Key Point

We found the Cisco definition of VLANs to be very clear: "[A] group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible."121

Figure 2-6 illustrates the VLAN concept. On the left side of the figure, three individual physical LANs are shown, one each for Engineering, Accounting, and Marketing. (These LANs contain workstationsEl, E2, A1, A2, M1, and M2and serversES, AS, and MS.) Instead of physical LANs, an enterprise can use VLANs, as shown on the right side of the figure. With VLANs, members of each department can be physically located anywhere, yet still be logically connected with their own workgroup. Thus, in the VLAN configuration, all the devices attached to VLAN E (Engineering) share the same broadcast domain, the devices attached to VLAN A (Accounting) share a separate broadcast domain, and the devices attached to VLAN M (Marketing) share a third broadcast domain. Figure 2-6 also illustrates how VLANs can span across multiple switches; the link between the two switches in the figure carries traffic from all three of the VLANs and is called a trunk.

Figure 2-6. A VLAN Is a Logical Implementation of a Physical

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Figure 2-6. A VLAN Is a Logical Implementation of a Physical

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