Alcatel Practice Test

VLANs divide broadcast domains, and 802.1Q tagging is the method that lets a single physical link carry traffic for multiple VLANs. It does this by inserting a 4-byte tag into Ethernet frames that identifies the VLAN ID the frame belongs to. This tag sits after the source MAC address and before the EtherType, and it carries the VLAN ID along with optional priority bits used for QoS. Because the tag distinguishes VLANs, switches can forward frames from different VLANs over the same trunk link without mixing them. Frames for a VLAN on a trunk are tagged so the receiving switch knows which VLAN to place the frame into. Frames belonging to the native VLAN on a trunk can be sent untagged, depending on the configuration, which is why you sometimes hear that native VLAN frames are untagged. This mechanism is distinct from ARP, which is a protocol for mapping IP addresses to MAC addresses, and it does not involve duplicating frames or disabling trunks. The statement that best describes 802.1Q tagging is that VLANs create separate broadcast domains; 802.1Q inserts a 4-byte tag into Ethernet frames to identify VLAN IDs; trunks carry multiple VLANs; native VLAN frames are untagged.