Cisco Inter-Switch Link (ISL) Explained

ISL stands for Inter-Switch Link. It is a Cisco proprietary trunking protocol. By default, a switch port works only with a single VLAN. A trunking protocol allows it to work with multiple VLANs.

A switch port can work in two modes: access and trunk. In access mode, it sends and receives frames without VLAN information. In trunk mode, it accepts and forwards them with VLAN information.

When a switch port works in access mode, it is called an access port. When it works in trunk mode, it is called a trunk port. By default, all switch ports are access ports. We use access ports to connect end devices such as PCs, Servers, and Printers to the network. We use trunk ports to connect switches.

The switchport mode trunk command converts an access port into a trunk port. This command runs in interface configuration mode. The following commands convert the FastEthernet 0/1 port into a trunk port.

Switch(config)#interface FastEthernet 0/1
Switch(config-if)#switchport mode trunk

A trunk port adds VLAN information to frames before forwarding them. It can use one of two trunking protocols to add VLAN information. These protocols are ISL and 802.1Q. 802.1Q is the default encapsulation protocol. The following command changes the default encapsulation method to ISL.

Switch(config-if)#switchport trunk encapsulation isl

ISL has been deprecated. Cisco has replaced it with the 802.1Q protocol in new devices. If you don't see this command on a new switch, it means the switch does not support it.

How ISL trunking protocol works

ISL trunking protocol uses a 26-byte header and a 4-byte trailer to carry VLAN information with frames. It uses the header to store VALN information. Each header contains a 10-bit VLAN ID. In addition to the header, it appends a 4-byte CRC to the end of each frame. This CRC is in addition to FCS that the Ethernet frame requires.

On the sending device, it adds a header and trailer to frames. On the receiving device, it uses the VLAN ID stored in the header to identify the VLAN the frame belongs to. It uses CRC to verify the state of the frame. It removes the header and trailer from frames before forwarding them to their native VLAN ports.

Let us take an example.

The following image shows a network. This network has two ethernet switches with VLAN configuration. Both are connected through a trunk link. Trunk ports on both switches using ISL trunking protocol. Both have end devices from VLAN-10.

PC-A sends a broadcast message. The broadcast message reaches the S1 switch's F0/1 port. The S1 switch checks the configured VLAN on the F0/1 port. VLAN-10 is configured on the F0/1 port. S1 forwards the broadcast message from all ports that belong to VLAN-10. A trunk port belongs to all VLANs. It forwards frames of all VLANs to the device connected on the other end of the trunk link. It encapsulates all frames with an ISL header and trailer before forwarding them.

In this example, S2 is connected to the other end of the trunk link. It receives encapsulated frames from S1 and reads the ISL header of each frame to know its VLAN. After reading VLAN information from each frame, it removes the ISL header and trailer from the frame and forwards it from the ports that belong to the frame's VLAN. For example, if the frame belongs to VLAN-10, it forwards the frame from ports that belong to VLAN-10.

Key points: -

• ISL is a Cisco proprietary protocol.
• ISL works only on Cisco routers and switches.
• ISL works only with Fast and Gigabit Ethernet links.
• ISL adds a 26-byte header and a 4-byte CRC trailer to the frame.
• ISL functions at the data-link layer of the OSI model.
• This protocol has been deprecated. Cisco has replaced it with the 802.1Q protocol.