Cisco created ISL before the IEEE standardized a trunking protocol. Thus, ISL is a Cisco proprietary
solution and can be used only between two Cisco switches. ISL fully encapsulates each original Ethernet
frame in an ISL header and trailer. The original Ethernet frame inside the ISL header and trailer remains
unchanged.
The ISL header includes a VLAN field that provides a place to encode the VLAN number. By tagging a
frame with the correct VLAN number inside the header, the sending switch can ensure that the receiving
switch knows to which VLAN the encapsulated frame belongs. Also, the source and destination addresses in
the ISL header use MAC addresses of the sending and receiving switch, as opposed to the devices that
actually sent the original frame.