VLAN Frame tagging is a technology which is used to identify the VLAN that the packet belongs to. The VLAN Frame tag is placed on the Ethernet frame when the Ethernet frame reaches a switch from an access port, which is a member of a VLAN. If the switch has a trunk port, the Ethernet frame can be forwarded out the trunk link port. This enables each switch to see what VLAN the Ethernet frame belongs to and can forward the frame to corresponding VLAN access ports or to another VLAN trunk port.
Before forwarding the frame to a VLAN access port, the switch removes the VLAN identifier and the VLAN membership information is hence transparent to the end devices.
There are different VLAN frame trunking technologies available.
• Inter-Switch Link (ISL): Cisco proprietary VLAN frame tagging. No much support from other vendors. Supported only in old Cisco Switch models.
• IEEE 802.1Q: IEEE industry standard VLAN frame tagging
• LAN Emulation (LANE): LANE is used to communicate with multiple VLANs over ATM.
• 802.10 (FDDI): Protocol for sending VLAN information over FDDI.
The CCNA Routing and Switching certification requires knowledge only in IEEE 802.1Q.