Before understanding VLANs, you must first have a very specific understanding of the definition of a LAN. Although you can think about LANs from many perspectives, one perspective in particular will help you with understanding VLANs:
A LAN includes all devices in the same broadcast domain.
As described in Chapter 9, "Cisco LAN Switching Basics," a broadcast domain includes the set of all LAN connected devices that can send a broadcast frame, and all the other devices in the same LAN get a copy of the frame. So, you can think of a LAN and a broadcast domain as being basically the same thing.
Without VLANs, a switch treats all interfaces on the switch as being in the same broadcast domain—in others words, all connected devices are in the same LAN. With VLANs, a switch can put some interfaces into one broadcast domain and some into another. Essentially, the switch creates multiple broadcast domains. These individual broadcast domains created by the switch are called virtual LANs.
This chapter focuses on VLANs and the concepts and configuration required to implement VLANs on Cisco switches. This chapter covers VLAN concepts, including VLAN trunking. Also, you will read about what types of devices can be used to forward traffic between different VLANs.
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