Cisco Inter Switch Link
A Cisco proprietary link layer protocol called Cisco Inter Switch Link (ISL) was created to preserve Virtual Local Area Network (VLAN) data inside Ethernet frames while traffic moves between switches or between switches and routers. It serves as Cisco’s VLAN encapsulation technology and is only compatible with specific Cisco devices via Fast and Gigabit Ethernet connections.
Purpose and Functionality
Trunking, which enables several VLANs to share a single physical link between network switches, is the main function of ISL. By doing this, a fast link is produced that transports data for multiple VLANs without combining them. ISL is used to control traffic for these several VLANs when a switch port is set up as a trunk port. ISL functions at the OSI model’s data-link layer and in a point-to-point VLAN environment.
How ISL Works: Encapsulation
ISL works by adding an extra header and trailer to an initial Ethernet frame. Because of this encapsulation technique, the original Ethernet frame inside the ISL packet is unaltered.
Header: ISL gives the Ethernet frame a 26-byte header. A 15-bit VLAN Identifier (VLAN ID), which identifies the particular VLAN to which the frame belongs, is one of the most important pieces of information contained in this header. The “colour” of the frame is another name for the VLAN ID. Additionally, the header contains information on the ISL frame itself, such as the source and destination addresses. One of two multicast MAC addresses unique to ISL (e.g., 01-00-0C-00-00-00) is the destination address, while the transmitting switch’s address is the source address in the ISL header.
Trailer: The frame ends with a 4-byte Cyclic Redundancy Check (CRC) or Frame Check Sequence (FCS) trailer. This CRC ensures that the ISL packet is not corrupted by doing an extra check on top of any frame checking that the Ethernet frame itself requires.
Each Ethernet frame gains 30 bytes of overhead due to this encapsulation process (26 bytes for the header and 4 bytes for the trailer). An Ethernet encapsulated ISL frame can therefore be anywhere from 94 and 1548 bytes in size.
ISL encapsulation is introduced when a frame is to be sent out a port that is set up as a trunk link. The ISL encapsulation is eliminated if the frame is sent out over an access link port.
Also Read About CISCO Switch Configuration: A Safe and Effective Setup Guide
Important Features and Protocol Areas
ISL can accommodate up to 1000 VLANs. In addition to the VLAN ID, the ISL protocol encapsulation contains the following fields:
- The destination address, or DA, is as follows: The multicast MAC address 01-00-0C-00-00-00 is used by the destination address. The receiver is informed that the packet is in Inter-Switch Link (ISL) format by the first 40 bits of the DA field.
- Type: The type of encapsulated frame is indicated by this parameter. Ethernet (0000), Token Ring (0001), FDDI (0010), or ATM (0011) could be the frame type.
- User: A 4-bit code makes up the USER field. The TYPE field’s meaning is expanded using the USER bits. “0000” is the default value in the USER field. The bits “0” and “1” in the USER field of Ethernet frames represent the packet’s priority as it moves across the switch.
- SA (Source Address): The Source Address is the address of the switch that is sending the Inter-Switch Link (ISL) packet.
- Len: The packet’s length is indicated by the Len field.
- SNAP (Subnetwork Access Protocol): “AAAA03” is a 24-bit constant number that is used in the AAAA03 SNAP field.
- HSA (High Bits of Source Address): The top three bytes of the SA field, or the manufacturer ID section, are represented by the 24-bit HSA field.
- BPDU: Determines whether the frame is a VTP, CDP, or BPDU frame.
- Index: The source’s port index.
- Res: A field set aside for extra data, such as the FDDI Frame Check Sequence or Token Ring fields. This field should be zero for Ethernet.
Related Protocol
By reducing the number of VLAN trunk configuration steps, the Cisco Dynamic Inter-Switch Link Protocol (DISL) makes it easier to create ISL trunks between two linked Fast Ethernet devices.
Advantages of ISL
- VLAN Tagging: Multiple VLANs can be multiplexed over a single trunk link to VLAN tagging, which also makes it simple for switches to identify VLAN membership.
- Interconnection: Allows several switches to be connected together while preserving VLAN data.
- Performance: Offers low latency and full wire-speed performance.
- Efficiency: Users can access servers quickly and efficiently since there is no longer a need for a router for inter-VLAN communication.
Disadvantages and Deprecation
ISL has serious disadvantages despite its benefits:
- ISL is only compatible with Cisco switches because it is a Cisco-proprietary protocol. It is necessary to use the open standard IEEE 802.1Q for trunking across devices from different suppliers.
- Overhead: In contrast to IEEE 802.1Q, which merely adds a 4-byte tag, it adds a greater overhead (26-byte header + 4-byte trailer).
- VLAN Limit: IEEE 802.1Q supports up to 4096 or 4094 VLANs, however ISL supports up to 1000.
Cisco has mostly discontinued ISL as a result of these restrictions and other difficulties with VLAN trunking. Nowadays, Cisco aggressively supports the usage of the industry-standard IEEE 802.1Q protocol for VLAN trunking, even if older Cisco switches frequently do not support it. Older Cisco networks may experience ISL, however 802.1Q is nearly always used in modern deployments.