Frame Tagging

Frame tagging is the method used by Cisco Catalyst switches to identify to which VLAN a frame belongs. As a frame enters the switch, the switch encapsulates the frame with a header that "tags" the frame with a VLAN ID. Any time a frame needs to leave one switch for another, the tagged frame is sent throughout the switching fabric. When the frame arrives at the destination switch, the tag tells the switch to which VLAN the frame belongs. This process is illustrated in Figure 9.3 using the VLAN IDs 10, 20, and 30. The tag is stripped off of the frame before the frame is sent out to the destination device. This process gives the illusion that all ports are physically connected to the same switch.

Figure 9.3. ISL and frame tagging.

Be sure to understand the function of frame tagging, which "tags" a frame with a user -defined VLAN ID.