In the VLAN Part 1 video, Don puts FA 0/1/21 into both the data and voice VLAN in the case of having both an VoIP phone and a workstation. How does the switch know what goes to the data VLAN and what goes to the voice VLAN? How does the switch know that VoIP traffic goes to the voice VLAN?
CCENT VLAN Question
The switch reads the vlan tags, both data vlans and voice vlan tags and uses CDP. So if you're using a Cisco IP Phone, the switch will use CDP to identify the IP phone as the IP phone and associate that phone to the voice vlan and voice data to the voice vlan. While regular vlan traffic will not get tagged with the voice vlan tag.
If you use a non Cisco phone, you must manually tag (change) the phone to put it's data on voice vlan.
So the short answer is the switch uses CDP to do what you asking about.