Implementing VLANs allows you to segment a single switch into multiple logical networks. This improves security (HR can't see Sales traffic) and performance (broadcast containment).
2. New Concept: Voice VLAN (Auxiliary VLAN)
In modern offices, computers often plug into the back of a VoIP phone, which then plugs into the wall.
This creates a unique problem: We need two VLANs on a single access port.
Access VLAN (Data): Untagged. Handled by the PC.
Voice VLAN: Tagged (802.1Q). Handled by the Phone.
When you type switchport voice vlan 110, the switch tells the phone via CDP/LLDP to tag its own traffic with ID 110, while passing PC traffic through untagged.
3. Deep Dive: IEEE 802.1Q Tagging
Trunks use Tagging to identify which VLAN a frame belongs to.
[ Dest MAC ] [ Src MAC ] [ 802.1Q TAG ] [ EtherType ] [ DATA ] [ FCS ]
The Untagged Rule: Traffic belonging to the Native VLAN (and Data VLAN on an endpoint) is sent across the trunk without a tag.
