Netis ST3326(ST-3302) [63/118] Mac address overview

MAC address Overview

Introduction to MAC Address Table

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 number 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, tthe switch broadcasts the packet to all ports except the one that

originally received the packet.

Introduction to MAC Address Learning

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.

Managing MAC Address Table

Aging of MAC address table

To fully utilize a MAC address table, which has a limited capacity, the switch uses an aging

mechanism for updating the table. That is, the switch starts an aging timer for an entry when

dynamically creating the entry. The switch removes the MAC address entry if no more packets

with the MAC address recorded in the entry are received within the aging time.

Entries in a MAC address table

Entries in a MAC address table fall into the following categories according to their characteristics

and configuration methods:

 Static MAC address entry: Also known as permanent MAC address entry. This type of MAC

address entries are added/removed manually by the network operator and cannot age out

by themselves. Using static MAC address entries can greatly reduce broadcast packets and

are suitable for networks where network devices seldom change.