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A Network Trunk is:

A point-to-pint link that transmits and receives traffic between switches or between switches and routers. Trunks carry the traffic of multiple VLANs adn can extend VLANs across an entire network. FastEthernet or Gigabit ethernet can use either ISL or 802.1Q to carry traffic for mulitple VLANs over a single link. Frames recieved from users in the administratively-defined VLANs are identified or taggged for transmission to other devices. Based on the rules you define, a unique identifier is inserted in each frame header before it is forwarded. The tag is examined and understood by each device before any broadcasts or transmission to other switches, rotuers, or end stations. When the frame reaches the last switch or router, the tag is removed before the rame is transmitted to the target end station. definition-of-a-trunk