Inter-Switch Link (ISL)

Because switches operate at Layer 2, a switch cannot forward data from one VLAN to another VLAN without a Layer 3 device, such as a router or internal route processor, to perform a Layer 3 resolution function. An Access layer port can carry the traffic of only one VLAN; a port that is trunked using ISL can carry more than one or all of the VLANs configured on the switches.

Inter-Switch Link (ISL) is a Cisco proprietary protocol used to interconnect two devices with a trunk link to carry multiple VLANs. ISL must be configured on a 10/100BaseTX port between two Cisco devices that support ISL. ISL operates in a point-to-point VLAN environment supporting up to 1000 VLANs. You can define VLANs as you would logical networks.

ISL is available on Cisco routers beginning with the 2600 series. Gigabit Ethernet trunk links default to using the IEEE 802.1Q encapsulation for creating a trunk port. Ethernet has the ability to negotiate whether ISL or 802.1Q encapsulation methods are being used. Fast Ethernet, however, defaults to using ISL as the trunking encapsulation method.

When you configure ISL between two devices, some specific rules must be followed:

• Use ISL only on 100Mbps or higher speed links. (A 10Mbps link can be used, but this choice is not recommended.)

• Verify that you have sufficient memory on the switch or router, because ISL encapsulates and de-encapsulates the frame needing higher processing overhead.

• Use ISL+ to encapsulate Token Ring.

• Configure ISL links as point-to-point links.