Frame Tagging Explained

To identify the VLAN a packet is belonging to, switches use tagging to assign a numerical value to each frame in a network with multiple VLANs. This is done to ensure that switches know out which ports to forward frames.

For example, consider the following network topology.

frame tagging

There are two VLANs in the toplogy pictured above, namely VLAN 3 and VLAN 4. Host A sends a broadcast packet to switch SW1. Switch SW1 receives the packet, tags the packet with the VLAN ID of 3 and sends it to SW2. SW2 receives the packet, looks up at the VLAN ID, and forwards the packet only out the port Fa0/1, since only that port is in VLAN 3. Host B and host C will not receive the packet because they are in different VLAN than host A.

NOTE
When forwarding a tagged frame to a host device a switch will remove the VLAN tag, since host devices don’t understand tagging and would drop the packet.

Download our Free CCNA Study Guide PDF for complete notes on all the CCNA 200-301 exam topics in one book.

We recommend the Cisco CCNA Gold Bootcamp as your main CCNA training course. It’s the highest rated Cisco course online with an average rating of 4.8 from over 30,000 public reviews and is the gold standard in CCNA training: