Identifying VLANs
VLANs can span multiple connected switches. Switches in this switchfabric must keep track of frames and which VLAN frames belong to. Frametagging performs this function. Switches can then direct frames to the appropriate port.
There are two different types of links in a switched environment: Access links Links that are only part of one VLAN and are referred to asthe native VLAN of the port. Any device attached to an access link isunaware of a VLAN membership. This device just assumes it is part of a broadcast domain, with no understanding of the physical network.Switches remove any VLAN information from the frame before it is set to an access link device. Access link devices cannot communicate with devices outside their VLAN unless the packet is routed through a router.
Trunk links
Trunks can carry multiple VLANs. Originally named aftertrunks of the telephone system, which carries multiple telephone conversations,trunk links are used to connect switches to other switches, to routers, or even to servers. Trunked links are supported on Fast or Gigabit Ethernet only. To identify the VLAN that a frame belongs to with Ethernet technology, Cisco switches support two different identification techniques: ISL and 802.1q. Trunk links are used to transport VLANs between devices and can be configured to transport all VLANs or just a few. Trunk links still have a native, or default, VLAN that is used if the trunk link fails.
Frame Tagging
The switch in an internetwork needs a way of keeping track of users and frames as they travel the switch fabric and VLANs. A switch fabric is a group of switches sharing the same VLAN information. Frame identification ( frame tagging) uniquely assigns a user-defined ID to each frame. This is sometimes referred to as a VLAN ID or color. Cisco created frame tagging to be used when an Ethernet frame traverses a trunked link. The VLAN tag is removed before exiting trunked links. Each switch that the frame reaches must identify the VLAN ID, then determine
what to do with the frame based on the filter table. If the frame reaches a switch that has another trunked link, the frame will be forwarded out the trunk link port. Once the frame reaches an exit to an access link, the switch removes the VLAN identifier. The end device will receive the frames without having to understand the VLAN identification.