13 MAC A DDRESS TABLE MANAGEMENTn This chapter describes the management of static, dynamic, and blackhole MACaddress entries. For information about the management of multicast MAC addressentries, refer to “Multicast Overview” on page 185.Introduction to theMAC Address TableAn 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 MACaddress of the packets. To forward packets quickly, a switch maintains a MACaddress table, which is a Layer 2 address table recording the MACaddress-to-forwarding port association. Each entry in a MAC address tablecontains the following fields:■ Destination MAC address■ ID of the VLAN which a port belongs to■ Forwarding egress port numbers on the local switchWhen forwarding a packet, an Ethernet switch adopts one of the two forwardingmethods based upon the MAC address table entries.■ Unicast forwarding: If the destination MAC address carried in the packet isincluded in a MAC address table entry, the switch forwards the packet throughthe forwarding egress port in the entry.■ Broadcast forwarding: If the destination MAC address carried in the packet isnot included in the MAC address table, the switch broadcasts the packet to allports except the one receiving the packet.Introduction to MACAddress LearningMAC address table entries can be updated and maintained through the followingtwo ways:■ Manual configuration■ MAC address learningGenerally, the majority of MAC address entries are created and maintainedthrough MAC address learning. The following describes the MAC address learningprocess of a switch:1 As shown in Figure 41, User A and User B are both in VLAN 1. When User Acommunicates with User B, the packet from User A needs to be transmitted toEthernet 1/0/1. At this time, the switch records the source MAC address of the
