It is offered as an alternative to the IEEE 802.1Q standard, a widely used VLAN tagging protocol, although the use of ISL for new sites is deprecated by Cisco.
[2] With ISL, an Ethernet frame is encapsulated with a header that transports VLAN IDs between switches and routers.
This is a key advantage for IEEE 802.1Q as it means tagged frames can be sent over standard Ethernet links.
A VLAN ID is added only if the frame is forwarded out a port configured as a trunk link.
If the frame is to be forwarded out a port configured as an access link, the ISL encapsulation is removed.