Introduction to the
MAC Address Table
Introduction to MAC
This chapter describes the management of static, dynamic, and blackhole MAC
address entries. For information about the management of multicast MAC address
entries, refer to "Multicast Overview" on page 185.
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
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