Inter Swich Link Protocol is a Cisco Propriety Protocol

Inter-Switch Link (ISL) protocol is a Cisco propriety protocol and Inter-Switch Link (ISL) is available and supported on Cisco products only. If you need a non-proprietary VLAN protocol, consider using the IEEE 802.1Q protocol.


Inter-Switch Link (ISL) protocol primarily is used for Ethernet media (Fast Ethernet or Gigabit Ethernet). Cisco has also included provisions to carry Token Ring, FDDI, and ATM frames over Ethernet ISL. Inter-Switch Link (ISL) protocol encapsulates the entire Ethernet frame (Fast Ethernet or Gigabit Ethernet) with a 26-byte header and a 4-byte frame check sequence (FCS) for a total of 30 bytes of overhead

The fields of the Inter-Switch Link (ISL) protocol encapsulation frame are given below.

• DA (Destination Address): The destination address uses the multicast MAC address 01-00-0C-00-00-00. The first 40 bits of the DA field signal the receiver that the packet is in Inter-Switch Link (ISL) format.

• Type: The type of frame encapsulated: Ethernet (0000), Token Ring (0001), FDDI (0010), and ATM (0011).

• User: The USER field consists of a 4-bit code. The USER bits are used to extend the meaning of the TYPE field. The default USER field value is “0000”. For Ethernet frames, the USER field bits “0” and “1” indicate the priority of the packet as it passes through the switch.
• SA (Source Address): Source address of the switch transmitting the Inter-Switch Link (ISL) outline.

• Len: The length of the bundle.

• SNAP: Subnetwork Access Protocol (SNAP) and Logical Link Control (LLC). The AAAA03 SNAP field is a 24-bit consistent estimation of “AAAA03”.

• HSA (High Bits of Source Address): The HSA field is a 24-bit esteem which speaks to the upper 3 bytes (the producer ID partition) of the SA field.

• VLAN (Destination VLAN ID): Indicates VLAN ID of the bundle. VLAN ID is a 15-bit esteem that is utilized to recognize outlines on various VLANs. VLAN ID is otherwise called the “shading” of the casing.


• BPDU: Indicate whether a BPDU, or CDP or VTP outline

• Index: The port file of the wellspring of the bundle.

• Res: Reserved field for extra data, for example, Token Ring or FDDI Frame Check Sequence field. For Ethernet, this field ought to be zero.

• Encapsulated Ethernet Frame: The genuine Ethernet outline.

• ISL CRC: Four-byte beware of the ISL bundle to guarantee it isn’t ruined.

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