To recognize VLAN a packet is belonging to which switches use tagging to assign numerical value to each frame in a network with multiple VLAN. This is done to ensure that switch know out which port to forward frame.
Such that, consider
the below network topology:
There are two VLAN in the above topology, namely VLAN 3 and VLAN 4. Host A send a
broadcast packet to switch SW1. Switch SW1 receive the packet and tags the packet
with the VLAN ID of 3 and send it to SW2. SW2 receive the packet, look up at
the VLAN ID, and forward packet only out the port Fa0/1, since only that
port is in VLAN 3. Host B and host C will not receive packet because they are
in different VLAN.
NOTE
When forward a tagged frame to a host device a switch will remove the VLAN
tag, since host devices do not understand tagging and would drop the packets.
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