Cisco Inter-Switch Link (ISL) is a Cisco proprietary link layer protocol that maintains VLAN information in Ethernet frames as traffic flows between switches and routers, or switches and switches. ISL is Cisco's VLAN encapsulation protocol and is supported only on some Cisco equipment over the Fast and Gigabit Ethernet links. It is offered as an alternative to the IEEE 802.1Q standard, a widely used VLAN tagging protocol, although the use of ISL for new sites is deprecated by Cisco.