Signamax
™
065-7921PoE 12-Port 10/100/1000BaseT/TX Web Smart PoE Switch
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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 limited, MAC address aging function is applied.
When the MAC address has resided and keeps no update in the table for a long time, this
means the traffic using that entry has yet 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 MAC.
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 put 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 is supported. QoS in a switch must associate a
transmission schedule to transmit the packet. This function is much to do with the priority level
a packet has. With the given priority, the scheduler will do the proper action on it. The
scheduler has many ways to implement, and different chips may support different schedule
algorithms. Most common schedulers are:
FCFS: First Come First Service.
Strictly Priority: All High before Low.
Weighted Round Robin:
Set a weight figure to the packet with a priority level, say 5-7, and next, set another
weight to the packet with a priority level, say 2-4 and so on. The WRR will transmit the packet
with the weight. So the packet of each priority level can be allocated a fixed bandwidth.
