Cisco created ISL before the IEEE standardized a trunking protocol. Because ISL is Cisco proprietary, it can be used only between two Cisco switches. ISL fully encapsulates each original Ethernet frame in an ISL header and trailer, with the encapsulated original Ethernet frame being unchanged. Figure 10-4 shows the framing for ISL.
Figure 10-4 ISL Header
The ISL header includes several fields, but most important, the ISL header VLAN field 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 which VLAN the encapsulated frame belongs to. 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. Other than that, the details of the ISL header are not that important.
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