Inter-Switch Link (ISL) and IEEE 802.1Q Frame tagging protocols

Inter-Switch Link (ISL)

VLAN tagging protocol is Inter-Switch Link (ISL). This protocol is Cisco proprietary protocol, means that, unlike 802.1Q, it can have used only between Cisco switch. It is considering to be despise, and newer Cisco switch do not even support it.

ISL work by encapsulating frame in ISL header and trailer. The encapsulated frames remain unchanged. The VLAN ID is included in ISL header.

Original frame:

ISL frame:

IEEE 802.1Q

IEEE 802.1Q is one of the VLAN tagging protocol and supported by Cisco switch. Dot1Q standard was develop by Institute of Electrical and Electronics Engineers (IEEE), so it is an open standard protocol and can be used on non-Cisco switch.

To determine to which VLAN a frame belongs, a field is inserted into the frame header.

Original frame:

802.1Q frame:

An example will attempt to simplify the concept of dot 1q. Let us say that we have network of 2 switch and 4 host. And hosts A and host D are in a VLAN 2, while host B and C are in VLAN 3.

On the network segment between two switch, a process called VLAN trunking. Let’s say a host A sends broadcast frame. SW1 “tags” frame by inserting VLAN ID in the header of frame before sending frame to SW2. SW2 receives frame and knows that the frame belongs to the VLAN 3, so it sends the frame only to host D, since that hosts is in VLAN 3.