time, the switch records the source MAC address of the