https://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html
Inter-Switch Link and IEEE 802.1Q Frame Format - Cisco
This document provides the basic information and a summary of the frame fields for Inter-Switch Link (ISL) and IEEE 802.1Q encapsulation.
inter switch linkieee 802 1qframeformatcisco