Cisco Inter-Switch Link

The Inter-Switch Link Protocol (ISL ) is a proprietary protocol from Cisco Systems that VLAN information as data flows between switches and routers regulates. The ISL protocol competes with the widely used, open the specified IEEE 802.1Q protocol, which is also supported by current Cisco devices.

ISL is a Cisco -developed method of encapsulation for VLANs, it works only with manufacturer-specific routers, switches and LAN cards, Cisco, or with products of licensees. ISL encapsulates the entire layer 2 data frame in an ISL packet ( header and trailer ). Since the entire frame is encapsulated is only possible between ISL - capable components communication. But this, the method works with very low latency and protocol- independent, they can continue any Layer 2 frames just as well as transport all higher packages. Since the encapsulated packets themselves can be transported not encroach on any frame types such as Ethernet or Token Ring. The size of an ISL frame between 94 bytes and can be up to 1548 bytes because of the overhead ( additional header fields ) by the protocol.

Another associated Cisco protocol, Dynamic Inter-Switch Link Protocol ( DISL ) simplifies the creation of ISL connection between two connected Fast Ethernet devices, as only one endpoint must be configured. Fast EtherChannel technology enables the collection of two full- duplex Fast Ethernet cables for broadband backbone connections.

The ISL protocol is to be no longer used in new installations as possible and be in the longer term replaced by the widespread, non-proprietary IEEE 802.1q protocol, according to Cisco.