Cisco Inter-Switch Link

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Cisco Inter-Switch Link (ISL) is a Cisco Systems proprietary protocol that maintains VLAN information as traffic flows between switches and routers.

ISL is Cisco's VLAN encapsulation method and supported only on Cisco's equipment through Fast & Gigabit Ethernet links. The size of an ISL frame can be expected to start from 94 bytes and increase up to 1548 bytes due to the overhead (additional fields) the protocol creates via encapsulation. ISL adds a 26-byte header (containing a 10-bit VLAN identifier) and a 4-byte CRC trailer to the frame. ISL functions at the Data-Link layer of the OSI model.

Another related Cisco protocol, Dynamic Inter-Switch Link Protocol simplifies the creation of an ISL trunk from two interconnected Fast Ethernet devices. Fast EtherChannel technology enables aggregation of two full-duplex Fast Ethernet links for high-capacity backbone connections. DISL minimizes VLAN trunk configuration procedures because only one end of a link needs to be configured as a trunk.

Cisco's ISL competes with the IEEE 802.1Q protocol, a widely used non-proprietary VLAN tagging protocol.