20
In chart
1
，
on a two layer switch which not set VLAN
，
any radio frame is forwarded to all the other ports in
addition to receiving port. e.g
，
PC A
（
port 1
）
After sending the broadcast information
，
will be
forwarded to the port 2
、
3
、
4.
In chart 2
，
the switch generate red and blue VLAN
；
Meanwhile set port 1
、
2 as red VLAN
、
port 3
、
4 are VLAN. Then A
（
port 1
）
send out broadcast frame
，
switches only will forward it to other port
belongs to a VLAN——port 2 which also belong to red VLAN
，
Will not be forwarded to the blue port
VLAN. As well
，
C
（
port 3
）
send out broadcast frame
，
Will only be forwarded to other belong to the
blue port VLAN
，
Will not be forwarded to belong to the red VLAN ports.
In this way
，
VLAN by limiting the broadcast frame forwarding range to partition the broadcast
domain. In the above chart for better illustration
，
use red, blue two colors to identify different VLAN
，
in the practice is using “VLAN ID" to distinguish.
