Cisco Inter-Switch Link
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ISL is Cisco's VLAN Encapsulation protocol and is supported only on some Cisco equipment over 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. In the case of ISL the tag is external to the Ethernet frame, which effectively is the same as encapsulating the Ethernet frame, whereas with IEEE 802.1Q the tag is internal. This is a key advantage for IEEE 802.1Q as it means tagged frames can be sent over standard Ethernet links.
The size of an Ethernet encapsulated ISL frame can be expected to start from 94 bytes and increase up to 1548 bytes because of the overhead (additional fields) the protocol creates via encapsulation. ISL adds a 26-byte header (containing a 15-bit VLAN identifier) and a 4-byte CRC trailer to the frame. ISL functions at the Data-Link layer of the OSI model. ISL is used to maintain redundant links.
Another related Cisco protocol, Dynamic Inter-Switch Link Protocol (DISL) 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.
- CCNA Exploration LAN Switching and Wireless course, v 4.0, sec 3.2.3