4.4 VLAN Identification
4.4.2 ISL
ISL is a vendor-specific, proprietary protocol used to interconnect multiple switches and maintain VLAN information as traffic travels between switches on trunk links.

With ISL, an Ethernet frame is encapsulated with a header that transports VLAN IDs between switches and routers. ISL does add overhead to the packet as a 26-byte header containing a 10-bit VLAN ID. In addition, a 4-byte cyclic redundancy check (CRC) is appended to the end of each frame. This CRC is in addition to any frame checking that the Ethernet frame requires.

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. Figure illustrates the ISL frame format. Figure lists the sizes of the various ISL fields and Figure describe the ISL fields contained within the frame.