Unlike ATM, ISL forwards the data across the high-speed link without breaking the frames into cells. The entire frame is sent intact across the ISL connection. An ISL port is considered a trunk port. A trunk is a physical link that carries the traffic of multiple VLANs between two switches or between a switch and a router, thereby allowing the VLANs to be extended across switches. The ISL protocol is a packet tagging protocol that contains a standard Ethernet or Token Ring frame and the VLAN information associated with that frame.
Some additional information is also present in the frame.
It is primarily intended for network managers who do not want an ATM backbone for the campus. Figure 3-1 shows an example of the ISL backbone design. The Catalyst 5000 family of switches should be used to provide the ISL backbone. The Catalyst 3900 switch can then be connected to this backbone via the dual 100-Mbps ISL expansion module. These NICs can be used for high-speed attachment to servers.