SignaMax 065-7931 User's Guide download pdf (Page 45)

Signamax 065-7931 16-Port 10/100/1000BaseT/TX WebSmart Switch

How does a switch operate?

A Layer 2 switch uses some features of the Data Link layer in the 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 is good or bad and extracts the source MAC address (SA) and destination

MAC address (DA) to find 1) if SA is existing in the MAC address table; if no, the

switch puts it in the MAC address table; if yes, 2) looks up DA and its associated

port to which the traffic is forwarded. If the DA does not exist, the switch will have

the packet broadcasted.

Due to the size of the MAC address table being limited, a 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 that traffic using that entry has not

arrived 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 addresses.

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

rules and egress rules 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.

Fig. 3-6

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SignaMax 065-7931 User's Guide Page 45