Inter-Switch Link Protocol
The Inter-Switch Link Protocol ( ISL ) is a proprietary protocol from Cisco Systems that regulates VLAN information such as data flows between switches and routers . The ISL protocol competes with the widely used, openly specified IEEE 802.1Q protocol, which is also supported by current Cisco devices.
ISL is a method of data encapsulation for VLANs developed by Cisco, but it only works on a manufacturer-specific basis with routers, switches and LAN cards from Cisco, or with products from licensees. ISL encapsulates the entire Layer 2 - data frame in an ISL packet ( header and trailer ). Since the entire frame is encapsulated, communication is only possible between ISL-capable components. However, the method works with very low latency and is protocol-independent; it can also transport any layer 2 frames as well as all higher packets. Since the encapsulated packets themselves cannot be touched, any frame types such as Ethernet or Token Ring can also be transported. The size of an ISL frame is between 94 bytes and can be up to 1548 bytes due to the overhead (additional header fields) caused by the protocol.
Another associated Cisco protocol, Dynamic Inter-Switch Link Protocol (DISL), simplifies the creation of an ISL connection between two connected Fast Ethernet devices, since only one endpoint has to be configured. Fast Etherchannel technology enables two full-duplex Fast Ethernet cables to be combined for broadband backbone connections.
According to Cisco, the ISL protocol should no longer be used for new installations if possible and should also be replaced in the long term by the widespread, non-proprietary IEEE 802.1Q protocol.