Why is vlan tagging used

And because not everything in this world is perfect, no matter how good the LAN Emulation was introduced to solve the need of creating VLANs over WAN links, allowing network managers to define workgroups based on logical function, rather than physical location.

With this new technology so to speak - it's actually been around since ! LANE is not very common and you will most probably never see it implemented in small to mid-sized networks, however, this is no reason to ignore it.

Just keep in mind that we won't be looking at it in much depth, but briefly covering it so we can grasp the concept. When implemented between two point-to-point links, the WAN network becomes totally transparent to the end users:. This implementation is usually found on Cisco's high-end switch models such as the Catalyst series where special modules are installed inside the switches, connecting them to an FDDI backbone. This backbone interconnects all major network switches, providing a fully redundant network.

When intalling the appropriate switch modules and with the use of the To provide further detail, the diagram below shows the IEEE It's okay if your impressed or seem confused with the structure of the above frame, that's normal: You'll be suprised to find out that the Cisco switch in the previous diagram must process the Ethernet II frame and convert it before placing it on the IEEE This conversion is required to maintain compatability and reliability between the two different topologies.

The most important bit to remember here is the SAID field and its purpose. This page introduced four popular VLAN tagging methods, providing you with the frame structure and general details of each tagging method.

Out of all, the IEEE Deal with bandwidth spikes Free Download. Why is VLAN tagging required? Ask Question. Asked 5 years, 1 month ago. Active 5 years, 1 month ago. Viewed 4k times. Improve this question. Danis Fermi Danis Fermi 1 1 gold badge 3 3 silver badges 15 15 bronze badges. Multicast MAC addresses are absolutely common to appear in a lot of vlans at the same time.

Add a comment. Active Oldest Votes. U can understand this better if you consider the data path of a frame in a switch. Suppose Switch A has 24 ports can be any number , this switch is has been configured to have 3 vlans, namely vlan 10 for ports vlan 20 for port and the default vlan - vlan 1 for ports Now if this switch receives a frame destined for port 11 vlan 20 , but the frame does not have a vlan tag.

Hope it helps. Improve this answer. Hey in this case my doubt is can't the switch identify the VLAN based on which port the data came in through? DanisFermi, If the frame comes into switch, then yes, that switch can identify which VLAN the frame is in by the interface with which it entered the switch.

Only if the frame leaves that switch via a trunk, does the switch tag the frame, because no other switch will know into which interface on the switch the frame entered, or to which VLAN, the frame belongs. If the frame comes into a different switch, it comes into your switch through a trunk, and that is where VLANs are tagged on a trunk in order to mark which frames are in which VLANs.

Switches can't look at the table of another switch. DanisFermi, that is a bad idea. If you do that then the VLAN could change. That is because the VLANs are not tagged on an access interface, only on a trunk interface. That's why it is a best practice to only connect switches with trunk links. When the frame reaches the switch port, the switch will add the VLAN tag. When a frame leaves an untagged port, the switch strips the VLAN tag from the frame.

The traffic is then forwarded as normal. An example of this is when two switches are connected, and pass tagged traffic. The sender will send a frame with a VLAN tag. For example, a broadcast may be received on VLAN In this case, the switch will flood the frame to all other ports configured with VLAN In some cases, an untagged frame will arrive on a tagged port.

The switch assigns any untagged frame that arrives on a tagged port to the native VLAN. Consider this Example. The ports that the hosts connect to are trunk ports, with native VLAN 15 configured. Carrying untagged traffic has its uses.

This happens when one switch wants to send information to another switch. An example of switch-to-switch communication is CDP. CDP is a Cisco protocol used to share information about connected devices. In this case, if there is a trunk link between two switches, how does the sending switch decide which VLAN to use? In short, it sends untagged traffic, which is on the native VLAN. Be aware that there are other VLAN types and uses, which fall outside the scope of this article. There are also different ways of using data VLANs.

This is sometimes used by service providers to keep customer traffic separate. It may also be used to extend the number of available VLANs. When two switches are connected via trunk ports, and the native VLAN between the two does not match, the switch logs an error like this:.

The question is, does this cause a problem? The short answer is; no. There are two types of frames that could pass through this link; tagged and untagged. Any tagged traffic will be passed without being changed, due to this being a trunk link. Therefore, the only concern here is for untagged traffic.

In this example, the two switches are connected with a trunk link. However, the native VLANs 15 and 20 do not match, resulting the error above. So what happens if an untagged packet crosses this link? As discussed earlier, when an untagged frame enters a switch port, the native VLAN is tagged on the frame.

This is CDP informing you that the current configuration is not best-practice, and may result in some unexpected behaviour. What would happen if two access ports were used to link two switches, instead of trunk ports? Incoming frames will be added to the VLAN on the access port, whatever it may be. The difference is that although traffic will flow, essentially only one VLAN is being allowed from one switch to the other. Some people may even say that this VLAN cannot be trunked.

Is this correct? These are generally misleading concepts.