Cisco Inter-Switch Link
Encyclopedia
Cisco Inter-Switch Link (ISL) is a Cisco Systems
proprietary protocol
that maintains VLAN information in Ethernet
frames as traffic flows between switches
and routers, or switches and switches.
ISL is Cisco's VLAN tagging protocol and is supported only on some Cisco equipment over Fast and Gigabit Ethernet links. It is offered as an option 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.
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
proprietary protocol
Proprietary protocol
In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.-Enforcement:Proprietors may enforce restrictions through patents and by keeping the protocol specification a trade secret...
that maintains VLAN information in Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
frames as traffic flows between switches
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
and routers, or switches and switches.
ISL is Cisco's VLAN tagging protocol and is supported only on some Cisco equipment over Fast and Gigabit Ethernet links. It is offered as an option to the IEEE 802.1Q
IEEE 802.1Q
IEEE 802.1Q is the networking standard that supports Virtual LANs on an Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames...
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
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
encapsulated ISL frame
Data frame
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
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
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
of the OSI
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
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
EtherChannel
EtherChannel is a port link aggregation technology or port-channel architecture used primarily on Cisco switches. It allows grouping of several physical Ethernet links to create one logical Ethernet link for the purpose of providing fault-tolerance and high-speed links between switches, routers and...
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.