It's hard to be a networker today and not work with virtual LANs (VLANs) and VLAN trunking. Almost every campus LAN uses VLANs, and almost every campus LAN with more than one switch uses trunking. In short, you have to know these topics.
VLANs allow a switch to separate different physical ports into different groups so that traffic from devices in one group never gets forwarded to the other group. This allows engineers to build networks that meet their design requirements, without having to buy a different switch for each group. Also, multiple switches can be connected together, with traffic from multiple VLANs crossing the same Ethernet links, using a feature called trunking.
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