A trunk is a switch link that carries traffic for many VLANs (virtual LANs) over one physical connection. Without trunks you would need a separate cable between switches for every VLAN. The IEEE 802.1Q standard makes trunking work by inserting a 4-byte tag into each Ethernet frame between the source MAC address and the EtherType. The tag includes a 12-bit VLAN ID, giving IDs up to 4094, and a 3-bit priority field used for class of service. The receiving switch reads the tag, removes it and forwards the frame in the right VLAN.
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