Inter-Switch Link. Protocol (ISL), a Cisco-proprietary protocol, maintains VLAN information as traffic flows between switches and routers.

Inter-Switch Link (ISL) tagging accomplishes the same task as 802.1Q trunking but uses a different frame format. ISL trunks are Cisco proprietary and define only a point-to-point connection between two devices, typically switches. The name Inter-Switch Link hints at this design. ISL frame tagging uses a low-latency mechanism for multiplexing traffic from multiple VLANs on a single physical path. ISL has been implemented for connections among switches, routers, and network interface cards (NICs) used on nodes such as servers. To support the ISL feature, each connecting device must be ISL-configured. A router that is ISL-configured can allow inter-VLAN communications. A non-ISL device that receives ISL-encapsulated Ethernet frames will most likely consider them protocol errors because of the format and size of the frames.

ISL functions at Layer 2 of the OSI model like 802.1Q, but it differs by encapsulating the entire Layer 2 Ethernet frame inside an ISL header and trailer. Because ISL encapsulates the entire frame, it is protocol-independent and can carry any type of Layer 2 frame or upper-layer protocol between the switches. The encapsulated frames may be token-ring or Fast Ethernet, and are carried unchanged from transmitter to receiver.

ISL has the following characteristics:

• Performed with application-specific integrated circuits (ASIC)
• Not intrusive to client stations; client does not see the ISL header
• Effective between switches, routers and switches, and switches and servers with ISL NICs

Dynamic Inter-Switch Link Protocol (DISL), also a Cisco protocol, 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.

Protocol Structure

ISL header structure:

DA-40-bit multicast destination address.

• Type-4-bit descriptor of the encapsulated frame types-Ethernet (0000), Token Ring (0001), FDDI (0010), and ATM (0011).
• User-4-bit descriptor used as the type field extension or to define Ethernet priorities. This is a binary value from 0, the lowest priority, to 3, the highest priority.
• SA-48-bit source MAC address of the transmitting Catalyst switch.
• LEN-16-bit frame-length descriptor minus DA type, user, SA, LEN, and CRC.
• AAAA03-Standard SNAP 802.2 LLC header.
• HSA-First 3 bytes of SA (manufacturer"s ID or organizational unique ID).
• VLAN-15-bit VLAN ID. Only the lower 10 bits are used for 1024 VLANs.
• BPDU-1-bit descriptor identifying whether the frame is a Spanning Tree bridge protocol data unit (BPDU). Also set if the encapsulated frame is a Cisco Discovery Protocol (CDP) frame.
• INDEX-16-bit descriptor that identifies the transmitting port ID. Used for diagnostics.
• RES-16-bit reserved field used for additional information, such as Token Ring and Fiber Distributed Data Interface (FDDI) frame Frame Check (FC) field.

Related Terms: IEEE 802.1Q, VTP, DTP, Fast Ethernet, Token Ring

Sponsor Source: ISP and DISL are Cisco protocols.

Reference Links:
http://www.cisco.com: Understanding and Configuring VLAN Trunking Protocol
http://www.cisco.com: Configuring VLAN Trunks