Inter-Switch Link (ISL) overview
Inter-Switch Link (ISL) is a Cisco proprietary protocol for frame tagging. Since it is a propriatery protocol, it can be used only between Cisco switches. It supports up to 1000 VLANs and can be used over Fast Ethernet and Gigabit Ethernet links only.
ISL works by encapsulating an Ethernet frame in an ISL header and trailer. The encapsulated frame remains unchanged. The VLAN ID is included in the ISL header.
ISL encapsulates the frame: