Virtual LANs form the basis of modern local-area networks today. VLANs provide many benefits, including alleviating performance problems due to excessively large broadcast domains and securing devices by separating them into multiple VLANs, even though the devices may be connected to the same physical switch. Cisco Catalyst switches support up to 4096 VLANs (using 802.1Q trunking) or 1024 VLANs (using Cisco ISL trunking).
Each VLAN is totally separated from other VLANs on a switch. Devices within the same VLAN can communicate with each other directly at Layer 2 level. Devices that are in different VLANs can communicate only via a Layer 3 router that is attached to each VLAN. All VLANs are stored in a VLAN database. In its simplest form, a VLAN consists of an ID and a name. The VLAN ID uniquely identifies the VLAN, while the name gives the VLAN a descriptive meaning. Other parameters exist that define a VLAN, for example, the media type (e.g., Ethernet or Token Ring) and MTU (maximum transmission unit). Ethernet switches exclusively use transparent bridging to forward frames within a VLAN. Transparent bridging is aptly named because end devices are not aware that they connect to a bridging device such as a switch.
A fundamental requirement for any networking device is the ability to manage the device, both locally and remotely. Cisco Catalyst switches possess a virtual management interface that can be placed into any VLAN, ensuring the appropriate management devices can gain remote management access to the switch. The virtual management interface on a CatOS-based switch is called sc0, while on Cisco IOS-based switches, it is called a switched virtual interface (SVI).