28 MAC A DDRESS TABLE MANAGEMENTn This chapter describes the management of static and dynamic MAC addressentries. For information on the management of multicast MAC address entries,refer to “Multicast Overview” on page 413.OverviewIntroduction to MACAddress LearningAn Ethernet switch maintains a MAC address table to forward packets quickly. AMAC address table is a port-based Layer 2 address table. It is the base for Ethernetswitch to perform Layer 2 packet forwarding. Each entry in a MAC address tablecontains the following fields:■ Destination MAC address■ ID of the VLAN which a port belongs to.■ Forwarding port number.Upon receiving a packet, a switch queries its MAC address table for theforwarding port number according to the destination MAC address carried in thepacket and then forwards the packet through the port.The dynamic address entries (not configured manually) in the MAC address tableare learned by the Ethernet switch. When an Ethernet switch learns a MACaddress, the following occurs:When a switch receives a packet from one of its ports (referred to as Port 1), theswitch extracts the source MAC address (referred to as MAC-SOURCE) of thepacket and considers that the packets destined for MAC-SOURCE can beforwarded through Port 1.■ If the MAC address table already contains MAC-SOURCE, the switch updatesthe corresponding MAC address entry.■ If MAC-SOURCE does not exist in the MAC address table, the switch addsMAC-SOURCE and Port 1 as a new MAC address entry to the MAC addresstable.
