A trunk port carries traffic for multiple VLANs simultaneously over a single physical link. Frames on a trunk include a VLAN tag so the receiving switch knows which VLAN they belong to. You use trunk ports to connect switches together or to connect a switch to a router doing inter-VLAN routing. Without trunks, you'd need a separate physical link for each VLAN between switches. This would quickly exhaust your available ports and add unnecessary cabling.
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$ curl repovive.com/roadmaps/network-design/switching-and-vlans/trunk-ports
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