Why Native VLAN exists on a Trunk?

Question

Basically, A Native VLAN carries untagged traffic on a trunk line.
A trunk line allows mutiple VLAN traffic ( tagged traffic). So Why Native VLAN exists on a trunk.
Why Native VLAN was created?
I'm pretty confused up with VLANs.

Answer 1

Hi,
The second question - why PC Network adapters support VLAN tags - is actually easier to answer :)
First of all, with regards to VLANs and frame tagging, there is actually nothing special to support on a network adapter! The VLAN tag itself is in fact stored in the payload of a tagged frame. Even to the most dumb network adapter, a tagged frame looks like any other - Destination MAC, Source MAC, EtherType (set to 0x8100), Payload (holding the rest of the VLAN tag, the original EtherType and the original Payload), and the FCS. Supporting VLANs and frame tags is possible on a purely software level. The fact that network adapters often do have hardware support for VLANs is related to performance reasons: With hardware VLAN support, the tagging, de-tagging, filtering and/or sorting frames based on the VLAN tag value is faster and it allows offloading these operations from the computer's CPU to the network card. However, even if the network adapter did not have any kind of VLAN support, the VLANs could still be implemented purely in the card's software driver.
Ordinarily, you would not see a common PC send and receive tagged frames. However, there are situations in which even a PC would send or receive a tagged frame. One of reasons is the presence of the Class-of-Service (CoS) bits in a VLAN tag. You surely know that basic Ethernet frame format does not include any kind of priority marking. There is no field in an Ethernet header that would allow you to indicate that this or that frame requires a preferential treatment. VLAN tags, on the other hand, have a 3-bit CoS field that allows you to indicate the priority of the tagged frame. Should a  PC need to send a frame that needs to be explicitly marked as more important than others, it can be done by inserting a VLAN tag into this frame and setting the CoS field to a non-zero value (with 3 bits, the maximum CoS value is 7).
Another reason for a computer to send and receive tagged frames would be when the computer itself would be intentionally placed into multiple VLANs. For example, the router-on-a-stick performing inter-VLAN routing is not a concept just for dedicated hardware routers. For example, any computer running Linux can be used in place of a Cisco router to perform inter-VLAN routing. Just like on a Cisco router, you would configure the Linux with subinterfaces for each VLAN it should be able to route from and to, assign IP addresses, and voila - you have a cheap and powerful inter-VLAN router. Yet another reason for receiving and sending tagged frames on a computer would be virtualization: You could have a server that runs multiple virtual operating systems in VirtualBox, VMWare, Xen or some other virtualization solution, and you want each of these virtual PCs to have a "separate" network card so that they can not talk to each other when they communicate with the outside world. You would do this again by creating subinterfaces on the physical machine, and bridging the individual virtual PCs with unique subinterfaces so that each virtual PC gets its own subinterface onto which it is bridged. As a result, the communication of individual virtual PCs would be tagged on the physical link depending on what virtual machine was speaking.
So, while not exactly a typical situation for an ordinary office PC, it is nonetheless quite normal to see a computer being connected to a trunk port. This, however, is always done with the prior knowledge that the computer will indeed need to talk to several VLANs simultaneously and directly. Otherwise there's no need for that.
Regarding the native VLAN on trunks - well, this is a neverending debate. I wish the native VLAN was never invented but well, it's here so we have to fight with it. Often, it is explained as "the VLAN that will save you if you happen to connect a normal PC to a trunk", and you have asked quite correctly - why on Earth would I want to connect a normal PC to a trunk, if not for reasons stated above? And you would be perfectly right - you wouldn't. The reason for native VLANs is different. If you try to study the IEEE 802.1Q standard you will learn that it does not recognize the terms access port and trunk port. It has no distinction for these port types. Instead, 802.1Q considers each port to be possibly associated with multiple VLANs at once. One of these VLANs is called the Primary VLAN, its number (ID) is called the Primary VLAN ID (PVID), and this VLAN is considered to be the one that is always associated with the port and thus does not need to use tags. Any other VLAN that is in addition associated with the port obviously has to use tags to be distinguishable. From this viewpoint, a port that is associated just with its PVID would be what Cisco calls an access port, and a port that is associated with VLAN IDs other than just its PVID would be what Cisco calls a trunk port, with the PVID being the trunk's native VLAN ID.
So in the way IEEE defines VLANs and their usage, PVID (= native VLAN ID) is a property of each port, so any implementation that claims compatibility with 802.1Q has to implement the PVID. Cisco decided to have a twist on the understanding of VLANs, and came up with access ports (i.e. ports associated just with their PVID and no other VLAN ID) and trunk ports  (i.e. ports associated with many VLAN IDs including PVID), and so each trunk port must have its PVID - and that is what we call native VLAN and why we need to at least support it - although we do not need to make use of the native VLAN on trunks.
Quite convoluted.
Best regards,
Peter