VLAN Trunks

Briefly define a VLAN trunk:
A VLAN trunk is an Ethernet point-to-point link between an Ethernet switch interface and an Ethernet
interface on another networking device, such as a router or a switch, carrying the traffic of multiple
VLANs over the singular link. A VLAN trunk allows you to extend the VLANs across an entire network.

When a switch receives a frame on a port configured in access mode and destined for a remote device
via a trunk link, the switch takes apart the frame and inserts a VLAN tag, recalculates the FCS, and
sends the tagged frame out the trunk port.

The VLAN tag field consists of an EtherType field and a tag control information field. Also, the FCS
is recalculated for tagged frames.

The EtherType field is set to the hexadecimal value of 0x8100. This value is called the tag protocol ID
(TPID) value. This is how the switch receiving the frame knows to look for information in the tag
control information field for VLAN information.

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