Another VLAN tagging protocol is Inter-Switch Link (ISL). This protocol is Cisco proprietary, which means that, unlike 802.1Q, it can be used only between Cisco switches. It is considered to be deprecated, and newer Cisco switches don’t even support it.
ISL works by encapsulating a frame in an ISL header and trailer. The encapsulated frame remains unchanged. The VLAN ID is included in the ISL header.