When An 802.1q Tag Is Added To An Ethernet Frame, Where Is It Placed?

When an 802.1q tag is added to an ethernet frame, where is it placed?
We insert it between the source’s address and the ethernet type field. Today, let us discuss the basics of tagging and trunking.

Let us talk about the different terminologies such as the dot1q VLAN tag, trunk ports, and excess ports. let us take one example of how it works. Normally we refer to 802.1q as a dot1q, it is a networking standard that supports the VLANs on the ethernet network. This standard defines a network between the two switches in order to tell which traffic belongs to which of these networks.

Let us take an example to understand the concept in detail. suppose we consist of the four PCs. Let say PC1, PC2, PC3, PC4. we also assume two switches i.e switch 1 and switch 2 which are labeled. And let us assume an airplane. Switch 1 has a port FA 0/24 and switches 2 has 0/24. Now all these PCs are tied into the switch straight through the cables.

The PC1 goes to port 1 on the switch, PC2 goes to port 2, PC3 goes to port 3 and PC4 goes to port 4 on the switch. When we connect a PC to the switch that switches port is going to assign as an access port. Let us say that we assign VLAN 2 to port 3, so it says that the switch port mode access switch is the member of the VLAN 2.

And the PC that connected to it also be part of the VLAN 2. similarly the PC 1, the PC 1, and PC3 are part of the VLAN 2 which are on separate switches. Now, let us take port 2 as the access port that is connected to the VLAN 3. Now, we have two VLANs on both the switches but if the pc1 and pc2 want to talk with each other it will make it a difficult task.

Let us say that the PC1 has the IP address as 10.0.0.1 and PC3 has an IP address as 10.0.0.3. We give completely different IP addressing space to the PC4 as 20.0.0.4, it is connected to the different VLANs. And take 20.0.0.2 for the PC2. Now, let us know what actually the problem is.

Here when the PC1 sends the broadcast out, it should go to every single other device in that same VLAN. so the broadcast comes into the switch and its other side. So we need to specify the interfaces. We are gonna set up the trunk ports. The trunk ports are expecting to carry the traffic that needs to go across for the multiple VLANs. The tagging keeps track of the broadcasts from the VLAN 2 or VLAN three.

When a PC1 sends a broadcast to the switch, it is received by the switch on the access port, the switch actually knows the broadcast belongs to which VLAN. Instead of broadcast to all, it simply sends it to through the trunk.