Frame Tagging
It is possible to build VLANs to pair number of connected switches. In the figure below you can see that the hosts from different VLANs are spread throughout number of switches. The biggest advantage to implement VLANs is its flexible and power-packed nature. The drawback is that it can become complicated—also for a switch— so it is important to have a way through which keeping the track of each one is easy i.e. all the users and frames when they travel the VLANs and switch fabric. (Switch fabric is actually referred to as a group of switches that shares the same information of VLAN.) This is actually the point from where the frame tagging actually starts. This method of frame identification is quite unique for assigning a userdefined ID to every single frame. It is also referred to as a “color” or “VLAN ID.” Now lets check out how it actually works: Determining the VLAN ID from the frame tag is quite essential for every single switch that the frame reaches to, so that it can discover the use of frame by seeing the info available in the filter table. In the case where the frame reaches a switch that has another trunked link, then the frame will be sent out the trunk-link port. As soon as the frame arrives at a point of exit to an access link, the switch deletes the identifier of VLAN. In this way destination device receives the frames without understanding the identification of VLAN.
Methods of VLAN Identification
VLAN identification is all about the switches keeping record of all the frames when they pass through a switch fabric.. In this manner the switches determine/identify which frame is related to which VLAN, and there are multiple methods of trunking:
Inter-Switch Link (ISL)
Inter-Switch Links or ISL is actually proprietary to Cisco switches, and it is utilized only for the Fast Ethernet and Gigabit Ethernet links. Some best uses of ISL routing includes server interface cards, switch port and router interfaces in order to trunk a server. In order to create functional VLANs without breaking the 80/20 rule this approach is considered best. Now the question is, what’s the 80/20 rule? It is actually a formula according to which 80 percent of the data traffic should remain on the local segment while the remaining 20 percent or less than 20 percent should cross a segmentation device. At the same time the trunked server is a component of all VLANs (broadcast domains), so the users can access it without crossig a layer-3 device. If you want more info about ISL, just look at this article: ISL Protocol
IEEE 802.1Q
As per the standard method of frame tagging it is actually created by the IEEE, and in order to identify the VLAN it inserts a field into the frame. If the trunking occurs between a Cisco switched link and an other brand of switch then in order to make the trunk work use 802.1Q.