A trunk is a switch link that carries traffic for multiple VLANs. On a trunk, IEEE 802.1Q inserts a 4-byte tag into each Ethernet frame, containing the 12-bit VLAN ID and the 3-bit CoS priority. The receiving switch reads the tag and places the frame in the correct VLAN. Trunks connect switches to each other, to routers doing router-on-a-stick, and to hypervisors and wireless controllers.
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