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A trunk port is a port that is assigned to carry traffic for all the VLANs that are accessible by a specific switch, a process known as trunking. Trunk ports mark frames with unique identifying tags – either 802.1Q tags or Inter-Switch Link (ISL) tags – as they move between switches. Therefore, every single frame can be directed to its designated VLAN.
An Ethernet interface can either function as a trunk port or as an access port, but not both at the same time. A trunk port is capable of having more than one VLAN set up on the interface. As a result, it is able to carry traffic for numerous VLANs at the same time.
To accurately deliver traffic on a trunk port with multiple VLANs, the device makes use of tagging, or the IEEE 802.1Q encapsulation method. In this method, a tag is inserted within the frame header. This tag carries details regarding the particular VLAN to which the packet and frame belong. This approach enables packets, which are encapsulated for multiple VLANs, to cross exactly the same port as well as retain the traffic separation among the VLANs. The encapsulated VLAN tag also permits the trunk to switch the traffic from one end to another via the network over the same VLAN.
A trunk port carries/receives traffic to/from all VLANs by default. All VLAN IDs are permitted on all trunks. However, it is possible to remove VLANs from this comprehensive list to stop traffic from particular VLANs from passing over the trunks.