The article written here describes the Inter-switch link (ISL) VLAN (Virtual Local Area Network) tagging and its uses in detail. For the functioning of the frame tagging in VLAN (Virtual Local Area Network) we use a proprietary protocol.

One such protocol is named Inter-Switch Link (ISL). Inter-Switch Link protocol can only be used with Cisco switches because they are a proprietary protocol. The Inter-Switch Link (ISL) has ISL header and ISL trailer.

The ISL (Inter-Switch Link) works by combining or wrapping the Ethernet frame in the ISL (Inter-Switch Link) header and the ISL trailer. The ISL (Inter-Switch Link) header contains the VLAN (Virtual Local Area Network) id. The wrapped Ethernet frames will not change, they remain constant.

The frame before wrapping contains destination MAC, source MAC, length/type, data; FCS after wrapping with the ISL header the frame contains the ISL header, destination MAC, source MAC, length/type, data, FCS and ISL FCS. About 1000 VLANS are supported by this ISL.

This ISL can only be used in the type like Fast Ethernet type and the Gigabyte Ethernet links. In the ISL header it contains the header part is 24 byte and the Frame Check Sequence (FCS) is 4 byte, the overall byte is 30 byte contains in the ISL header section.

Mainly ISL (Inter-Switch Link) are function occurs in the OSI (Open Systems Interconnection) model. In the OSI Open Systems Interconnection) model data link layer is the layer used for ISL functioning. ISL manages the VLAN (Virtual Local Area Network) information’s in the frames used.

Inter-Switch Link Protocol (ISL) fields are described briefly in the below part, they are:

Destination Address (DA) – multicast MAC addresses are used in the Destination address. 40 bits of fields are available in the destination address field. The packets send the signal in the ISL format.

Type- the type of field describes which type of frame is used to the combination. Examples provided are Ethernet (0000), Token Ring (0001), FDDI (0010), and ATM (0011).

User- four-bit codes are used in the user field. This field is used to expand the type of field available. 0000 is the default user value.

Source Address (SA) – this field contains the address of the source and is used in ISL (Inter-Switch Link).

Len- packet length is described in this field.

HSA- High bits of Source Address is a 24-bit field which represents the upper 3 layers.

VLAN (Destination VLAN id)-VLAN (Virtual Local Area Network) contains the VLAN id of the packet. It contains a 15-bit field and this field is used to identify different VLANs (Virtual Local Area Network) used.

Also Read: 

BPDU- which frame is used is identified.

Index- it contains the source address index number.

Res- For the additional information, to store it the field used is the reserved field.

ISL CRC- to check that whether the information’s in the packets are corrected or any error occurs, for knowing it such fields are used. It is a 4-byte field.