Frame tagging is used when VLAN traffic travels over a trunk link. Trunk links carry frames for multiple VLANs. Therefore, frame tags are used for identification of frames from different VLANs. Inter Switch Link (ISL) and Institute of Electrical and Electronics Engineers (IEEE) 802.1q are the two frame tagging methods supported on Cisco devices.

The purpose of frame tagging is not inter-VLAN routing. A Layer 3 device, such as a router or multilayer switch, is used for inter-VLAN routing. To configure
interVLAN routing a logical or subinterface for each VLAN must be created on the single physical interface used to connect to the switch. An IP address is NOT
applied to the physical interface; instead, each subinterface is configured with an IP address that will become the default gateway of all devices residing in that VLAN. Consequently, each subinterface and its VLAN devices must reside a different subnet as well. If a subinterface on the router is NOT configured with an IP address that resides in the same network as the hosts that reside in the VLAN that the subinterface serves, the hosts in that VLAN will be isolated from the other VLANs. The hosts in the VLAN served by the subinterface should also use this address as their default gateway, or the hosts in the VLAN will likewise be isolated form the other VLANs