User`s manual

Signamax

™

065-7840C-WS 24-Port 10/100/1000BaseT/TX WebSmart Switch

Publication date: June, 2011

Revision 1.0

How does a switch operate?

A Layer 2 switch uses some features of the Data Link layer in OSI model to

forward the packet to the destination port(s). Here we introduce some important

features of a switch and how they work.

MAC address table

When a packet is received on a port of switch, the switch first checks if the

packet good or bad and extracts the source MAC address (SA) and destination

MAC address (DA) to find 1) if SA is existed in the MAC address table, if no, puts it

in the MAC address table, if yes, 2) looks up DA and its associated port to which the

traffic is forwarded. If DA does not exist, have the packet broadcasted.

Due to the size of the MAC address being limited, the MAC address aging

function is applied. When the MAC address has resided and has not been updated

in the table for a long time, this means the traffic using that entry has not come for a

while. If this time period is more than the aging time, the entry will be marked invalid.

The vacancy is now available for other new MACs.

Both learning and forwarding are the most important functions in a switch.

Besides that, VLAN can be one of the rules to forward the packet. There are ingress

rule and egress rule applied. The ingress rule is used to filter the incoming packet

by VLAN ID and so on and to decide whether the packet is allowed to enter the

switch or not. The egress rule is used to forward the packet to the proper port.

Mac address aging

There is a field in MAC address table used to set the entry’s Age time, which

determines how long a MAC entry can reside in a switch. The age time is refreshed

when a packet with that SA. Usually, the age time is programmable.

Transmission schedule

In most layer 2 switches, the QoS feature is supported. QoS in a switch must

associate a transmission schedule to transmit the packet. This function has much to

do with the priority level to which a packet has been assigned. With the given

Fig. 3-6