Cisco Inter-Switch Link (ISL) Explained

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.

Access ports connect the switch to end devices such as PCs, Servers, and Printers to the network. Trunk ports connect it to other switches or routers. The switchport mode trunk command converts an access port into a trunk port. This command runs in interface configuration mode. For example, 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. ISL stands for Inter-Switch Link. It is a Cisco proprietary trunking protocol. 802.1Q is the default encapsulation protocol. The following command changes the default encapsulation protocol to ISL.

Switch(config-if)#switchport trunk encapsulation isl

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

How does the ISL trunking protocol work

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 the FCS that the Ethernet frame requires.

The sending device adds a header and trailer to frames. The receiving device uses the VLAN ID stored in the header to identify the VLAN to which the frame belongs. 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.

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

PC-A sends a broadcast message. The broadcast message reaches the F0/1 port of S1. S1 checks the configured VLAN on the F0/1 port. F0/1 port belongs to VLAN-10. S1 forwards the broadcast message from all ports in 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. S2 receives encapsulated frames from S1 and reads the ISL header of each frame to know its VLAN ID. After reading VLAN information from each frame, it removes the ISL header and trailer from the frame and forwards it using ports functioning in 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.
• Cisco has replaced it with the 802.1Q protocol.

Conclusion

ISL is an encapsulation protocol. It allows switches to exchange VLAN information. It works only on trunk ports. On the sending device, it inserts VLAN information in the frame header. On the receiving device, it removes that information and reassembles the original frame header. This protocol is no longer in use. Cisco has replaced it with 802.1Q.