1-11 MAC Address Table ManagementThis chapter describes the management of static, dynamic, and blackhole MAC address entries. Forinformation about the management of multicast MAC address entries, refer to the part related tomulticast protocol.OverviewIntroduction to MAC Address TableAn Ethernet switch is mainly used to forward packets at the data link layer, that is, transmit the packetsto the corresponding ports according to the destination MAC address of the packets. To forward packetsquickly, a switch maintains a MAC address table, which is a Layer 2 address table recording the MACaddress-to-forwarding port association. Each entry in a MAC address table contains the following fields:z Destination MAC addressz ID of the VLAN which a port belongs toz Forwarding egress port numbers on the local switchWhen forwarding a packet, an Ethernet switch adopts one of the two forwarding methods based uponthe MAC address table entries.z Unicast forwarding: If the destination MAC address carried in the packet is included in a MACaddress table entry, the switch forwards the packet through the forwarding egress port in the entry.z Broadcast forwarding: If the destination MAC address carried in the packet is not included in theMAC address table, the switch broadcasts the packet to all ports except the one receiving thepacket.Introduction to MAC Address LearningMAC address table entries can be updated and maintained through the following two ways:z Manual configurationz MAC address learningGenerally, the majority of MAC address entries are created and maintained through MAC addresslearning. The following describes the MAC address learning process of a switch:1) As shown in Figure 1-1, User A and User B are both in VLAN 1. When User A communicates withUser B, the packet from User A needs to be transmitted to GigabitEthernet 1/0/1. At this time, theswitch records the source MAC address of the packet, that is, the address “MAC-A” of User A to theMAC address table of the switch, forming an entry shown in Figure 1-2.
