VLAN is used for divide the switch into different logical parts you can say it can segregate the broadcast domain into different parts.
VLANs simplify network management:
Network adds, moves, and changes are achieved with ease by just configuring a port intothe appropriate VLAN.
A group of users that need an unusually high level of security can be put into its own VLAN so that users outside of the VLAN can’t communicate with them.
VLANs can be considered independent from their physical or geographic locations.
VLANs greatly enhance network security.
VLANs increase the number of broadcast domains while decreasing their size.
Most of the time, VLANs are created by a sys admin who proceeds to assign switch ports to each VLAN. VLANs of this type are known as static VLANs
Creating static VLANs is the most common way to create a VLAN, and one of the reasons for that is because static VLANs are the most secure. This security stems from the fact that any switch port you’ve assigned a VLAN association to will always maintain it unless you change the port assignment manually.
A dynamic VLAN determines a node’s VLAN assignment automatically.Using intelligent management software, we can base VLAN assignments on hardware (MAC)addresses, protocols, or even applications that create dynamic VLANs.
An access port belongs to and carries the traffic of only one VLAN. Traffic isboth received and sent in native formats with no VLAN tagging whatsoever. Anything arriving on an access port is simply assumed to belong to the VLAN assigned to the port
isa point-to-point link between two switches, between a switch and router, or even between a switch and server, and it carries the traffic of multiple VLANs—from 1 to 4,094 at a time (though it’s really only up to 1,005 unless you’re going with extended VLANs).
Trunking can be a real advantage because with it, you get to make a single port part of a whole bunch of different VLANs at the same time.
VLAN Identification Methods
VLAN identification is what switches use to keep track of all those frames as they’re traversing a switch fabric. It’s how switches identify which frames belong to which VLANs, and there’s more than one trucking method.
Inter-Switch Link (ISL)
Inter-Switch Link (ISL)is a way of explicitly tagging VLAN information onto an Ethernet frame. This tagging information allows VLANs to be multiplexed over a trunk link through an external encapsulation method (ISL), which allows the switch to identify the VLAN membership of a frame over the trunked link.This is proprietary to Cisco.
Created by the IEEE as a standard method of frame tagging, IEEE 802.1Q actually inserts afield into the frame to identify the VLAN.