Switchport trunk

A trunk is a port on a switch that is configured to carry traffic from multiple VLANs, and is typically implemented within the framework of an Ethernet network.

There are various types of Switchport trunk encapsulation, but the most commonly used one is the IEEE 802.3q standard. In this standard, each frame that traverses a trunk includes a VLAN tag that identifies the VLAN to which that particular frame belongs. The tag is placed within the Ethernet header.

Once that frame is received on the other end, the tag is read and removed, and the frame is placed on the appropriate VLAN.

Some additional concepts involved with trunks can be found at the following notes:

• Native VLANs and untagged frames
• VLANs - when a tagged frame arrives on an access port
• Private VLANs - across a trunk
• Switchport trunk configuration ignores access commands
• VLANs - 802.1Q tunneling (QinQ)
• DTP
• VoIP - Configuring a switch port for use with an IP phone

Links:

https://networklessons.com/switching/how-to-configure-trunk-on-cisco-catalyst-switch