Cisco Inter-Switch Link (ISL) is a
Cisco Systems
Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational digital communications technology conglomerate corporation headquartered in San Jose, California. Cisco develops, manufactures, and sells networking hardware, ...
proprietary protocol
In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.
Intellectual property rights and enforcement
Ownership by a single organization gives the owner the ability to place restricti ...
that maintains
VLAN
A virtual local area network (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer ( OSI layer 2).IEEE 802.1Q-2011, ''1.4 VLAN aims and benefits'' In this context, virtual, refers to a ph ...
information in
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...
frames as traffic flows between
switches
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
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
In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use ...
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.
[CCNA Exploration LAN Switching and Wireless course, v 4.0, sec 3.2.3]
With ISL, an Ethernet frame is encapsulated with a header that transports VLAN IDs between switches and routers. 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.
ISL does add overhead to the frame as a 26-byte header containing a 10-bit VLAN ID. In addition, a 4-byte CRC is appended to the end of each frame. This CRC is in addition to any frame checking that the Ethernet frame requires. The fields in an ISL header identify the frame as belonging to a particular VLAN.
A VLAN ID is added only if the frame is forwarded out a port configured as a trunk link. If the frame is to be forwarded out a port configured as an access link, the ISL encapsulation is removed.
The size of an Ethernet encapsulated ISL
frame
A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent.
Frame and FRAME may also refer to:
Physical objects
In building construction
*Framing (con ...
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
The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer pr ...
of the
OSI model.
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.
See also
*
Dynamic Trunking Protocol
The Dynamic Trunking Protocol (DTP) is a proprietary networking protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, and for negotiating the type of trunking encapsulation to be u ...
(DTP)
References
{{reflist
External links
Inter-Switch Link and IEEE 802.1Q Frame Format
Inter-Switch Link