Switchport - IEEE 802.1q

IEEE 802.1Q is a standard that is used to maintain VLAN information on Ethernet frames that traverse a trunk link between switches.

802.1Q sometimes denoted as dot1q, is a type of trunk encapsulation that adds a VLAN ID to the Ethernet header. It is currently the de facto standard for all networking equipment vendors for creating trunk links between switches. For Cisco devices it has largely superseded the proprietary ISL protocol that is no longer available in newer switches.